If you're suggesting they can induce criticality by poking debris with a claw, you're dumber than I give you credit for.
Of course you can induce criticality that way. You can hit a small chunk of uranium with a hammer and reach criticality, at least for a moment. U-235 can reach criticality with a mass as small as 780 g under the right circumstances. And the presence of water, potentially
with some amount of uranium in solution, greatly raises the risk. Of course, it would only remain critical while compressed, and so such a small criticality event would likely be a risk only to the robots, because it would be small and self-contained.
Perhaps you meant that it cannot cause a nuclear explosion (which requires not just enough material and moderation to sustain a reaction, but also for it to increase exponentially and not burn itself out in a fraction of a second).
But since it was a 60's style LWR, there wouldn't be pure U-235 in the reactor. It would be LEU which is probably 4% U-235 and 96% U-238 or so as new fuel. Exactly what the isotopic distribution of this material is, I'm not exactly sure, but I'm 100% certain it doesn't contain more than a few percent of U-235. Since U-238 is a neutron poison, I seriously doubt that criticality could be induced accidentally. You probably couldn't induce criticality in this material without explosives (or lots of a moderator material) but I can't be sure of that. The hazard with this material is the fission products and all but the medium lived ones like Cs-137 and Sr-90 will have already burned out by now to stable elements. The long lived stuff like U-235 isn't very radioactive (but it is poisonous). The short lived stuff is now stable isotopes, mostly heavy metals. Its still a hazard to human life, but not in the dramatic fashion you imagine. Its more dangerous because of the amount of heavy metals which are poisonous to humans in high amounts and less dangerous because of nuclear issues at this point.