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BES-5 is an acronym for a Russian thermo-electric generator design in which the heat source is a U 235 fast fission nuclear reactor (FNR).
Usually radioisotope thermal generators use Pu 238 as a heat source because it has the lowest shielding requirements. Nonetheless, its decay over time makes it inappropriate for use in deep-space exploration applications, where spaceships need to travel for decades.
The FNR activates just when the critical mass is attained. Then, using a mechanical control system, the reactor can be dormant for centuries and, when necessary, re-activated.
The design of a FNR is similar to an atomic bomb where there is a sub-critical mass (≈90% of the critical mass) where it is inserted a rod of fissile material.
The fuel core of the reactor was 0.2 m in diameter, 0.6 m long and weighed, as an assembly, 53 kg. The 30 kg of uranium was more than 90% enriched U235  It generated 3 kW of electrical power  created by thermoelectric conversion of 100 kW of thermal output. The reactor weighed 385kg including the radiation shielding.
The fission of 2.6 kg of U 235 (5% of the critical mass) is able to produce a constant output of 28 kW for 250 years (2 kW of electricity). Although the thermal output of a 52 kg mass of Pu238 would be identical it would decline through time and, after 250 years would be reduced to 4 kW due to its half-life of 87.7 years.
- Special illustrated presentation by the delegation of the Russian Federation at the XXXIII session of the scientific and technical subcommittee of COPOUS on collisions of nuclear power sources with space debris, Vienna, February 16, 1996
- G.M. Gryaznov, V.S. Nikolayev, V.I. Serbin, V.M. Tyugin, "Radiation safety of the space nucler power systems and its realization on the satellite Cosmos-1900", Chapter 45 of the book Space Nuclear Power Systems 1989, Orbit Book Company, Malabar, Florida 1992.
- A.V Zrodnikov, V.Y Poupko, G.M. Gryaznov, "Experimental detection of neutron gas pressure on the control rods of a nuclear reactor under microgravity conditions", Proceedings of 11 th symposium on space nuclear power and propulsion, January 9-13, 1994, Albuquerque, American Insitute of Physics, new York, 1994.
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