Core of Stricken Reactor #2 Probably Leaked, U.S. Says; Nitrogen Injection for #1
By Matthew L. Wald and Andrew Pollack, New York Times, April 6, 2011

WASHINGTON — The United States Nuclear Regulatory Commission said Wednesday that some of the core of a stricken Japanese reactor had probably leaked from its steel pressure vessel into the bottom of the containment structure, implying that the damage was even worse than previously thought.

The statement came as the Tokyo Electric Power Company, the operator of the Fukushima Daiichi plant, started to inject nitrogen into the reactor containment vessel of unit No. 1 to prevent a possible explosion.

The Nuclear Regulatory Commission’s statement regarded unit No. 2, and the agency underscored that its interpretation was speculative and based on. high radiation readings that Tokyo Electric had found in the lower part of unit No. 2’s primary containment structure, called the drywell. The statement said that the commission “does not believe that the reactor vessel has given way, and we do believe practically all of the core remains in the vessel.”

The agency’s statement was issued after Representative Edward J. Markey, Democrat of Massachusetts, told a House hearing on Wednesday morning that the commission had told him that the core had melted through the vessel.

He based that on a question his staff had asked the agency. But the agency responded to him by e-mail on Tuesday without directly addressing possible melting, saying only that it speculated that “part of the Unit 2 core may be out of the reactor pressure vessel and may be in the lower space of the drywell.” After the hearing, in response to numerous questions, the agency said that “there are possible leakage paths from the reactor vessel into the drywell.”

It did not say whether the fuel was molten or solid. If molten fuel has left the reactor’s pressure vessel...