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BES-5 is an acronym for a Russian thermoelectric generator where the heat source is a U235 fast fission nuclear reactor - FNR.
Usually RTGs use Pu 238 as heat source because it has the lowest shielding requirements. Nonetheless, its decay over time turns it inappropriate to be used in deep-space exploration - DSE where spaceships need to travel for centuries.
The FNR turns-on just when the critical mass is attained. Then, using a mechanical control system, the reactor can be dormant for centuries and, when necessary, turned on.
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 fission of 2.6 kg of U235 (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.