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Cambridge University Press978-1-107-40845-6 - Scientific Basis for Nuclear Waste Management XXXII: MaterialsResearch Society Symposium Proceedings: Volume 1124Editors: Neil C. Hyatt, David A. Pickett and Raul B. RebakExcerptMore information

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Mater. Res. Soc. Symp. Proc. Vol. 1124 © 2009 Materials Research Society 1124-Q01-01

Long-Term Performance of the Proposed Yucca Mountain Repository, USA

Peter N. Swift1, Kathryn Knowles1, Jerry McNeish1, Clifford W. Hansen1, Robert L. Howard2,Robert MacKinnon1, S. David Sevougian1

}Sandia National Laboratories, Department of Energy Office of Civilian Radioactive WasteManagement Lead Laboratory for Repository Systems, 1180 North Town Center Drive, MailStop LL423, Las Vegas, NV 891442University of Nevada Las Vegas National Supercomputing Center for Energy and theEnvironment, 4505 Maryland Parkway, Box 454028, Las Vegas, NV 89154

ABSTRACT

This paper summarizes the historical development of the United States Department ofEnergy's (DOE's) 2008 performance assessment for the proposed high-level radioactive wasterepository at Yucca Mountain, Nevada, and explains how the methods and results meetregulatory requirements specified by the United States Nuclear Regulatory Commission (NRC)and the United States Environmental Protection Agency (EPA). Topics covered include (i)screening of features, events and processes, (ii) development of scenario classes, (iii)descriptions of barrier capability, and (iv) compliance with applicable quantitative standards forindividual protection, individual protection following human intrusion, and ground waterprotection.

INTRODUCTION

The United States Department of Energy Office of Civilian Radioactive WasteManagement (DOE-OCRWM) submitted a license application on 3 June 2008 to the UnitedStates Nuclear Regulatory Commission (NRC) seeking authorization to construct a repository atYucca Mountain, Nevada, for the permanent disposal of spent nuclear fuel (SNF) and high-levelradioactive waste (HLW) [1]. On 8 September 2008, the NRC accepted the DOE's applicationfor technical review and docketed it to begin the formal regulatory process of review andhearings. The three- to four-year regulatory process is expected to lead to a licensing decisionregarding construction authorization no sooner than September 2011.

The total system performance assessment (TSPA) model developed to support the DOE'slicense application provides quantitative estimates of long-term performance of the repository,including estimates of future radiation doses to a hypothetical "reasonably maximally exposedindividual" (RMEI) [2], and is based on more than two decades of scientific investigations.Modeling work performed for this TSPA builds on prior iterations of system-level analyses [3-8]beginning in the early 1990s and continuing through the 2002 Yucca Mountain SiteRecommendation [9] and Final Environmental Impact Statement [10].

REGULATORY FRAMEWORK

The management of SNF and HLW is governed in the United States by the provisions ofthe Nuclear Waste Policy Act of 1982 (NWPA), as amended [11]. As required by the NWPA,the United States Environmental Protection Agency (EPA) has issued public health and






environmental radiation protection standards for Yucca Mountain [12], and the NRC has issuedfinal and proposed regulatory requirements [13,14] at 10 Code of Federal Regulations (CFR)Part 63 that establish criteria for the implementation of the EPA standards. Additional guidancerelevant to evaluating compliance with the NRC regulations is provided in the Yucca MountainReview Plan [15].

The EPA and NRC regulations define the overall framework for the TSPA as an analysis thatidentifies and evaluates relevant features, events, and processes (FEPs) that could affectrepository performance, and then estimates performance taking into account uncertaintiesassociated with significant FEPs and weighting their consequences by their probabilities ofoccurrence. In practice, this regulatory direction is implemented as a probabilistic uncertaintyanalysis using numerical models for components of the repository and the geologic setting,linked into a system-level model suitable for Monte Carlo uncertainty analysis [16-21].

