Establishment of low density MOX pellet fabrication process

One of the important subjects to be considered in fuel design phase is to reduce Pellet Cladding Mechanical Interaction, PCMI, which might be caused by swelling of MOX pellet at higher burn up stage. Reduction of smear density of MOX fuel is the practical resolution to improve PCMI resistance. In general, two methods such as low density pellet by using organic additive (hereinafter referred to as 'pore former') and annular pellet can be utilized so as to decrease smear density of MOX fuel. Before the launch of the operation of Plutonium Fuel Production Facility, PFPF, which was designed and constructed with the purpose to fabricate MOX fuel for the proto-type Fast Breeder Reactor, FBR, 'MONJU', a laboratory scale examination had conducted to determine the process to decrease pellet smear density adopted for PFPF. This examination had concluded provisional scheme of fabrication process for low density MOX pellet by using paraffin type additive as pore former. In 1989, JNC started low density MOX pellet fabrication for MONJU initial load fuel. At the beginning of pellet fabrication, densities of sintered pellets widely spread over because pore former was deformed by decay heat of plutonium, and it distributed heterogeneously in the matrix of sintered pellet. In order to resolve these problems, JNC examined alternative pore former with higher heat resistance than paraffin type, and selected cellulose type. At the same time, JNC introduced new designed powder transfer container to decline temperature of MOX powder. In connection with these improvements, granulation method was modified to distribute pore former uniformly in the blended MOX powder. Through these modification and improvements, low density MOX pellet fabrication process at PFPF had been established and about 5.4 millions of low density MOX pellets had been fabricated by the end of 1996. (author)