Two weeks after the earthquake and tsunami, Japan's crippled Fukushima Daiichi nuclear power complex continues to spread both radiation and distrust of nuclear power as the plant's situation lurches from hopeful to harrowing and back again.
This week, Tokyo Electric Power restored grid power to much of the plant's equipment, bringing instrumentation back to life and, in a few cases, restoring cooling to overheated reactors and spent fuel pools. But on Wednesday, Japan's Nuclear and Industrial Safety Agency reported that black smoke from Fukushima Daiichi's reactor unit 3 and a spike in radiation around reactor 2 had forced workers to temporarily abandon work to restart the cooling systems.
The struggle to regain control at Fukushima Daiichi has made the global nuclear industry jittery. Italy's cabinet has decided to put its plans for a return to nuclear energy on hold for one year. Earlier this week, U.S. utility NRG Energy and Japan's Toshiba said they were slowing plans to build new reactors at NRG's Bay City, Texas, nuclear plant next year. Their reasons for the delay were the possibility of shifting requirements from the U.S. Nuclear Regulatory Commission (NRC) and uncertainty over Tokyo Electric Power's ability to take a planned stake in the project.
The International Atomic Energy Agency in Vienna says it has yet to identify any "significant risk to human health." But in Japan there have been reports of radiation in fresh foods and water, and Tokyo residents with infants were warned by municipal authorities Wednesday not to use tap water after radioactive iodine at roughly double Japan's safety limit for infants was detected at a Tokyo water treatment plant.
Meanwhile, radiation trackers have measured the fission products over North America to reveal the extent of radiation released by the Fukushima accident. The measurements showed that in the first four days following the March 11 quake and tsunami, Fukushima Daiichi released Iodine-131 packing 4x1017 Becquerels of radiation, says Gerhard Wotawa, a radiation tracker at the Central Institute for Meteorology and Geodynamics in Vienna. His team generated the estimates using data from the global detectors installed to enforce nuclear weapons test bans.
Wotawa says Fukushima Daiichi released about one-fifth as much Iodine-131 as the 1986 Chernobyl accident. Iodine-131 is short-lived, with a half-life of just eight days, but the resulting carcinogenesis plays out over decades. Elevated thyroid cancer rates linked to Chernobyl's radioactive iodine have yet to decline 25 years later, according to a U.S. National Cancer Institute study published last week.
Fukushima Daiichi released over 3x1016 Becquerels-worth of Cesium-137 in the incident's first four days—about half as much as Chernobyl, according to Wotawa. Cesium-137 has a half-life of 30 years; land contaminated by it forced the permanent relocation of more than 100,000 residents from around Chernobyl.
The relatively high level of Cesium-137 is one of several factors suggesting that loss of cooling water protecting spent nuclear fuel in pools above Fukushima Daiichi's reactors caused some of the radioactive releases. Spent fuel contains little Iodine-131, which quickly breaks down once fuel is pulled from a reactor, but it retains lots of Cesium-137.
On Monday, NRC chief of operations William Borchardt said its experts had concluded that "radiation releases and the dose rates" observed at Fukushima Daiichi were "primarily influenced" by loss of cooling water at the fuel storage pools above reactor units 3 and 4.
Nuclear power experts and regulators say measures to reduce risks from fuel storage pools in the U.S. are a likely outcome of a 90-day review of safety requirements announced by the NRC on Monday. Borchardt said one item the NRC would likely review was backup power and cooling systems for spent fuel pools at the 104 reactors in the United States.
Spent fuel pools lack the hardened steel and concrete containment structures designed to limit the impact of a nuclear reactor meltdown. That means that fuel in a pool that catches fire, explodes, or even resumes chain reactions is more likely to widely disperse its radioactive material. The risk of such spent fuel accidents may be particularly acute in the United States. Unlike France and Japan, the U.S. government rejected recycling of spent fuel in centralized reprocessing facilities. It subsequently failed to implement its plan B: shipping spent fuel to a national repository at Nevada's Yucca Mountain. The result is that spent fuel pools designed to cool fuel bundles for five years are instead packed with decades' worth of fuel bundles.
Charles Forsberg, an MIT research scientist and executive director of a nuclear fuel cycle report issued last fall by MIT's Energy Initiative, predicts that Fukushima Daiichi will accelerate the movement of U.S. spent fuel from pools to dry casks. The casks are cylindrical steel, lead, and concrete containers designed to hold 10 to 15 metric tons of spent fuel. Cooling is passive, relying on natural air circulation. "Dry cask storage is a big dumb can with thick walls. There isn't much that can go wrong with it," says Forsberg.
Dry casks are already used at several dozen U.S. reactors to accept fuel that no longer fits in the pools. Putting even more spent fuel in dry casks, says Forsberg, would make the fuel that's left in the pool less vulnerable to the loss of cooling that is thought to have partially melted down some of Fukushima Daiichi's spent fuel. "You've got less decay heat, and more water to boil off, if bad things happen," says Forsberg.
A 2006 report by the U.S. National Research Council affirmed the advantages of dry cask storage, suggesting that it would be "prudent" to accelerate the shift to dry casks. The year before President Obama nominated him as the chairman of the Nuclear Regulatory Commission, Gregory Jaczko called a mandated move to dry cask storage the "most clear-cut" opportunity to expand the U.S. nuclear power industry's margin of safety.
Whether that happens now may come down to who is willing to foot the bill. The cost of moving most fuel to dry casks is on the order of $43 million to $109 million, according to estimates prepared for the Massachusetts attorney general in 2006 (the figures supported a petition to mandate dry cask storage at New England's Pilgrim and Vermont Yankee nuclear plants).
Forsberg says that investment would represent an "insignificant" increase in the price of nuclear power. But nuclear utilities in the U.S. are reluctant to absorb any costs associated with spent fuel waste management, citing nearly $18 billion in fees they have collected from ratepayers for a federal trust fund that was supposed to cover the cost of Yucca Mountain. "It remains the U.S. government's responsibility and contractual obligation to remove used fuel from reactor sites," says a spokeswoman for Southern Company, which operates six reactors in the southeastern United States.
David Lochbaum, a nuclear safety expert with the Union of Concerned Scientists, says he hopes reactor operators will come around, and make the move to protect their power plants. "The owners ... have billion-dollar assets to protect. One of the cheapest ways to protect that asset is to get the spent fuel into a less vulnerable position. I would hope that the NRC wouldn't have to make this happen," he says.
sources : here and here
No comments:
Post a Comment