Update: A meltdown at this point is possible, since the decay heat is pretty significant. However, as long as the core remains submerged, it shouldn't be an issue. In the worst case, you could more or less stick a hose into the primary system (one of the chief reasons that the Three Mile Island meltdown occurred is that the operators were unaware of the falling water levels in the core).

The main concern at this point is definitely the steam pressure.