Tokyo (CNN) -- Crews at a nuclear plant struck by an earthquake, then a tsunami and then an explosion in the span of 36 hours resorted Saturday to flooding a feverish nuclear reactor with sea water in hopes of preventing a meltdown of its core.
An explosion that sent white smoke rising above the Fukushima Daiichi plant Saturday afternoon buckled the walls of a concrete building that surrounded one of the plant's nuclear reactors, but did not damage the reactor itself, Chief Cabinet Secretary Yukio Edano told reporters.
The explosion was caused, he said, by a failure in a pumping system as workers tried to prevent the reactor's temperature from racing out of control.
The International Atomic Energy Agency, citing Japanese authorities, said the explosion occurred outside the plant's primary containment vessel and that the vessel remained intact. The explosion injured four workers, it said.
To limit damage to the reactor core, Tokyo Electric Power Company began injecting sea water mixed with boron into the primary containment vessel in an operation that got under way Saturday night, IAEA said.
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Gallery: Massive quake hits Japan The use of sea water and boron was described as a "Hail Mary pass" by Robert Alvarez, senior scholar at the Institute for Policy Studies focused on energy policies and a former senior policy adviser to the U.S. secretary of energy.
"My understanding is that the situation has become desperate enough that they apparently don't have the capability to deliver fresh water or plain water to cool the reactor and stabilize it and now, in an act of desperation, are having to resort to diverting and using sea water," he said.
Boron, a chemical element, was being added to the water "to sort of stymie other potential nuclear reactions," he said.
But, he acknowledged to reporters in a conference call, "There's a lot that we don't know about what's happening with these reactors. It's trying to piece together a picture where you're dealing with just a few pieces of the puzzle."
Another expert said enough was known to conclude that Saturday's nuclear events in Japan rank high on the list of similar incidents. "If this accident stops right now it will already be one of the three worst accidents we have ever had at a nuclear power plant in the history of nuclear power," said Joseph Cirincione, an expert on nuclear materials and president of the U.S.-based Ploughshares Fund, a firm involved in security and peace funding.
He said only the 1979 partial meltdown of a reactor core at the Three Mile Island nuclear plant in Pennsylvania and the 1986 Chernobyl disaster in the Soviet Union were worse.
If the effort to cool the nuclear fuel inside the reactor fails completely -- a scenario experts who have spoken to CNN say is unlikely -- the resulting release of radiation could cause enormous damage to the plant or release radiation into the atmosphere or water. That could lead to widespread cancer and other health problems, experts say.
Japan's Nuclear and Industrial Safety Agency (NISA) was dribbling out information slower than some experts wanted: it said Saturday that it detected cesium-137 and other isotopes near Unit 1 early in the day. Cesium is a byproduct of the nuclear fission process that occurs in nuclear plants.
While Cabinet Secretary Edano said later in the day that radiation levels appeared to be falling, the government nevertheless ordered an evacuation of residents within a 20-kilometer radius of the Daiichi plant, as well as a second facility where the cooling system had failed -- the Fukushima Daini plant.
An estimated 170,000 people have been evacuated, though the process was ongoing, the International Atomic Energy Agency said Saturday.
Japanese authorities have classified the event at Fukushima Daiichi Unit 1 as a level 4 "accident with local consequences" on the International Nuclear and Radiological Event Scale intended to communicate to the public the significance of radiation-linked events. The scale runs from 0 to 7, with the latter being classified as a major accident.
On Saturday night, three patients at a hospital tested positive for radiation exposure, according to the Japanese public broadcasting station NHK, citing a statement from Fukushima Prefecture.
The three were randomly selected from a group of 90 hospital workers and patients who were outside the hospital -- about three kilometers from the Daiichi plant -- awaiting evacuation at the time of the explosion. The patients had already been hospitalized at the medical facility prior to Friday's quake.
While the three showed signed of exposure, "no abnormal health conditions have been observed," NHK quoted the prefecture as saying.
Meanwhile, two experts from the U.S. Nuclear Regulatory Commission with expertise in boiling-water nuclear reactors like those affected by the disaster have been sent to Japan as part of a U.S. Agency for International Development (USAID) team.
The NRC's Maryland-based headquarters operations center has been operating around the clock since the beginning of the emergency.
Japanese authorities, meanwhile, appeared to be preparing for the possibility of a nuclear release. Japan public broadcaster NHK reported the country's defense ministry had sent a unit that specializes in dealing with radioactive contamination to a command post near one of the stricken plants.
The government was also preparing to distribute iodine tablets to residents, the IAEA said. Iodine is commonly prescribed to help prevent the thyroid gland from taking in too much radioactivity, according to the U.S. Environmental Protection Agency website.
In all, the earthquake prompted the automatic shutdown of 10 reactors at three nuclear plants near the quake site, Japan's nuclear agency said. Problems have been reported at all three plants, although the fire reported Friday at the Onagawa nuclear plant was quickly extinguished and it has not been a focus of concern since.
At the Fukushima Daini plant, problems had been detected with the pressure and cooling systems at three of the four reactors that shut down, but plant owner Tokyo Electric Power Company reported all of the reactors were stable Saturday.
Japan's nuclear agency said there is a strong possibility that the radioactive cesium the monitors detected was from the melting of a fuel rod at the plant, adding that engineers were cooling the fuel rods by pumping water around them.
A spokesman for the agency said atomic material had seeped out of one of the five nuclear reactors at the Daiichi plant, located about 160 miles (260 kilometers) north of Tokyo.
The problems at the Daiichi plant began Friday when the 8.9-magnitude quake struck off the eastern shore of Miyagi Prefecture. The quake forced the automatic shutdown of the plant's nuclear reactors and knocked out the main cooling system, according to the country's nuclear agency.
A tsunami resulting from the quake then washed over the site, knocking out backup generators that pumped water into the reactor containment unit to keep the nuclear fuel cool, according to the agency.
As pressure and temperatures rose inside the reactors at the Daiichi and Daini plants, authorities ordered the release of valves at the plants -- a move that experts said was likely done to release growing pressure inside as high temperatures caused water to boil and produce excess steam.
As crews were working to pump additional water into the reactor containment unit to lower the temperature, the pumping system failed, Edano said, causing an explosion that injured four workers and brought down the walls of the building containing the reactor.
The team then reverted to a plan to flood the reactor with sea water, which Edano said would lower the temperature to acceptable levels. That work began Saturday night and was expected to take two days, Edano said.
Before Edano's announcement, Malcolm Grimston, associate fellow for energy, environment and development at London's Chatham House, said the explosion indicated that "it's clearly a serious situation, but that in itself does not necessarily mean major (nuclear) contamination."
"This is a situation that has the potential for a nuclear catastrophe. It's basically a race against time, because what has happened is that plant operators have not been able to cool down the core of at least two reactors," said Robert Alvarez, a senior scholar at the Institute for Policy Studies in Washington.
If damage from the explosions or aftershocks have compromised the structural integrity of the reactor complex, it could make efforts to cool the reactors more difficult, Cirincione said.
"The big unanswered question here is whether there's structural damage to this facility now," he said.
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