contribution from subrata chakraborty, ph.d. (affiliation ... · 4, songs task force (technical...

1, SONGS Task Force (Technical Committee), Contributed by Subrata Chakraborty

Contribution from Subrata Chakraborty, Ph.D. (Affiliation: University of California, San

Diego)

Contributed to Rep. Mike Levin’s Task Force (Technical Committee) on San Onofre

Decommissioning (Process, Material and Natural Factors)

Disclaimer: I address these questions based on my analysis of relevant peer-reviewed and

published literature and government documents as referred in the document and listed at the end.

Since we are independently evaluating the entire system, these analyses are NOT based on Holtec

provided analysis, I have only taken information from Holtec’s documents when physical

parameters were required. This is my independent research and UCSD is not directly or indirectly

involved. Figures and Tables are sequentially numbered following the question area and question

number (i.e., Figure 1 of Material Science question 1 is numbered as MS-1-1).


MS-1: What are fuel and canister system (including the concrete) failure mechanisms,

processes, and consequences: Embrittlement, cracking/gouging, corrosion (water, galvanic)?

There are several parts to this question and they are addressed separately as follows.

Fuel failure: There are two fundamental reactions that take place in a reactor that determine the

composition of the fuel: (i) fission of fissile nuclides, such as U-235 and Pu-239, and (ii) neutron

capture and sequential -decay reactions that create transuranium isotopes, mainly Pu-239 from

U-238. Hence, the Pu concentration in the fuel increases with time, and fissile Pu-239 provides up

to one-third of the energy generated in a typical light-water reactor (LWR, SONGS had pressurized

water reactor (PWR), that is a variety of LWR). The final composition of the fuel depends on the

fuel type, chemical composition, level of initial enrichment in U-235, neutron energy spectrum

and the extent of fission of the fuel (the burn-up level).

Figure MS-1-1. Understanding the microstructure of the spent nuclear fuel (SNF) after irradiation

inside the reactor. (a) schematic showing the grain boundary, gas bubbles, oxide layers. (b) Cross-

section of a fuel pellet (diameter = 1 cm), the fractures are created by the steep thermal gradient from

the center of the pellet to its edge. Volatile fission product elements segregate in these fractures and

become part of the instantaneous release fraction. (c) Higher-magnification image of individual grains

in a reactor fuel sample, demonstrating the segregation of fission product gases into bubbles that

decorate the surfaces of grains of UO2 in the fuel. Figure taken from Ewing, Nature Materials, 2015.


Figure MS-1-1 shows a structure of reactor irradiated fuel [Ewing, 2015] and the inventory is

shown in Figure MS-1-2 [Bayssie et al., 2009]. The initial level of radioactivity of the irradiated

fuel is very high, caused mainly by the presence of the 3 to 4 atom-% of fission products (e.g., I-

131, Cs-137 and Sr-90) and activation products (for example, Co-60, Ni-63), with a longer-lasting

contribution from long-lived transuranium elements (e.g., Pu-239, Np-237 and Am-241). At the

end of the fuel’s useful life in the reactor, about 96% of the spent nuclear fuel (SNF) is UO2. The

remaining balance consists of fission products, transuranium elements and activation products, but

these elements occur in many different phases, and over differing structural length scales: (a)

fission product gases, such as Xe, I and Kr, occur as finely dispersed bubbles in the fuel grains;

(b) metallic fission products, such as Mo, Tc, Ru, Rh and Pd, form immiscible metallic precipitates

(ε-particles) that are nanometres to micrometres in size; (c) fission products form oxide precipitates

of Rb, Cs, Ba and Zr; (d) some fission product elements, such as Sr, Zr, Nb and lanthanides, can

form solid solutions with the UO2 fuel; and (e) transuranium elements can substitute for U in the

UO2.

The distribution of elements is not homogeneous within a single pellet (Figure MS-1-1) because

of the steep thermal gradients that exist (as high as 1,700 °C at the center of the pellet and

decreasing to 400 °C at its rim). Thermal excursions during reactor operation can additionally

cause a coarsening of the grain size, as well as extensive micro-fracturing (Figure MS-1-1).

Volatile elements, such as Cs and I, may also migrate to grain boundaries, fractures and the gap

between the edge of the fuel pellet and the surrounding metal cladding. The extent of burn-up is

also not uniform across the fuel pellet; higher burn-up at the edge of the pellet leads to higher

Figure MS-1-2. Pie chart showing the inventory in spent nuclear fuel-pellets in moderate burn-up

(taken from Bayssie et al., 2009).


concentrations of Pu-239, an increase in porosity and a reduction in grain size of the UO2 grains

(~0.15 to 0.3 μm). Thus, the SNF has complex chemistry and phase distribution, resulting from its

thermal history, burn-up and initial composition. All these factors have to be considered when

understanding the long-term evolution of fuel [Ewing, 2015].

Summary: The SNF is a complex and actively evolving material, with changing radiotoxicity

and chemical toxicity over time and due care required in handling. Based on the available

literature, it does not seem possible to start a self-sustaining chain reaction within the

canister while stored in normal conditions (i.e., would not reach criticality) and while only

the fuel degradation is considered. However, cladding failure could release radioactivity

inside the canister. If canister systems are deemed to be 100% fail-proof, there will be no

issue with the radioactivity release inside the canister as that would not expose the

surroundings. However, as we will discuss, the canister failure issues in the following section,

the release of radioactivity to the environment remains to be a matter of major concern.

Canister failure:

United States Nuclear Waste Review Board nicely summarized the potentials of dry storage

canister failure modes [Rigby and Members, 2010]. This analysis mostly adopts Review Board’s

analysis (with references therein) with the addition of a few recent pieces of literature and focused

only on the outer surface of the canister (Figure MS-1-3). It is mentioned in NRC’s letter [NRC,

2019] that the canister surfaces were laser peened. This is a new technique and reduces the

possibility of stress corrosion cracking (SCC) [Hackel et al., 2018; Sathyajith and kalainathan,

2015]. In this technique, the outer layer (about a millimeter (mm)) goes through plastic

deformation and reduces the stress. However, according to a white paper [MPR_White_Paper,

2018] prepared by MPR, Inc for Edison, the welds of the canisters are all laser peened, and not the

entire canister. This is contrary to the NRC’s letter [NRC, 2019], which states that the canister

surfaces are laser peened and thus creates a protective coating and resist againt scratching and

gouging during the downloading process. This descriptional difference is significant, while laser

peened welds are robust against SCC as potential corrosion site, as the total surface is not peened,

hence any scratch would be a new potential site for SCC.


Atmospheric Chemistry Corrosion Environment. A dry-storage nuclear waste cask or canister

exposed to the atmosphere for decades to a few centuries will be exposed to a

corrosiveenvironment determined by virtually all chemical species in the atmosphere and the

effects of gamma radiation. This exposure will be enhanced by the natural convection cooling of

the canister where air, along with dust, will flow along the metal surface. The flow of air will

continually bring dust into contact and deposition with the metal surface, and the dust will contain

chlorides, nitrates, salts, insoluble inorganics, and organics. Gamma radiation will be present and

will result in chemical reactions occurring in the local air and dust; the result will be the production

of a spectrum of chemical species. Hydrogen chloride, sulfuric acid, nitric acid and virtually all

oxides of nitrogen occur in the atmosphere as a result of chemical reactions. These three acids will

react with calcite and dolomite (magnesium, present in dust) and produce the corresponding

chloride, nitrate and sulfate salts. Along with inorganic compounds, organics are ubiquitous in

atmospheric dust. It is documented that biofilms are capable of influencing electrochemical

processes at the metal surface, often leading to the deterioration of metals referred to as

biocorrosion or microbiologically influenced corrosion. Biofilms typically consist of microbial

cells and their metabolic products including extracellular polymers, and inorganic precipitates.


