The other fact to remember about fission explosives is that smaller == dirtier. The tiny W54 got a 1 kiloton yield from 5kg of plutonium. The contemporary Mar

The other fact to remember about fission explosives is that smaller == dirtier. The tiny W54 got a 1 kiloton yield from 5kg of plutonium. The contemporary Mark 18 Nuclear Bomb got 500 kilotons out of 60kg of uranium. 12 times the fissile material gave 500 times the explosive yield. Less energy converted into heat means more energy left in the form of radioactive material. Much of said radioactive material is going to end up deposited on the inside of your engine.