Aspects of Plutonium Fuel Design for a Large Boiling Water Reactor

The effects of fuel designs on plutonium recycle in a large boiling water reactor are investigated using the LEOPARD, SKAT-4 and BOLERO code. Some of the limitations within which any substitute fuel must operate in an existing reactor are imposed on the design of plutonium-bearing fuel. These are: the external dimensions of the substitute fuel assembly must be the same as the original assembly to avoid vessel modification; the void reactivity, the initial excess reactivity and the burn-up reactivity swing must be of the same order as the original one to avoid control rod modification; the heat transfer condition must be similar to the original one and the reactor must always be capable of rated power output. The design to satisfy these limitations and to give near optimum moderation is found to employ the smaller diameter rods or higher water-to-fuel ratio for the plutonium recycle case. The fuel element of constant surface area has a tendency to possess a positive void reactivity coefficient at the end of burn-up. However, the smaller fuel rod diameter and slightly increased heat transfer area of the fuel element can improve this tendency. This design has the advantage that less fissile inventory is needed to give the same burn-up level and the disadvantage of incremental fabrication costs of the smaller diameter rods. The plutonium value analysis is calculated on the selected fuel designs on the basis of fuel cycle cost compared with that of 235U fuel core. It appears from these results that the plutonium recycle in a large boiling water reactor can be economical, based on the present analysis of plutonium value. (author)