Plutonium is difficult to handle, it is a radioactive metal so it disintegrates. But it's not just that. It is very fragile, easily broken into smaller pieces. Those who made the atomic bomb in the Manhattan project found that if we mix with the gallium, plutonium is not fragile and is easily manipulated. They didn't know why and didn't care. Some American physicists have now understood why: the relationships between plutonium atoms are very irregular, but when gallium atoms are interspersed they become regular.