Nature/ 自然： 福岛面临有史以来的核污染清理
最新数据表明，切尔诺贝利式的（封埋）努力是必要的。
杰夫 布朗菲 （ Geoff Brumfiel ）
由于从福岛东电核反应堆已经损坏，伴随不断升级的核污染直接威胁，工程师和科学家正在面临一个清理过程，它可能持续几十年，甚至一个世纪。
处理过以往核事故的专家们说，要把核材料和及其损坏污染的废物从事故现场清除，这些已经核污染的废物将使福岛核电站清理工作面临独特的挑战。该核电站的损坏反应堆，有接近 1000 吨核燃料和成千上万吨的放射性污染的水（见图表，在线链接。略去）。
上周，东芝公司初步提出一个简略建议，用十多年清理事故现场。但是，曾经参与宾夕法尼亚州三哩岛核事故清理工作的老兵们说，它（福岛核电站）可能需要更长时间。转移清除放射性材料需要精心策划和可靠精细的技术方案，这对于已经发生部分熔堆和爆炸严重破坏的现场是更加艰巨的任务。
通常，直到反应堆稳定后才可以开始清理。核电站周边辐射开始减弱，但进一步泄漏的威胁仍然存在。在 4 月 7 日和 11 日，附近发生剧烈余震，使人更加担忧， 三个破损的反应堆状况可能进一步恶化。但是负责管理经营核电站的东京电力公司（东电， The Tokyo Electric Power Company (TEPCO) ）说没有发现进一步的损坏。
3 月 26 日，一份来自美国核管理委员会（ US Nuclear Regulatory Commission ， NRC ）的报告泄露给纽约时报，报告说，在三月份发生的福岛核电站现场大规模爆炸分散了核燃料，其中包括贮存在水池里的反应堆 ' 乏燃料池。 核管理委员会官员还认为，在 2 号核反应堆内的铀燃料一部分可能已泄漏出不锈钢容器，渗入到水泥地板下 , 但是至今为止日本政府还没有证实这一点。该文件还表明，在损坏的反应堆中心，循环水系统不能正常工作。来自于纽约，特洛伊的伦斯勒理工学院的核工程名誉教授理查德雷希说：因此有必要继续使用水浸没法。
这一策略存在自身的问题。该反应堆的冷却系统通常是一个闭合循环体系。水浸没核反应堆意味着被放射同位素污染的水将不断地排泻到环境中。东京电力公司已经报告了在工地附近的建筑物和壕沟里发现高强度放射性积水。
杰克迪瓦恩（ Jack DeVine ）是一个独立的核咨询顾问，他认为，在任何清理操作中，水的处理和清理将是一个迫切的优先选项。
杰克迪瓦恩曾花了六年拆除三哩岛 1979 年发生部分熔化的反应堆 2 号机组。三哩岛事故遗留下了上千吨含有放射性铯 -137 在污染水， 渗入到反应堆的的地下室周围。 经过数月时间， 美国组队建立了一个系统， 可以提吸出污水，并经过抗放射性的分子筛过滤。分子筛吸附铯和其他放射性同位素，使其从水中脱除，而留下几近干净的水，然后在现场蒸发彻底去污。针对福岛核事故，杰克迪瓦恩提议可以使用相类似的装置，由于损坏的核反应堆仍然不断泄漏，清理工作是一个与时间赛跑的过程，日本已经把储罐里的 10 ， 000 多吨含低放射性污水不得不被倾泻到太平洋海中，以备贮存高放射性污染的冷却水。 见第 145 页（链接）。
清理反应堆本身是一个更大的挑战。碎片，高辐射强度使其无法进行任何适当的损坏调查。红惠特克（ Red Whittaker ）说， 在近期内，将需要机器人探索反应堆厂房，了解其内的辐射情况。他是机器人专家， 在宾夕法尼亚州匹兹堡的卡耐基梅隆大学曾开发了用于其他核事故的机器人系统。
很可能需要数年时间， 人们才可以认知了解损坏的反应堆的内部。 在三哩岛核事故后，工程师们不得不等了 3 年，直到辐射强度下降到足够低的水平， 允许他们通过一个驱动控制棒把一个照相机安放在反应堆的中心。迪瓦恩进而说，在此次福岛核事故中，它可能需要更长的时间。沸水反应器（ BWR ）设计中使用了坚实的不锈钢盖，只能由一重型，装载在反应堆上方的，自带燃烧驱动的起重机拆除密封。然而， 三座核反应堆发生了爆炸，也包括了用于起重机的燃料，这些起重设备明显地破坏了。所以核电站人员必须想另外的方法进入反应器内。
该反应堆的设计也存在其他问题。迪瓦恩说： “ 沸水堆内像老鼠窝一样的狭小空间，挤满了管道和阀门 “ 。 为了有效移除核燃料，迪瓦恩认为，必须在每个反应堆周围新建一个或多个专用起重机， “ 这不是一两个月内可以完成的 ” 。 惠特克补充说，机器人和人将需要有条不紊，协调一致地共同承担其中的工作， “ 这些工作的性质是需要足够的耐心和持久性 ” 。
事实上，与三哩岛核泄露处理相比， 福岛核事故清理所付出的努力似乎更类似于乌克兰切尔诺贝利核电站的清理策略，切尔诺贝利核电站的工程师们已经开始花费巨款，约合 1.4 亿美元）筑建石棺， 其工作用自动起重机进行，把 1986 年 4 号反应堆发生爆炸后数月内匆匆丢弃的石头钢筋水泥吊离。 在 2001 年 , 原则上同意，新建筑寿命将使用一个世纪。 可是新石棺至少要到 2015 年完成。清洁过程预定计划持续到 2065 年 – 事故发生到此几乎 80 年。