Interaction of biofilms and exopolymers with metal ions has long been proposed as one of the

mechanisms of metal biodeterioration [Beech, 2004].

Coupled with the inorganic and organic chemistry, there is a gamma radiation field at the metal

surface of a dry-storage container. This gamma field should be of interest for susceptibility to

stress corrosion cracking

because of gamma-field-induced chemistry changes and there is

observational evidence regarding that. On top of the radiation field effect, it should be noted that

the chemical reaction rates increase with increasing temperature. All these atmospheric corrosion

factors outlined above are potentially viable for SONGS ISFSI canisters and supply the ingredient

for different types of corrosion outlined below with their effects.

Pitting and Crevice Corrosion. Pitting corrosion is a localized form of corrosion by which

cavities or "holes" are produced in the material. Pitting is considered to be more dangerous than

uniform corrosion damage because it is more difficult to detect, predict and design against. A

small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering

system. There are several pathways for the initiation of this kind of corrosion- (i) Localized

chemical or mechanical damage to the protective oxide film, water chemistry factors which can

Figure MS-1-3. Schematics of canister cooling systems: (left) canister wall natural diffusive cooling

(inlet and outlet air vents are at the top; (right) flow-dynamics of inner canister cooling (figure taken

from Holtec International).


cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend

to render a protective oxide film less stable) and high concentrations of chloride (as in seawater).

(ii) The presence of non-uniformities in the metal structure. Non-uniformities might come from

external treatment of the surface (scratches and gouges).

Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to,

the gap or crevice between two joining surfaces. The gap or crevice can be formed between two

metals or a metal and non-metallic material. Outside the gap or without the gap, both metals are

resistant to corrosion. Crevice corrosion is initiated by a difference in concentration of some

chemical constituents, usually oxygen, which set up an electrochemical concentration cell. Outside

of the crevice (the cathode), the oxygen content and the pH are higher - but chlorides are lower.

Chlorides concentrate inside the crevice (the anode), worsening the situation. Ferrous ions form

ferric chloride and attack the stainless steel rapidly. The pH and the oxygen content are lower in

the crevice than in the bulk water solution, just as they are inside a pit.

Stress corrosion cracking (SCC). SCC in metals can occur from the combined effect of tensile

stress and the presence of a water-soluble chloride salt. Bulk liquid water need not be present

because an aqueous environment will always exist on metal surfaces exposed to the atmosphere

due to sorption of moisture from the air. This moisture sorption phenomenon is sometimes called

"physisorbed" water. This type of corrosion also is referred to as chloride-induced stress corrosion

cracking. SCC is considered corrosion with local slip at the crack tip and is often found to initiate

where pitting or crevice corrosion has occurred. Air cooling of stainless steel canister walls means

that the wall is in direct contact with moist air that will contain salt, including chlorides. Austenitic

stainless steels are susceptible to chloride stress corrosion cracking starting on the outside surfaces

in certain humid marine air environments under tensile stress. Residual tensile stresses in the

storage canister are mostly derived from welding between the wall and lids and cold working of

the metal. As the temperature of the canister decreases, condensation can occur resulting in the


accumulation of corrosive media such as chlorides, particularly in marine or industrial

environments.

A metal canister’s corrosion lifetime for CSCC corrosion can be estimated as the time needed for

the canister temperature to decline to the point where condensation is possible, which can be above

100ºC, plus the time for SCC corrosion to initiate, plus the time needed for SCC corrosion to

propagate through the canister wall thickness. Figure MS-1-4 shows the predicted rate of

propagation of cracks once formed in stainless steel (SS316) based on measured values [Ilgen et

al., 2015].

A series of natural exposure and accelerated corrosion tests of conventional stainless steel used

spent fuel canisters were conducted recently by the Central Research Institute of Electric Power

Industry in Japan [Kosaki, 2008]. Natural exposure tests were conducted at Miyakojima, an island,

one of the most corrosive areas in Japan, and accelerated corrosion tests were conducted in an

environment filled with NaCl steam mist at 60ºC and humidity of 95 percent. One set of

experimental tests on types 304 and 316 stainless steel yielded CSCC corrosion initiation times

ranging from about 1.6 to 3 years under natural exposure conditions. Pitting or crevice corrosion

Figure MS-1-4. SCC propagation rates for atmospheric corrosion of 304SS and 316SS. Time to

failure corresponds to the time required to penetrate a 0.625' thick canister wall (llgen et al., 2015).


was found to be a trigger to CSCC because CSCC initiation was found to start from the bottom of

the corrosion area by pitting or crevice corrosion. Under natural exposure conditions, the CSCC

corrosion rate varied from about 0.04 to 0.6 mm/year over a range of residual tensile stresses,

which would take from about 25-375 years to penetrate through a thickness of thin-wall canister

(~0.6 inches).

Cooling system failure via vent blockage. One of the selling points of dry cask storage is its

passive cooling system. The outer surface is being cooled via the diffusive flow of marine airmass

of ambient temperature through inlet vents and relatively hot air exit from the outlet vent (location

of both of these vents are located at the top as shown in Figure MS-1-3). There is a significant

chance of clogging the vents (partially or fully). There is genuine potential for such situations.

There are screens installed in the vents to prevent clogging of the vents and the latest FSAR [FSAR,

2018] proposed mitigation of this situation by cleaning the screen as quickly as possible. This

mitigation strategy only works if the screens accumulate debris over time. Presence of a big

tsunami wave or even mud-slide (as this is a flood basic as mentioned in the geology section) a

huge amount of mud and silt/sand could get pass these screens and accumulate at the bottom of

the UMAX hole and could potentially block the airflow system (partially or fully). This kind of

scenario is not out of question as the region saw extreme landslides in the past decades (especially

during 1997-’98 El-Nino event as described in the Geology section). Such extreme but likely

scenarios are not addressed in FSAR [FSAR, 2018] and if such an event does occur that would

block the air vents and simple cleaning of screen would not be effective mitigation. There are

modeling studies to evaluate the dry cask cooling/ventilation system, one of which shows that

clogging the outlet vents is more crucial than clogging the inlet vents and without the proper

cooling, the fuel temperature might rise above the threshold point (~673 K) degrading the cladding,

Thermal simulation of a container with SNF accident conditions shows that the maximum

temperature can be higher than safety criteria limits [Alyokhina, 2018].

Helium leakage from the steel canister. According to the standard operational procedure, after

loading SNF inside the canister under thespent fuel pool water, canisters are thoroughly dried and

filled with 2-atmospheric pressure of helium and welded shut and leak tested. There always

remains a practical possibility for an undetected leak (tiny and under the detection limit). Also,

there is always a chance of development of micro-level leakage in the canister. Any leak in the


canister would result in helium loss, which is the active material inside the canister to maintain the

effective heat transfer from the spent fuel rod to the canister body [Penalva et al., 2017]. Therefore,

any leaks from the container would result in a cooling degradation (might be over a few years of

time scale) of fuel that might jeopardize fuel integrity if the temperature exceeded a threshold

value.

Summary: The canister failure could occur in several ways- corrosion cracking and air inlet

blockage are the two major ones. The research shows that even SS316L is not corrosion-

resistant. The welds of the canisters used in SONGS are treated with laser peening and that

process is supposed to modify the weld-surfaces and improve the performance against stress

corrosion cracking. However, the entire body of the canisters are not treated via this process,

the scratches and gouges created during the downloading process might create the potential

sites for pitting crevice corrosion. Althrough these canisters are exposed to the corrosive

marine environment and thus prone to atmospheric corrosion. One of the major ways of

blocking air cooling vents is through the flash flood (if not unexpected Tsunami waves).

