The USSR manufactured a number of RTGs using Sr-90. Sr-90 is roughly as energetic as Pu-238, but requires more shielding. It has the advantage that it doesn't

The USSR manufactured a number of RTGs using Sr-90. Sr-90 is roughly as energetic as Pu-238, but requires more shielding. It has the advantage that it doesn't need to be manufactured, because it is one of the more abundant fission products.