The earthquake-crippled Fukushima No. 1 nuclear plant suffered a dangerous radiation leak Tuesday in the wake of two new explosions and a fire as officials on site scrambled to avert a meltdown.

At 10:22 a.m., a radioactivity monitoring post near the No. 3 reactor showed 400 millisieverts per hour, 400 times the amount an ordinary person is exposed to in a year.

The figure was 100 millisieverts per hour near the No. 4 reactor and 30 millisieverts per hour between the No. 2 and No. 3 reactors.

Radiation exposure of 7,000 to 10,000 millisieverts per hour is considered a lethal dose, said an official at the Institute of Applied Energy. A millisievert is 1,000 microsieverts.

“There is no doubt it is an amount that would have (a harmful) effect on the human body,” Chief Cabinet Secretary Yukio Edano said during a morning news conference. “But that is the amount right near the leak. The farther away, it drops.”

Later in the day, Edano said the government was still trying to determine whether the sharp rise in the radiation was caused by an one-time event, pointing out that the radiation detected at the compound’s main gate — a sample point often used to measure the radiation level there — rapidly fell to 596.4 microsieverts at 3:30 p.m. after peaking at 11,930 at 9 a.m.

“We are still observing carefully, but this may not be a continuing phenomenon,” he said.

The situation, however, definitely remained severe for workers trying to stop a possible meltdown of fuel rods by cooling the reactors.

The nation’s worst nuclear crisis dramatically worsened as four of the six reactors at the Fukushima No. 1 nuclear plant have experienced hydrogen explosions since Friday’s historic temblor hit the Tohoku region.

A small explosion Tuesday took place at the No. 2 reactor, causing partial damage to the chamber. Attached below the vessel, the suppression chamber’s function is to cool the steam flowing from the vessel and thus relieve its internal pressure.

Fears were heightened over whether a containment vessel might be compromised, after the suppression chamber of the No. 2 reactor’s containment vessel failed.

Tokyo Electric Power Co. ordered about 750 of its employees to evacuate the plant site but kept 50 there to pump seawater into the reactor cores in a desperate race to cool down three of the four reactors to prevent meltdowns.

But during a news conference at 4:25 p.m., Edano said workers were successfully injecting cooling seawater into the No. 1 and No. 3 reactors.

Seawater — the last-resort coolant to bring the heating reactors under control — was also being successfully pumped into the No. 2 unit, but the government still needs to keep watching the situation carefully, Edano said.

Kazuhiko Maekawa, professor emeritus at the University of Tokyo, said Tuesday that although a meltdown is possible — at least at present — such a case would be different from the 1986 Chernobyl core meltdown in Ukraine that involved a large-scale nuclear explosion.

Even if the fuel rods melt down, they would remain in a liquid state, Maekawa said. If the primary reactor containment vessels “are not totally destroyed,” lethal radioactive materials would remain within the reactor compound, he said.

“In the Chernobyl accident, the explosion spread nuclear materials containing ‘death ash’ and contaminated many people,” Maekawa, a counselor at the Nuclear Safety Research Association, said, ruling out such an explosion in Fukushima.

“If you succeed in preventing melted fuel from leaking, the possibility of large-scale radioactive contamination is low. Local residents should remain calm and not panic,” he said.

Tuesday’s radiation release was believed triggered by a hydrogen explosion at 6:14 a.m. at the No. 4 reactor, which later caught fire. The reactor had been shut down for a regular check when the earthquake hit. But spent fuel stored inside heated up and generated hydrogen that was the apparent cause of the explosion. The fire was later brought under control.

Tepco continued to pump seawater into the Nos. 1, 2 and 3 reactors, trying to cool off the heating fuel rods inside.

On Monday, the No. 2 reactor’s fuel rods were fully exposed for more than two hours, threatening a meltdown. After Tepco pumped in seawater, the water levels rose at one point to the level where the bottom half of the fuel rods were in the coolant.

However, the water level dropped in the night, exposing the rods for a second time.

The hydrogen explosions at the Nos. 1, 2 and 3 reactors, which blew off their housings, came after their cooling systems failed.

Early Tuesday, the government, irritated by Tepco’s slow reporting to the prime minister’s office, set up a joint crisis headquarters at the utility’s head office in Chiyoda Ward, Tokyo. “We are still experiencing a worrisome situation, but we will overcome this crisis, whatever it takes,” Kan told reporters. “I will take the lead. I will resort to every possible means to prevent the damage from spreading.”

Japan has asked the United States to provide more cooling equipment to help deal with the crisis, Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission, said in Washington.

Information from Kyodo added