These events carry silt and sands and the present first line of defense to protect the air vents

are the installed screens, which are inadequate for such debris. In such an event the debris

could fill the bottom of the UMAX cavity and block the airflow. The research shows that the

air vent blockage could potentially raise the temperature of the canister exceeding the safety

limit. There is no mitigation strategy existing in Edison’s FSAR, as the FSAR does not

include such scenario. The SONGS’ ISFSI is located in a flood plain and in the past decades

such an event took place (during 1997-’98 El-Nino year) and must be considered. No study

was found which calculates the internal pressure of the canister due to fuel failure and fission

gas leakage and hydrogen desorption inside the canister, thus it was not possible to perhaps

state the status of the canister if the design criticality is reached due to an off-normal

condition. Moreover, these canisters are welded shut with at least 2-atmosphere of helium.

Though initially leak checked, any undetected leak (tiny and under the detection limit, initial

or post-developed) of the canister would yield a helium loss, which is the active material

inside the canister to maintain the effective heat transfer from the spent fuel rod to the


canister body. Thus, potential leaks from the container would result in a cooling degradation

of fuel that might jeopardize fuel integrity if temperature exceeded a threshold value.

Concrete failure:

United States Nuclear Waste Review Board nicely summarized the potentials of dry storage

canister failure modes [Rigby and Members, 2010]. I mostly adopted their analysis (with references

therein) with the addition of a few recent pieces of literature.

Concrete is used in the overpack of the canisters and in the foundation pads for the stored canisters.

SONGS ISFSI is an underground system and basically a concrete block with cavities for canister

storage. The degradation of concrete is caused by many physical and chemical mechanisms, some

of which act in concert. The age-related mechanisms may degrade concrete and reinforcing steel

during dry storage period, in a couple of decades. Other degradation mechanisms may be

experienced by concrete in contact with the ground, such as the foundation pads, because of

groundwater and chemicals in the soil as outlined below.

Elevated temperature. Concrete that is exposed to high temperatures will lose moisture and

eventually can induce thermomechanical destruction of both the internal cement structure and the

bonds between cement and aggregate. Undesirable chemical reactions between the cement and

aggregates also can occur. All of these types of degradation significantly decrease concrete

strength, elastic stiffness, and toughness. For example, at 150ºC (300ºF), the mean concrete

strength is about 80 percent of its normal value. Generally, the threshold of degradation in the

concrete is at a temperature range of 66ºC to 95ºC (150ºF to 180ºF). Because the concrete in

SONGS ISFSI will reach the temperature in this range [FSAR, 2018], the concrete would need to

handle such temperatures, as well as conditions of natural cooling mechanisms failure. Exposure

to elevated temperature could be a significant age-related degradation mechanism for the concrete

enclosures. Creep of concrete at an elevated temperature, under a sustained load could have an

adverse effect on the concrete structure over time. Furthermore, if air vents are plugged because of

some natural or human-caused off-normal scenario, concrete temperatures could rise significantly

above the normal operating range. Degradation of concrete would affect its strength and might not

be able to withstand earthquake-related shock.


Freeze-Thaw Cycles. In warm climates, such as at SONGS, this degradation mechanism will not

be effective.

Leaching of Calcium Hydroxide.

Climates with significant rainfall provide a source of flowing

water on concrete surfaces and in cracks or joints that can leach out calcium hydroxide (white

hydrated lime) from the concrete. Removal of calcium hydroxide can significantly weaken

concrete mechanically and leaching over long periods increases the porosity and permeability of

concrete, making it more susceptible to leaching and freeze-thaw damage. This kind of damage is

also not of importance for SONGS.

Chemical Attack. Chemical attacks on concrete derive from hydrated carbon dioxide in the air

and hydrated sulfates in the soil. Both can lead to local acid solutions in concrete that react with

the high alkalinity calcium hydroxide (lime) of concrete (pH > 12.5) to convert the calcium

hydroxide to calcium carbonate (a process called “carbonation”). Fully “carbonated” concrete

results in concrete pH levels slightly above 8. The carbonation progression inward in concrete

proceeds at a slow but fairly steady rate, on the order of 1-2 mm per year.

Whereas carbonation

tends to decrease porosity and increase concrete density and strength, sulfate attacks usually

increase the porosity and permeability of concrete and can lead to decreased density and

compressive strength. Sulfate reactions are accompanied by expansive stress within the concrete,

which can lead to spalling, cracking, and strength loss.

The presence of chlorides also lowers the

pH of concrete and so can lead to corrosion of reinforcing steel. For concrete that is obscured from

view, soil sulfate attacks may not be noticeable until damage is significant, which is one of the

concerns for the underground storage that is being employed at SONGS.

Corrosion of Reinforcing Steel. For the reinforcing steel bars in concrete to corrode, diffusion of

both chloride ions and carbon dioxide into the concrete is required at particular concentration

thresholds. The high alkalinity of concrete (pH >12.5) protects the embedded reinforcing steel

from corrosion. However, when the local pH near the steel is decreased below the threshold level

of about 11.5, then chemical corrosion initiates more easily. Initiation of chemical rebar corrosion

also requires a chloride threshold concentration, which is on the order of 0.06 percent by weight

of concrete for black steel. The pH reduction occurs by the intrusion of aggressive ions, primarily

chlorides, in the presence of oxygen. The transport of chloride ions in intact concrete occurs by

ionic diffusion according to Fick’s Second Law of Diffusion. This means that the higher the


chloride concentration in the seawater next to concrete, the faster the chloride ion diffusion will

occur in the concrete. Accordingly, concrete surfaces that are continually exposed to salty marine

or industrial chemical environments are most at risk, particularly if cracks are present. Calcium

chloride accelerates the corrosion more than sodium chloride. In addition to the corrosive agents,

the severity of corrosion is influenced by the quality of concrete (cement type, properties of

aggregates, and moisture content), depth of concrete cover over steel, and the permeability of

concrete.

The key outcome of this degradation mechanism is the creation of steel corrosion products that

cause high swelling pressures within the concrete, which often leads to concrete spalling and

cracking. Once rebar corrosion initiates, significant concrete near-surface damage can proceed

quickly (on the order of months or a few years). This minor spalling and cracking usually will not

affect the structural performance of the reinforced concrete significantly until a critical level of

rebar tensile strength is lost. The service life of concrete structures where this is the dominant

degradation mechanism, and it often is, can be predicted by models based on the chloride ion

diffusion. Because rebar is hidden from view by the concrete cover, any initiating degradation of

concrete structures by this mechanism can be detected by making electrochemical measurements

of the reinforcing steel that indicates corrosion, if this capability was designed in advance (none

of the current dry-storage systems relying on concrete have this capability).

The reaction of Aggregates with Alkalines. The alkaline aggregate reaction is a common

concrete-degradation mechanism identified by an irregular cracking pattern on the surface of

concrete. The mechanism can be greatly minimized by a careful selection of aggregate that does

not contain reactive materials. Two types of reaction, alkali-silica and alkali-carbonate have been

identified. Moisture must be available for the chemical reactions to occur. Thus, concrete that is

either consistently wet, or that experiences wet and dry periods, is susceptible.

Creep and Shrinkage. A normal part of concrete behavior, as the concrete dries out, is that it

shrinks in volume, inducing tensile strains and, possibly, shrinkage cracks. Most of the shrinkage

(98 percent) typically occurs during the first few (e.g., five) years of service. The significance of

a shrinkage crack as a potential contributor to degradation depends primarily on its size and

environmental exposure conditions. A crack can allow aggressive agents access to the reinforcing

steel, promoting the possibility of corrosion.


