Pu is an unusual metal. It has at least 6 different solid allotropes (phases), each of which have different density, hardness etc. The phase can change over ti

Pu is an unusual metal. It has at least 6 different solid allotropes (phases), each of which have different density, hardness etc. The phase can change over time, kind of like how an old chocolate bar can become white and crumbly or soft and chewy. You build your bomb expecting your Pu to be in the δ phase (delta phase). If your pit has changed into a different phase due to age, the explosive lenses will not compress it correctly and you will get a dud or reduced yield. https://en.wikipedia.org/wiki/Allotropes_of_plutonium