required fuel (uranium blanket, BR>1).

Fermi was not convinced of his preliminary estimates, and the changeover to enriched uranium was not unlikely. Besides, he felt that “the public may not accept an energy source that is encumbered by vast amounts of radioactivity, and that produces a nuclear explosive, which may fall into hostile hands.”¹⁶⁹

Before long, the uranium enrichment process, used for weapons production matured. This was further developed for nuclear submarine and thermal power reactors in the 1950s. Even simple assessments show that it would be better to start fast reactors on enriched uranium if only to reduce uranium consumption, to say nothing of the safety implications (estimates for modern light water reactors are given below).¹⁷⁰ A breeding ratio of approximately 1 would be sufficient (with BR~1.05 being optimal),¹⁷¹ and fast reactors of moderate power density would naturally go into equilibrium ‘burning’ of U238, Pu, and minor actinides (MA). This would facilitate the resolution of safety problems (NPP, waste, proliferation) with the ensuing reduction of NPP costs. Although he was present during the start-up of EBR-I in 1951, Fermi himself never returned to the development of fast reactors. Instead, he delegated their development to ANL, where his outline evolved into the fast breeder concept, including:

• Uranium blanket with weapons-grade Pu, and BR>1, which led to the reactivity margin ∆K>>β_eff, with the risk of a prompt criticality excursion, and to separation of uranium and Pu in reprocessing

• high fuel power density P and breeding rates ω~(BR-1)P

• heat removal by light-weight and heat-conducting (but combustible and neutron-moderating) Na, which has a relatively low boiling point (T_boil ~ 900°C), close-packed lattice of fuel rods in tight shrouds; worse thermal hydraulics; flow blockage danger

Consequently, the inherent safety properties of FRs were left untapped. As with thermal reactors, the present-day fast neutron machines are also potentially prone to severe accidents, involving a prompt criticality excursion, loss of coolant with the additional hazards of Na exposure to air and water, and positive void effect in the event of rapid Na boiling. Moreover, the problems of waste and proliferation remain unresolved, and the FRs cost even more than the expensive thermal reactor facilities. Nevertheless, the idea of Pu breeding, which appeared correct at first glance, was embraced by major physicists. Eventually, this came into general use, was included in educational programs, and became a universally ingrained stereotype.