Corrosion Behavior of Spent MTR Fuel Elements in a Drowned Salt Mine Repository
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CORROSION BEHAVIOR OF SPENT MTR FUEL ELEMENTS IN A DROWNED SALT MINE REPOSITORY BERT-G. BRODDA AND J. FACHINGER Research Center Julich KFA, Institute for Safety Research and Reactor Technology, Nuclear Waste Management, D-52425 Julich, Germany. ABSTRACT Spent MTR fuel from German Material Test Reactors will not be reprocessed, but stored in a final salt repository in the deep geologic underground. Fuel elements will be placed in POLLUX containers, which are assumed to resist the corrosive attack of an accidentally formed concentrated salt brine for about 500 years. After a container failure the brine would contact the fuel element, corrode the aluminum plating and possibly leach radionuclides from the fuel. A source term for the calculation of radionuclide mobilization results from the investigation of the behavior of MTR fuel in this scenario, which has to be considered for the long- term safety analysis of a deep mined rock salt repository. Experiments with the different plating materials show that the considered aluminum alloys will not resist the corrosive attack of a brine solution, especially in the presence of iron, under the conditions in a drowned salt mine repository. Although differences in the corrosion rates of about two orders of magnitude were observed when applying different parameter sets, the deterioration must be considered to be almost instantaneous in geological terms. Radionuclides are mobilized from irradiated MTR fuel, when the "meat" of the fuel element becomes accessible to the brine solution. It seems, however, that the radionuclides are effectively trapped by the aluminum hydroxide formed, as the activity concentrations in the brine solution soon reach a constant level with the progressing corrosion of the cladding alu-
minum. In the presence of iron a more significant initial release was observed, but also in this case an equilibrium activity seems to be reached as a consequence of radionuclide trapping. INTRODUCTION Spent fuel elements from German Material Test Reactors (MTRs) have in the past been reprocessed in DOE facilities in the USA. Since this was discontinued in 1988, and because no proprietary reprocessing capacity is available in Germany, such material is being considered for final storage in a rock salt repository in the deep geologic underground. Long-term safety analysis for the storage of highly radioactive waste in a final repository of this type is based on a mobilization scenario which assumes an accidental water ingress, the formation of a concentrated salt solution, which corrosively penetrates the container walls, comes in direct contact with the fuel elements, corrodes them also, and finally mobilizes radionuclides into the biosphere. The subject of our investigations is MTR fuel of the DIDO type, which is used in the J01ich FRJ-2 reactor. It contains, on average, 20 wt% U-Al alloy with an initial enrichment of 80% in 235U. The fuel alloy plates with a thickness of 0.6 mm (meat) are covered on both sides with aluminum claddings of 0.38 mm, the total thickness is t
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