Neutronic feasibility study of U–Th–Pa based high burnup fuel for pebble bed reactors

•Neutronic feasibility study of U–Th–Pa based fuel for pebble bed reactors has been conducted.•The fuel contains low enrichment of 233U and an additional 231Pa content within a few percent.•Target burnup of 130–170 GWd/T is achieved for selected fuel compositions.•Negative temperature coefficient is assured during burnup.

Neutronic feasibility study of U–Th–Pa based high burnup fuel for the pebble bed reactors has been carried out. An additional content of P231a in a mixed U233–T232h fuel pebble was optimized for simultaneously controlling long-term excess reactivity and attaining high fuel burnup. Because of a large thermal absorption cross section, P231a has a potential to absorb thermal neutrons and act as a burnable absorber in the early stage of burnup. In the latter stage, P231a is transmuted to U233 and act as a fertile fuel. Parametric study was performed for heavy metal (HM) amount, U233 enrichment and P231a content. The objective is to attain a target burnup greater than 100 GWd/T with a low uranium enrichment and a small content of P231a. Six fuel compositions with the HM amount of 9–18 g per pebble, the U233 enrichment of 8 and 10%, and the P231a content of 3.2–5.5% were selected for discussion and comparison. The burnup levels of about 130–170 GWd/t are achieved for the six fuel compositions. The results of temperature coefficient show that negative reactivity feedback during burnup with the increase of temperature is assured, which is one of the important safety parameters.