In addition to defining the overall method to be used for the performance assessment,regulations also define the quantitative limits on long-term performance that are to be met fordemonstrations of regulatory compliance. Specifically, 10 CFR 63.311 defines an individualprotection standard that limits the mean annual dose to the RMEI during 10,000 years afterdisposal to 0.15 mSv (15 mrem). The rule also provides separate standards for allowablereleases of radiation to ground water (10 CFR 63.331), and a limit for mean annual doses to theRMEI following an assumed and stylized human intrusion event (10 CFR 63.321). The finalEPA rule for implementing a dose standard after 10,000 years [12] extends the individualprotection and human intrusion standards to 1,000,000 years, setting a limit on the mean annualdose to the RMEI of 1 mSv (100 mrem) for the period between 10,000 and 1,000,000 years.

The EPA and NRC regulations define the scope of the long-term analysis by definingcriteria for FEPs. Specifically, performance assessments shall not consider features, events, orprocesses "that are estimated to have less than one chance in 100,000,000 per year of occurring."Impacts of FEPs that have a higher probability of occurrence need not be evaluated if overallperformance in the initial 10,000 years "would not be changed significantly" by their occurrence(40 CFR 197.36(a)(l)). Regulatory requirements call for the use of "multiple barriers, consistingof both natural barriers and an engineered barrier system" (10 CFR 63.113(a)). Although noquantitative limits apply to the performance of components of the multiple barrier system, theDOE is required to describe the capability of barriers and to provide the technical basis for thatdescription, consistent with the performance assessments used to demonstrate compliance withthe system-level standards.

As required by 10 CFR 63 Subpart G, all scientific and engineering work that directlysupports the performance assessment for the license application is performed and documented inaccordance with appropriate quality assurance standards [22].

THE TSPA MODEL

The TSPA is a system-level model that integrates submodels for each of the variouscomponents of the natural and engineered barriers, simulating the various processes that canaffect repository performance, including water movement, material degradation, andradionuclide transport [17-19]. Because of the complexity of the individual processes and theneed for a large number of system-level simulations to support the Monte Carlo uncertaintyanalysis, the TSPA model relies on simplifications (abstractions) of some of the major processes.As described in Ref. 2, section 6, the TSPA model relies both on linked external process models






that simulate specific processes as well as a set of abstractions that run within GoldSim software[23].

The Yucca Mountain TSPA identified the scenarios to be analyzed for the licenseapplication by following a five-step approach shown in Figure 1. A comprehensive list ofpotentially relevant FEPs was identified based on input from past Yucca Mountain performanceassessments and input from other programs internationally, and the FEPs were evaluated andscreened according to criteria specified by the NRC, e.g., at 10 CFR 63.114 [24,25]. Of the 374FEPs identified for evaluation, 222 were excluded from the quantitative TSPA modeling on thebasis of either low probability, low consequence, or incompatibility with regulatoryrequirements. The remaining 152 FEPs were included in one or more of the performanceassessment analyses for evaluating compliance with the regulatory standards.

FEP Analysis J

Identify and Classify FEPs PotentiallyImportant to PostcJosure Performance,

Including Input from International RadioactiveWaste Disposal Programs

Screen Ust of FEPs Using Probability,Consequence, art®* NRC Regulations to

Determine Inclusion and Exclusion

ScenarioDevelopment

Construct Nominal and Disruptive EventsScenario Classes from Retained FEPs

Construct Calculation of TotalMean Annual Dose

Implementation *Specify the Implementation of Nominal

and Disruptive Events Scenario Classesin TSPA

00»17OCJB4Oai

Figure 1. Steps in the Development of Scenarios for the Yucca Mountain TSPA (from Ref. 2,figure 6.1.1-1)






