An explosion Monday rocked the second of three reactors at earthquake-ravaged Japan's Fukushima Dai-ichi nuclear complex, but officials insisted that radiation levels near the facility were safe, according to news reports.

Authorities had been scrambling to cool the reactor with seawater after Friday's devastating earthquake and tsunami knocked out water systems and backup generators used to regulate temperatures inside the reactor. A similar explosion at a second reactor at the complex occurred on Saturday. The explosions prompted an order for hundreds of people to stay indoors, the Associated Press reported.

The plant's operators are being forced to periodically release radioactive steam into the atmosphere as part of the make-shift emergency cooling process that could last up to a year or more. That's because the operators must constantly flood the reactors with seawater, then release the resulting radioactive steam, according to experts familiar with the design of the nuclear complex, about 170 miles northeast of Toyko, The New York Times reported.

Soon after Monday's explosion, the owner of the plant, Tokyo Electric, warned it had lost the ability to cool the third reactor at the site. Workers were once again preparing to flood the reactor with seawater, which could lead to an explosion there as well, the AP reported.

Following Monday's explosion, Japan's Chief Cabinet Secretary, Yukio Edano, said the containment vessel holding radioactive nuclear rods at the reactor was intact, allaying some fears of health threats to the public. Television coverage of the building housing the reactor appeared to show damage similar to Saturday's blast at another reactor at the site, with outer walls blown off, leaving only a skeletal frame, the AP said.

Workers are desperately trying to avoid a complete meltdown at the reactors -- the melting of the radioactive core -- that could release radioactive contaminants into the environment and pose major, widespread health risks. Edano said he was confident of escaping the worst scenarios, the AP reported.

A "meltdown" isn't a technical term, but instead a layman's description of a serious collapse of a power plant's systems and ability to control temperatures, the news service reported.

International scientists said that, while there are serious dangers posed by the stricken Japanese reactors, there's little risk of a Chernobyl-style catastrophe. The Chernobyl reactor, in the former Soviet republic the Ukraine, had no outer containment shell when it was destroyed in 1986, the scientists noted, the AP said.

"The likelihood there will be a huge fire like at Chernobyl or a major environmental release like at Chernobyl, I think that's basically impossible," said James F. Stubbins, a nuclear energy professor at the University of Illinois.

Although more than 200,000 people had been evacuated from the area around the Japanese reactors as a precaution, Edano said the radioactivity released into the environment so far was small and didn't pose any health threats, the AP said.

A complete meltdown could release uranium and dangerous contaminants into the environment and pose major, widespread health risks, the news service said.

Almost any increase in released radiation can raise long-term cancer rates, and authorities were planning to distribute iodine to residents in the area, according to the International Atomic Energy Agency. Iodine counteracts the effects of radiation, the AP said.

As many as 160 people, including 60 elderly patients and medical staff who had been waiting for evacuation in the nearby town of Futabe, and 100 others evacuating by bus, might have been exposed to radiation, said Ryo Miyake, a spokesman for Japan's nuclear agency. The extent of their exposure -- or whether it had reached dangerous levels -- was not clear. They were being taken to hospitals, the AP said.

Friday's earthquake and tsunami that pounded Japan's northeastern coast has left at least 2,800 people dead and hundreds missing, according to government officials. But police in one of the hardest-hit areas said the death toll there alone could eventually top 10,000, the AP reported.

Radiation expert Jacqueline Williams, a research professor in the department of radiation oncology at the University of Rochester in New York, said depending on the type of explosion at the reactor site, there could be a radiation risk to those at the plant.

"Anybody who is going in will be exposed to radiation -- and it will be whole-body," she said. "That's where you can get a lot of injuries to emergency personnel and maintenance personnel, depending on the degree of protection they go in with," she added.

High levels of radiation can be lethal because "radiation disrupts your cells and you die," she said.

The danger to people outside the immediate area could come from inhaling radioactive particles, Williams said. The type of radiation released into the air depends on the type of fuel used at a plant, she added.

Often the big components of released radiation are radioactive iodine and radioactive cesium, Williams said.

Breathing in or eating food contaminated with radioactive iodine can cause thyroid cancer. Food can become contaminated as the radioactive dust settles on crops and even grass that cows or other animals eat, she explained.

Radioactive cesium can cause more damage long-term, including cancer and lung problems, Williams said.

How far the radioactivity might spread would depend on weather conditions such as wind and rain, Williams said. These factors also need to be taken into account when deciding how far to move people from potential danger.

"The best protection from radiation is to get inside," she said. "Get something between you and the radiation."

In addition, all food should be washed and people should avoid any contaminated milk and meat. Radiation can also affect the water supply, Williams said.

"If it affects the water supply, then you are in more serious trouble," she said.

In Tokyo late Saturday afternoon, word of the explosion prompted people to hoard supplies of bottled water, the Washington Post reported.

"I saw a chain letter e-mail from my friend telling about the explosion in Fukushima," said one shopper who, as is the custom, wanted only to give his first name, Masahito. "Right now they're saying it's a nuclear accident. I have been trying to buy enough water for one week, just in case, but I can't find it anywhere. I've already been to four places, including a supermarket."

Williams noted that Japan relies on nuclear power for much of its energy needs, since it has no natural power resources. "But they are in an earthquake-prone area, and they have nuclear power stations where they shouldn't be," she said.