New assessments of the explosion at Unit 2 of Japan’s stricken Fukushima Dai-ichi nuclear plant Tuesday heightened fears that it will begin spewing large amounts of radiation.

The explosion probably damaged the main protective shield around the uranium-filled core inside one of the plant’s six reactors. Such a breach would be the first at a nuclear power plant since the Chernobyl catastrophe 25 years ago in what was then the Soviet Union.

The latest explosion – compounded by a fire in a different unit this morning – marked yet another setback in the five-day battle to stabilize the Dai-ichi facility, which suffered heavy damage to its cooling systems after Friday’s earthquake and tsunami. Other explosions occurred earlier at two of the plant’s reactors.

The blast Tuesday at Unit 2 was not outwardly visible, but potentially more dangerous because it may have created an escape route for radioactive material bottled up inside the thick steel-and-concrete reactor tube. Radiation-laced steam is probably building up between that tube and the building that houses it, experts said, triggering fears that the pressure would blow apart the structure, emitting radiation from the core.

“They’re putting water into the core and generating steam, and that steam has to go somewhere,” said Arnie Gunderson, a nuclear engineer with 40 years of experience overseeing the Vermont Yankee nuclear facility, whose reactors are of the same vintage and design as those at the Fukushima Dai-ichi plant. “It has to be carrying radiation.”

Nuclear experts have stressed that radiation releases on the scale of Chernobyl are unlikely or even impossible, given the Japanese plant’s heavier engineering and additional layers of containment.

Still, Tokyo Electric Power Co., which operates the Dai-ichi facility, said radiation briefly rose to dangerous levels at the plant Tuesday morning.

This morning, the Japanese government raised the permitted radiation exposure for plant workers by 2.5 times to allow them to work longer, according to NHK TV.

Crews noted a drop in pressure inside the reactor and also within a doughnut-shaped structure below, called a suppression pool. The simultaneous loss of pressure in those two places indicates serious damage, nuclear experts said.

The explosion probably happened after the streams of seawater that crews have been pumping into the reactor faltered. The fuel rods were left completely exposed to the air for some time, Tepco said in a statement. Without water, the rods grew white-hot and possibly melted through the steel-and-concrete tube.

Tepco said a skeleton crew of 50 to 70 employees – far fewer than the 1,400 or more at the plant during normal operations – was working in shifts to keep seawater flowing to the three reactors now in trouble.

The removal of most of the plant’s workers “is a sign to me that they have given up trying to prevent a disaster and gone into the mode of trying to clean up afterward,” Gunderson said.

Also Tuesday, and again this morning, fires temporarily flared up in Unit 4, causing fear that spent uranium fuel sitting in a pool above the reactor was burning. Such a conflagration would generate intense concentrations of cesium-137 and other dangerous radioactive isotopes.

A spokesperson for the Nuclear Energy Institute, an industry lobbying group, said that Tepco concluded that the first fire in Unit 4 was not in the spent fuel pool, “but rather in a corner of the reactor building’s fourth floor.”

The company briefly considered spraying water into holes in the Unit 4 building – caused by the previous explosions at the site – with helicopters. The company abandoned that plan, but still may use fire trucks to shoot water into the building.

Such a measure would be a last-ditch effort to prevent the spent fuel from burning and to keep radioactive isotopes from being released into the air.

“This is scary,” said Lake Barrett, a nuclear engineer who directed the cleanup of the Three Mile Island nuclear facility in Pennsylvania after the 1979 accident there. “The plans in a severe accident are to just get a fire hose in there, get any kind of water to keep water in the pool above the fuel. “

With the outer containment building at Unit 2 primed for a possible explosion, any fire crews would be in grave peril.

During normal plant operations, uranium fuel rods that can no longer produce enough heat for generating electricity are periodically removed from a reactor and placed into the spent fuel pools above the reactors. These rods continue to generate heat and radioactive isotopes for many years.

Keeping this material covered with water is sufficient to cool it. But water levels may have fallen dramatically during the crisis, exposing fuel rods to the air.

Robert Alvarez, an analyst at the Institute for Policy Studies who has long warned of the dangers of spent fuel pools, said that – unlike the reactors themselves – the fuel pools typically do not have backup pumps to maintain water flow. “They were so overwhelmed,” he said of the workers straining to contain the disaster, that they were unable to maintain enough water in the pool to prevent boiling.

If the fuel pools are exposed to the air, the radiation doses coming from them could be life-threatening up to 50 yards away, Alvarez said.

Late Tuesday, Tepco said water levels were “low” in Unit 4’s used fuel pool. Japanese officials said today that the water level in Unit 5 was slightly low but that they plan to use a generator to add more.

Satellite photos show steam rising from the facility. The amount of radioactivity carried by the plume is unknown, but small increases in radiation – not enough to affect human health – were reported in Tokyo, about 150 miles to the southwest of the facility, and in other parts of Japan.

In response, NHK television reported that the Japanese government had ordered the country’s 47 prefectures to publicly report recorded radiation levels twice a day.