Physics study for a long-cycle, high-burnup prismatic fuel assembly

For a prismatic VHTR fuel assembly, a physics study has been done to maximize the fuel performances in terms of the cycle length and the discharge burnup. The U.S. NGNP prismatic fuel block is considered as a reference design in this work. The objective of this study is to maximize the cycle length and the fuel burnup for a given fuel enrichment. Basic physics studies have done to identity the relationship between the fuel reactivity and the fuel configurations in terms of TRISO packing fraction, diameter of the fuel kernel, and moderating power of the fuel block. Both a typical uranium fuel and a TRU fuel are considered in this paper. It is clearly shown that for a long-cycle and a high burnup, the fuel loading needs to be increase and at the same time the moderating power of the fuel block should also be increased. Two ways are considered for a higher moderation of the fuel block: a larger pitch of holes and an extra graphite layer along the hexagon. Impact of the increased pitch on the fuel temperature is also evaluated with a thermal analysis code. We have shown that a TRU fuel can be utilized very efficiently in a VHTR core. (author)