Study on transmutation technology of long-lived-fission-products (LLFP) using commercial fast reactors. Loading type of LLFP target assembly and transmutation performances of cores designed in FS phase-II

In the Feasibility Study on Commercialized Fast Reactor Cycle System in Japan (FS), transmutation technology of Long-Lived Fission Products (LLFP) using commercial fast reactors have been studied to reduce the environmental burden of radio-active waste. In this report, the following contents, loading type of LLFP target assembly', 'transmutation performances of the cores designed in FS phase-II' and 'capability of transmutation core with high support factor (SF, that is defined as transmutation/production ratio for LLFP nuclide)', were studied. Those results are summarized below. Design studies for two loading types of transmutation cores (in-core and ex-core loading type cores) were conducted to give quantitative comparison of those performances. As a result, in the in-core loading type core, LLFP inventories were found to be decreased to 1/7 for 99Tc and 1/3 for 129I compared with those of the ex-core loading core. The small inventory leads fast introduction pace of transmutation cores, therefore, we selected the in-core loading type as the reference of FS phase-II. LLFP transmutation performances were evaluated for typical FS phase-II designed cores by applying the above in-core loading method. As a result, every core was confirmed to have the capability to attain SF > 1.0 in keeping design conditions of commercial fast reactor (such as breeding ratio of over 1.03). In keeping the design conditions of commercial fast reactor, SF was found to be limited to around 1.0. Thus, we carried out sensitivity evaluations to SF to examine how much SF was improved by relaxation of some design conditions. As a result, since sensitivities of every condition to SF were found to be very small, we concluded that large SF to transmute LLFP amount generated in plural cores may be impossible for the commercial reactors. (author)