[轉貼]為什麼我不擔心日本的核電站?

immanuel

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那只是文章標題，我個人很擔心~

僅供參考！

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http://mitnse.com/2011/03/13/why-i-am-not-worried-about-japans-nuclear-reactors/

Modified version of original post written by Josef Oehmen

This post originally appeared on Morgsatlarge. It has been migrated to this location which is hosted and maintained by the MIT Department of Nuclear Science and Engineering. Members of the NSE community have edited the original post and will be monitoring and posting comments, updates, and new information. Please visit to learn more.

***Note that the title of the original blog does not reflect the views of the authors of the site.  The authors have been monitoring the situation, and are presenting facts on the situation as they develop.  The original article was adopted as the authors believed it provided a good starting point to provide a summary background on the events at the Fukushima plant.***

The original post written by Dr Josef Oehmen “Why I am not worried about Japan’snuclear reactors.” are being reposted in different languages. They have not been checked / verified.

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We will have to cover some fundamentals, before we get into what is going on.

Construction of the Fukushima nuclear power plants

The plants at Fukushima are Boiling Water Reactors (BWR for short). A BWR produces electricity by boiling water, and spinning a a turbine with that steam. The nuclear fuel heats water, the water boils and creates steam, the steam then drives turbines that create the electricity, and the steam is then cooled and condensed back to water, and the water returns to be heated by the nuclear fuel. The reactor operates at about 285 °C.

The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with a very high melting point of about 2800 °C. The fuel is manufactured in pellets (cylinders that are about 1 cm tall and 1 com in diameter). These pellets are then put into a long tube made of Zircaloy (an alloy of zirconium) with a failure temperature of 1200 °C (caused by the auto-catalytic oxidation of water), and sealed tight. This tube is called a fuel rod. These fuel rods are then put together to form assemblies, of which several hundred make up the reactor core.

The solid fuel pellet (a ceramic oxide matrix) is the first barrier that retains many of the radioactive fission products produced by the fission process.  The Zircaloy casing is the second barrier to release that separates the radioactive fuel from the rest of the reactor.

The core is then placed in the pressure vessel. The pressure vessel is a thick steel vessel that operates at a pressure of about 7 MPa (~1000 psi), and is designed to withstand the high pressures that may occur during an accident. The pressure vessel is the third barrier to radioactive material release.

The entire primary loop of the nuclear reactor – the pressure vessel, pipes, and pumps that contain the coolant (water) – are housed in the containment structure.  This structure is the fourth barrier to radioactive material release. The containment structure is a hermetically (air tight) sealed, very thick structure made of steel and concrete. This structure is designed, built and tested for one single purpose: To contain, indefinitely, a complete core meltdown. To aid in this purpose, a large, thick concrete structure is poured around the containment structure and is referred to as the secondary containment.

Both the main containment structure and the secondary containment structure are housed in the reactor building. The reactor building is an outer shell that is supposed to keep the weather out, but nothing in. (this is the part that was damaged in the explosions, but more to that later).

Fundamentals of nuclear reactions

The uranium fuel generates heat by neutron-induced nuclear fission. Uranium atoms are split into lighter atoms (aka fission products). This process generates heat and more neutrons (one of the particles that forms an atom). When one of these neutrons hits another uranium atom, that atom can split, generating more neutrons and so on. That is called the nuclear chain reaction. During normal, full-power operation, the neutron population in a core is stable (remains the same) and the reactor is in a critical state.

It is worth mentioning at this point that the nuclear fuel in a reactor can never cause a nuclear explosion like a nuclear bomb. At Chernobyl, the explosion was caused by excessive pressure buildup, hydrogen explosion and rupture of all structures, propelling molten core material into the environment.  Note that Chernobyl did not have a containment structure as a barrier to the environment. Why that did not and will not happen in Japan, is discussed further below.

In order to control the nuclear chain reaction, the reactor operators use control rods. The control rods are made of boron which absorbs neutrons.  During normal operation in a BWR, the control rods are used to maintain the chain reaction at a critical state. The control rods are also used to shut the reactor down from 100% power to about 7% power (residual or decay heat).

The residual heat is caused from the radioactive decay of fission products.  Radioactive decay is the process by which the fission products  stabilize themselves by emitting energy in the form of small particles (alpha, beta, gamma, neutron, etc.).  There is a multitude of fission products that are produced in a reactor, including cesium and iodine.  This residual heat decreases over time after the reactor is shutdown, and must be removed by cooling systems to prevent the fuel rod from overheating and failing as a barrier to radioactive release. Maintaining enough cooling to remove the decay heat in the reactor is the main challenge in the affected reactors in Japan right now.

