Assembly-level analysis of heterogeneous Th–Pu PWR fuel

Abstract

This study compares homogeneous and heterogeneous thorium–plutonium (Th–Pu) fuel assemblies (with high Pu content – 20wt%), and examines whether there is an increase in Pu incineration in the latter. A seed-blanket configuration based on the Radkowsky thorium reactor concept is used for the heterogeneous assembly. This separates the thorium blanket from the uranium seed, or in this case a plutonium seed. The seed supplies neutrons to the subcritical thorium blanket which encourages the in situ breeding and burning of 233U, allowing the fuel to stay critical for longer, extending burnup of the fuel.While past work on Th–Pu seed-blanket units shows superior Pu incineration compared to conventional U–Pu mixed oxide fuel, there is no literature to date that directly compares the performance of homogeneous and heterogeneous Th–Pu assembly configurations. Use of exactly the same fuel loading for both configurations allows the effects of spatial separation to be fully understood.It was found that the homogeneous fuel with and without burnable poisons was able to achieve much higher Pu incinerations than the heterogeneous fuel configurations, while still attaining a reasonably high discharge burnup. This is because in the heterogeneous cases, 233U breeding is faster, thereby contributing to a much larger fraction of total power produced by the assembly. In contrast, 233U build-up is slower in the homogeneous case and therefore Pu burning is greater. This 233U begins to contribute a significant fraction of power produced only towards the end of life, thus extending criticality, allowing more Pu to burn.