日本核泄漏威脅持續(xù)核事故引發(fā)安全系統(tǒng)質(zhì)疑

Fears rise over second nuclear reactor

Radiation continued to leak from an earthquake-stricken nuclear power station in Japan on Sunday as engineers worked to cool its overheated fuel, while thousands of people along the country’s northeastern coast remained missing after Friday’s quake and tsunami.

Conditions at the Number Three reactor at Tokyo Electric Power’s (Tepco) Fukushima Daiichi nuclear power facility, 240km north of Tokyo, deteriorated overnight as its coolant systems failed, forcing operators to pump seawater into the reactor vessel. The same emergency measure was employed on Saturday at the Number One reactor.

Yukio Edano, Japan’s chief cabinet secretary, said there was a “significant possibility” that a meltdown had occurred at both the Number One and Number Three reactors at the facility. “We are acting under the assumption that it has,” he told a televised news conference.

Radiation levels at the site spiked as engineers vented contaminated steam from both reactors to relieve pressure building up inside. “Tiny amounts” of radioactive material had escaped into the air during the ventilations, Mr Edano said.

At least 1,500 people were reported to have died and thousands more were still unaccounted for. Nearly 400,000 people had been evacuated to temporary shelters.

On Saturday, the steel and concrete building housing the Fukushima Daiichi Number One nuclear reactor was blown apart by an explosion, but officials said the blast had not breached the reactor’s core. Scientists said the use of sea water in the attempt to cool the Number One reactor was a highly unusual step, suggesting that more conventional options for bringing the situation under control had been exhausted.

There were reports that about 160 local residents may have been exposed to radiation. Thirteen people were diagnosed with possible radiation exposure, according to local media.

The Japanese government doubled the radius of the evacuation zone around the power plant to 20km after the blast.

People living and working near the plant were being evacuated on Saturday night and the International Atomic Energy Agency said the government was preparing to distribute iodine tablets, a treatment that can prevent the body from absorbing radiation in the event of a leak.

Television footage showed a large explosion at 3:36pm local time on Saturday, with plumes of grey smoke pouring from the site. Tepco said four workers at the site were injured in the explosion.

At the time Mr Edano said the blast had “destroyed the roof of the outer building but the reactor containment vessel did not explode”.

The level of radiation was higher than normal before the blast because engineers had been venting contaminated steam from the reactor vessel, in an effort to relieve pressure that had been building since the cooling system failed on Friday. The venting process triggered the explosion when combustible hydrogen released by the steam mixed with oxygen in the air, officials said.

Japan’s Nuclear and Industrial Safety Agency (Nisa) confirmed the presence of caesium and iodine, both radioactive elements, in the vicinity of Fukushima Daiichi Unit 1. Nisa reported an initial increase in levels of radioactivity around the plant earlier on Saturday, but these levels have been observed to lessen in recent hours.

Reactor Failures Spark Questions On Safety Systems

The stricken Fukushima Daiichi facility has six reactors in total, but three were shut for maintenance when the earthquake occurred. The other plant, Fukushima Daini, also experienced coolant problems, and authorities have instructed people living within 10km of that facility to evacuate.

Earlier on Saturday, Naoto Kan, Japan’s prime minister, flew to the Fukushima power plant.

 

The accident at the Fukushima Daiichi nuclear-power plant casts doubt on the fundamental premise that has guided the global nuclear industry for five decades: that engineers can build enough redundancy into plant safety systems to overcome dangers. In Fukushima prefecture, Tokyo Electric Power Co. found that the layers of redundancy in the plant's electric-supply and cooling systems weren't sufficient to nullify the power of nature, which took the form of a massive, magnitude 8.9 earthquake followed by large aftershocks and tsunami waves. Plant operators were reduced to using seawater in a desperate attempt to cool an overheated reactor core and prevent a meltdown that could result in a radioactive release if all protective barriers were breached and the containment building leaked. Although the exact chronology of the accident won't be known for some time, the earthquake severed grid electricity, and backup diesel generators failed to furnish sufficient power to the plant's Unit 1 reactor, the oldest and smallest at the site. Battery power was called upon, but there are indications that it and other plant systems that use waste heat to operate vital pumps and valves were insufficient for operators to maintain control of the plant. Diesel generators appear to have been knocked out by tsunami waves that followed the earthquake. Peter Bradford, a member of the U.S. Nuclear Regulatory Commission at the time of the Three Mile Island accident in Pennsylvania in 1979, said the accident exposes shortcomings in risk analysis as well as engineering. 'The redundancy, such as it was, obviously was inadequate to the event that actually happened,' he said. He said the problem is that certain risks always are discounted in the licensing process as 'so highly unlikely that you don't have to plan for them.' He said that may be the case in Japan, with an earthquake that apparently exceeded the level that the plant was designed to withstand, possibly compounded by other unexpected technical problems and tsunami waves. It isn't yet known if operator error may have played a role, as it did three decades ago at Three Mile Island. 'The really important question,' Mr. Bradford said, 'is to ask how different licensing bodies decide what risks have to be guarded against and see if that analysis was adequate.' Richard Meserve, a physicist and former NRC chairman, said the Japanese reactors experienced a 'one-two punch of events beyond what anyone could expect or what was conceived.' A reassessment of safety threats to boiling-water reactors, in particular, and also to coastal reactors will result, he said. Operators of similar early-vintage General Electric Co. boiling-water reactors in the U.S. said they are trying to understand events in Japan. The NRC sent boiling-water-reactor experts to Japan over the weekend and agency Chairman Gregory Jaczko said, 'We stand ready to assist in any way possible.' Two kinds of nuclear plants are most likely to be affected by the accident- those of similar reactor type and those that also are located in coastal areas around earthquake faults. In earthquake-prone California, attention immediately turned to PG&E Corp.'s Diablo Canyon nuclear plant and Southern California Edison's San Onofre plant, both of which are on the Pacific Ocean, near fault zones. Companies with boiling water reactors similar to the ones in Japan include Exelon Corp., Entergy Corp. and Xcel Energy Inc. There are nearly two dozen such plants in the U.S. Marshall Murphy, spokesman for Exelon's nuclear business, said his company 'is not in a position to talk about' the Japan accident or its similar units, a list that includes its Quad Cities plant in Cordova, Ill., its Dresden plant in Morris, Ill., and its Oyster Creek plant in Forked River, N.J. Entergy has several similar units, including its Pilgrim plant in Plymouth, Mass., its Vermont Yankee plant in Vernon, Vt., and its FitzPatrick plant in Scriba, N.Y. Xcel's Monticello plant in Monticello, Minn., also is similar. Xcel and Entergy referred calls to an industry association. The Japanese nuclear accident has come at a delicate time for the nuclear-power industry, growing globally, for the first time in decades as a new generation of so-called 'passive' designs is being embraced as offering more safety. The passive feature means they rely less on electrically driven pumps and valves, for safety, and more on systems designed to shut down plants safely in emergencies and to prevent operator error.