It is important to note that many of these fission products decay (produce heat) extremely quickly, and become harmless by the time you spell “R-A-D-I-O-N-U-C-L-I-D-E.”  Others decay more slowly, like some cesium, iodine, strontium, and argon.

[ 本文最後由 immanuel 於 2011-3-15 17:06 編輯 ]

immanuel

What happened at Fukushima (as of March 12, 2011)

The following is a summary of the main facts. The earthquake that hit Japan was several times more powerful than the worst earthquake the nuclear power plant was built for (the Richter scale works logarithmically; for example the difference between an 8.2 and the 8.9 that happened is 5 times, not 0.7).

When the earthquake hit, the nuclear reactors all automatically shutdown. Within seconds after the earthquake started, the control rods had been inserted into the core and the nuclear chain reaction stopped. At this point, the cooling system has to carry away the residual heat, about 7% of the full power heat load under normal operating conditions.

The earthquake destroyed the external power supply of the nuclear reactor. This is a challenging accident for a nuclear power plant, and is referred to as a “loss of offsite power.” The reactor and its backup systems are designed to handle this type of accident by including backup power systems to keep the coolant pumps working. Furthermore, since the power plant had been shut down, it cannot produce any electricity by itself.

For the first hour, the first set of multiple emergency diesel power generators started and provided the electricity that was needed. However, when the tsunami arrived (a very rare and larger than anticipated tsunami) it flooded the diesel generators, causing them to fail.

One of the fundamental tenets of nuclear power plant design is “Defense in Depth.” This approach leads engineers to design a plant that can withstand severe catastrophes, even when several systems fail. A large tsunami that disables all the diesel generators at once is such a scenario, but the tsunami of March 11th was beyond all expectations. To mitigate such an event, engineers designed an extra line of defense by putting everything into the containment structure (see above), that is designed to contain everything inside the structure.

When the diesel generators failed after the tsunami, the reactor operators switched to emergency battery power. The batteries were designed as one of the backup systems to provide power for cooling the core for 8 hours. And they did.

After 8 hours, the batteries ran out, and the residual heat could not be carried away any more.  At this point the plant operators begin to follow emergency procedures that are in place for a “loss of cooling event.” These are procedural steps following the “Depth in Defense” approach. All of this, however shocking it seems to us, is part of the day-to-day training you go through as an operator.

At this time people started talking about the possibility of core meltdown, because if cooling cannot be restored, the core will eventually melt (after several days), and will likely be contained in the containment. Note that the term “meltdown” has a vague definition. “Fuel failure” is a better term to describe the failure of the fuel rod barrier (Zircaloy).  This will occur before the fuel melts, and results from mechanical, chemical, or thermal failures (too much pressure, too much oxidation, or too hot).

However, melting was a long ways from happening and at this time, the primary goal was to manage the core while it was heating up, while ensuring that the fuel cladding remain intact and operational for as long as possible.

Because cooling the core is a priority, the reactor has a number of independent and diverse cooling systems (the reactor water cleanup system, the decay heat removal, the reactor core isolating cooling, the standby liquid cooling system, and others that make up the emergency core cooling system). Which one(s) failed when or did not fail is not clear at this point in time.

Since the operators lost most of their cooling capabilities due to the loss of power, they had to use whatever cooling system capacity they had to get rid of as much heat as possible. But as long as the heat production exceeds the heat removal capacity, the pressure starts increasing as more water boils into steam. The priority now is to maintain the integrity of the fuel rods by keeping the temperature below 1200°C, as well as keeping the pressure at a manageable level. In order to maintain the pressure of the system at a manageable level, steam (and other gases present in the reactor) have to be released from time to time. This process is important during an accident so the pressure does not exceed what the components can handle, so the reactor pressure vessel and the containment structure are designed with several pressure relief valves. So to protect the integrity of the vessel and containment, the operators started venting steam from time to time to control the pressure.

As mentioned previously, steam and other gases are vented.  Some of these gases are radioactive fission products, but they exist in small quantities. Therefore, when the operators started venting the system, some radioactive gases were released to the environment in a controlled manner (ie in small quantities through filters and scrubbers). While some of these gases are radioactive, they did not pose a significant risk to public safety to even the workers on site. This procedure is justified as its consequences are very low, especially when compared to the potential consequences of not venting and risking the containment structures’ integrity.

