Improving fuel utilization in SmAHTR with spectral shift control design: Proof of concept

Highlights: • Improving the fuel utilization in a graphite moderated reactor by adopting the 'spectral shift' concept. • The feasibility of this concept was tested in the Small Advanced High-Temperature Reactor. • At BOL, the reactor is under-moderated, with excess neutrons being primarily breeding ²³⁹Pu. • Graphite is continuously inserted thermalizing the neutron spectrum and increasing reactivity. • The extra ²³⁹Pu bred during the cycle is then burned, allowing the cycle to be extended. - Abstract: This paper presents a spectral shift design based approach to improve the fuel utilization factor or alternatively to increase the cycle length in a graphite moderated reactor. The feasibility of this concept was tested in the Small Advanced High-Temperature Reactor (SmAHTR). This is a small sized Fluoride-salt-cooled high-temperature reactor (FHR) that uses tri-isotropic (TRISO)-coated particle fuels and graphite moderator materials. A major benefit of the TRISO particles is the ability to mitigate fission product release in the case of an accident. However, the fabrication costs associated with TRISO particles are expected to be significantly higher than the traditional UO₂ fuel. The preliminary studies presented in the paper are focused on extending the achievable irradiation period without increasing the value of the enrichment. In order to increase the discharge burnup, the design includes graphite structures that are initially removed from the core. This imposes a harder spectrum, which enhances the breeding of ²³⁹Pu. Then, the graphite structures are gradually and continuously inserted into the core to sustain criticality. This procedure shifts the hard spectrum into a more thermal one and enables a more efficient utilization of ²³⁹Pu. The preliminary results indicate that this design achieves considerably longer irradiation periods and hence lower fuel cycle costs than the reference design.