Spent nuclear fuel from reactors doesn't have usable quantities of the right plutonium isotope. RTGs use the uncommon Pu-238. Plutonium in spent nuclear fuel is

Spent nuclear fuel from reactors doesn't have usable quantities of the right plutonium isotope. RTGs use the uncommon Pu-238. Plutonium in spent nuclear fuel is mostly Pu-239, 240, 241, and 242.