During this time, mobile generators were transported to the site and some power was restored.  However, more water was boiling off and being vented than was being added to the reactor, thus decreasing the cooling ability of the remaining cooling systems. At some stage during this venting process, the water level may have dropped below the top of the fuel rods.  Regardless, the temperature of some of the fuel rod cladding exceeded 1200 °C, initiating a reaction between the Zircaloy and water. This oxidizing reaction produces hydrogen gas, which mixes with the gas-steam mixture being vented.  This is a known and anticipated process, but the amount of hydrogen gas produced was unknown because the operators didn’t know the exact temperature of the fuel rods or the water level. Since hydrogen gas is extremely combustible, when enough hydrogen gas is mixed with air, it reacts with oxygen. If there is enough hydrogen gas, it will react rapidly, producing an explosion. At some point during the venting process enough hydrogen gas built up inside the containment (there is no air in the containment), so when it was vented to the air an explosion occurred. The explosion took place outside of the containment, but inside and around the reactor building (which has no safety function).  Note that a subsequent and similar explosion occurred at the Unit 3 reactor. This explosion destroyed the top and some of the sides of the reactor building, but did not damage the containment structure or the pressure vessel. While this was not an anticipated event, it happened outside the containment and did not pose a risk to the plant’s safety structures.

Since some of the fuel rod cladding exceeded 1200 °C, some fuel damage occurred. The nuclear material itself was still intact, but the surrounding Zircaloy shell had started failing. At this time, some of the radioactive fission products (cesium, iodine, etc.) started to mix with the water and steam. It was reported that a small amount of cesium and iodine was measured in the steam that was released into the atmosphere.

Since the reactor’s cooling capability was limited, and the water inventory in the reactor was decreasing, engineers decided to inject sea water (mixed with boric acid – a neutron absorber) to ensure the rods remain covered with water.  Although the reactor had been shut down, boric acid is added as a conservative measure to ensure the reactor stays shut down.  Boric acid is also capable of trapping some of the remaining iodine in the water so that it cannot escape, however this trapping is not the primary function of the boric acid.

The water used in the cooling system is purified, demineralized water. The reason to use pure water is to limit the corrosion potential of the coolant water during normal operation. Injecting seawater will require more cleanup after the event, but provided cooling at the time.

This process decreased the temperature of the fuel rods to a non-damaging level. Because the reactor had been shut down a long time ago, the decay heat had decreased to a significantly lower level, so the pressure in the plant stabilized, and venting was no longer required.

***UPDATE – 3/14 8:15 pm EST***

Units 1 and 3 are currently in a stable condition according to TEPCO press releases, but the extent of the fuel damage is unknown.  That said, radiation levels at the Fukushima plant have fallen to 231 micro sieverts (23.1 millirem) as of 2:30 pm March 14th (local time).

***UPDATE – 3/14 10:55 pm EST***

The details about what happened at the Unit 2 reactor are still being determined.  The post on what is happening at the Unit 2 reactor contains more up-to-date information.  Radiation levels have increased, but to what level remains unknown.

immanuel

中譯文應該是original version的翻譯，僅供參考！

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http://bbs.wenxuecity.com/japan/949551.html

我在這裡寫下這些文字，是為了讓大家對在日本發生的事情——核反應堆的安全問題，感到放心。事態確實嚴重，但是已經在控制範圍內。這篇東西很長！但是你讀完之後，你會比世界上任何記者都明白核反應堆究竟是怎麼回事。
核泄漏確實已經發生，但是在將來不會有任何顯著的泄露。

“顯著泄露”大概會是個什麼程度？打個比方說，可能比你乘坐一趟長途飛行，或是喝下一杯產自本身具有高程度自然輻射地區的啤酒，所受到的輻射要多一些。

我讀了自從地震發生以來的所有新聞報道。可以說幾乎沒有一篇是準確或是無誤的（當然也可能是因為地震發生之後在日本的通訊問題）。關於“沒有一篇是無誤的” 我並不是指那些帶有反核立場的採訪，畢竟這在現在也挺常見的。我指的是其中大量的關於物理和自然規律的錯誤，及大量對於事實的錯誤解讀——可能是因為寫稿子的人本身並不了解核反應堆是如何建造和運營的。我讀過一篇來自 CNN 的三頁長度的報道，每一個段落都至少包含一個錯誤。

