Development of spent fuel cleaning method for JSFR

In the design study of Japan sodium cooled Fast Reactor (JSFR), we have been developing a dry spent fuel cleaning method in place of the conventional steam-water cleaning method which are applied in the existing sodium cooled fast reactor (SFR) such as MONJU. In the conventional steam-water cleaning method, spent fuel subassemblies are rinsed to remove residual sodium almost completely. But, because of the rinse process, it provides a certain amount of radioactive liquid waste. In the conventional storage system, spent fuel subassemblies are accommodated in a closed can in the water pool. Then the spent fuel must be cleaned completely so that residual sodium could not affect water conditions inside the can. In the case of JSFR, spent fuel subassemblies are accommodated directly in the water pool. Then certain amount of residual sodium on the spent fuel subassemblies are acceptable as long as the water pool cooling system can handle it. The amount of the radioactive liquid waste of the dry cleaning method is less than that of the steam-water cleaning method because the dry cleaning method does not have rinse procedure which spends a lot of fresh water. If the dry cleaning method is adopted in JSFR, the radioactive liquid waste could be decrease to tens of percent compared with the steam-water cleaning method. The schematic diagram of the JSFR dry cleaning concept is shown in Fig.1.When the decay heat of a spent fuel subassembly becomes lower enough in the ex-vessel sodium storage (EVST), the subassembly is transported from EVST to the water pool storage by the ex-vessel transfer machine (EVTM). During the transportation, the dry cleaning is capable to be done right above the EVST using the EVTM gas cooling system. Then the EVTM carry the spent fuel to the water pool storage. In this study, a full-scale mockup of a JSFR fuel pin bundle has been manufactured and dry cleaning performance tests after sodium immersion have been conducted. The subassembly mockup involves fuel pin bundle, wrapper tube and inner duct. To simulate decay heat of the spent fuel, the fuel pin bundle have heater inside fuel pins. Since the JSFR fuel subassembly has an inner duct, sodium drain from the inner duct have been also evaluated. The test results showed that measured sodium amounts on the mockup fuel pin bundle are less than 200g including residual sodium in the inner duct. And this result is thought to demonstrate basic feasibility of the dry cleaning method. Now, we are evaluating the results of the test to gain the design data for the spent fuel storage water pool cooling and filtering system