Japanese officials denied that the water is gone from the spent-fuel pool, the Associated Press reported.  Jaczko, testifying before the House Energy and Commerce Committee on what his agency has been told about the crisis, said the plant’s unit 4 reactor appeared to have suffered a hydrogen explosion.  The reactor at unit 4 was shut down at the time of the earthquake last Friday, meaning that crews had transferred all of the radioactive fuel from the reactor’s core to the pool. The building housing the pool was damaged when two nearby reactor buildings exploded Saturday and Monday.  “It’s unprecedented,” said David Helwig, a retired nuclear engineer who spent 40 years working on boiling water nuclear reactors of the same design as those at Fukushima Daiichi. “That’s never happened before.”  Left exposed to the air, the fuel rods will start to decay and release radioactivity into the air.  Severe structural damage is the only way the fuel pool could be emptied, Helwig said. The 50-foot-deep pools have no outlets at the bottom, thus preventing them from draining in case of an accident.  The spent-fuel pool at another reactor, unit 3, also appeared compromised, Jaczko said.  The increasingly desperate picture of the struggle at the stricken nuclear plant emerged after Japanese helicopter crews abandoned an attempt to dump water on the pools of uranium fuel after detecting dangerous radiation above the plant.  Also Wednesday, the director general of the International Atomic Energy Agency confirmed damage to three of the facility’s six nuclear reactors, which contain radioactive fuel that is hotter, and more radioactive, than the fuel stored in the damaged pools.  In another sign of the worsening crisis, the U.S. ambassador to Japan warned Americans to stay at least 50 miles from the plant — more than four times the distance recommended in the Japanese government’s evacuation plan. Japanese authorities confirmed that crews at the plant had to temporarily abandon their posts as radiation readings spiked.  Jaczko’s testimony suggested that damage to the plant is worse than the Japanese government and the plant’s operator, Tokyo Electric Power Co., has acknowledged. On Tuesday, the company said water levels in the three of the site’s seven fuel pools were dropping, but did not say that the fuel rods themselves had been exposed.  Such exposure has been a huge concern among some longtime critics of the standard practice at nuclear facilities of storing used, or “spent,” fuel in structures that lack the heavy concrete-and-steel shields surrounding the reactor cores themselves.
The United States on Wednesday urged Americans who live within 50 miles of Japan’s  earthquake-damaged Fukushima Daiichi nuclear power plant to evacuate, and the top U.S. nuclear regulatory official indicated that Japan faces an increasingly dangerous situation at one of the plant’s reactors.