fissionable

Critical mass is the minimum mass of 'fissionable' material required to sustain a nuclear fission reaction.

If we use more nuclear power (either because we are forced to, or in pursuit of lower CO2 emissions), we could do this for about 100 years using current nuclear reactors, before 'fissionable' uranium runs out.

The fact remains that many, if not most, countries can't be trusted with the technology, if only because of carelessness with 'fissionable' materials or nuclear waste.

Why is uranium 'fissionable' and not, say, aluminum?

the 'fissionable' material with which to construct nuclear weapons

Acquisition of 'fissionable' materials is a substantial technical prerequisite for the manufacture of nuclear explosive devices.