Total System Performance Assessment Research Articles

Cladding Evaluation in the Yucca Mountain Repository Performance Assessment

The Yucca Mountain Project (YMP) 1998 Total System Performance Assessment Viability Assessment (TSPA-VA) analyzed the degradation of Zircaloy clad commercial fuel rods and the resulting exposure of the fuel in the event of a waste package failure. The cladding degradation mechanisms considered were damage before emplacement, mechanical failure from drift collapse, localized corrosion, general corrosion, delayed hydride cracking (DHC), hydride reorientation, creep rupture, and stress corrosion cracking (SCC). The potential for further cladding degradation due to cladding rupture as a result of fuel oxidation was also considered in the modeling effort. These models have been improved for use in future TSPAs. The current cladding degradation model divides the analysis into two phases, cladding failure (perforation) and cladding unzipping (crack propagation caused by the expansion of UO2 fuel after reaction with water). Cladding failure occurs during reactor operation, from creep strain failure during high temperature periods in dry storage or in the early periods in the repository, or localized corrosion. After a Waste Package (WP) containing spent nuclear fuel in the repository fails, moisture is assumed to enter the waste package and the failed cladding starts to unzip (tear open) from the formation of secondary uranium phases. This slowly exposes the fuel. In addition, the inventory of fission products located in the gap between the cladding and fuel pellet is rapidly released. The cladding model limits the amount of fuel that is exposed to moisture and becomes available for dissolution. As a result, the doses to the affected population are reduced (factor of 20 to 50 in TSPA-VA) from the case where cladding is not considered.

Total System Performance Assessment for Waste Disposal Using a Logic Tree Approach

The Electric Power Research Institute (EPRI) has sponsored the development of a model to assess the long‐term, overall “performance” of the candidate spent fuel and high‐level radioactive waste (HLW) disposal facility at Yucca Mountain, Nevada. The model simulates the processes that lead to HLW container corrosion, HLW mobilization from the spent fuel, and transport by groundwater, and contaminated groundwater usage by future hypothetical individuals leading to radiation doses to those individuals. The model must incorporate a multitude of complex, coupled processes across a variety of technical disciplines. Furthermore, because of the very long time frames involved in the modeling effort (>104 years), the relative lack of directly applicable data, and many uncertainties and variabilities in those data, a probabilistic approach to model development was necessary. The developers of the model chose a logic tree approach to represent uncertainties in both conceptual models and model parameter values. The developers felt the logic tree approach was the most appropriate. This paper discusses the value and use of logic trees applied to assessing the uncertainties in HLW disposal, the components of the model, and a few of the results of that model. The paper concludes with a comparison of logic trees and Monte Carlo approaches.

A BIOSPHERE MODELING METHODOLOGY FOR DOSE ASSESSMENTS OF THE POTENTIAL YUCCA MOUNTAIN DEEP GEOLOGICAL HIGH LEVEL RADIOACTIVE WASTE REPOSITORY

Recent developments in performance standards for proposed high level radioactive waste disposal at Yucca Mountain suggest that health risk or dose rate limits will likely be part of future standards. Approaches to the development of biosphere modeling and dose assessments for Yucca Mountain have been relatively lacking in previous performance assessments due to the absence of such a requirement. This paper describes a practical methodology used to develop a biosphere model appropriate for calculating doses from use of well water by hypothetical individuals due to discharges of contaminated groundwater into a deep well. The biosphere model methodology, developed in parallel with the BIOMOVS II international study, allows a transparent recording of the decisions at each step, from the specification of the biosphere assessment context through to model development and analysis of results. A list of features, events, and processes relevant to Yucca Mountain was recorded and an interaction matrix developed to help identify relationships between them. Special consideration was given to critical/potential exposure group issues and approaches. The conceptual model of the biosphere system was then developed, based on the interaction matrix, to show how radionuclides migrate and accumulate in the biosphere media and result in potential exposure pathways. A mathematical dose assessment model was specified using the flexible AMBER software application, which allows users to construct their own compartment models. The starting point for the biosphere calculations was a unit flux of each radionuclide from the groundwater in the geosphere into the drinking water in the well. For each of the 26 radionuclides considered, the most significant exposure pathways for hypothetical individuals were identified. For 14 of the radionuclides, the primary exposure pathways were identified as consumption of various crops and animal products following assumed agricultural use of the contaminated water derived from the deep well. Inhalation of dust (11 radionuclides) and external irradiation (1 radionuclide) were also identified as significant exposure modes. Contribution to the total flux to dose conversion factor from the drinking water pathway for each radionuclide was also assessed and for most radionuclides was found to be less than 10% of the total flux to dose conversion factor summed across all pathways. Some of the uncertainties related to the results were considered. The biosphere modeling results have been applied within an EPRI Total Systems Performance Assessment of Yucca Mountain. Conclusions and recommendations for future performance assessments are provided.

