Evaluation on the Release of Key Nuclides in Cemented Waste Form in a Rock Cavern Disposal Site

Abstract

It is an effective and feasible way to dispose cemented waste form from nuclear power plant in the rock cavern disposal site. Simulation work with Ecolego software was conducted in this paper to better quantify the transfer process of nuclides in cemented waste form in the rock cavern disposal site over a long time scale. A rock cavern disposal site of low and intermediate radioactive waste was taken as an example. For the transfer process of the nuclides crossing the metal steel drums to the concrete disposal container, 3 sets of compartment models were established respectively. One was for the convective transfer process, and the other two were for the diffusion process in the not deteriorated or partially deteriorated cemented waste form. Release activities of Co-60, Sr-90, Cs-137, Ni-63 and I-129 were calculated with a period of 1000 years beginning at the closing point of the disposal site. In the case of convective transfer, the total peak release time occurs in 120 years after the closure of the disposal site, and the dominant nuclide is Cs-137. During the 1000 years, almost all of the I-129 is released into the concrete disposal container. In the case of diffusion transfer, the total peak release rate occurs in 103 years after the closure of the disposal site, and the dominant nuclide is still Cs-137. During the 1000 years, the total release ratio of I-129 head the list. The total release ratio of Co-60 is almost zero in both processes. Results show that Cs-137 wins top places in the peak release rate and total release activity in both convective and diffusion transfer cases, indicating that Cs-137 is the most concerned nuclide in the release of cemented waste form. The peak release rate and total release activity of each nuclide under diffusion process were lower than those under convective transfer process, regardless of whether the cemented waste form was degraded or not, indicating that the release of transfer under convection transfer was more significant. The total release activity of all nuclides in the diffusion process of the partially deteriorated cemented waste form is 161.434% higher than that of the non-degraded cemented waste form, which shows that the deterioration of the cemented waste form has a significant impact on the diffusion process. This paper is the very first domestic evaluation study on the transfer process of nuclides in the cemented waste form and the metal barrel under the condition of rock cavern disposal. The results can provide an effective basis for the safety assessment and the engineering scheme design of the disposal site.