The SNAP-27 fuel was made up of plutonium oxide microspheres, divided into two inner capsules each with a little under one and a half kilograms of PuO2. The inner capsules resided in an outer capsule. Both inner and outer clads were made of Haynes 25 superalloy. (Do I get a T-shirt after all these years?) PuO2 is pretty insoluble in water.

Some earlier generators used plutonium metal and were designed to burn up on reentry, dispersing the fuel harmlessly. Precisely this happened with Transit 5-B-N3 in 1964. Although the generator did exactly what it was supposed to do, and the material dispersed as predicted, later heat sources and fuel forms were designed for intact retrieval, even from extraorbital trajectories.

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