接下來我們會告訴大家一些關於核反應堆的基本原理，然後解釋目前正在發生的是什麼。

福島核電站的反應堆屬於“沸水反應堆”（Boiling Water Reactors），縮寫 BWR。沸水反應堆和我們平時用的蒸汽壓力鍋類似。核燃料對水進行加熱，水沸騰後汽化，然後蒸汽驅動汽輪機產生電流，然後蒸汽冷卻後再次回到液態，然後再把這些水送回核燃料處進行加熱。蒸汽壓力鍋內的溫度通常大約是 250 攝氏度。

上文提到的核燃料就是氧化鈾。氧化鈾是一種熔點在 3000 攝氏度的陶瓷體。燃料被製作成小圓柱（想像一下就像樂高積木尺寸的小圓柱）。這些小圓柱被放入一個用鋯錫合金（熔點 2200 攝氏度）製成的長桶，然後密封起來。這就是一個燃料棒（fuel rod）。然後這些燃料棒被放到一起組合為一個更大的單元，然後這些燃料單元被放入反應堆內。所有的這些，就是一個核反應堆核心（core）的內容。

鋯錫合金外殼是第一層護罩，用來將具有放射性的核燃料與世隔絕。

然後核心被放入“壓力容器”中，也就是我們之前提到的蒸汽壓力鍋的比喻。壓力容器是第二層護罩。這是一個堅固結識的大鍋，設計用於容納一個溫度可能達到數百攝氏度的核心。在核心降溫措施恢復前，壓力容器起到一定的保護作用。

一個核反應堆的所有的這些“硬件”——壓力容器，各種管道，泵，冷卻水，然後被封裝到第三層護罩中。第三層護罩是一個完全密封的，用最堅固的鋼和混凝土製成的非常厚的球體。第三層護罩的設計，建造和測試只是為了一個目的：當核心完全熔融時，將其包裹在其中。為了實現這個目的，在壓力容器（第二層護罩）的下方，鑄造了一個非常巨大厚實的混凝土大碗，這一切都在第三層護罩的內部。這樣的設計就像是為了“抓住核心”。如果核心熔融，壓力容器爆裂（並且也最終融化的話），這個大碗就可以裝下融化了的燃料及其他一切。這個大碗設計成讓融化的燃料能夠向四周鋪開，從而實現散熱。

在第三層護罩的周圍包裹的是反應堆廠房。反應堆廠房是一個將各種風吹雨打擋住的外殼。（這也是在爆炸中被毀壞的部分，我們稍後再說）

福島第一核電站一號機確實是通用電氣的Mark I型沸水堆。新聞裡露出鋼筋的部分是最外部的廠房，裡面的安全殼應該沒事。——@鐵公雞zq

核反應的一些基本原理

鈾燃料通過核分裂產生熱量。大的鈾原子分裂成更小的原子，這樣就產生熱量及中子（構成原子的一種粒子）。當中子撞擊另外一個鈾原子時，就觸發分裂，產生更多的中子並一直繼續下去。這就是核裂變的鏈式反應。

而現在的情況時，當一堆燃料棒湊在一起時就會很快導致過熱，然後在 45 分鐘後就會導致燃料棒融化。但是值得指出的是，在核反應堆內的燃料棒是絕對不可能導致像原子彈那樣的核爆炸的。製造一顆原子彈實際上是相當困難的（不信你們可以去問問伊朗）。當年切爾諾貝利的情況是，爆炸是由於大量的壓力積攢，氫氣爆炸然後摧毀了所有的護罩，然後將大量的融化的核心揮灑到了外界（就像一顆 “髒彈”）。這樣的情況為什麼在日本沒有發生，及為什麼不會發生，請繼續看下面。

為了控制鏈式反應的發生，反應堆操作員會用到“控制棒”。控制棒可以吸收中子，從而瞬間停止鏈式反應。一個核反應堆是這樣設計的：當一切正常運轉時，所有的控制棒是不會用到的。冷卻水會在核心產生熱量的同時帶走熱量（並轉化為蒸汽和電力），並且在常規的 250 攝氏度的運轉溫度下還有許多餘地。

而挑戰在於將控制棒插入並停止鏈式反應後，核心依然在產生熱量。雖然鈾元素的鏈式反應已經停止，但是在鈾元素的核裂變過程中會產生一些具有放射性的副產品，比如銫和碘同位素，這些元素的放射性同位素會最終衰變為更小的原子，然後失去放射性。在這些元素的衰變過程中，也會產生熱量。因為它們不會再從鈾元素中產生（在控制棒插入之後鈾元素就停止衰變了），所以它們的數量會越來越少，然後在衰變結束的過程中，大約幾天時間內，核心就會最終冷卻下來。

