Short-term irradiation behavior of low-density americium-doped uranium–plutonium mixed oxide fuels irradiated in a fast reactor

A short-term irradiation test (B14) was conducted using the experimental fast reactor Joyo. The objective of the test was to investigate the early thermal behavior of americium-doped uranium–plutonium mixed oxide (Am-MOX) fuels such as fuel restructuring and redistribution of actinides. Four fuel pins which contained 2.4 wt.% americium-doped uranium–plutonium mixed oxide (Am, Pu, U)O2−x fuels were supplied for testing. Pellet-cladding gap width and oxygen-to-metal (O/M) molar ratio of oxide fuels were determined as experimental parameters. The respective amounts of plutonium and americium in the fuel pellets were 31 and 2.4 wt.%. The fuel pins were irradiated at the maximum linear heating rate of ∼470 W cm−1 in the B14 test. After the irradiation, post-irradiation examinations were conducted. The influences of both pellet-cladding gap width and O/M molar ratio on the fuel restructuring were confirmed, and no fuel melting was observed. Americium and plutonium were similarly redistributed depending on the O/M molar ratio. In the fuel specimen of the highest O/M molar ratio, the difference in contents between americium and plutonium around the central void was significant. From the examination results, the margins to the criteria in the thermal design of the fuel pin would be suitable under the expected operating conditions of a conventional fast breeder reactor.