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Spent Fuel and Waste Science and Technology

Probabilistic Assessment of Stress Corrosion Cracking of SNF Dry Storage Canisters

C.J. O’Brien, C. Alexander, E. Schindelholz, C. Bryan, R. DingrevilleSandia National Laboratories

[email protected]

SFWST Annual Working Group Meeting University of Las Vegas, Las Vegas NV5/22/2018Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the presentation do not necessarily represent the views of the U.S. Department of Energy or the United States Government. SAND2018-5527C.

Spent Fuel and Waste Science andTechnology

n Sandia has been working to evaluate SCC of SNF interim storage canisters for several years. This model was developed to assess the importance of parameters controlling SCC penetration, in order to guide experimental research.

n The current model is built on previous versions, with the following additions and improvements

– Working maximum pit size model– Thermal model for vertical canister– Additional weld locations– Stress profiles based on weld residual stress data form the SNL mockup (previous model

used NRC model results)– Dust/salt deposition model– Verification tests added

CAVEAT: Modifications and additions to the SCC model capture mechanistic processes better; however, they include additional parameters that require specification

Improvements to Prior Work

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nWhat is the probability of a crack penetrating a SNF storage canister at given time?

nHow can we quantify the risk of a through-wall crack penetrating a canister at candidate sites for long-term storage?

nHow can this information be used to guide inspection and maintenance schedules?

nHow can we use such an approach to identify knowledge and data gaps critical to interim storage?

Motivating Questions

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Outline

n Problem Descriptionn Model Outlinen Detailed Description of Modelsn Deterministic Analysis for Generic Sites n Assessing Sensitivity of Model Parameters

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SCC Model Outline

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Salt Deposition

Weather

Pit Initiation Crack Initiation Crack Penetration

Incubation Time Pit Growth Crack Growth Time

• Weather Model• Canister Temperature

Model• Salt Deposition Model

• Weather Model• Canister Temperature Model• Salt Deposition Model• Corrosion Model

(Maximum Pit Depth)

• Weather Model• Canister Temperature Model• Crack Growth Model• Residual Stress Model

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Outline


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n Weather model is fit to functional form that accounts for daily temperature and humidity extrema (2x a day)

n Model is fit to NOAA weather station data from 2012 at closest sites to 64 storage sites

n Weather model captures:– Daily Max/min temperature– Daily Max/min dewpoint– Annual cycles– Stochastic variation is tuned to represent measured

fluctuationsn Salt load in atmosphere is not typically known

at storage sites due to lack of data– EPA’s CASTNET data is available nationwide, but

not at storage sites– CASTNET data varies from 5 µg/m3 in coastal FL to

0.012 µg/m3 in SD

Environment Model

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-5

0

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50 100 150 200 250 300 3500

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Temperature[℃]

RelativeHumidity[%]

Time Step [dy]

Inland TemperatureInland RH

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-5

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50 100 150 200 250 300 3500

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Temperature[℃]

RelativeHumidity[%]

Time Step [dy]

Coastal TemperatureCoastal RH

Inla

ndC

oast

al

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Max Mean Temp

Min Mean Temp

Max Mean RH

Min Mean RH


Canister Model

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Canister Surface Temperature Initial decay heat load of 24kW

COBRA-SFS Model

Horizontal OrientationVertical Orientation

Top

Underside

Top

Bottom

Weld 4

Weld 3

Weld 2

Weld 1

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Suffield et al., 2012, PNNL-21788Cuta and Adkins, 2014, FCRD-UFD-2014-00050


Deposition Model 1 of 3:Fluid-Flow Effects

n Salt deposition is a competition between gravity and friction

– Gravity has the greater effect– Particles always stick upon contact– No removal of deposited particles – Flow assumed to be turbulent

n Horizontal– Surface orientation dominates deposition rate

and location

n Vertical– Slightly higher deposition rate near top due to

friction effects

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Horizontal

Max

Weld 2

Weld 1

Gravity

🍎

Airflow

Vertical

Weld 2

Weld 1

AirflowMinimal

Deposition

Detailed flow models for canisters in concrete overpack needed!