Alternative Assessments of the Performance of the Yucca Mountain Candidate HLW Repository

AbstractThis paper summarizes some of the most recent total system performance assessment (TSPA) work supported by EPRI for the proposed repository at Yucca Mountain, Nevada. In Part I of the paper, (standard) TSPA analyses are presented. In Part II examples of two types of analyses that augment standard TSPAs are provided that suggest: (1) many components of the Yucca Mountain system contribute to overall hazard reduction; and (2) the biosphere dose conversions used, based on reasonably maximizing assumptions about future human behavior, provide a reasonable upper bound. These additional analyses should provide further confidence when considering more (standard) TSPA analyses.

Multi-Scale Near-Field Thermohydrologic Analysis of Alternative Designs for the Potential Repository at Yucca Mountain

AbstractA multi-scale, thermohydrologic (TH) modeling methodology has been developed that integrates the results from 1-, 2-, and 3-D drift-scale models and a 3-D mountain-scale model to calculate the near-field TH variables affecting the performance of the engineered barrier system (EBS) of the potential repository at Yucca Mountain. This information was used by Total System Performance Assessment—Viability Assessment (TSPA-VA) and is being used by the ongoing TSPA, supporting the License Application Design Selection, to assess waste-package (WP) corrosion, waste-form dissolution, and radionuclide transport in the EBS. Line-load WP spacing, which places WPs nearly end to end in widely spaced drifts, results in more locally intensive and uniform heating along drifts, causing hotter, drier, and more uniform conditions on WPs than point-load spacing, which is used in the VA design. Backfilling drifts with a granular material with coarse, well-sorted, nonporous grains (e.g., a coarse quartz sand) results in a large, persistent reduction in RH on WPs; point-load spacing allows only the medium-to-high-heat-output WPs to benefit from RH reduction, but line-load spacing enables all WPs to benefit.

Total system performance assessment for waste disposal using a logic tree approach.

The Electric Power Research Institute (EPRI) has sponsored the development of a model to assess the long-term, overall "performance" of the candidate spent fuel and high-level radioactive waste (HLW) disposal facility at Yucca Mountain, Nevada. The model simulates the processes that lead to HLW container corrosion, HLW mobilization from the spent fuel, and transport by groundwater, and contaminated groundwater usage by future hypothetical individuals leading to radiation doses to those individuals. The model must incorporate a multitude of complex, coupled processes across a variety of technical disciplines. Furthermore, because of the very long time frames involved in the modeling effort (>> 10(4) years), the relative lack of directly applicable data, and many uncertainties and variabilities in those data, a probabilistic approach to model development was necessary. The developers of the model chose a logic tree approach to represent uncertainties in both conceptual models and model parameter values. The developers felt the logic tree approach was the most appropriate. This paper discusses the value and use of logic trees applied to assessing the uncertainties in HLW disposal, the components of the model, and a few of the results of that model. The paper concludes with a comparison of logic trees and Monte Carlo approaches.

Alternate Source Term Models for Yucca Mountain Performance Assessment Based on Natural Analog Data and Secondary Mineral Solubility

AbstractPerformance assessment calculations for the proposed high level radioactive waste repository at Yucca Mountain, Nevada, were conducted using the Nuclear Regulatory Commission Total-System Performance Assessment (TPA 3.2) code to test conceptual models and parameter values for the source term based on data from the Peña Blanca, Mexico, natural analog site and based on a model for coprecipitation and solubility of secondary schoepite. In previous studies the value for the maximum constant oxidative alteration rate of uraninite at the Nopal I uranium body at Peña Blanca was estimated. Scaling this rate to the mass of uranium for the proposed Yucca Mountain repository yields an oxidative alteration rate of 22 kg yr−1, which was assumed to be an upper limit on the release rate from the proposed repository. A second model was developed assuming releases of radionuclides are based on the solubility of secondary schoepite as a function of temperature and solution chemistry. Releases of uranium are given by the product of uranium concentrations at equilibrium with schoepite and the flow of water through the waste packages. For both models, radionuclides other than uranium and those in the cladding and gap fraction were modeled to be released at a rate proportional to the uranium release rate, with additional elemental solubility limits applied. Performance assessment results using the Peña Blanca oxidation rate and schoepite solubility models for Yucca Mountain were compared to the TPA 3.2 base case model, in which release was based on laboratory studies of spent fuel dissolution, cladding and gap release, and solubility limits. Doses calculated using the release rate based on natural analog data and the schoepite solubility models were smaller than doses generated using the base case model. These results provide a degree of confidence in safety predictions using the base case model and an indication of how conservatism in the base case model may be reduced in future analyses.

