AN ASSESSMENT OF RADIONUCLIDE RELEASE INVENTORY BASED ON LONG-TERM FIELD TESTS OF NUCLEAR WASTE GLASS K-26

摘要

Long-term field test data are reported on environmental behaviour of a high-alkali-borosilicate nuclear waste glass K-26 in an experimental shallow ground repository. This glass was under continuous monitoring during 16-years. The groundwater that had been in contact with the waste glass was collected for chemical analyses and for the content of radionuclides leached out of corroding glass. It has been determined that the average leaching rate of 137{sup left}Cs gradually diminished during the test period from 0.94μg/cm{sup}2 day over the first year to 0.22μg/cm{sup}2 day over 16 years of tests. Two basic mechanisms of radionuclide releases e.g. diffusion-controlled ion exchange and hydrolysis were taken into account in analysing glass corrosion mechanisms. The rate of radionuclide release via ion-exchange was determined by Doremus' theory of interdiffusion. The rate of radionuclide release via hydrolysis was determined by transition state theory of Agaard and Helgeson. Analysis of field test data revealed that radionuclide leaching rates obey a square root time dependence indicating a prevailing diffusion-controlled release mechanism. The effective diffusion coefficient, which controls the leaching of radioactive Cs from the glass K-26, was determined as D{sub}(Cs) = 4.5·10{sup}(-12) cm{sup}2/day. The ion exchange phase will eventually be followed by the glass network hydrolysis. Processing of field test data revealed an approximate rate of K-26 glass hydrolysis in the near surface repository as high as r{sub}h ≈ 0.1μm/year. Results obtained were used to predict the leached inventory of radionuclides from the glass K-26 with time. The total release of radionuclides from 190 kg of radioactive glass K-26 will not exceed 20 kBq over 300 years.