Irradiation. The degradation of concrete exposed to neutron and/or gamma radiation is manifested

in many ways. Fast and slow neutrons usually cause aggregate growth, decomposition of water,

and warming of concrete. Gamma radiation affects the cement paste portion of the concrete,

producing heat and causing water migration. The degradation due to nuclear heating and water

loss is more serious than degradation associated with direct radiation damage. This is because

nuclear heating causes the free water within the concrete to evaporate, and both the neutron

shielding, and structural characteristics of the concrete become impaired. As a consequence, the

concrete could experience a decrease in its strength (compressive, tensile, and bonding strengths)

and stiffness (modulus of elasticity) from shrinkage and cracking if the thermal gradient is

excessive. According to the American National Standard ANSI/ANS-6.4-1985, nuclear heating

can be neglected if the incident energy fluxes are less than 1,010 MeV/cm2sec. Under typical dry-

storage conditions, the energy flux is negligible and decreases with time, making this aging

mechanism insignificant.

Summary: As is evidenced by deteriorating concrete structures, atmospheric corrosion and

degradation of concrete structures do occur. The additional influence of heat and radiation

damage can compound environmental damage. Usually, monitoring the condition of

concrete overpacks to identify damage before it becomes significant will be important. Once

the damage is found, a follow-up maintenance program will correct the damage and help

minimize future degradation problems. However, the design of ISFSI at SONGS is such that

visual inspection of the concrete is not possible, as it is an underground system and it is a

block structure, which deters from any kind of inspection. There are already reported

observations of visible metal and concrete degradation of dry-storage systems in areas near

the sea in different storage locations. The dust and chemical species that comprise an

atmospheric-corrosion environment can affect several different mechanisms. The heating

degradation of concrete is more severe for SONGS because of its block structure. The

independently standing concrete overpacks would have better heat dissipation efficiency

than the underground systems as that of SONGS.


MS-2: Current knowledge on Hydride formation, hydride reorientation, H2 formation,

radiation damage of the fuel rods and cladding

The US Nuclear Waste Technical Review Board’s 2010 assessment of dry storage [Rigby and

Members, 2010] is comprehensive documentation on this subject. The present description and

analysis are heavily based on that review and updated accordingly with the most current literature

data. This seems to be a complex subject and many research papers available [Adrych-Brunning

et al., 2018; Cha et al., 2015; Cha et al., 2018; Chu et al., 2008; Colas et al., 2013; Jang and Kim,

2017; Y-J Kim et al., 2015; Lee et al., 2018; Min et al., 2014; Ronald Adamson, 2017; Won et al.,

2014]. There are several mechanistic pathways for fuel and cladding degradation of the spent fuel

during dry storage as shown in Figure MS-2-2.

The Zirconium alloy cladding (the outer coating of the fuel pellets) goes through two heating-

cooling cycles. It gets hot when in the reactor, at that time H2 (produced in the surrounding) get

dissolved in this cladding as the fuel comes out from the reactor to the pond, it starts cooling from

400 °C to 50 °C. During that time hydrogen precipitates as hydride in the outer edge of the circular

cladding. This is the first cycle. During placing the fuel for dry storage, the temperature goes up

very quickly and after the cask is welded-shut, the temperature starts to decrease very slowly, at

this cooling phase, the formed hydrides reorient itself. How much reorientation will take place

depends on several parameters: dissolve hydrogen amount, cooling rate, terminal temperature,

Figure MS-2-1. Sketch showing the mechanisms effecting spent fuel cladding in dry storage (taken

from US Nuclear Waste Technical Board’s Review document (2010).


H2/O2 ratio and neutron damage during its

lifetime. All these factors are

experimentally proven, and the maximum

damage of the cladding happens after 5

years of dry storage as shown in Figure

MS-2-1 [Adrych-Brunning et al., 2018;

Cha et al., 2015; Jang and Kim, 2017; Y-J

Kim et al., 2015; Min et al., 2014; Won et

al., 2014]. Additionally, with increased

burnup, more corrosion-produced

hydrogen will lead to more hydrogen

being absorbed by the cladding. More

hydrogen pickup will lead to more hydride

precipitation and possibly other effects

such as more embrittlement, delayed

hydride cracking, and acceleration of the

corrosion rate [NRC, 2007a]. Many of

these factors may significantly affect the

physical state of the used fuel and cladding

during dry storage.

What happens when hydrides reorient radially? It becomes brittle. Ductility decreases

significantly and technically the fuel is damaged at that point. Since the reorientation depends on

so many factors and not every canister is the same, the reorientation of hydrides would be different

for different canisters and it is hard to make sure that hydride orientation has not taken place

(individually for each canister).

Figure MS-2-1. Micrograph showing orientation of

hydrides in Zircaloy-4 cladding of 230 ppm H2: (a) as-

hydride and (b) after 8 cycles of thermal treatment

(taken from Chu et al., 2008).


Table MS-2-1. Cladding degradation effects. The table summarizes the degradation effects of

cladding materials during prolonged dry storage and each degradation effects are also described

in the table [Spykman, 2018].

A quote from NRC document- CNWRA-2012-001 [C NRC, 2012], “No breach of canister was

considered in this analysis, following a cladding breach, internal gas pressures within SNF rods

represent a driving force for the release of gas and fuel particulates into an SNF canister. Initial

and damaged states of SNF, including fuel pellet fracturing. Because the breach sizes and number

of breach sites are small compared to the total surface area of cladding, pathways for the flow of

gas and particulates out of SNF rods are confined, and aerosol dynamics considerations are

necessary to estimate the extent to which SNF particulates, generated within the fuel rod cladding,

can be released from the fuel rods into the canister. Fission product gas and SNF particulates can

both contribute to the radiological source term”.

The formation of hydrites in the zirconium alloy cladding is very common and and in some

condition these hydrides will reorient in the radial direction and that would alleviate creep

deformation and eventually make the cladding material brittle [Rigby and Members, 2010]. The

above description (from NRC) of cladding breach would lead to radiological leaks inside the


canisters as NRC have not considered any canister breach [C NRC, 2012]. Another issue

intertwined with the degradation of zirconium cladding is the allowed peak temperature, because

the tensile hoop stress of the cladding increases with the extreme thermal history of the fuel. Figure

MS-2-3 explains the relationship between the hoop stress and temperature [Kook et al., 2013].

Recent experimental results show that the hgher heat-up temperature and larger tensile hoop stress

generated larger radial hydride fraction. Authors explained the radial hydride fraction by the

combined effects of a hydrogen solubility

difference between heat-up and cool-down

temperatures [Cha et al., 2018]. Unfortunately,

there is no agreed upon internationally allowed

peak cladding temperature. NRC established a

regulation on a cladding peak temperature during

interim dry storage in which maximum

temperature should not exceed 400 °C for all fuel

burn-ups under normal conditions, however, a

higher short-term temperature limit is allowed for low burn-up fuel. Contrary to that, Japan

Nuclear Regulation Authority (NRA) established a different regulation that specifies the allowable

cladding peak temperatures of 250 and 275 °C, depending on fuel burnup [Cha et al., 2018]. This

discrepancy in regulation standard between two leading countries indicates the lack of

understanding of the time evolution of the cladding morphology and chemistry while in dry

storage. The radiation damage of the fuel cladding (by beta, alpha and neutrons [In general, β-

decay is the primary source of radiation during the first 500 years of storage, as it originates from

the shorter-lived fission products]) further complicates the fuel degradation process and that the

results in cladding swelling and He bubble trapping [Adrych-Brunning et al., 2018; J-S Kim et al.,

2017; Ronald Adamson, 2017]. The stress due to rod internal pressure can also induce cladding

degradation such as, stress corrosion cracking, hydride reorientation, and delayed hydride cracking

[Alam and Hellwig, 2008; J-S Kim et al., 2017]. If the fuel degrades during extended storage, it

could be susceptible to damage from the vibration and shocks encountered during transport

operations. The consequences may include release of fission-product gases into the canister or the

cask interior [Rigby and Members, 2010]. This is an important factor and fission-product gases

must be contained during a transportation accident.