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n The rate at which particles deposit is governed by deposition velocity model

– Function of: flow rate, temperature, particle size, atmospheric aerosol load, and surface orientation

n Flow rate through overpack is a critical parameter

– Determines coefficient of friction around canister – Flow rate determines dashed line in top right figure

n No known direct measurements of particle size/aerosol load at sites

– Atmospheric particle size can vary locally due to road salt, distance to ocean, and proximity to cooling tower

– Distance to ocean plays a critical role in size distribution

– Values are right are estimates (except Fukushima)– Size has largest impact on amount of deposition to

vertical surfaces

Deposition Model 2 of 3:Particle Size Effects

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0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.01 0.1 1 10 100

MassFraction

Particle Diameter [µm]

Inland SitesCooling Tower

FukushimaDiablo Canyon

Fit to Inland SitesFit to Diablo Canyon

Fit to Fukushima

1.0e-10

1.0e-08

1.0e-06

1.0e-04

1.0e-02

1.0e+00

1.0e-02 1.0e-01 1.0e+00 1.0e+01 1.0e+02

DepositionVelocity[m/s]

Particle Diameter [μm]

Horizontal: u* = 1e-4Vertical: u* = 1e-4Ceiling: u* = 1e-4

u* = 0.01u* = 0.21u* = 0.4

Dia

blo

Can

yon

Inla

nd S

ite

Decreasing airflow

Horizontal Vertical Ceiling

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n Deposition is shown for horizontal and vertical canister at “Coastal” site

– Assuming 24kW initial decay heat load

– 1 year of deposition shown

– Constant atmospheric salt load of 6 µg/m3

– Far field flow rate of 1.2 m/s

– Mean particle size ~20µm

n Deposition strongly dependent on surface orientation (gravitational settling)

n Cooler surfaces receive more deposition (all else being equal)

11

Deposition Model 3 of 3:Orientation effects

Coastal Horizontal Canister

Coastal Vertical Canister

Airfl

owAi

rflow

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n Cracks are assumed to start from corrosion pits

– Pit growth (anodic current need) is limited by the maximum available cathodic current

– Pit depth at given conditions is calculated by Chen & Kelly Model† that accounts for:• Cl– brine layer thickness (deliquescence)

as a function of temperature & RH• Temperature • Brine conductivity• Material susceptibility

– Extended parameterization to apply to concentrated brines (low RH) at elevated temperatures• Brine properties• Cathodic polarization data

(experimental source)• Repassivation potential‡ (Erp) a function of

temperature, Cl– concentration, and pH

Maximum Pit Depth Model

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Brine

Steel

M+x(OH–)xCl–

0 10 20 30 40 50Radius of Pit [µm]

0

10

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30

40

50

Cur

rent

[µA

]

Imax

ILC

Maximum Stable Pit Size

Erp

EL

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†Chen and Kelly, J. Elec. Chem. Soc. 157 (2) C69‡Anderko, Corr. Sci. 45 (2004) 1583-1612


n Initiation occurs upon reaching Kondo Criterion (K > K1C)– Assumes mode 1 crack – SCC can only occur in with tensile residual stress– Critical stress intensity factor is an uncertain variable– Criterion for initiation is reached when maximum pit depth exceeds critical value rc

• Kondo criterion (1985): crack initiates when stress intensity factor (K) for equivalent-depth crack exceed K1C

n Upon pit reaching critical depth rc– Crack forms and grows according to

crack growth law:

– Pit corrosion no longer tracked ascrack growth rates exceed effective pit growth rates

Crack Initiation

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PitDepth[µm]

Elapsed Time [yr]

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Maximum Pit Depth Plot

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n As crack initiation location is uncertain, the residual stress will be sampled between stress at the WC (weld centerline) and in the HAZ (heat-affected zone)

n Data from measurements on SNL full-diameter canister mockup will be used (Bryan, Enos)

n Crack propagation rates taken from detailed review of available data

Crack Growth Model

SFWST 2018 14

-100

0

100

200

300

400

500

0 0.2 0.4 0.6 0.8 1

Stress[MPa]

Normalized Depth

WC Hoop StressHAZ Hoop StressWC Axial StressHAZ Axial Stress

From: D. G. Enos and C. R. Bryan. SAND2016-12375R, Sandia National Laboratories, United States, November 2016.

SAND2016-xxxxxxx 24

2.8 Crack growth rate data collected under immersed conditions, for high chloride brines

Figure 2.8-1. Data from chloride-rich brine immersed experiments.

