Michael Apted

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)

PERFORMANCE ANALYSIS OF COPPER CANISTER CORROSION UNDER OXIDIZING OR REDUCING CONDITIONS

The finite-difference CAMEO code for modeling general corrosion of copper canisters is described. CAMEO represents the engineered barrier system and surrounding fractured host rock in 3-dimensional cylindrical coordinates. The time of containment failure is evaluated using CAMEO, as constrained by transport rates of corrodants to the canister or by transport rates of corrodant products away from the canister. Additional chemical processes explicitly modeled in CAMEO include 1) copper corrosion, and 2) kinetics of Cu(I) oxidation to Cu(II), both as a function of near-field pore water chemistry, specifically pH, Eh, and chloride concentration. The diffusional transport and sorption behavior of Cu(I) and Cu(Il) are also separately incorporated.

Source-Term Comparison Using the Arest and Syvac-Vault Models: Effects of Decay-Chain In-Growth and Precipitation

A series of calculations of radionuclide release was performed with the AREST and SYVAC-Vault models (SVM) in order to assess concurrance. Specifically, the effects of precipitation and decay chain in-growth on the predicted release of nuclides from waste packages containing spent nuclear fuel were compared between each code. The results for maximum release rates generally agreed within a factor of 10. The differences in results can be explained based on the differences in geometry and boundary conditions between the two codes. Both codes showed nearly identical enhancement factors in release rates of uranium-series nuclides (U-238, U-234, Th-230, Ra-226) arising from the effect of decay-chain in-growth. Calculated enhancement factors in release rates for precipitation of a new uranium-bearing solid within the waste package were also in good agreement between AREST and SVM.

The Effect of Precipitation Fronts Induced by Radionuclide Chain Decay and Elemental Solubility Limits on Near-Field Mass Transport

The formation and impact of precipitation fronts on the diffusional mass transport of radionuclides from a high-level nuclear waste canister through a bentonite buffer has been investigated in a series of numerical simulations. The precipitation fronts arise from chain decay and ingrowth, coupled with differences in elemental solubility limits and sorption properties. The fronts influence particularly the behavior of uranium, plutonium and neptunium isotopes. The isotopic concentration profiles across the buffer differ considerably from results obtained with models that employ elemental solubility limits simply as a boundary condition at the waste-bentonite interface.