Thorium-based plutonium incineration in the I$^2$S-LWR

Abstract

This paper presents an analysis of a homogeneous thorium-plutonium fuel cycle developed for the Integral Inherently Safe LWR (I$^2$S-LWR). The I$^2$S-LWR is an advanced 2850 MWt integral PWR with inherent safety features. Its baseline fuel and cladding materials are U$_3$Si$_2$ and advanced FeCrAl steel, respectively. The advanced steel cladding can withstand longer exposure periods with significantly lower degradation rates compared to traditional Zr-based alloys. However, longer fuel cycles would require higher fuel enrichment, and this is currently limited to 5$^w$% in the I$^2$S-LWR. Therefore, an alternative thorium-plutonium mixed oxide (TOX) fuel cycle is investigated. In principle, the TOX fuel cycle has no fissile content limitation and becomes even more attractive for long irradiation periods, due to the efficient build-up of $^{233}$U, which increases its cumulative energy share and hence decreases the initial Pu requirements per unit of energy produced by the fuel. Current Pu recycling practice in the form of U–Pu mixed oxide (MOX) fuel is not well-suited for Pu disposition due to continuous Pu production from $^{238}$U. This study compares the TOX and MOX cores in terms of efficiency of Pu disposition. The results show that the burnt Pu fraction in the TOX cycle is much higher, and could be further enhanced for longer irradiations (100 MWd/kg or more).