Investigations of the Possible Separation Distance Between Spent Fuel/High-Level Waste and a Low/Intermediate-Level Waste Repositories due to Gas and Heat Generation

Abstract

This study is part of a generic investigation for the assessment of the required minimum distance between a Spent Fuel/High-Level Waste/ Intermediate-Level Waste (SF/HWL/ILW) repository and a Low/ Intermediate-Level Waste (L/ILW) repository. For this, a large-scale numerical model was constructed to investigate the two-phase flow behavior for such a repository configuration in a low-permeability claystone formation. The modeling focused on the pressurization mechanisms associated with (a) resaturation of backfilled underground facilities, (b) thermal effects caused by heat generation from the SF/HLW canisters, and (c) gas generation from corrosion and degradation of different wastes in the L/ILW and ILW caverns and in the SF/HLW emplacement tunnels. The model accounts for gas generation from corrosion and degradation of both L/ILW and ILW wastes indicating decreasing rates with time, and from corrosion of the SF/HLW canisters characterized by a constant rate. Heat generation from radioactive decay of radionuclides of MOX/UO2 wastes is described by an exponential decay with time. The preceding operational phases of the different repository components were simulated representing the transient initial conditions for the post-closure phase. The simulated pressure buildup in the L/ILW repository shows a near linear increase between 10 and 4,000 years when the peak pressure of 6.5 MPa is reached for a repository at about 370 m bg. This is followed by a similar decline, recovering to near hydrostatic pressures after 1 million years. The SF/HLW repository (repository level 600 m bg) indicates a pressure rise between 100 and 1,000 years affected by the early thermal effects, followed by a steep increase between 3,000 and 100,000 years when the pressures level off to a maximum of 6.5 MPa after 160,000 years (corresponding to a steel corrosion rate of 1 μm/year). This is the time when all the metal is corroded and the gas generation stops resulting in a sudden decline, and the pressures level off to about 4.5 MPa in the SF/HLW emplacement tunnel after 1 million years. The numerical modeling demonstrates that the main pressurization mechanism is from gas generation in the different repository components. The pressure histories show a distinct separation of the pressure peaks between the L/ILW repository and the SF/HLW/ILW repository. Moreover, the thermal phenomena affect the pressures in the SF/HLW repository at early time only (prior to about 2,000 years). The thermal expansion of the pore water in the nearfield around the SF/HLW tunnels does produce a relatively steep pressure buildup after 100 years, but it dissipates rapidly prior to the main pressure buildup caused by the gas generation and gas accumulation in the SF/HLW repository. The thermally induced pressure buildup is restricted to the vicinity of the SF/HLW emplacement tunnels (decameter range) and thus, significant interference of the thermally induced pressure perturbation around the SF/HLW/ILW repository with the early gas pressure buildup in the L/ILW repository can be excluded.