This article, originally published in Nature in 1980, seeks to provide a discreet, selective, but adequate physical basis for understanding the scope for using reactor-grade plutonium in fission bombs at some of the diverse levels of sophistication open to various potential users. With modest design sophistication, high-burn-up plutonium from power reactors can produce powerful and predictable nuclear explosions. The author argues that denaturing plutonium by adding to it is not possible or valid. Taking all effects on weapons physics into account, a high plutonium content could reduce expected yield to a level that could devastate only a modest portion of a city, and make that yield much less predictable. The author concludes that the greater technical difficulty of using power-reactor plutonium for effective military bombs may be more than counterbalanced by the greater political and economic ease of obtaining that plutonium.