John H. Kessler

Flow and transport in fractured tuff at Yucca Mountain : numerical experiments on fast preferential flow mechanisms

Recent discovery of bomb-related 36Cl at depth in fractured tuff in the unsaturated zone at the Yucca Mountain candidate high-level waste (HLW) repository site has called into question the usual modeling assumptions based on the equivalent continuum model (ECM). A dual continuum model (DCM) for simulating transient flow and transport at Yucca Mountain is developed. In order to ensure properly converged flow solutions, which are used in the transport simulation, a new flow solution convergence criteria is derived. An extensive series of simulation studies is presented which indicates that rapid movement of solute through the fractures will not occur unless there are intense episodic infiltration events. Movement of solute in the environs of the repository is enhanced if the properties of the tuff layer at the repository horizon are modified from current best-estimate values. Due to a large advective–dispersive coupling between the matrix and fractures, the matrix acts as a major buffer which inhibits rapid transport along the fractures. Consequently, fast movement of solutes through the fractures to the repository depth can only be explained if the matrix–fracture coupling term is significantly reduced from a value that would be calculated on the basis of data currently available.

Biosphere modelling for the assessment of radioactive waste repositories; the development of a common basis by the BIOMOVS II reference biospheres working group

Performance criteria for radioactive waste repositories are often expressed in terms of dose or risk. The characteristics of biosphere modelling for performance assessment are that: (a) potential release occurs in the distant future, (b) reliable predictions of human behaviour at the time of release are impracticable, and (c) the biosphere is not considered to be a barrier as the geosphere and the engineered barriers. For these and other reasons, differences have arisen in the approaches to biosphere modelling for repository dose and risk assessment. The BIOMOVS II Reference Biospheres Working Group has developed (a) a recommended methodology for biosphere model development, (b) a structured list of features, events and processes (FEPs) which the model should describe, and (c) an illustrative example of the recommended methodology. The Working Group has successfully tested the Interaction Matrix (or Rock Engineering Systems, RES) approach for developing conceptual models. The BIOMOVS II Working Groups on Reference Biospheres and Complementary Studies have laid the basis for considerable harmonisation in approaches to biosphere modelling of long term radionuclide releases.

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.

The Thermal-Hydrological Impact on Increased Spent-Fuel Storage Capacity in Yucca Mountain Repository

Abstract This paper describes the recent work to evaluate the technical storage capacity for spent fuel in the Yucca Mountain repository. To increase the capacity from the current statutory limit of 63000 tonnes HM commercial spent nuclear fuel (CSNF), two alternative repository designs are proposed and analyzed, which add two additional emplacement drifts adjacent to each current-design drift. All designs assume the same waste package inventory, or heat generation rate, and drift ventilation as the current design. As both alternative designs would fit the well-characterized repository footprint, no additional site characterization at Yucca Mountain would be necessary. The work also examines extended ventilation and phased waste-loading assumptions in anticipation of an expanded role for nuclear power in electricity generation. The key parameter to the storage capacity in the Yucca Mountain site is water movement. To study the thermal and hydrological responses to increased storage capacity, series of two-dimensional models were used to simulate coupled heat and mass (water and air) transfer within the repository system and the near-field subsurface environment, including all geological formations above and below the repository horizon from the surface to the water table. A three-dimensional model was applied to investigate the effect of axial heat transfer and fluid flow. The results show that the current repository footprint can accommodate three times the currently legislated 63000 tonnes HM of CSNF without compromising repository performance.

Room at the Mountain: Estimated Maximum Amounts of Commerical Spent Nuclear Fuel Capable of Disposal in a Yucca Mountain Repository

The purpose of this paper is to present an initial analysis of the maximum amount of commercial spent nuclear fuel (CSNF) that could be emplaced into a geological repository at Yucca Mountain. This analysis identifies and uses programmatic, material, and geological constraints and factors that affect this estimation of maximum amount of CSNF for disposal. The conclusion of this initial analysis is that the current legislative limit on Yucca Mountain disposal capacity, 63,000 MTHM of CSNF, is a small fraction of the available physical capacity of the Yucca Mountain system assuming the current high-temperature operating mode (HTOM) design. EPRI is confident that at least four times the legislative limit for CSNF ({approx}260,000 MTHM) can be emplaced in the Yucca Mountain system. It is possible that with additional site characterization, upwards of nine times the legislative limit ({approx}570,000 MTHM) could be emplaced. (authors)

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

The 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.