They would be concerned about maintaining coolability of the cores according to their mandatory requirements. They should have a plan for "what if".
It appears from this article that Units 1, 2 and 3 were operating, but shutdown because of the quake.
The emergency power system for Units 1 and 2 failed. Hopefully they can restore the system, or use contingency plans.
Without power, the would be concerned about the coolant getting too hot, which means higher than normally allowed, but which still could be within design parameters for emergency situations. However, we prefer not to allow the core to get outside of normal or anticipated conditions.
Basically, they don't want the primary cooling systems to over-pressurize.