Analysis of Total System Performance Assessment

References:

• Letter dated September 18, 2001, from George M. Hornberger, Chairman, ACNW, to Richard A. Meserve, Chairman, NRC, Subject: Total System Performance Assessment-Site Recommendation.
• Letter dated January 17, 2002, from George M. Hornberger, Chairman, ACNW, to Richard A. Meserve, Chairman, NRC, Subject: Total System Performance Assessment and Conservatism
• Nuclear Waste Technical Review Board. 2002a.  Letter Report to Congress and the Secretary of Energy, January 24, 2002. Arlington, Virginia.
• Nuclear Waste Technical Review Board. 2002b. Report to the Secretary of Energy and the Congress, April 2002. Arlington, Virginia.
• Nuclear Energy Agency (NEA). 2002. An International Peer Review of the Yucca Mountain Project TSPA-SR. Paris, France.

The following are summaries of analyses of the TSPA-SR by federal regulatory agencies, such as the NWTRB and the USNRC, and an international organization, the NEA.

NEA Evaluation of the TSPA-SR

·        Overall system methodology

• A comprehensive methodology for classifying uncertainties should be created.  Uncertainties should be clustered into aleatory uncertainties (uncertainties associated with statistical variability) and epistemic uncertainties (those associated with a lack of knowledge).  The former can be specified quantitatively while the latter are difficult to calculate and can lead to non-conservative results.  In addition, reducing uncertainty should be a primary goal of future performance assessments.
  
  
• Risk dilution may be a problem in TSPA-SR results and should be addressed.  Risk dilution occurs when uncertainty in certain parameters is overestimated and this increase in the variability of input leads to a decrease in the mean of the output.  Thus, the use the conservative parameter values could lead to a reduced mean dose of radiation in the TSPA models.  NEA recommends that DOE study the importance of risk dilution in their models in addition to evaluating the overall limits of the probabilistic method used in the TSPA.
  
  
• The NEA approved of the sensitivity analysis in the TSPA-SR.  Sensitivity analysis determines the relative significance of uncertainties in different parts of the simulation with respect to repository performance.
  
  
• A more comprehensive safety case should be developed.  Normally, a safety case integrates all relevant arguments in support of the long-term safety of the repository.  In the case of the TSPA-SR, the safety case consisted mostly of the analysis of the results of the computer simulations, which demonstrated compliance with EPA regulations.  NEA recommends that the safety case be better developed with a focus on communication to relevant stakeholders and acknowledgement of remaining technical issues.
  
  
• The DOE should improve its understanding of the repository system behavior as a whole.  While the TSPA-SR demonstrates regulatory compliance, it places little emphasis on contextualizing model conceptualizations and results within the body of knowledge acquired during the site investigation and repository design.  In order to accomplish this goal, a realistic analysis of repository performance should be completed where data from natural analogues, sites with geology similar to the proposed repository, and other sources are fully utilized.  In other words, there should be a clear distinction between an analysis containing conservative assumptions and one based on realistic and empirically supported assumptions. 
  
  
• During the history of the Yucca Mountain Project, the TSPA has undergone a number of different changes and revisions.  However, the rationale for these changes is not clearly documented in the TSPA-SR.  A history of repository design changes and the corresponding technical concerns driving them would make the project’s maturation more transparent.
  
  
• The NEA also recommends further research in a number of areas: initiation of long-term studies of corrosion rates of the waste canister material alloy-22; experimental verification of thermodynamic models and parameters such as solubility constants; independent evaluation of the engineered barrier transport model; and further study of natural analogues including the Peña Blanca uranium deposit in northern Mexico.

·        International Perspective

• The geologic conditions of Yucca Mountain are significantly different from those found in many other proposed repositories.  For example, Yucca Mountain is located in a closed groundwater basin above the water table in oxidizing conditions.
  
  
• Much of the recent emphasis of DOE performance assessments has been on the engineered barriers instead of the geologic barrier.  Many other national repository programs focus more on the performance of the geologic barrier.
  
