Np/Pu Seeding of U in the Breeder Regions in an ASTRID-type Sodium Fast Reactor to Avoid Production of Very High-quality Fissile Material

UK has been investigating scenarios in which fast reactors are deployed in a breeding cycle that is not dependent on external uranium supply. Although it not certain whether there will ever be a driver for strategic independence in the UK, it is nevertheless important that all the options are examined and their implications understood. An ASTRID-type Sodium Fast Reactor (SFR) with an iso-breeder core (overall breeding ratio marginally above 1.0) would be a plausible option in such a scenario and has been assumed in some of the reference scenarios. However, there is a theoretical vulnerability to a proliferation scenario in which the axial breeder regions are sheared separately in the head end of a reprocessing plant and used to recover very high isotopic quality plutonium. This paper describes an assessment of the effectiveness of Np-237 and plutonium seeding of the breeder blankets in order to avoid the production of such high isotopic quality fissile materials. The study was initially intended to focus on Np-237 seeding, but it was found that plutonium seeding would be more practicable. The purpose of the study was not to demonstrate that an ASTRID iso-breeder could be operated with a high degree of intrinsic proliferation resistance, but rather to investigate whether seeding the breeding blankets could avoid the production of virtually pure Pu-239 that otherwise is produced in the breeder region. The study demonstrates that seeding the breeder regions is effective to a limited extent by ensuring that pure Pu-239 production is avoided. Unseeded breeder regions produce plutonium which is in the weapons grade category and could be regarded as incompatible with the IAEA INPRO guideline that: the attractiveness of nuclear material and nuclear technology in an Innovative Nuclear System for a nuclear weapons program should be low.