Figure MS-2-3. Relationship between

Temperature and Hoop Stress in Cladding.


What is unknown is the degree of residual water remaining in dried fuel, and its ultimate fate and

effects. Radiolysis of the water could allow fuel pellet, cladding, or metal component degradation

if the water or oxygen/hydrogen gas were to escape cladding through pinholes or cracks in

canisters.

Summary: According to available literature the zirconium alloy cladding of SNF are

susceptible to degradation, more so compared to the fuel rods itself. Cladding is an integral

part of the fuel and it is hard to dissociate one from the other. The cladding failure is a major

issue and will result in the release of radioactivity inside the canister. There is a high

possibility that the embrittled cladding would dissociate while retrieving the canister from

the ISFSI for transportation and that would result in unconfined radioactivity inside the

canister. The maximum cladding degradation happens 5 years after the dry storage begins.

Moreover, there is no international consensus of allowable peak cladding temperature

(higher temperature degrades the cladding faster). The US NRC regulation allows peak

temperature to be below 400 °C, whereas the Japan Nuclear Regulation Authority (NRA)

established the regulatory limit to 275 °C. This regulatory discrepancy highlights the lack of

knowledge on time evolution of cladding morphology and chemistry.


Two topics (under the GE section) are combined in the following discussion.

GE-1: What is the timeline, vulnerability of ISFSI to coastal processes under current and

future, including: coastal erosion, groundwater, sea level, flood (tsunami), humidity/fog and

weather-related?

GE-2: What is current and future earthquake frequency and magnitude and what is the

impact on the site?

Geomorphology, Coastal Geology and Hydrology of San Onofre State Beach: The

geomorphic features of the southern California coast are the raised marine terraces that rise in steps

from the beach up to the slopes of adjacent mountains. Between Dana Point on the north and the

Mexican Border on the south, marine terraces and beach-ridges record a series of Quaternary sea-

level high-stands superimposed on tectonically rising segments of the California coast. The lowest

terraces are fronted by sea cliffs exposed to both marine and subaerial processes [Kuhn, 2000;

Survey, 1889a]. The San Onofre Area, a 2.5-mile segment of coastline approximately 12.5 miles

southeast of Dana Point, is characterized by reddish-brown Quatenary marine or nonmarine terrace

and alluvial fan deposits, which range from 15 to 30 meters thick. Thick, horizontally-bedded

Pleistocene strata unconformably overly the Monterey Formation (Miocene) throughout this

region. A cross-sectional beach profile of the San Onofre State beach is shown in Figure GE-1-1.

SONGS is located on the coastal terrace.

Erosion of the coastal terrace at San Onofre State Beach and Camp Pendleton is seen to occur

under both natural conditions and, as a result of man-induced accelerated erosional processes.

Subaerial erosional processes, which occur along this coastal segment are: (a) rainfall-induced

landslides, (b) lateral and headward erosion along canyons accelerated by man’s alteration of

existing drainage patterns; (c) gullying of the coastal terrace and cliff face by rain wash, and (d)

surface erosion of the top of the coastal terrace [Kuhn and Shepard, 1984]. Since 1987, the beach

width most markedly increased following the storms of 1992-93 and 1997-98 during two of the

biggest El-Nino events were recorded in these years in recent history [Kuhn, 2000; Young, 2015;

2018]. More than 80% of the cliffs between the SONGS and Target Canyon (Camp Pendleton)

consists of landslides and the landslides appear to be directly related to periods of intense sediment

saturation and large storm swell.


There are no perennial streams in the general vicinity of the SONGS site. However, ephemeral

streams and watercourses exist. The major streams are San Mateo Creek, located approximately 2

miles to the northwest, and San Onofre Creek located approximately 1 mile to the northwest [NRC,

2007b]. The total Groundwater Storage Capacity for this basin is estimated to be 6,500 acre-feet

(DWR 1975; SDCWA 1997). The historical average groundwater production is about 750 acre-

feet /yr and average recharge of reclaimed water is about 500 acre-feet/yr (SDCWA 1997).

Sea Level Rise, High Tide, and Coastal Aquifers: Thermal expansion of the ocean and glacier

melting have been the dominant contributors to 20th-century global mean sea level rise.

Observations since 1971 indicate that thermal expansion and glaciers (excluding Antarctic

glaciers) explain 75% of the observed rise. The contribution of the Greenland and Antarctic ice

sheets has increased since the early 1990s, partly from increased outflow induced by warming of

Figure GE-1-1. Cross-sectional beach profile of San Onofre State beach. SONGS is located at the

coastal Terrace shown in the sketch (not in scale). Figure taken from Kuhn (2000).


the immediately adjacent ocean. Since 1993, when observations of all sea-level components

become available, the sum of contributions equals the observed global mean sea level rise within

uncertainties. Changes in ocean currents, ocean density, and sea level are all tightly coupled such

that changes at one location impact local sea level and sea level far from the location of the initial

change, including changes in sea level at the coast in response to changes in open-ocean

temperature. Although both temperature and salinity changes can contribute significantly to

regional sea-level change, only temperature change produces a significant contribution to global

average ocean volume change due to thermal expansion or contraction [Church et al., 2013].

The global energy balance is a fundamental aspect of the Earth’s climate system. At the top of the

atmosphere, the boundary of the climate system, the balance involves shortwave radiation received

from the Sun, and shortwave radiation reflected, and longwave radiation emitted by the Earth. The

rate of storage of energy in the Earth system must be equal to the net downward radiative flux at

the top of the atmosphere. This energy imbalance is rising due to increased greenhouse gases in

the atmosphere and hence the global increase in temperature. The Intergovernmental Panel on

Climate Change (IPCC) adopted a set of emission scenarios known as ‘representative

concentration pathways’, or RCPs. RCPs consist of four future pathways, named for the associated

radiative forcing (the globally averaged heat-trapping capacity of the atmosphere measured in

watts/square meter) level in 2100 relative to pre-industrial values: RCP 8.5, 6.0, 4.5 and 2.6,

respectively. RCP 8.5 is consistent with a future in which there are no significant global efforts to

limit or reduce emissions. RCP 2.6 is a stringent emissions reduction scenario and assumes that

global greenhouse gas emissions will be significantly curtailed. Under this scenario, global CO2

emissions decline by about 70% between 2015 and 2050, to zero by 2080, and below zero

thereafter. RCP 4.5 and 6 are the two intermediate scenarios and resulted in a similar way.


There is another extreme sea-level rise scenario in the Fourth National Climate Assessment

(known as an H++ scenario), which predicts much higher sea-level rise in the long run compared

to RCP 8.5 [Griggs et al., 2017]. Figure GE-1-1. shows the predicted sea-level rise in different

scenarios at La Jolla (40 miles south of SONGS). By 2030 and 2050, the predicted sea-level rise

for all the scenarios is similar (around 15 cm by 2030 and 25 cm by 2050), but the predicted rise

differs significantly among different scenarios in 2100 and 2150 (Figure GE-1-2). Although long-

term mean sea-level rise by itself will provoke increasing occurrences of coastal lowlands

flooding, over the next several decades it is highly likely that short-term increases in sea level will

continue to be the driver of most of the strongest impacts along the coast of California. Short-term

processes, including Pacific Basin climate fluctuations (Pacific Decadal Oscillation, El Niño

Southern Oscillation, and North Pacific Gyre Oscillation), King tides (perigean high tides),

seasonal cycles, and winter storms, will produce significantly higher water levels than sea-level

rise alone.

