Neptunium in the Fuel Cycle: Nonproliferation Benefits Versus Industrial Drawbacks

Introducing neptunium into the nuclear fuel cycle has been proposed in the past as a way to impede the diversion or the direct use of plutonium to fabricate a nuclear explosive device. This paper aims to technically analyze the industrial consequences should this proposal be implemented. Two scenarios are considered: (a) adding neptunium to fresh uranium oxide (UOX) fuel before irradiation in a light water reactor (LWR) and (b) separating neptunium together with plutonium from used UOX fuel and using this combined oxide to fabricate mixed oxide (MOX) fuel before subsequent irradiation in an LWR. In both cases, assembly calculations for a pressurized water reactor using fresh fuel doped with neptunium are presented for a wide range of neptunium proportions. The consequences on the core and fuel performance and the fuel cycle are analyzed. The analysis shows that while irradiating neptunium-doped UOX fuel can offer significant proliferation-resistance benefits because of the increased quantity of the plutonium isotope ²³⁸Pu in the discharged fuel, it entails heavy industrial penalties even at 1% Np content. The use of neptunium with MOX fuel is limited to 0.5% in order to maintain a negative void coefficient. At this proportion, it offers minimal increase in ²³⁸Pu content, and it is unlikely that detectability through gamma-ray emissions of the resulting plutonium-neptunium oxide mixture is increased. The fact that neptunium itself may pose a proliferation risk must be carefully weighed in any decision to use neptunium as a tool to increase proliferation resistance.