BREEDER REACTORS:
0n1 + 92U238 92U239 decays to 94Pu239 which is a fissile material with a half life of 24,360 years. However, Pu-239 can absorb a neutron to become Pu-240 which is NOT a fissile material (half life of 6,580 years)!
Thus if we leave the fuel in the reactor too long, some of the Pu-239 will burn up and some will convert to Pu-240, and we will have too low a fraction of Pu-239 to make into a bomb.