Figure GE-1-2. Predicted sea level rise at La Jolla, CA (40 miles south of SONGS) based on different

climatic scenario (emission estimates) described in the text. In all the scenarios the predicted rise is

about the same for 2030 and 2050 and diverges thereafter. The plot shows the increases in different

scenarios in different shades of green. The H++ scenario is the extreme case (shown in dark green)

where the predicted sea level increase is dramatic, about over 260 inches by 2150 (Data taken from [4].

In a conservative scenario, the expected sea-level rise by 2050 is by about 11 inches.


Over the recorded era of the 20th and early 21st centuries, most of the significant storm damage to

California’s coastline has occurred during major El Niño events, when elevated sea levels

coincided with storm waves and high tides. The most prominent of those cases were major El Niño

events, for example, 1940-41, 1982-83, and 1997-98, when sea levels were elevated 8 to 12 inches

for several months at a time [Feely, 1987; J. J. Barsugli and P. D. Sardeshmukh, 1999; Yeh et al.,

2009].

High tides along the California coast occur twice daily, typically of uneven amplitude, and are

caused predominantly by the gravitational attraction of the moon and the sun on the Earth’s oceans.

Extreme tides, called spring tides, occur in multi-day clusters twice-monthly at times of the full

and new moon. Additionally, even higher tides occur several times a year and are designated as

perigean high tides, or more popularly “King tides”. These events are now recognized as producing

significant coastal flooding in some well-known areas. The Earth-moon-sun orbital cycles also

amplify tidal ranges every 4.4 and 18.6 years, producing peaks in the monthly high tide that are

about 15 cm and 8 cm, respectively, higher than in the intervening years [Griggs et al., 2017] (see

Figure GE-1-3).

Recently, there are several studies addressing the intrusion of seawater in coastal aquifers [Chun

et al., 2018; Hoover et al., 2017; Singaraja et al., 2018] in the context of sea-level rise. Sea-level

rise related impacts on coastal systems can occur in several ways. Marine inundation will shift the

Figure GE-1-3. Recorded tide surge from San Clemente tide-gauge for last two years. The plot

shows the monthly variation of the tide level along with occasional surges, which are as are as

high as 7 feet from the mean-sea level.


coastline landward, erode beaches, accelerate cliff failure, degrade some coastal habitats, and

potentially damage coastal infrastructure [Young, 2015; 2018]. Recent studies have demonstrated

that sea-level rise also could contribute to saltwater intrusion in coastal regions by raising the

interface between intruding saltwater and overlying freshwater.

Sea-level rise and tidal forcing will cause the water table level to rise in the coastal areas, and the

mean high sea level could approach and ultimately rise above the ground surface [Chun et al.,

2018] specifically in southern California [Hoover et al., 2017]. Moreover, the tides are also known

to affect the groundwater fluctuation in the coastal aquifers [Singaraja et al., 2018].

Geological Setting of the Site and Earthquakes: The SONGS site is located in a seismically

active zone on the Pacific coast, which is part of active Pacific-North America transform plate

boundary, and where the seafloor is deformed by several large oblique-slip fault systems [Legg et

al., 2015]. The offshore Southern California Borderland has undergone dramatic adjustments as

conditions changed from subduction tectonics to transform tectonics, including major Miocene

oblique extension, followed by transpressional fault reactivation. A moderately landward-dipping

San Mateo–Carlsbad (SMC) fault converges downward with the steeper, right-lateral Newport-

Inglewood/ Rose Canyon (NIRC) fault forming a fault wedge as shown in Figure GE-2-1. The

NIRC fault zone is an active strike-slip fault system within the easternmost of the offshore Inner

Continental Borderlands (ICB) fault system and an active component of the Pacific-North America

plate boundary. The best-estimated geologic slip rate of this fault is 1.5 ± 0.5 mm/yr, which is


approximately 5-15% of the estimated 50 mm/yr plate boundary deformation in southern

California [D M Singleton et al., 2018; D M A Singleton, D. C.; Maloney, J. M.; Rockwell, T. K.,

2017]. NIRC poses a significant hazard to coastal Southern California because of its proximity to

some of the most densely populated regions of North America (e.g., San Diego, Orange, and Los

Angeles counties, as well as Tijuana, Mexico). There have been several moderate-sized

(magnitude > 5.5) earthquakes in this region in the last 100 years and the estimated epicenters are

marked in Figure 1 [Legg et al., 2015; Sahakian et al., 2017; Sorlien et al., 2015]. Recently, the

NIRC fault has been mapped in detail and the rapture magnitudes for three different scenarios are

estimated [Sahakian et al., 2017]. These authors assigned a magnitude value up to 7.4 in the

Figure GE-2-1. Geological location map of SONGS. Data taken from [1-5]. The locations

of the epicenters of the previous earthquakes are shown with year of occurrence and

magnitude in parenthesis. The location of the faults and the locations of the epicenters of

the previous earthquakes are shown for illustration purpose only (and might not be precise).

The relative motions of the two plates are also shown.


Richter scale for an earthquake from this fault alone without an estimate of the error in this

analysis. The prediction of an earthquake is a fascinating area of research, and the prediction of

the magnitude is a daunting task. As mentioned, given the wedged-shaped fault system between

NIRC and SMC faults, the prediction is more complicated than that from a single fault. In addition

to the offshore faults, there are multiple faults east of the SO-NPP. These include the San Jacinto

fault and Elsinore fault on the east and the famous San Andreas fault northeast of the SO-NPP.

Considering all these complex geological features together, an earthquake ≥ 8 in the Richter scale

is well within the possibility and could not be ruled out for this reason.

Tsunamis are the consequence of oceanic earthquakes where long ocean waves sustained by

gravity that increase in amplitude as water depth decreases. Therefore, such waves are particularly

hazardous along populated coastlines near offshore faults that produce a vertical displacement of

the seafloor and water column. The hazard from earthquake-generated tsunamis offshore of

Southern California has received relatively little attention, however, there is a historical record of

Tsunamis in this region, two of them within the last 100 years [Lander et al., 1993]. A report

prepared by Intersea Research Corporation (for Southern California Edison, 1976) estimated the

maximum height of Tsunami waves of 15-feet height from the mean low sea level [Intersea

Research Corporation, 1976]. The existence of the borderland shelves (Figure GE-2-1) are

considered to be the main barrier towards high Tsunami waves for the SONGS site. About 133

feet high waves were detected during 2011 Japan earthquake [WikiPedia, 2011], however, the

predicted wave heights are about 33 feet at the open ocean [NOAA, 2011]. This discrepancy shows

that the predictions could be significantly off from reality. A recent study [Ryan et al., 2015]

updated the possibility of tsunamis in Southern California triggered by an earthquake of magnitude

7.4 in the Ventura basin (a few tens of miles north of the SONGS), which has the similar dip-slip

faults as described for the SONGS location. With the new studies of the seafloor fault mapping

and the computer simulation of the tectonic movements and triggering of a tsunami, the

understanding of the natural hazards present at the coastal region of southern California has

significantly improved in recent years.

