low burn up magnox reactors. Plutonium produced in the fuel of a breeder reactor generally has a higher fraction of the isotope plutonium-240, than that produced in other reactors, making it less attractive for weapons use, particularly in first generation nuclear weapon designs similar to Fat Man. This offers an intrinsic degree of proliferation resistance, but the plutonium made in the blanket of uranium surrounding the core, if such a blanket is used, is usually of a high Pu-239 quality, containing very little Pu-240, making it highly attractive for weapons use. "Although some recent proposals for the future of the ALMR/IFR concept have focused more on its ability to transform and irreversibly use up plutonium, such as the conceptual PRISM (reactor) and the in operation(2014) BN-800 reactor in Russia, the developers of the IFR acknowledge that it is 'uncontested that the IFR can be configured as a net producer of plutonium'." As mentioned above, if operated not as a burner, but as a breeder, the IFR has a clear proliferation potential "if instead of processing spent fuel, the ALMR system were used to reprocess irradiated fertile (breeding) material (that is if a blanket of breeding U-238 was used), the resulting plutonium would be a superior material, with a nearly ideal isotope composition for nuclear weapons manufacture." Reactor design and construction A commercial version of the IFR, S-PRISM, can be built in a factory and transported to the site. This small modular design (311 MWe modules) reduces costs and allows nuclear plants of various sizes (311 MWe and any integer multiple) to be economically constructed. Cost assessments taking account of the complete life cycle show that fast reactors could be no more expensive than the most widely used reactors in the world – water-moderated water-cooled reactors. Liquid metal Na coolant Unlike reactors that use relatively slow low energy (thermal) neutrons, fast-neutron reactors need nuclear reactor coolant that does not moderate or block neutrons (like water does in an LWR) so that they have sufficient energy to fission actinide isotopes that are fissionable but not fissile. The core must also be compact and contain the least amount of neutron moderating material as possible. Metal sodium (Na) coolant in many ways has the most attractive combination of properties for this purpose. In addition to not being a neutron moderator, desirable physical characteristics include: Low melting temperature Low vapor pressure High boiling temperature Excellent