Reactor Neutronics: Impact of Fissile Material

Given a wide variety of reactor designs and fuel types, it can be difficult to identify the underlying cause of basic performance differences such as flux level and enrichment requirement. In this paper, using solely the definitions of the core multiplication factor and core power, simple relations have been derived allowing estimates of the flux ratio and fissile material concentration ratio for any reactor concept when ²³⁵U is replaced with ²³⁹Pu or vice versa. These relations are functions of the neutron nonleakage probability, and one only needs to know the number of neutrons emitted per fission and the fission cross-section ratio between the ²³⁵U system and the ²³⁹Pu system. It is found that for a reactor concept having significant leakage, the achievable flux level when using ²³⁹Pu as fissile material can be up to 45% larger than when using ²³⁵U as fissile material, and the required fissile concentration of ²³⁹Pu is up to 48% lower than that of ²³⁵U to achieve criticality.