Study of the Consequences of Secondary Water Radiolysis within and Surrounding a Defective Canister
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Study of the Consequences of Secondary Water Radiolysis within and Surrounding a Defective Canister Jinsong Liu1, Bo Strömberg2 and Ivars Neretnieks1 1 Department of Chemical Engineering and Technology, Royal Institute of Technology, S-100 44, Stockholm, Sweden 2 Swedish Nuclear Inspectorate (SKI), S-106 58. Stockholm, Sweden ABSTRACT A model has been developed to study the effects of secondary water radiolysis caused by dispersed radionuclides in a bentonite buffer surrounding a copper canister. The secondary radiolysis is the radiolysis caused by radionuclides that have been released from the spent fuel and are present either as solutes in the pore-water, as sorbed species on the surface of other minerals, or as secondary minerals. The canister is assumed to be initially defective with a hole of a few millimeters on its wall. The small hole will considerably restrict the transport of oxidants through the canister wall and the release of radionuclides to the outside of the canister. The dissolution of the spent fuel is assumed to be controlled by chemical kinetics at rates extrapolated from experimental studies. Two cases have been considered with the purpose to illustrate the behaviors of both conservative and non-conservative nuclides. The nuclides that are most relevant are those expected to be the dominant α-emitters in the long-term (e.g. 239Pu, 240Pu, 241Am). In the first case it is assumed that there is no precipitation of secondary minerals of the relevant radionuclides inside the canister. In the second case it is assumed that the radionuclide concentration within the canister is controlled by its respective solubility limit. The radionuclide released to the surrounding buffer is then predicted using a mass balance model. The modelling results show that in both cases, the spent fuel will not be oxidized at a rate significantly faster compared to the case where secondary radiolysis is completely neglected. In the first case, however, a large domain of the near-field can be oxidized due to a much faster depletion of reducing minerals in the buffer, compared to the case where secondary radiolysis is neglected. In the second case, the effects of the secondary water radiolysis will be quite limited. INTRODUCTION In the concept of deep geological disposal of spent nuclear fuel, the near-field of the repository is chemically reducing [1] and the reducing environment favors the retardation of radionuclides in the spent fuel. The radionuclides initially imbedded in the spent fuel matrix will, however, emit ionizing radiation and cause water radiolysis [2, 3]. This is probably the most important mechanism that could alter the reducing environment in the near-field. During oxidative dissolution of the spent fuel, radionuclides (fission products and actinides) are released. If the metal canister has some manufacturing defects, like a small hole on its wall, the released radionuclides will be transported out from the canister and dispersed in the bentonite buffer [4]. The dispersed radionuclides can also cause water ra
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