Too well shielded for the blast to get them, but the heat melted the fissionables down to critical mass.
What we need is some publicity for it, some critical mass, and some way that we can get ordinary people involved.
The plutonium bomb worked by compressing the plutonium into a critical mass which sustains a chain reaction.
Claire always knew what to do when his temper started climbing to critical mass.
If only they had a tube of some sort that would withstand the chemical explosion—the one that brought the critical mass together!
First, two or more pieces of plutonium large enough to form a critical mass.