Summary: Effect of the Natural Causes On SONGS ISFSI (GE-1 and GE-2)

Based on the above information, the following points emerge as a concern for underground

dry storage: (i) SONGS is located at a coastal terrace supported by high-erosion prone sea


cliff and coastal bluff. The foundation of the 8-feet sea wall (the average total height of the

barrier is 28-feet from the mean low water level), built to protect SONGS from the ocean

wave action, is on the fragile bluff and, therefore, vulnerable during high erosion events, e.g.,

following El-Nino fueled storm surge (as that happened during 1997-’98 El-Nino). There is

published data on San Onofre state beach and the surrounding area that these areas are

prone to sporadic land slides and erosion.

(ii) SONGS is located in the groundwater catchment basin, which indicates groundwater

activity. The ISFSI is an underground system, the base of which is only 18 inches above the

ground water-table, but surrounded by groundwater activities (because of the catchment

area). Any breach of the storage system potentially would contaminate the groundwater

system specially with long-lived fission-product radionuclides [e.g., Cs-137 (t1/2 = 30 years),

Sr-90 (t1/2 = 29 years), Pu- 239/240 (t1/2 = 24,000 years, 6500 years), I-129 (t1/2 = 16 million

years), Se-79 (t1/2 = 327,000 years), Zr-93 (t1/2 = 1.53 million years), , Pd-107 (t1/2 = 6.5 million

years)] [Chiba et al., 2017]] because owing to their high solubility in water and mobility in

the geosphere [Salvatores et al., 1998] and potentially contaminating the regional watersheds

and estuaries.

(iii) Off-Shore Earthquakes and Tsunamis: SONGS is located in the seismically active zone.

Though a recent study computed a maximum earthquake intensity of 7.3 in Richter scale

[Sahakian et al., 2017], other studies documented earthquake magnitude over 8 for the

similar kind of slip-fault as that exist near San Onofre [Legg et al., 2015]. Tsunamis are the

consequence of oceanic earthquakes where long ocean waves sustained by gravity that

increase in amplitude as water depth decreases. A tsunami triggered by a large earthquake

at the southern California coast cannot be ignored based on the available study as discussed.

As noted, there were huge (more than 4 times) discrepancies between the predicted wave-

height and the actual one for the 2011 Japan tsunami and proves the non-fail-proof model

outcomes for the natural systems. To provide a quantitative estimate, one could

conservatively consider a recurrence interval of 1 in 300 years for such a great tsunami, then

the probability of occurrence of such an event in the next 50 years is about 15 % (i.e., 1- (1-

1/300)50). Since the seawall height is measured from the mean low water level, in a futuristic

scenario of swollen sea (or if the tsunami happened during those high tide seasons, the

effective seawall height would be close to 20 to 21 feet and might be ineffective for stopping


the tsunami waves. The tsunami waves carry sands and debris onshore in high velocity, and

that debris might impact the cooling mechanism of the underground ISFSI and the potential

consequences are discussed in canister failure section (MS-1).

Two topics (under the GE section) are combined in the following discussion.

RM-1: How is risk being defined, what is the probability cut off for considering scenarios

RM-2: What are assumptions used in the risk analysis and are they still valid today? E.g.

transport readiness of canisters, canister requirements after Yucca shut down, etc.

The risk tree analysis is the preferred method of analyzing risk in the nuclear industry while

assessing the risk of dry SNF transportation and storage [Chen et al., 2010; NRC, 2007a; Yun et

al., 2017]. The NRC uses the risk triplet, in which the elements of risk are the scenarios, the

frequencies of the scenarios, and the consequences of the scenarios, where the measure of risk is

the consequences multiplied by the frequency of the consequences and mathematically defined

(measure risk is the consequences multiplied by the frequency of the consequences) [NRC, 2007a]

as shown in Eq. 1.

𝑅 = 𝑓∑ 𝑃𝑛𝐾𝑛𝑚𝑛=1 …….. (Eq. 1)

where R = total risk, f= frequency (1/time), the conditional probability of nth accident scenario,

and consequence of the nth accident. The risk of all plausible accident scenarios in all stages of the

dry cask storage operation can also be derived in the same way.

Some of the recent literature followed a similar risk-free analysis and found negligible risk in the

entire process [Chen et al., 2010; Yun et al., 2017]. Because of the multiplicative nature, the overall

risk of failure of the dry cask storage comes out to be negligibly small. They use a risk-free

approach, which is multiplicative. As an example, suppose a task has 4 steps, and the risk of each

step is between 0 and 1. If only one step of extreme risk (e.g., 1), and other steps are of little risk

(e.g., 0.1, 0.2, and 0.1). The total risk would be 0.002, which means very low risk even though the

first step is of extremely high risk. This kind of analysis does not consider any risk related to the

degradation of the canister [NRC, 2007a].


Another method for performing risk analysis is by failure mode and risk analysis framework [Yang

et al., 2011], which is a semi-quantitative method. In this type of analysis, every stage of the

process could be evaluated separately by determining the Risk Probability Number (RPN). RPN

is obtained by multiplying three different factors for every stage: Severity, Occurrence, and

Detection. The three factors are ranked on a scale of 1 to 10. For severity, 10 is extreme; for

occurrence 10 means the high frequency of occurrence and for detection, 1 means low risk due to

high detection ability, and 10 means high risk due to low detection probability. The method is

semi-quantitative because, for most cases, the number (scale factor) assigned is subjective

(intelligent guess based on some prior experience), however, it identifies processes of different

risk levels. The higher the RPN number, the higher is the risk of catastrophe. As an example, Table

I shows estimated RPN values for spent nuclear fuel loading and storage in a marine environment.

Table I. Failure Mode and Risk Analysis for SNF Loading and Storage in the Marine Environment

(This table should be used just as an outline as it is not yet been published).

The risk analysis of these two methods described here yield different outcomes. As an example,

during an incident like dropping a canister during transfer, the risk tree analysis identifies as the


high-risk process, but the failure mode and risk analysis identifies as one of the low-risk ones,

because the detection risk is very low (well-trained personnel could easily detect it without any

technological aids) [Chen et al., 2010; NRC, 2007a; Yun et al., 2017]. When using this analysis

for the SONGS underground storage (ISFSI), tsunamis, followed by the internal degradation of

the canister by different processes (as described), was identified as one of the greatest risks because

the detection of risk is high for these situations. Though the risk estimation methods are subjective,

however, they are important as they provide a semi-quantitative guideline for mitigating risk

factors in SNF handling and storage.

This is an active area of research and there were several papers presented in the Waste Management

Conference (Phoenix, AZ, March 2019) and the common theme of these papers is that the risk tree

analysis is not the perfect method to capture the real risk as being currently followed.

Summary: According to new research, the present nuclear industry-standard of risk analysis

schemes does not represent the real risk. NRC performs risk studies based on risk-tree

analysis and provides three steps of safety criteria: absolutely safe, reasonably safe, unsafe.

These criteria are arbitrary and based on biased judgments.


SH-1: What are the root causes of the near-drop incident and gouging of canisters? Is there

a mitigation strategy that could, could have been, or could be applied for canisters that would

alleviate concerns?

The process of downloading Holtec (MPC 37) canisters inside the UMAX cavity is one of the

many initial processes in storing dry casks. There was a “near-miss” at the SO-NPP on August 3,

2018 and it was investigated by NRC. As workers were lowering one of the 54-tonne (US ton)

canisters packed with SNF (37 fuel assemblies), the canister got caught on the lip of an inner ring

(Figure SH-1-1), hanging by about 0.6 cm. The drop-restraining system was not in place and the

canister was about to fall by about 5.5 m (18 ft). During downloading operation, the canister system

(with overpack) was not visible to the crane operator (due to line of sight radiation shielding).

