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Thermohydraulic Analysis of Gas Generation in a Disposal Facility for Vitrified High-Level Radioactive Waste in Boom Clay

Janez Perko, Eef Weetjens

Nuclear Technology / Volume 174 / Number 3 / June 2011 / Pages 401-410

Technical Paper / TOUGH2 Symposium / Radioactive Waste Management and Disposal

Assessment of gas generation and transport is inevitable for evaluation of the safety of nuclear waste disposal in deep geological formations. The long-term safety of the geological disposal facility is guaranteed by several engineered and natural barriers. The reference disposal concept in Belgium consists of a concrete-based repository situated in Boom Clay, which is a low-permeability plastic clay. Hence, the mobility of gas and liquid within these barriers is very small and may lead, in combination with increased temperatures due to decay heat of the waste, to pressure buildup and the potential structural failure of barriers. The focus of this study is on coupling two-phase water and gas flow with a heat source, originating from the heat dissipating waste. The main gas production mechanism within the considered geological repository system is (anaerobic) corrosion of metal barriers, generating H2 gas. The corrosion process itself and therefore the intensity of the gas source is temperature dependent. Furthermore, the heat source is time dependent due to the decaying nature of the radioactive material. This property, in turn, makes the gas generation rate time dependent as well. The cases presented in this work couple variable gas generation with a time-variable heat source and are modeled with TOUGH2. Because of large uncertainties associated with the yet-uncharacterized engineered materials (e.g., concrete), two bounding material permeabilities with a span of two orders of magnitude are chosen for comparison. Results demonstrate that the peak pressures for the isothermal and nonisothermal cases do not differ considerably in the case of high-permeability buffer material. On the contrary, the peak pressures differ considerably for low-permeability material, which hinders the flow of water induced by thermal expansion of water with temperature increase. This peak pressure is not related to the gas-generation process and occurs a little earlier than the gas pressure peak, which is in this case comparable to the high-permeability case. Overall, this near-field analysis showed that the effect of pressure increase remains relatively localized and should not affect the structural integrity of the host formation. The behavior of the system is additionally refined by the implementation of temperature-dependent hydrogen solubility within the numerical code, which slightly modifies the transition to H2 gas phase.

 
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