  
• The EPA regulations and other regulatory requirements governing Yucca Mountain are “more prescriptive than in many other countries” (NEA, 2002).  For instance, the 10,000 year period over which the repository must demonstrate a “reasonable expectation” of compliance and the specification of the representative volume of groundwater to be used in dose rate calculations are more specific than analogous requirements in other countries.  This specificity and rigidity has caused the TSPA-SR “to focus more on demonstrating numerical compliance with quantitative criteria than on demonstrating an understanding of repository performance.

NWTRB Review of Scientific and Technical Work Related to Discussion of a Site Recommendation for Yucca Mountain

• Viewed as a whole, the NWTRB believes the technical and scientific work performed as of January 2002 provides a weak to moderate basis for the DOE’s repository performance estimates.  This is a result of the many gaps in data and basic understanding of the relevant processes, which results in a number of corresponding uncertainties.  However, the NWTRB believes that “at this point, no individual technical or scientific factor has been identified that would automatically eliminate Yucca Mountain from consideration as the site of a permanent repository” (NWTRB, 2002b).
  
  
• The NWTRB sees room for improvement in the understanding of a number of components the repository system.  These components include the hydrologic properties of rock faults, rates of corrosion of alloy-22 at long time scales and high-temperatures relevant to repository conditions, colloid-facilitated transport of radionuclides, and intrusive igneous activity into the repository.
  
  
• It is important to realize that there “is no fully satisfactory way of validating the results of the TSPA” and there much to be done to develop confidence in performance estimates and repository design (NTWRB 2002a).  Studying natural and engineered analogues to repository system elements can “offer insights into long-term…repository processes” that are not observable in the laboratory.  In addition, such analogues bolster a “defense-in-depth” argument, where the safety of the repository does not depend on any single system component.
  
  
• In the TSPA-SR and previous performance assessments, the DOE has often mixed conservative assumptions and models with optimistic or realistic ones.  This has left unclear the level of conservatism and uncertainty in the models.  Thus, assessments of repository performance and future risks become questionable.  The NWTRB has recommended that the DOE quantify the levels of uncertainty and conservatism in their performance assessments.  Although there is still much work to be done, a significant effort at quantifying uncertainty has been made in the Supplemental Science and Performance Analyses issued in July 2001.
  
  
• The DOE’s base case design for Yucca Mountain has been a “hot design” where the temperature of the repository is kept high enough to boil water.  Ideally, water would boil away before it could contact waste containers, but the high temperature would also increase corrosion rates of the drip shield and waste canister.  In the opinion of the NWTRB, the DOE should perform a “full evaluation and comparison of the base-case (high-temperature) and low-temperature designs.”  Moreover, there is little data for corrosion rates at the high-temperatures of a hot design.
  

Review of the TSPA-SR by the USNRC’s Advisory Committee on Nuclear Waste (ACNW)

• ACNW believes that the TSPA-SR does not present realistic and risk-informed conclusions.  Many of the input data and parameters are based on conservative rather than realistic assessments of the available scientific evidence.  There are few connections between many of the assumptions in the TSPA-SR models and scientific data.  In other words, computations and analyses are based on assumptions that rely little on available evidence.  Moreover, conservative assumptions are mixed with non-conservative assumptions and this mixing obscures a realistic assessment of risk.
  
  
• Also, ACNW believes that motivation of the TSPA-SR is to demonstrate regulatory compliance rather than provide a risk informed assessment of repository performance.
  
  
• The DOE claims to choose conservative parameter distributions, but defining what is or is not conservative when non-linear processes are involved is difficult and often counterintuitive.  Thus, the practice of using conservative assumptions may introduce even more uncertainty into the computer models.
  
  
• “The complexity of the TSPA-SR model compromises the ability to comprehend and develop confidence in the results” (ACNW, 2001).  It would be prudent for the DOE to develop a simple model for dominant dose contributors that clearly defines the connections between major components of the TSPA-SR.
  
  
• “The use of assumption-based conservative analysis for performance assessment compromises the regulator’s ability to quantify defensible safety margins” (ACNW, 2002).