Data for SCC crack growth rates under immersed conditions in chloride-rich brines have been compiled and are provided here as a potential talking point. The data extend to higher temperatures than data collected for atmospheric SCC. An aqueous brine cannot be present at temperatures higher than 60-70ºC, based on the possible range of absolute humidities in air circulating through a SNF interim storage system; however, these data may provide insights into the mechanism of the corrosion process.

The following data sets have been identified:

# Spiedel [1981] — Solution annealed 304L, in 42% MgCl2 (130ºC), and in 22% NaCl (105ºC). Also, solution annealed and sensitized 304 samples, in 22% NaCl, over a range of temperatures from 23ºCto 105ºC.

# Shaikh et al., [2013] — 316LN base and weld metal samples, in 5M NaCl + 0.15 M Na2SO4 + 2.5 ml/L HCl. Also, solution annealed, sensitized, or 10% cold worked 304LN samples, at 90ºC, 100ºC, and 108ºC, in the same solution.

# Russell and Tromans, [1979] — Cold worked (25 and 50%) 316 SS in 44.7% MgCl2, at 116 and 154ºC.

# Hawkes et al., [1963]. — Solution annealed 316 SS in 42% MgCl2, at 154ºC.

# Nakayama and Takano, [1986]. — Solution annealed 304 SS, in 42% MgCl2, at 143ºC.

1.E-13

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

0.0023 0.0025 0.0027 0.0029 0.0031 0.0033 0.0035

Cra

ck p

rop

agat

ion

rat

e, m

/s

1/T, K–1

Shaikh et al., 2013, 316 weldShaikh et al., 2013, 316Russell and Tromans, 1979 - 316Hawkes et al., 1963 - 316 SAShaikh et al. 2013 - 304LN SAShaikh et al., 2013, 304LN sens.Shaikh et al. 2013, 304LN 10% CWSpiedel, 1981 - 304L SASpiedel, 1981 - 304 SASpiedel 1981 - 304 sens.Nakayama and Takano, 1986 - 304 SA

90ºC

80ºC70ºC 60ºC

22ºC50ºC

100ºC

116ºC

154ºC

143ºC

40ºC

130ºC

C. R. Bryan and D. G. Enos, SAND2016-2992R

5/22/2018


15

Outline


SFWST 20185/22/2018


n First year of deposition is shown

– Initial decay heat load: 24kW– Airflow rate in overpack: 1.2

m/s– Atmospheric Salt Load:

• 6µg/m3 (coast)• 0.2µg/m3 (inland)

– Assumed particle size distribution:• 20µm (coast)• 2µm (inland)

n Horizontal Canisters have significant deposition on upper surface only (Radial position>0.25)

n Surface orientation (gravity) has greater impact than surface temperature

16

Examples of Generic Sites:Weather & Salt Load

Coastal Horizontal Canister

Inland Horizontal Canister

Airfl

ow

SFWST 20185/22/2018n Note: particle size distribution dependence…


Examples of Generic Sites:Crack Initiation & Propagation

SFWST 2018 17

Site Coastal Inland

Orientation Horizontal Vertical Horizontal Vertical

Pit Initiation Time [yr] 0.008 0.253 0.717 0.703

Crack Initiation Time [yr] 0.938 >1.654 >1.654 >1.654

Penetration Time [yr] 1.000 >1.654 >1.654 >1.654

Maximum Deposition Density after 1st year [g/m2]

10.97 0.08 0.01 1.40×10-6

5/22/2018

n Note that predicted times are for comparison only!n Results normalized by penetration time for coastal horizontal results.


n Environmental Data– Site specific salt particle-size distribution– Site specific atmospheric salt concentration

n Airflow Data– Friction velocity (u*) values over the canister surface within the overpack as a

function of decay heat – Alternatively, free stream velocity as a function of decay heat in overpack to

generate estimates of friction velocity (u*)n Chemistry

– Measurements of cathodic polarization behavior of steels for relevant temperature and brine compositions

– Model for corrosion potential (EL) as a function of temperature and brine composition

– Reparameterization of Anderko’s Erp model for 316L SS

Data Needs

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n Exercise deterministic and probabilistic for each site (sites provide different weather conditions)

n Develop inspection schedule

n Guidance on future data needs

n Guidance on inspection schedule

Milestone Plan

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