" 

 == Critical mass == 

 Under ""Applications"", it says, ""Californium-251 has a very small critical mass (about 5 kg)...."" It should probably be stated that this is a calculation/extrapolation, as enough californium to directly test this has probably never been produced, and probably shouldn't be, because nuclear blasts are devastating and, in this case, would spray highly radioactive californium into the environment. Also, how credible are such estimates of critical mass?  "