The large majority of FEPs included in the performance assessment analyses are featuresand processes that are relevant to essentially all possible future states of the disposal system.Some included FEPs, however, represent rare events that may or may not occur in the future, andtherefore can be used to define scenario classes for analysis. The TSPA analyzes four discretescenario classes that characterize the range of future states of the system, in addition to a stylizedhuman intrusion scenario prescribed by regulation for evaluation under a separate standard (10CFR 63.311) [13,15]. The four scenario classes that are treated probabilistically include an earlyfailure scenario class, in which one or more waste packages or overlying drip shields failsprematurely due to undetected manufacturing or emplacement defects; an igneous disruptionscenario class in which a volcanic event causes magma to intersect the emplacement region, withor without an accompanying eruption; a seismic disruption scenario class, in which groundmotion or fault displacement damages waste packages and drip shields; and a nominal scenarioclass in which none of these three types of events occurs. Each of the event-based scenarioclasses is further subdivided into separate modeling cases that simulate consequences of specificevents: i.e., early drip shield and waste package failure cases, an igneous intrusion case (withouteruption), a volcanic eruption case, a seismic ground motion damage case, and a faultdisplacement damage case. The total mean annual dose for 10,000 years is developed bysumming the mean annual doses for each modeling case, noting that calculated releases fromnominal performance in this analysis are zero during the first 10,000 years, and taking intoaccount the probability of occurrence of each type of event. Analyses include both aleatoryuncertainty associated with the timing and characteristics of the events and epistemic uncertaintyassociated with the physical properties of the disposal system [16,20,21]. For analyses of1,000,000-year performance, when releases from nominal processes (e.g., general corrosion)become significant and damage from seismic ground motion becomes relatively likely to occursimply because of the very long time under consideration, the seismic ground motion andnominal modeling cases are combined into a single modeling case.

BARRIER CAPABILITY

Consistent with the requirements of 10 CFR 63.113(a), the DOE has identified threebarriers that are important to waste isolation at Yucca Mountain: an upper natural barrier thatlimits the amount of water reaching the drift environment; the engineered barrier system thatboth limits water flow and restricts radionuclide migration from the waste; and the lower naturalbarrier that significantly delays radionuclides during ground water transport away from therepository. Each of these barriers is composed of multiple components that contribute to theoverall capability of the system.

The upper natural barrier includes the surface soils and topography that limit the amountof precipitation that infiltrates into the bedrock, the 200 to 400 meters of unsaturated rock thatdamp the episodic effects of weather-related infiltration, and the capillary barrier provided by thedrift walls that tends to divert flow around the emplacement regions. The overall effect of theupper natural barrier is to reduce water flow (percolation flux) in the mountain to a small fraction(5 to 7 percent) of the mean annual precipitation above the repository, and to further reducemean seepage (the amount of water that enters a drift) to a small fraction of percolation flux (2-11%) in intact drifts (Ref. 2, Section 8.3.3.l.l[a]).

The engineered barrier system includes: the drift environment; the titanium drip shield,the waste package, including the Alloy 22 outer corrosion barrier, the inner stainless steel






container; the waste forms and associated shipping containers; the emplacement pallet; and theinvert material that forms the floor of the drift. The drip shield contributes to barrierperformance by providing a robust barrier to the flow of water: modeled failures of the dripshield due to corrosion occur between 270,000 and 340,000 years after closure, and the dripshield effectively prevents flowing water from reaching the waste until then. Earlier failures ofthe drip shield may occur due to emplacement errors, igneous disruption, fault displacement, orextreme ground motion events, but all are anticipated to occur at very low probabilities.Similarly, the Alloy 22 outer corrosion barrier of the waste packages provides a very effectivebarrier to both water flow and radionuclide transport. Under nominal conditions (without groundmotion damage), cracks in the closure welds of waste package, which would allow diffusivetransport of radionuclides, are modeled to occur only rarely before approximately 170,000 yearsafter closure. Taking ground motion damage into account, stress corrosion cracks can occurearlier in the repository history, but releases are still anticipated to occur by diffusion only.Larger penetrations of the waste package outer barrier due to general corrosion of the Alloy 22,which would allow advective transport of radionuclides in flowing water, rarely occur in themodel before about 560,000 years. By 1,000,000 years, approximately 9 percent of the wastepackages have penetrations due to general corrosion, regardless of damage due to seismic groundmotion. Seismic damage that could result in advective transport of radionuclides is accountedfor in the model, but rarely occurs (Ref. 2, Section 8.3.3.2 [a]).

