RTGs don't use things with 10,000 year half lives, because they need to have meaningful amounts of decay happening to make heat. 238Pu is pretty common, becaus

RTGs don't use things with 10,000 year half lives, because they need to have meaningful amounts of decay happening to make heat. 238Pu is pretty common, because it produces just alpha decay and has a half life of 87 years. Within 1000 years, the main remaining activity is 234U with a half life of 240,000 years: still an alpha emitter but much, much less active.