SR 97: Spent Fuel Alteration/Dissolution and the Influence of Near Field Hydrogen
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SR 97: Spent Fuel Alteration/Dissolution and the Influence of Near Field Hydrogen. Kastriot Spahiu and Patrik Sellin Swedish Nuclear Fuel and Waste Management Co. (SKB), Box 5864, S-102 40, Stockholm, Sweden. ABSTRACT A discussion of the evaluation of the source term in the SR 97 safety assessment of a deep repository for spent nuclear fuel is presented. Since the majority of the radionuclides are embedded in the uranium dioxide fuel matrix, they will be released only after the alteration/dissolution of the matrix. Therefore a description of the process of alteration/dissolution of the spent fuel matrix is needed in a safety assessment. Under normal repository conditions, i.e. reducing environment and neutral to alkaline pH, uranium dioxide has a very low solubility in water. If solubility is assumed to be the limiting factor, the dissolution of the fuel matrix will proceed very slowly due to the low water exchange in the defective canister. On this basis, a solubility-limited model for the release of the radionuclides from the fuel may be formulated. The reducing conditions can be upset by the radioactivity of the spent fuel, which generates oxidizing products through water radiolysis. This causes the oxidative alteration/dissolution of the UO2(s) matrix. A model for fuel matrix conversion resulting from radiolytic oxidative dissolution is discussed, as well as parameter variations and the associated uncertainties. In a repository the spent fuel will come in contact with groundwater after the copper canister has breached. Large amounts of hydrogen are then produced through the anoxic corrosion of the cast iron insert. Recent data on spent fuel leaching in presence of repository relevant hydrogen pressures and the implications on the actual and future spent fuel dissolution modeling will also be discussed. INTRODUCTION In the Swedish concept for disposal of high level waste [1], the spent nuclear fuel will be interim stored for 30 to 40 years, before it is encapsulated in copper canisters with an inner castiron insert. The canisters will then be placed in a deep repository (KBS-3 type) built at a depth of about 500m in granitic bedrock. Each canister will be surrounded by compacted bentonite clay. This arrangement constitutes a multiple barrier system, where the barriers support and complement each other to isolate the waste from the biosphere for a very long time. Recently, SKB has carried out the SR 97 long-term safety assessment [2] of a KBS-3 type repository. In the SR 97 safety assessment, the future evolution of the repository system is analyzed in the form of five scenarios. A description of the process of alteration/dissolution of the spent fuel is needed in all scenarios where spent fuel comes into contact with groundwater, and is discussed mainly in the defective canister scenario [3]. Light water reactor fuel is a heterogeneous material, consisting of a matrix of nearly stoichiometric uranium dioxide in which the higher actinides produced by capture of epithermal
neutrons are generally viewed as forming
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