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). It was mainly used in the US-A radar reconnaissance satellites.
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.
Spacecraft nuclear reactors must be fast reactors for the following reasons. First, normal moderator materials (carbon, water) add bulk and mass which is not desirable in a spacecraft. Second, for reasons of nucleonics the fuel must be enriched, but not to the point where it is weapons grade (about 5 percent U-235 in this design, whereas weapons grade is some figure above 75 percent). Note that some of the U-238 (which is fertile and not fissile) will be converted to Pu-239 during operation, and this is taken into consideration during the design and while estimating the power output and design life expectancy.
The design of the BES-5 FNR is such that a sub-critical assembly exists into which a rod of fissile material is inserted. Careful feedback and monitoring of the power level will keep the reactor delayed critical and not prompt critical, which can be done by a mechanical control system. The reactor can be dormant for centuries if needed, and reactivated when required.
This design differs from the US SNAP series of spacecraft reactors in significant ways.
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.
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). The thermal output of a 52 kg mass of Pu238 would initially be identical, but would decline through time and, after 250 years would be reduced to 4 kW due to its half-life of 87.7 years.