Currently, Esa uses an isotope called plutonium-238 for these batteries, but this is only available from Russia and America.

BBC: Nuclear space batteries could boost Sellafield jobs

The truth is radically different: Unless the United States is able in the future to obtain additional plutonium (either through isotope separation of existing reserves or through new production) and to resume production of highly enriched uranium, this country will be unable to meet its currently projected military requirements.

CENTERFORSECURITYPOLICY: Center for Security Policy | ��The Limitation Of Nuclear Materials As A Means Of Controlling Armaments��

Cesium remains the most widely dispersed isotope while concerns remain over long-term contamination from strontium and plutonium.

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