TOKYO, March 16, Kyodo

A nuclear crisis at the quake-hit Fukushima No. 1 nuclear plant deepened Tuesday as fresh explosions occurred at the site and its operator said water in a pool storing spent nuclear fuel rods may be boiling, an ominous sign for the release of high-level radioactive materials from the fuel.

The government ordered the plant’s operator, Tokyo Electric Power Co., on Tuesday night to inject water into the pool at the No. 4 reactor to cool it down ”as soon as possible to avert a major nuclear disaster.”

TEPCO said the water level in the pool storing the spent fuel rods at its Fukushima No. 1 nuclear plant’s No. 4 reactor may have dropped, exposing the rods. Unless the spent fuel rods are cooled down, they could be damaged and emit radioactive substances.

The firm said it has not yet confirmed the current water level or water temperature in the pool and will try to pour water into the facility from Wednesday through holes that were created following an explosion earlier Tuesday in the walls of the building that houses the reactor.

Due to high radiation levels at the No. 4 reactor, workers on Tuesday were unable to prepare for the pouring of water into the troubled pool. Difficult conditions have led the utility to evacuate around 730 of the 800 workers from the site, according to TEPCO.

The firm said its workers were only able to remain in the central control rooms at the Fukushima plant for 10 minutes to avoid exposure to excessive radiation levels. They have retreated to a remote site to monitor data on the reactors, it added.

At 6:14 a.m. on Tuesday, a blast occurred at the No. 4 reactor and created two square-shaped holes about 8 by 8 meters in the walls of the building that houses the reactor. At 9:38 a.m., a fire broke out there and smoke billowed from the holes.

The utility said it could not deny the possibility that the early morning explosion was caused by hydrogen generated by a chemical reaction involving the exposed spent nuclear fuel and vapor.

Chief Cabinet Secretary Yukio Edano said at a press conference, ”We believe very high-level radioactive substances have not been emitted continuously from the No. 4 reactor,” citing radiation monitoring data at the plant.

The nuclear agency said the water temperature in the pool stood at 84 degrees C as of 4 a.m. Monday, higher than the normal level of 40 to 50 degrees. Usually, the upper tip of the fuel rods is at a depth of 10 meters from the surface of the pool, it said.

Agency officials said the fuel rods will not reach criticality again as they have been stored in racks containing boron to prevent it.

Edano said water temperatures in the pools at the No. 5 and No. 6 reactors at the Fukushima plant have been rising as well.

The three reactors were not in service when a magnitude 9.0 earthquake jolted Fukushima Prefecture and other areas in northeastern Japan on Friday.

The agency said among the three, the situation is the severest at the No. 4 reactor because all the fuel rods are stored in the pool due to the change of the reactor’s shroud. At the No. 5 and No. 6 reactors, up to one-third of the rods are being kept in the pools. The more fuel rods are kept in a pool, the more radioactive substances could be emitted.

The new development followed a critical situation at the No. 2 reactor at the Fukushima plant earlier in the day, in which part of the reactor’s containment vessel was damaged following an apparent hydrogen explosion at 6:10 a.m.

TEPCO said the problem could develop into a critical ”meltdown” situation, in which fuel rods melt and are destroyed, emitting massive amounts of radioactive materials into the air.

Prime Minister Naoto Kan urged people living within 20 to 30 kilometers of the plant to stay indoors, after radiation equivalent to 400 times the level to which people can be safely exposed in one year was detected near the No. 3 reactor at the plant.

Residents within a 20-km radius have already been ordered to evacuate the area following Saturday’s hydrogen blast at the plant’s No. 1 reactor. The transport ministry also banned aircraft from flying within 30 kilometers of the nuclear plant to prevent possible radiation exposure.

”The danger of further radiation leaks (from the plant) is increasing,” Kan warned the public at a press conference, while asking people to ”act calmly.”

Edano said the high radiation level detected at 10:22 a.m. after the explosions at the No. 2 and No. 4 reactors ”would certainly have negative effects on the human body.”

The utility firm said later in the day the massive radiation amount of 400 millisievert per hour, or 400,000 microsievert, was recorded around debris in front of the No. 3 reactor and that the material may have come from the nearby No. 4 reactor.

TEPCO has been continuing operations to pour seawater into the troubled No. 1, 2 and 3 reactors to prevent overheating and further damage to their containment vessels. But despite the injection of water, fuel rods in the three reactors remain partially exposed.

The cores of those three reactors at the plant are believed to have partially melted following the devastating quake.

The country’s strongest recorded quake, also one of the largest in global history, caused the three reactors, which were all operating at the time, to automatically shut down.