The Cosmic Computer by H. Beam Piper

It didn't take as long as Mohammed Matsui expected. They found the fissionables magazine, and in it plenty of plutonium, each subcritical slug in a five-hundred-pound collapsium canister. There were repair-robots, and they only had to replace the cartridges in the power units of three of them. They sent them inside the collapsium-shielded death-to-people area -- transmitter robots, to relay what the others picked up through receptors wire-connected with the outside; foremen-robots, globes a yard in diameter covered with horns and spikes like old-fashioned ocean-navy mines; worker-robots, in a variety of shapes, but mostly looking like many-clawed crabs.

Neither the converter nor the reactor had sustained any damage while the fissionables were burning out. So the robots began tearing out reactor-elements, and removing plutonium slugs no longer capable of sustaining chain reaction but still dangerously radioactive. Nuclear reactors had become simpler and easier to service since the First Day of the Year Zero, when Enrico Fermi put the first one into operation, but the principles remained the same. Work was less back-breaking and muscle-straining, but it called for intense concentration on screens and meters and buttons that was no less exhausting.