Dude, it's a complicated sludge of radioactive material that is flowing and interacting with itself in complicated ways, not a pure uranium sample on a pedestal.

For instance, cooling it will make the molten metals harden, meaning that there will no material transport. So once some piece of the sludge goes out of criticality, cooling ensures that it will stay sub-critical and decay along a more preferable path.

There is also the possibility that water will do something funky to the chemistry beyond the cooling.