During the NRC hearing (November 2018) we learned that there was no guide system for

downloading and the crane operator was verbally instructed by few of the downloading crew.

Seeing the slack in the lifting slings, the crew mistakenly concluded that the canister was inside

the cavity. Later from the high radiation reading, they realized that the canister was caught in the

guide-ring of the cavity.

The root cause of this mishap is the improper and inadequate equipment and technology available

to the crew to perform this delicate task and the inadequate training of the crew. This task was

delicate because of several reasons- the weight (54-tonne) and dimension (6 ft in diameter and 18

ft height) pf the cylindrical-shaped canister; the clearance between the canister outer wall and the

inner diameter of the guide-ring was only about ¼- of an inch on all the sides. Figure SH-1-1

schematically shows the situation and also estimates the force vectors using Newtonian physics.

English et al. [English et al., 2019] examined this “near miss” scenario from a basic physics

standpoint. By examining the dropping of the canister in free fall, we have estimated the upper

value of the velocity, kinetic energy, and momentum when the canister crashes into the concrete

at the bottom of the UMAX cavity. The falling canister could hit the concrete floor at a speed of

40 km/hour with the impact force of over 3 x 108 N. This situation is equivalent to that of a fully-

loaded 18-wheeler truck with a gross weight of 54-tonne crashing into reinforced concrete at 40

km/hour. This impact could ruin the canister’s cooling system and potentially cause a large

radiation release. There was no physical drop test performed on this particular kind of canister.

Holtec’s FSAR [FSAR, 2018] states in section 3.4.4.1.4, “Tipover is not an applicable load case


for HI-STORM UMAX. The VVM (i.e., Vertical Ventilated Module) is situated underground and

cannot be moved; therefore, drop and tip-over events are not credible accidents for this design

configuration.” This assertion in safety analysis report indicates that the vendor-Holtec was not

prepared for this kind of accident and thus mitigation policy was not in place. NRC’s letter to the

Edison Company (EA 18-155, dated July 9, 2019) stated, “As part of the equipment enhancements,

the licensee installed a camera on the side of one of the VCT towers. The camera was positioned

to provide an overhead view of the top of the canister as it passed through the transfer cask into

the ISFSI vault. The camera wirelessly displayed the video feed to a monitor that was located next

to the Holtec cask loading supervisor and the SCE oversight specialist”.

There are few studies available regarding drop test of canister from a significant height to

understand the deformation of the canister [Choi et al., 2011; Lin et al., 2015; Wu et al., 2012].

Most of these studies are computer simulation (with a limited drop height) and there are handful

dealing with a physical drop of a reduced size canister with no load condition [Witte et al., 1998].

The motive behind these studies is to acquire knowledge about the deformation of internal

Figure SM-1-1. Schematically shows the stuck situation of the “near-miss” event. Applying the laws

of Newtonian physics, the force vectors are estimated and applying that estimated force the depth and

gouges are also estimated.


channels and heat flow conditions. The heat removal from the active SNF depends on complex

heat-flow dynamics and any deformation might result in insufficient heat removal and that leads

the canister towards criticality. The model simulations show that for a small drop, the canister’s

internal deformation is not severe, however, this conclusion might not readily be applicable for an

18 foot drop (the potential drop height of August 2018 incident).

Based on our analysis [English et al., 2019], installation of a camera as stated above is NOT the

adequate remedy or a mitigation strategy for future loading. Any prevention of metal to metal

scratching during the loading stage should be the goal to avoid scratching and gouging. These

artifacts turned out to be the potential site for chemical corrosion, especially of the hot and the

highly radiative surface and marine environment [Bayssie et al., 2009; Beech, 2004; Lv et al., 2016;

Nilsson, 2012; Padovani et al., 2017; Robertson, 1991; Ul-Hamid et al., 2017].

Another important point is the development of scratches and gouges during downloading because

of metal-to-metal interaction via indentation. The recent visual inspection by Edison (as stated in

the NRC letter to Edison, July 2019) [NRC, 2019] which was performed by a robotic crawler

equipped with navigational cameras and a borescope. The borescope was a flexible camera with

interchangeable tips (a GE equipment). All surface irregularities were recorded and compared to

post-fabrication photos to determine whether the surface irregularities were a result of

downloading operations. They determined that the maximum wear depth of 0.026 inches (0.66

mm). Though the results seem comforting, however the technology measures the depth based on

the computer model and there is no direct measurement. Therefore, these simulated depth

measurements should associate with a large uncertainty which was not stated in the NRC letter

[NRC, 2019] as well as in the GE’s (manufacturer) website. Even if we consider the measured

depth number at face value, the measured depth of 0.026 inches is 160% greater than the ‘worst-

case predicted scratch depth’ as published by Edison [Edison, 2019] using a proprietary computer

simulation model.

Holtec and Edison remotely measured 8 canister surface out of 29 and through a statistical analysis

“determined that the deepest scratch at one location resulting from insertion followed by

withdrawal with a 95 percent probability and 95 percent confidence to be 0.0584 inches (58 mils,

i.e., 1.47 mm), which was still below the ASME code limit of 10 percent (0.0625 inches)”[NRC,

2019]. The above statement is scientifically incorrect because as stated Wikipedia, “a 95%


confidence level does not mean that for a given realized interval there is a 95% probability that the

population parameter lies within the interval (i.e., a 95% probability that the interval covers the

population parameter). According to the strict frequentist interpretation, once an interval is

calculated, this interval either covers the parameter value or it does not; it is no longer a matter of

probability. The 95% probability relates to the reliability of the estimation procedure, not to a

specific calculated interval.” [NIST, 2008; Wikipedia, 2019]. Moreover, they have determined

these statistics with a very small population (i.e., 8) which indicated this 95% confidence limit is

not realistic.

Summary: Imperfect design, inadequate equipment and untrained crew were the root causes

of the August 2018 near-miss accident. A drop during canister loading was not even

considered as a potential accident issue by the contractor as stated in their FSAR. Insufficient

foresight of the vendor/contractor about this kind of potential accident scenario resulted in

no mitigation policy. At present the vendor installed camera (after the incident) to better

monitor the loading process. This mitigation strategy is still inadequate because it would not

prevent metal-to-metal contact during misalignment. This kind of metal-to-metal contact

should be avoided to decrease the risk of scratching and gouging of the stainless-steel surface

(see MS section for further detail). These later created imperfections are potential sites for

degradation of stainless steel through environmental processes (chemical, biofilm, etc)

especially when these stainless-steel surfaces are exposed to high temperatures in a

radiological environment with marine environment surroundings (these canisters are cooled

passively by thermal diffusion of marine air). The consequence of such a huge drop of SNF

loaded canister is not well known. Most studies considered a small drop (of 3 to 4 inches and

not 18 feet). Such a drop would potentially deform the internal channels for fluid flow for

adequate heat transfer from SNF to the surface of the canister. A contact sensor must be

installed to avoid any metal to metal grinding. One has to remember that when a 54-tonne

canister is being downloaded inside a UMAX hole through a sling system, there is no way

that the canister would not swing (basic principle of pendulum) and since the clearance is

small, a small swing would result in metal to metal contact and scratch. The scratch depth

measurement provided by Holtec/Edison is 160% more than their own predicted value, even

though the technique they used does not provide the associated uncertainty. The provided

statistic is erroneous because of the incorrect assertion as stated and due to a very small


sample size (i.e., population). Any real drop of a canister during the downloading process

would be catastrophic because it would deform the internal channels that would disrupt the

fluid dynamics and the system might run towards criticality.


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