Spent fuel dissolution in Belgian Supercontainer conditions: source term and compatibility
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Spent fuel dissolution in Belgian Supercontainer conditions: source term and compatibility K. Lemmens1, Th. Mennecart1 and C.Cachoir1 SCK•CEN, Boeretang 200, B-2400 Mol, Belgium
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ABSTRACT In the frame of the Safety and Feasibility Case 1 development, the Belgian agency for radioactive waste (ONDRAF/NIRAS) has to demonstrate that the geological disposal of spent nuclear fuel in the Boom Clay host rock following the Supercontainer design provides sufficient radiological safety in the long term. This safety relies on the containment of the radionuclides by the overpack, the limitation of radionuclide release from the waste matrix, the diffusion controlled transport of the radionuclides, and their low solubility and sorption on the Boom Clay host rock. This paper presents an evaluation of the main processes considered for the limitation of radionuclide release by the Spent Fuel in Supercontainer conditions, characterized by the presence of a concrete buffer and hyperalkaline pore water. We present a description of the main expected processes with their potential impact on the fuel alteration mechanisms, and potential reference fuel matrix alteration rates and effective fuel surface areas for use in safety and performance assessment models. INTRODUCTION The Supercontainer design with an Ordinary Portland Cement (OPC) buffer and limestone aggregates is the current reference design for the geological disposal of spent nuclear fuel in Belgium [1]. In the case of spent UOX fuel, four fuel assemblies are placed inside a Supercontainer with a 30 mm thick carbon steel overpack and a concrete buffer about 540 mm thick (see figure 1). A steel insert prevents dislocation of the assemblies.
Figure 1. Cross section of the disposal gallery with a Supercontainer containing four UOX fuel assemblies (for MOX fuel, one assembly is foreseen per container).
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The overpack has to prevent contact of the spent fuel with the cementitious pore water at least during the thermal phase (about 2500 years). This is a key safety requirement. After perforation of the overpack, the high pH of the infiltrating water may have an impact on the lifetime of the spent fuel. Hence, a research programme was started at the Belgian Nuclear Research Centre to assess the rate at which the radionuclides will be released in such an environment, taking into account the expected evolution of the fuel and the near field components. This paper presents the preliminary results of this evaluation. KEY PROCESSES AND THEIR EFFECT ON FUEL STABILITY The disposal system will evolve as a result of thermal, hydraulic, mechanical, chemical, radiation and biological processes. At the time of the release of radionuclides from the spent fuel, the chemical effects are expected to be dominant. This paper will therefore focus on the chemical evolution of the Supercontainer disposal system. We distinguish the chemical evolution of the concrete, of the metallic components, and of the spent fuel (SF). The evolution of the effective fuel surface area is discussed as well. Chemical
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