Developments in the fabrication technology of low density MOX pellets for fast breeder reactor fuel

The fabrication technology of low density MOX pellets was studied using some organic compounds as pore formers. The pore former, K3 showed the best performance among five organic compounds in the 7.5 kg-UOX runs, but K3 lost this in the 36 kg-MOX run, possibly because plutonium decay heat affected its performance. Comparison of thermal stability between K3 and a newly introduced pore former, Avicel, of high softening temperature, showed that K3 lost its spherical particle shape and consequently, its pore forming ability at 70 °C which was below its reported melting point (84-88 °C) while Avicel could maintain its spherical particle shape up to 150 °C. The pore forming performance of Avicel was also confirmed in the 36 kg-MOX run which is the same scale as for mass production of MOX fuel pellets.