Nature/ 自然： 福島麵臨有史以來的核汙染清理
最新數據表明，切爾諾貝利式的（封埋）努力是必要的。
傑夫 布朗菲 （ Geoff Brumfiel ）
由於從福島東電核反應堆已經損壞，伴隨不斷升級的核汙染直接威脅，工程師和科學家正在麵臨一個清理過程，它可能持續幾十年，甚至一個世紀。
處理過以往核事故的專家們說，要把核材料和及其損壞汙染的廢物從事故現場清除，這些已經核汙染的廢物將使福島核電站清理工作麵臨獨特的挑戰。該核電站的損壞反應堆，有接近 1000 噸核燃料和成千上萬噸的放射性汙染的水（見圖表，在線鏈接。略去）。
上周，東芝公司初步提出一個簡略建議，用十多年清理事故現場。但是，曾經參與賓夕法尼亞州三哩島核事故清理工作的老兵們說，它（福島核電站）可能需要更長時間。轉移清除放射性材料需要精心策劃和可靠精細的技術方案，這對於已經發生部分熔堆和爆炸嚴重破壞的現場是更加艱巨的任務。
通常，直到反應堆穩定後才可以開始清理。核電站周邊輻射開始減弱，但進一步泄漏的威脅仍然存在。在 4 月 7 日和 11 日，附近發生劇烈餘震，使人更加擔憂， 三個破損的反應堆狀況可能進一步惡化。但是負責管理經營核電站的東京電力公司（東電， The Tokyo Electric Power Company (TEPCO) ）說沒有發現進一步的損壞。
3 月 26 日，一份來自美國核管理委員會（ US Nuclear Regulatory Commission ， NRC ）的報告泄露給紐約時報，報告說，在三月份發生的福島核電站現場大規模爆炸分散了核燃料，其中包括貯存在水池裏的反應堆 ' 乏燃料池。 核管理委員會官員還認為，在 2 號核反應堆內的鈾燃料一部分可能已泄漏出不鏽鋼容器，滲入到水泥地板下 , 但是至今為止日本政府還沒有證實這一點。該文件還表明，在損壞的反應堆中心，循環水係統不能正常工作。來自於紐約，特洛伊的倫斯勒理工學院的核工程名譽教授理查德雷希說：因此有必要繼續使用水浸沒法。
這一策略存在自身的問題。該反應堆的冷卻係統通常是一個閉合循環體係。水浸沒核反應堆意味著被放射同位素汙染的水將不斷地排瀉到環境中。東京電力公司已經報告了在工地附近的建築物和壕溝裏發現高強度放射性積水。
傑克迪瓦恩（ Jack DeVine ）是一個獨立的核谘詢顧問，他認為，在任何清理操作中，水的處理和清理將是一個迫切的優先選項。
傑克迪瓦恩曾花了六年拆除三哩島 1979 年發生部分熔化的反應堆 2 號機組。三哩島事故遺留下了上千噸含有放射性銫 -137 在汙染水， 滲入到反應堆的的地下室周圍。 經過數月時間， 美國組隊建立了一個係統， 可以提吸出汙水，並經過抗放射性的分子篩過濾。分子篩吸附銫和其他放射性同位素，使其從水中脫除，而留下幾近幹淨的水，然後在現場蒸發徹底去汙。針對福島核事故，傑克迪瓦恩提議可以使用相類似的裝置，由於損壞的核反應堆仍然不斷泄漏，清理工作是一個與時間賽跑的過程，日本已經把儲罐裏的 10 ， 000 多噸含低放射性汙水不得不被傾瀉到太平洋海中，以備貯存高放射性汙染的冷卻水。 見第 145 頁（鏈接）。
清理反應堆本身是一個更大的挑戰。碎片，高輻射強度使其無法進行任何適當的損壞調查。紅惠特克（ Red Whittaker ）說， 在近期內，將需要機器人探索反應堆廠房，了解其內的輻射情況。他是機器人專家， 在賓夕法尼亞州匹茲堡的卡耐基梅隆大學曾開發了用於其他核事故的機器人係統。
很可能需要數年時間， 人們才可以認知了解損壞的反應堆的內部。 在三哩島核事故後，工程師們不得不等了 3 年，直到輻射強度下降到足夠低的水平， 允許他們通過一個驅動控製棒把一個照相機安放在反應堆的中心。迪瓦恩進而說，在此次福島核事故中，它可能需要更長的時間。沸水反應器（ BWR ）設計中使用了堅實的不鏽鋼蓋，隻能由一重型，裝載在反應堆上方的，自帶燃燒驅動的起重機拆除密封。然而， 三座核反應堆發生了爆炸，也包括了用於起重機的燃料，這些起重設備明顯地破壞了。所以核電站人員必須想另外的方法進入反應器內。
該反應堆的設計也存在其他問題。迪瓦恩說： “ 沸水堆內像老鼠窩一樣的狹小空間，擠滿了管道和閥門 “ 。 為了有效移除核燃料，迪瓦恩認為，必須在每個反應堆周圍新建一個或多個專用起重機， “ 這不是一兩個月內可以完成的 ” 。 惠特克補充說，機器人和人將需要有條不紊，協調一致地共同承擔其中的工作， “ 這些工作的性質是需要足夠的耐心和持久性 ” 。
事實上，與三哩島核泄露處理相比， 福島核事故清理所付出的努力似乎更類似於烏克蘭切爾諾貝利核電站的清理策略，切爾諾貝利核電站的工程師們已經開始花費巨款，約合 1.4 億美元）築建石棺， 其工作用自動起重機進行，把 1986 年 4 號反應堆發生爆炸後數月內匆匆丟棄的石頭鋼筋水泥吊離。 在 2001 年 , 原則上同意，新建築壽命將使用一個世紀。 可是新石棺至少要到 2015 年完成。清潔過程預定計劃持續到 2065 年 – 事故發生到此幾乎 80 年。
幾乎肯定東京電力公司不能承受如此規模的清理支出。 在華盛頓政策研究所工作的羅伯特 阿爾瓦雷斯說： “ 我認為，歸根結底，政府將不得不為此付出代價 ”. 羅伯特在比爾克林頓任期時曾經監管清理以前的美國核武器工廠。 日本 政府似乎已經醞釀是否采取措施接管核泄露電站， 自核泄露事故以來，東電的股票大幅下跌。
鑒於麵臨任務的複雜艱巨性，一些人認為完全放棄福島可能更好 - 至少暫時如此。曾是英國 塞拉菲爾德 ( Sellafield) 核處理基地的負責人， 已經退休的反應堆物理學家阿蘭約翰遜說： “ 我敢打賭：你封起來，並等待一百年 ” 。
眾所周知，塞拉菲爾德 ( 曾為 Windscale) 在 1957 年發生了英國曆史上最嚴重的核事故，當時反應堆的石墨芯失火。至少還要 20 年的時間等待反應堆最後熄火，這段時間裏放射性物質繼續衰變，同時給予工程師們足夠的時間來發展可能是最好的清理策略。約翰遜反問： “ 何必著急呢，有何用 ？ “
但是，在英格蘭自然災害是罕見的， 而在日本則完全不同。在未來數十年間，日本還可能受到大地震，海嘯和台風的襲擊， 因此，迪瓦恩懷疑應用同樣的策略，他說： “ 對我來說這是一個糟糕的選擇，如同把瓶子蓋擰緊放在那 ” 。
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.