The lower natural barrier contains 250 to 380 meters of unsaturated rocks (estimated tobe up to 120 meters less during wetter future climates) between the repository and the watertable, and approximately 18 km laterally along the potential transport pathway ground waterfrom the repository to the location of the RMEI. Processes that will delay radionuclide transportthrough the lower natural barrier include the slow rate of water flow, diffusion of dissolved-phase radionuclides from fractures into the rock matrix where water moves more slowly,sorption of dissolved species, dispersion and dilution of radionuclides transported in colloidalphases, and reversible filtration of colloidal species. Estimated transport times through theunsaturated portion of the lower natural barrier vary considerably, depending on the degree ofsorption and the extent to which transport occurs in fractures or in matrix porosity. Fornonsorbing radionuclides (e.g., 99Tc), median transport times through the unsaturated zone rangefrom tens of years (for cases in which transport is largely in fractures) to many thousands ofyears (for cases in which transport is largely in matrix porosity) (Ref. 26., Figure 6.6.2-8[b]).Transport times for sorbing species are longer. Similar ranges of transport times are estimatedfor the saturated zone. For nonsorbing radionuclides (e.g., Tc), median transport times rangefrom 100 years to 22,000 years, and strongly sorbing species (e.g., 226Ra) show median transporttimes from 18,000 years to more than 1,000,000 years (Ref. 27, section 6.7.1 [a]). Overall, thelower natural barrier effectively delays transport of many radionuclides sufficiently thatradioactive decay greatly reduces their concentrations by the time they reach the location of theRMEI.

Figure 2 summarizes overall barrier performance for the seismic ground motion modelingcase (which is a major contributor to the total mean annual dose) by displaying the meaninventory through time that has been released from the disposal system for selectedradionuclides, together with the total inventory that was initially present when the simulationbegan. All values are corrected for radioactive decay and ingrowth; thus, the total inventorycurve drops through time as radioactive decay reduces the overall radioactivity of the waste.






(a) LA_y5 005LAj/5 005^SM^009000^003 gsm.

f

. TotalInventory

• Total SZ

, 9§Tc

, , , , , * 230Th

M w « 234U

237Np_ _ . 239puT

239pu!. . . . . . 240puT. . „ . , 240pul

_ 242puT

» . . 242pul

243AmT

100 1,000 10,000Time (years)

100,000 1,000,000

Figure 2. Mean activity released from the saturated zone at the location of the RMEI, seismicground motion modeling case (from Ref.2, figure 8.3-26[a]a).

The release curves for individual radionuclides show a characteristic pattern of increasingas the probability of having an event that causes releases from the disposal system increases, andthen eventually declining as radioactive decay removes the species from the overall inventory ofmaterial released. Interpretation of this figure shows that 1,000,000 years after closure thedisposal system still contains, on average, approximately 95 percent of the total activityremaining at that time, and the large majority of the activity that has been released is in the formof 99Tc, which transports essentially without retardation. Short-lived species (e.g., 90Sr and137Cs) do not appear at all on the figure because they are fully contained within the disposalsystem until after decay has essentially removed them from the inventory. Radionuclides withintermediate half-lives (e.g., most isotopes of Pu and Am) show mean releases beginning after10,000 years, but releases are small and radioactive decay results in declining mean release totalsbefore 1,000,000 years. Radionuclides for which released activity is flat or still climbing at1,000,000 years are those that have very long half lives (99Tc, 242Pu, 237Np, 234U), and 226Ra, forwhich the release rate is determined by the concentration of its precursor species 234U.