几乎肯定东京电力公司不能承受如此规模的清理支出。 在华盛顿政策研究所工作的罗伯特 阿尔瓦雷斯说： “ 我认为，归根结底，政府将不得不为此付出代价 ”. 罗伯特在比尔克林顿任期时曾经监管清理以前的美国核武器工厂。 日本 政府似乎已经酝酿是否采取措施接管核泄露电站， 自核泄露事故以来，东电的股票大幅下跌。
鉴于面临任务的复杂艰巨性，一些人认为完全放弃福岛可能更好 - 至少暂时如此。曾是英国 塞拉菲尔德 ( Sellafield) 核处理基地的负责人， 已经退休的反应堆物理学家阿兰约翰逊说： “ 我敢打赌：你封起来，并等待一百年 ” 。
众所周知，塞拉菲尔德 ( 曾为 Windscale) 在 1957 年发生了英国历史上最严重的核事故，当时反应堆的石墨芯失火。至少还要 20 年的时间等待反应堆最后熄火，这段时间里放射性物质继续衰变，同时给予工程师们足够的时间来发展可能是最好的清理策略。约翰逊反问： “ 何必着急呢，有何用 ？ “
但是，在英格兰自然灾害是罕见的， 而在日本则完全不同。在未来数十年间，日本还可能受到大地震，海啸和台风的袭击， 因此，迪瓦恩怀疑应用同样的策略，他说： “ 对我来说这是一个糟糕的选择，如同把瓶子盖拧紧放在那 ” 。
Fukushima set for epic clean-up, Latest data suggest a Chernobyl-like effort will be needed.
Geoff Brumfiel
Published online 11 April 2011 | Nature 472, 146-147 (2011) | doi:10.1038/472146a
http://www.nature.com/news/2011/110411/full/472146a.html
As the immediate threat from Fukushima Daiichi's damaged nuclear reactors recedes, engineers and scientists are facing up to a clean-up process that could last for many decades, or even a century.
Experts on previous nuclear accidents say that the sheer quantity of nuclear material that needs to be removed from the site, together with the extent of the damage, makes Fukushima a unique challenge. The plant's damaged reactors are home to just under 1,000 tonnes of nuclear fuel and thousands of tonnes of radioactive water (see graphic).
Last week, the Toshiba Corporation floated a rough proposal to clean up the site in a decade. But veterans of clean-up operations at sites such as Three Mile Island in Pennsylvania say that it will probably take much longer. The removal of the radioactive material will require a carefully planned and technologically sophisticated programme, made all the more challenging by the devastation left after partial core meltdowns and explosions.
No clean-up can begin until the reactors are stabilized. Radiation around the plant is beginning to wane, but the threat of further releases has not yet passed. On 7 and 11 April, severe aftershocks struck nearby, raising fears that the three crippled reactors could be damaged further. The Tokyo Electric Power Company (TEPCO), which manages the plant, says that no additional damage has been detected.
