The most available fission resource is the large amount of Pu, U238 and U235 in spent reactor fuel. Fast spectrum reactors can completely consume the common U238 which is more than 100 more common than the rare U235 which current nuclear reactors are fueled with.
Current 'nuclear waste' could fuel our civilization for hundreds if not thousands of years. The energy content of uranium with fast cycle reactors is so high that we could run on uranium from low-grade sources like seawater. Uranium from granite rock in New Hampshire has an energy content similar to coal.