BES-5

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 385 kg including the radiation shielding.