The Potential of Plutonium as a Fuel in Near-Thermal Converter Reactors

The potential of plutonium as a fuel in near-thermal converter reactors is investigated. Over certain ranges of fuel loading and/or moderation, it is shown that the effective absorption cross section (averaged over the entire neutron spectrum) of Pu²⁴⁰ decreases with fuel burnup; i.e., decreases with the associated softening of the neutron spectrum. The plutonium, therefore, behaves as a self-stabilizing or self-compensating fuel with the decrease in Pu²⁴⁰ cross section balancing fissionable material burnup and fission product build-up. Thereby long core lives are attainable with nominal shim control requirements. The strong neutron temperature dependence of the effective Pu²⁴⁰ absorption cross section also results in a highly negative temperature coefficient of reactivity and thereby in the feasibility of spectral shift shim control. Economics evaluation indicates that fuel cycle costs of between 1.5 and 2.5 mills/kw-hr may be attainable with these plutonium fueled systems.