An alternative to Pu-238 is Am-241 https://www.world-nuclear-news.org/Articles/Can-americium-replace-plutonium-in-space-missions https://www.world-nuclear-news.

An alternative to Pu-238 is Am-241 https://www.world-nuclear-news.org/Articles/Can-americium-replace-plutonium-in-space-missions https://www.world-nuclear-news.org/Articles/Can-americium-re... . The UK has around 140t of trans-uranics in its civilian stockpile, of which an estimated 5.6 tonnes of this is Am-241 per https://www.repository.cam.ac.uk/bitstreams/627b4440-37c9-4e99-aef3-195b3416b29d/download https://www.repository.cam.ac.uk/bitstreams/627b4440-37c9-4e....