SCOLES REPLIES: It’s true that the isotope of plutonium used in weapons has a significant half-life. But it is also decaying throughout that time, and as it does, it leaves behind other elements that contaminate the pit. This gives the pit a different composition than when it was made. The uranium atoms that are produced by the decay process also knock plutonium atoms out of their place in the structure, similarly changing the pit’s properties. And all of this occurs while the plutonium is subject to more mundane chemical processes such as oxidation. So the gist is that with such changes, over time, a pit may not function as well or as safely as when it was new.