Modelling the Gas Generation of Intermediate and Low Level Radioactive Wastes

Gas generation is a safety issue for repositories for radioactive waste because of its potential impact on barrier integrity and radionuclide transport. Therefore, the amount of gas that might be produced and the generation rate has to be assessed. For a qualified prediction of the gas generation potential and rates under repository conditions for radiolysis, corrosion and microbial degradation, gas generation rates concerning real waste packages, laboratory experiments as well as natural analogues for microbial degradation have been evaluated. These data were applied to long-term modelling of gas generation at two sites. The modelling includes aerobic, anaerobic, dry and saturated phases and variations of pH depending on the degradation of cement in order to assess the range of gas generation rates. The results show very low gas generation by radiolysis. At one site, radiolytic gas generation was dominated by short-lived nuclides. The other site had a larger inventory of long-lived radionuclides, yielding higher long-term radiolytic gas generation. In contrast to previous calculations, the gas generation potential by microbial degradation was significantly lower than calculated by stoechiometry. The microbial gas generation potential is reduced by microbial degradation before closure of the site. Furthermore a saturation effect, known from natural analogues, waste dumps and experiments, was modelled, resulting in a significantly lower yield of gas. Overall, the gas generation potential and rate were dominated by corrosion processes of iron.