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Radiation levels near a quake-hit nuclear plant are now harmful to human health, Japan’s government said following two explosions and a fire at the crippled facility yesterday.

The crisis at the Fukushima No. 1 plant, 250 kilometers northeast of Tokyo, has now spread to four out of its six reactors following Friday’s quake and tsunami which knocked out cooling systems.

Prime Minister Naoto Kan said radiation had spread from four reactors.

“The level seems very high, and there is still a very high risk of more radiation coming out,” he said.

Thyroid cancer is the most immediate risk, and the Japanese government made plans to distribute potassiumiodide pills to prevent it. Worse case scenarios – lots ofradioactive fallout – can lead to other cancers years later.

“There is no doubt that unlike in the past, the figures are the level at which human health can be affected,” said chief government spokesman Yukio Edano.

Tens of thousands have already been evacuated from a zone within a radius of 20 kilometers from theaging plant, but the government urged people living within 10 km of the exclusion zone to stay indoors.

The government has stockpiled and is making plans to give out potassium iodide – pills that can keep radioactive iodine from being taken up by the thyroid gland and causing cancer.

“Those are all preventable cancers” if the protective pills are taken right after exposure, said University of New Mexico radiologist Dr. Fred Mettler. He led an international group that studied health effects of the Chernobyl disaster and is a US representative to the United Nations on radiation safety.

At Chernobyl “they had millions of square kilometers to cover and it was all rural areas and they didn’t really have anything stockpiled,” he said.

A higher than normal level of radiation was detected in Tokyo yesterday, but a city official said it was not considered at a level harmful to human health.

A blast early yesterday hit the No. 2 reactor at the crippled plant, and Edano later said there was also an explosion at the No. 4 reactor, which started a fire.

Although the No. 4 reactor was shut for maintenance when the quake and tsunami struck last Friday, “spent nuclear fuel in the reactor heated up, creating hydrogen and triggered a hydrogen explosion.”

Edano said radioactive substances were leaked along with the hydrogen.

The plant operator Tokyo Electric Power Co. (TEPCO) said the fire was extinguished later in the morning.

Similar hydrogen blasts had hit the No. 1 and No. 3 reactors on Saturday and Monday.

Edano said radiation levels as of 10:22 a.m. (0122 GMT) were 30 millisieverts between the No. 2 and the number-three reactors, 400 millisieverts near No. 3 and 100 millisieverts near No. 4.

A single dose of 1,000 millisieverts – or one sievert – causes temporary radiation sickness such as nausea and vomiting. A dose of 5,000 millisieverts would kill about half those receiving it within a month.

The government spokesman said radioactive substances might spread outside the 20-30 kilometer area but would dissipate the farther they spread.

It was still unclear whether the container sealing the No. 2 reactor had been breached.

The plant operator initially told the nuclear safety agency that it had not been holed, but later told AFP it was still checking for any breach.

The UN’s nuclear watchdog, the International Atomic Energy Agency, said Tokyo had asked for expert assistance in the wake of the disaster caused by a quake now measured at 9.0-magnitude.

Officials have struggled to prevent meltdowns at the damaged reactors, saying fuel rods may have been critically damaged by overheating.

A meltdown occurs when fuel rods melt at extremely high temperature, creating the potential for an explosion that breaches the reactor and spews large amounts radioactive material into the air.

But authorities have not reported the kind of radiation leakage that would accompany a major meltdown.

The continuing nuclear crisis has unnerved regional residents already struggling with the aftermath of the quake and tsunami.

“There are very few people out in the streets,” said Mako Sato, a cafe waitress in the town of Miharumachi just outside the evacuation zone. “They are either staying at home or in the evacuation centers.

“Since conditions surrounding the nuclear plant are so uncertain, I am worried. Food supplies are low and all that customers talk about is the quake and how scary it is, because we still feel aftershocks.”

An employee at the Hotel Chisun in Koriyama said there were no visible signs of panic despite the nuclear crisis.

“Everyone is reacting calmly. But due to safety concerns after the quake we aren’t accepting new business,” the employee said.

“There is very little food and convenience stores nearby are all closed. We are doing the best we can with our reserves.”

Within safe limits

Japanese officials said radiation levels at the plant are within safe limits, and international scientists said that while there are serious dangers, there is little risk of a catastrophe like the deadly 1986 blast in Chernobyl in Ukraine, where there were no containment shells to hold back the radiation.

Japanese authorities have been injecting seawater as a coolant of last resort, and advising nearby residents to stay inside to avoid contamination.

“It’s like a horror movie,” said 49-year-old Kyoko Nambu as she stood on a hillside overlooking her ruined hometown of Soma, about 40 kilometers from the plant. “Our house is gone and now they are telling us to stay indoors.

“We can see the damage to our houses, but radiation? … We have no idea what is happening. I am so scared.”

Outside experts were trying to piece together the level of risk based on the limited public pronouncements so far by Japanese officials. While scientists said the situation was serious, it didn’t seem to be careening toward Chernobyl-like total meltdown.

The reactor that exploded at Chernobyl, which sent a cloud of radiation over much of Europe and was blamed directly for nearly 50 deaths, was not housed in a sealed container as those at Daiichi are.

But Donald Olander, professor emeritus of nuclear engineering at the University of California at Berkeley, said having uranium fuel rods exposed was a crucial problem. Water is what keeps them from getting too hot, and melting the uranium fuel pellets inside.

He calculated that each of the three reactors at risk probably require 50 gallons (190 liters) a minute of new water being pumped in to cool down the enormous heat involved.

The more time that passes with the reactor containment vessels intact, the less the chance of a catastrophe because “nuclear decay” lessens some of the enormous heat in the reactor, said James Stubbins, head of nuclear engineering at University of Illinois.

Meanwhile, 17 US military personnel involved in helicopter relief missions were found to have been exposed to low levels of radiation after they flew back from the devastated coast to the USS Ronald Reagan, an aircraft carrier about 160 kilometers offshore.

US officials said the exposure level was roughly equal to one month’s normal exposure to natural background radiation, and the 17 were declared contamination-free after scrubbing with soap and water.

As a precaution, the carrier and other 7th Fleet ships involved in relief efforts had shifted to another area, the US said.