EVALUATION OF COMPLIANCE WITH QUANTITATIVE LONG-TERMSTANDARDS

Figures 3 and 4 show estimated expected annual doses, summed over all scenario classes.Individual curves on these plots represent different samplings of epistemic uncertainty associatedwith the characteristics of the physical system, and are averaged over the aleatory uncertainty






LA_v5.005_ED_003000_001.g»m; LA_v5.OO5_EW_0060CX3_001 gsm;LA V5005JG 003000 001 .gam; IAvS.005 SF 010800 001 .gem,

LA V5.005 SM 009000 001.gmt; vE1 004 GS 9.60.100JOKyrJfT[evefrtttme}.gsm.LA v5 005 10kyf Total Dose Calcs RevOi.gsm; LA vS.OOS 10Kyr Total Dose_RevO1 JNS

2,000 8,000 10,0004,000 6,000Time (years)

Figure 3. Total expected annual dose, summed over all scenario classes, for 10,000 years,(from Ref. 2, Figure 8.1-1 [a])

LA_v5 O05.EDJ)030GO OOO.gsm; LA V5.00S EW 006000 OOO.gsm;LAj/5 005JGJ»3000_000 gsm; LA v5.005 SF 0i080G_QQ0.gsm,

U_v5.005_SM_009000_003,gsm; vE1 004_GS_8.60 lOOJMyr ETfevent timel gsm;^ 3 LA^v$ 005,1 Myr^TotaLDose GatesJtevOO.gsm; LA vS,00$_1Myr Total Dose RovQOJNB

111

• Mean> Median> 95th Percentile> 5th Percentile

800,000 1,000,000200,000 400,000 600,000

Time (years)Figure 4. Total expected annual dose, summed over all scenario classes, for 1,000,000 years

(from Ref. 2, Figure 8.1-2[a]).






associated with the time and characteristics of events. Summary curves show the mean, median,5th, and 95th percentile values of the expected annual dose. For the 10,000-year individualprotection standard (10 CFR 63.311), the mean curve shown on Figure 3 is the appropriatemeasure for comparison to the regulatory limit of 0.15 mSv/yr (15 mrem/yr). For the 1,000,000-year standard, the mean curve on Figure 4 is the appropriate curve for comparison to the EPAlimit of 1 mSv/yr (100 mrem/yr). The maximum mean annual dose during 10,000 years is 0.24mrem (0.0024 mSv), and the maximum mean annual dose during 1,000,000 years is 2.0 mrem(0.02 mSv); both values are well below the regulatory limits. Figure 5 shows the meancontribution to the total mean annual dose from each of the major modeling cases included in theanalysis.

LA_v5.005_ED_003000_000.gsm; LA_v5.005_EW_006000_000.gsm ;LA_v5.005_IG_p03000_000.gsm; LA_v5.005_SM_009000_003.gsm;

LA_v5.005_SF_010800_OOO.gsm;vE1.004_GS_9.60.100_1Myr_ET[eventtime].gsm;LA_v5.005_1Myr_Total_Dose_Mean_Contributions_RevOO.JNB

Total

Drip Shield Early FailureWaste Package Early FailureSeismic Ground Motion

Seismic Fault Displacement

Igneous Intrusion

Volcanic Eruption

10-6

200000 400000 600000 800000 1000000

Time (years)Figure 5. Mean annual dose contributions from individual modeling cases to the total meanannual dose for 1,000,000 years (from Ref. 2, figure 8.1-3b[a]).

Compliance with the 10,000-year ground water protection standards (10 CFR 63.331) isdemonstrated by estimating concentrations of radionuclides in ground water at the location of theRMEI, excluding human intrusion and unlikely natural events (i.e., igneous disruption).Specifically, the combined activity of 226Ra and 228Ra in ground water must be less than 5picocuries per liter, and the total gross alpha activity, including 226Ra but excluding uranium andradon, must be less than 15 picocuries per liter, with both values including natural background.The maximum mean ground water concentration of combined radium derived from the proposedrepository is estimated to be 1.3 x 10~7 pCi/L, far below the natural background level of 0.5pCi/L, which in turn is well below the regulatory limit. Similarly, the mean maximum grossalpha activity resulting from releases from the proposed repository is estimated to be 6.7 x 10~5

pCi/L, far below the natural background of 0.5 pCi/L, which in turn is well below the regulatorylimit. Estimated mean annual doses from beta and photon emitting radionuclides in 2 liters per

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