AbstractIn the early 1980s, tests of the leaching behavior of spent light water reactor fuel were conducted in Sweden by SKB and in the USA by the NNWSI Project. Both organizations used fuels with similar burnup, leaching solutions with similar chemical compositions, and conducted the tests at ambient hot cell temperature. Most of the test results were closely similar. The exception was in the recovery of actinide elements at the end of leaching cycles. In the NNWSI tests, the test vessels were stripped with nitric acid at the end of the leaching cycle. When the actinide inventories recovered in the original leaching cycle plus vessel rinse solutions were added to the amount recovered from the acid stripping, the relative abundances of uranium, plutonium, and other actinides were approximately the same as their inventories in the fuel samples. In the SKB tests, the materials recovered from stripping the leaching vessels were low in amount and interpreted to be fine fuel fragments. Only a few percent of the plutonium inventory that would correspond to the uranium recovered in solution was accounted for in the solution samples. To investigate the reasons for this difference in recovery of the actinides, a new test was undertaken using a fuel sample that had been leached for several years using the SKB methods. The fuel was removed from the cladding after a total of a bit more than two years of leaching inside the cladding. The bare fuel was then leached for several cycles using a geometry that simulated the NNWSI tests and a test procedure that was similar to that used in the NNWSI tests. The cladding from which the fuel was removed was also leached in a separate vessel in parallel with the bare fuel leaching. This paper presents the results of the test series and discusses the effects of specimen geometry on the mobility of actinides during leaching. During leaching of spent fuel inside the original cladding, a secondary phase containing Pu seems to form. This phase appears to be less easily dissolved than spent fuel itself.