To pump in the water, the Japanese have apparently tried used firefighting equipment -- hardly the usual procedure. But forcing the seawater inside the containment vessel has been difficult because the pressure in the vessel has become so great.

One American official likened the process to "trying to pour water into an inflated balloon," and said that on Sunday it was "not clear how much water they are getting in, or whether they are covering the cores."

The problem was compounded because gauges in the reactor seemed to have been damaged in the earthquake or tsunami, making it impossible to know just how much water is in the core.

And workers at the pumping operation are presumed to be exposed to radiation; several workers, according to Japanese reports, have been treated for radiation poisoning. It is not clear how severe their exposure was.