A 26 March report from the US Nuclear Regulatory Commission (NRC), leaked to The New York Times, says that massive explosions at the plant in March scattered fuel from the reactors' spent-fuel pools around the site. NRC officials also believe that a portion of the uranium fuel inside the unit 2 reactor may have escaped its stainless steel containment vessel and fallen onto the concrete floor below, although the Japanese government has yet to confirm this. In addition, the document indicates that water is not circulating properly through the cores of the damaged reactors, so it will be necessary to continue to flood them, says Richard Lahey, an emeritus professor of nuclear engineering at Rensselaer Polytechnic Institute in Troy, New York.
This strategy creates its own problems. The reactors' cooling systems are normally a closed circuit. Flooding the cores means that water contaminated with radioisotopes will continue to spill out into the environment. TEPCO has already reported highly radioactive water in buildings and trenches around the site.
Dealing with the water will be a pressing priority for any clean-up operation, according to Jack DeVine, an independent nuclear consultant who spent six years dismantling the unit 2 reactor at Three Mile Island after it partially melted down in 1979. The accident left thousands of tonnes of water laced with radioactive caesium-137 swilling around in the reactor's basement. Over the course of months, the US team built a system that could suck the water out and pass it through radiation-resistant zeolite filters. The zeolite removed the caesium and other radioisotopes, leaving almost pristine water, which was eventually evaporated at a facility on the site. A similar system could work at Fukushima, says DeVine, although the constant leakage from the damaged cores means that any clean-up is a race against time. More than 10,000 tonnes of low-level radioactive water has already had to be dumped from storage tanks into the Pacific Ocean to make way for more-radioactive cooling water (see page 145).
Cleaning up the reactors themselves presents an even greater challenge. Debris and high radiation levels are making it impossible to conduct proper surveys of the damage.
In the near term, robots will need to explore the reactor buildings and map the radiation inside, says Red Whittaker, a robotics expert at Carnegie Mellon University in Pittsburgh, Pennsylvania, who has developed systems for other nuclear accidents.
It could be years before anyone can look inside the cores themselves. At Three Mile Island, engineers had to wait three years before radiation levels had fallen sufficiently to allow them to lower a camera through a control rod drive shaft into the heart of the reactor. At Fukushima, it could take longer still. The boiling water reactor (BWR) design used there is sealed with a solid stainless-steel cap that can only be removed by a heavy, fuel-loading crane located above the reactor. Explosions at three of the units with fuel in their cores mean that "their cranes are clearly toast", says DeVine. Plant operators will have to find another way in.
The reactor's design also presents other problems. "The BWR is just a rat's nest of tight spaces, pipes and valves," says DeVine. To effectively remove the fuel, DeVine thinks that one or more new buildings with dedicated cranes must be built around each reactor. "It's not something that will be up in a month or two," he says. Whittaker adds that robots and humans will need to share the work in a methodical, coordinated way. "The nature of these operations is that they are patient and persistent," he says.
Indeed, the effort required seems likely to be more akin to the clean-up strategy at Chernobyl in the Ukraine than that of Three Mile Island. Engineers at Chernobyl are beginning to lay the foundations for a massive US$1.4-billion enclosure, complete with automated cranes, that will eventually lift apart the sarcophagus of steel and concrete hastily thrown up around the stricken unit 4 reactor in the months after it exploded in 1986. The new building, which is intended to last a century, was agreed in principle in 2001, but will not be completed until at least 2015. Clean-up of the site is scheduled to last until 2065 — almost 80 years after the accident.
TEPCO almost certainly cannot afford a clean-up on this scale. "I think that, ultimately, the government is going to have to pay for it," says Robert Alvarez at the Institute for Policy Studies in Washington DC, who oversaw clean-up of former US nuclear weapons plants during the administration of President Bill Clinton. The government already seems to be mulling whether to take over the utility, shares in which have plunged since the accident.
Given the complexity of the task ahead, some think it may be better to abandon Fukushima entirely — at least for the time being. "My bet would be: you seal it and wait a hundred years," says Alan Johnson, a retired reactor physicist who was head of Britain's Sella¬field nuclear processing site in the late 1980s. Sellafield, once known as Windscale, was in 1957 the site of the United Kingdom's worst nuclear accident, when a reactor's graphite core caught fire. Final decommissioning of the reactor is still at least 20 years away, but the hiatus has allowed radioactive materials to decay and given engineers time to develop the best clean-up strategies possible. "What's the rush in doing it quicker?" asks Johnson.
But natural disasters are rare in England. Given the threat of major earthquakes, tsunamis and typhoons that could strike Japan in the decades to come, DeVine has his doubts about applying the same strategy. "Bottling it up and leaving it seems to me to be a really bad choice," he says.