Nuclear Repository Performance Assessment: Insights into Critical Models and Parameters Affecting Projected Future Doses

ABSTRACTThe Phase 3 Total System Performance Assessment (TSPA) sponsored by the Electric Power Research Institute (EPRI) has led to new insights into critical models and parameters affecting estimated doses to humans from a potential repository of high-level radioactive wastes at Yucca Mountain, Nevada. The Phase 3 model has been extended to encompass time-varying climate and infiltration, detailed modeling of the source term and hydrology, and detailed specification of possible interaction between percolating ground water and waste containers. The model estimates doses to a time of one million years.The three key radionuclides contributing to estimated total doses are Tc-99,1–129, and Np-237. Five other nuclides contributing to dose in lesser (but significant) amounts are U-233, Th-229, Pa-231, Ac-227, and Se-79. These results are consistent with other TSPAs.From sensitivity studies, the most critical models and parameters are as follows. Infiltration and percolation assumptions, including the amount of lateral diversion of infiltration water, are important and need verification with site data and/or more detailed modeling. Parameters of the unsaturated zone (UZ) and saturated zone (SZ) determine dilution and delay of concentrations and peak doses downstream. The fraction of containers that become wet are not critical in our model, but this lack of sensitivity reflects our coupling of the fraction with a model of focused flow past the containers; an different model might indicate higher sensitivity. Also, the degree of coupling between fracture and matrix flow is important in affecting the times of peak doses but not their magnitudes.Other critical design assumptions that could lead to reduced and/or delayed doses are a more robust container design, a capillary barrier around each container, the dilution during hydrologie transport from the repository to a potential agricultural community downstream, and the characteristics of an “average” individual in that community who might receive a dose.

Incorporation of “Corrosion-Time” and Effects of Corrosion-Product Spalling in Waste Package Degradation Simulation in the Potential Repository at Yucca Mountain

ABSTRACTA two-layer waste package (carbon steel outer barrier and Alloy 825 inner barrier) is specified to dispose of high-level nuclear waste at the potential repository at Yucca Mountain. A set of improvements and more realism have been added to a stochastic waste-package degradation model which was developed for a recent total system performance assessment of the potential repository [1]. The waste-package surface is divided into “patches” to better represent the general corrosion of the carbon-steel outer barrier. The “corrosion-time” concept is developed to represent the corrosion of the carbon-steel outer barrier in changing exposure conditions with time such as those expected in the potential repository. With the patches approach and the corrosion-time concept implemented into the waste-package degradation model, sensitivity of the waste package degradation (failure and pitting degradation) to different threshold spalling thicknesses of the corrosion products from the carbon-steel outer barrier is analyzed. The results show that the waste-package pitting degradation is sensitive to the corrosion-products spalling thickness of the carbon-steel outer barrier. A greater pitting degradation of the waste packages is predicted with a smaller spalling thickness. Further understanding of the corrosion-products spalling in different exposure conditions (i.e., water chemistry, water contact mode, etc.) and its effects on carbon steel corrosion is needed to enhance the confidence in the waste-package performance modeling in the potential repository.

Preliminary Analysis of Important Site-Specific Dose Assessment Parameters and Exposure Pathways Applicable to a Groundwater Release Scenario at Yucca Mountain

AbstractTo develop capabilities for compliance determination, the Nuclear Regulatory Commission (NRC) conducts total system performance assessment (TSPA) for the proposed repository at Yucca Mountain (YM) in an iterative manner. Because the new Environmental Protection Agency (EPA) standard for YM may set a dose or risk limit, an auxiliary study was conducted to develop estimates of site-specific dose assessment parameters for future TSPAs. YM site-relevant data was obtained for irrigation, agriculture, resuspension, crop interception, and soil. A Monte Carlo based importance analysis was used to identify predominant parameters for the groundwater pathway. In this analysis, the GENII-S code generated individual annual total effective dose equivalents (TEDEs) for 20 nuclides and 43 sampled parameters based upon unit groundwater concentrations. Scatter plots and correlation results indicate the crop interception fraction, food transfer factors, consumption rates, and irrigation rate are correlated with TEDEs for specific nuclides. Influential parameter groups correspond to expected pathway behavior of specific nuclides. Results for nuclides that transfer more readily to plants, such as 99Tc, indicate crop ingestion pathway parameters are most highly correlated with the TEDE, and those that transfer to milk (59Ni) or beef (79Se, 129I, 135Cs, 137Cs) show predominant correlations with animal product ingestion pathway parameters. Such relationships provide useful insight to important parameters and exposure pathways applicable to doses from specific nuclides.