Minimal Nuclear Device

What is the minimum mass of pure U-235 or Pu-239 required in a device for a nuclear event? How would you estimate this value?

Answer

Pure U-235 can be accumulated into a critical mass for a sustained nuclear chain reaction, but pure Pu-239 cannot start itself because the loss rate of neutrons exceeds the production rate. On the average, each U-235 fission produces 2.5 neutrons for every incident neutron. At the critical mass of fissile material the chain reaction will be sustained. For U-235 this critical mass is about 7 kilograms for ideal behavior, requiring a sphere about the diameter of a baseball of pure U-235. Surely this baseball would be too hot to handle!

Diffusion problems of an expanding material would require a neutronnreflecting strong tamper material surrounding the U-235 sphere to delay the expansion for a few microseconds to achieve additional fissions before exploding. At 100 percent efficiency the explosion would be equivalent to about 120 kilo tonnes of TNT. However, no nuclear device is that efficient.

Declassified records indicate that about 60 kilograms of highly enriched uranium was used in the nuclear device that was released over Hiroshima, Japan, in 1945. The explosive charge for the device detonated over Nagasaki three days later was provided by about 8 kilograms of plutonium-239 (>90 percent Pu-239).