目前讓人頭痛的就是這些余熱。

核反應堆內的第一類放射性物質就是燃料棒中的鈾元素，及放射性副產物銫和碘同位素。這些物質都在燃料棒內部。

而除此之外，還存在第二類放射性物質，產生於燃料棒外部。而首先需要說明的是，這些外部的放射性物質的半衰期都非常短，這意味著它們會在很短的時間內衰變為沒有放射性的物質。“很短”的意思就是幾秒。所以即使這類放射性物質被釋放到自然環境中，他們也是毫無危害的。為什麼呢？因為大約就你在讀完“R-A- D-I-O-N-U-C-L-I-D-E”的這幾秒內，這類物質就衰變到完全不具有放射性了。這類放射性物質就是氮-16（N-16），也就是氮氣（構成大氣的氣體之一）的具有放射性的同位素。另外就是一些稀有氣體比如氬。但是這些物質是如何產生的呢？當鈾原子裂變時，會產生一個中子。大部分的這些中子都會撞擊到其他的鈾原子由此鏈式反應就一直持續發生。但是其中的一些會離開燃料棒並撞擊到水分子，或是冷卻水中的空氣。然後，一個不具有放射性的元素就會 “捕獲”這個中子，並變得有放射性。而就如前文所述，在數秒內它就會衰變到它本來的面目。

上面所描述第二類的放射性物質在我們接下來要討論的核泄露中非常重要。

immanuel

福島到底發生了什麼

接下來我會試著去總結目前的主要事實。衝擊核電站的地震的威力是核電站設計時所能承受的威力的五倍（裡氏震級之間的放大倍數是對數關係，所以 8.9 級地震的威力是 8.2 級，即核電站的設計抗震威力的 5 倍，而不是 0.7 的差異）。所以我們首先為日本的工程技術水平喝彩，至少一切目前是保下來了。

當 8.9 級地震衝擊核電站時，所有的反應堆就自動關閉了。在地震開始後的數秒內，控制棒就插入到了核心內，鏈式反應即刻中止。而此時，冷卻系統就開始帶走余熱。這些余熱相當於反應堆正常運轉時產生的 3% 的熱量。

地震摧毀了核反應堆的外部電力供應。而這是核反應堆能夠遇到的最嚴重的故障之一，因此，在設計核反應堆的備用系統時，“電站停電”是一種被高度關注的可能性。因為核反應堆的冷卻泵需要電力以維持運轉。而反應堆關閉後，核電站本身就不能產生任何電力。

在地震發生後的一小時內一切情況是平穩的。為緊急情況而準備的多組柴油發電機中的一組啟動，為冷卻泵提供了所需的電力。然後海嘯來了，比核電站設計時所預料的規模要更巨大的海嘯，摧毀了所有的柴油發電機組。

在設計核電站時，工程師們所遵循的一個哲學就是“縱深防禦”。這意味著你首先需要為了你能夠想象到最災難的情況設計防衛措施，然後為了你覺得可能絕對不會發生的子系統故障設計方案，以確保即使這樣的可能絕對不會發生的故障發生後，核電站依然可以安全。而一場巨大的摧毀所有柴油發電機組的海嘯就是這樣的一種極端情況。而所有的防衛的底線就是前面提到過的第三層護罩，將一切可能發生的最糟糕情況——控制棒插入或者未插入，核心融化或者未融化——容納於其中。

當柴油發電機組被衝走後，反應堆操作員將反應堆切換到使用緊急電池。這些電池被設計為備用方案的備用方案，用於提供給冷卻系統 8 個小時所需的電力，並且也確實完成了任務。

而在這 8 個小時內，需要為反應堆找到另外一種供電措施。當地的輸電網絡已經被地震摧毀。柴油發電機組也已經被海嘯衝走。所以最後通過卡車運來了移動式柴油發電機。

整個事件從這一刻起開始變得糟糕。運來的柴油發電機無法連接到電站（因為接口不兼容）。所以當電池耗盡後，余熱就無法再被帶走。

在這個點上反應堆操作員開始按照“冷卻失靈”的緊急預案進行處理。這是“縱深防禦”中的更進一層。理論上供電系統不至於徹底失效，但是現實如此，所以操作員們只能退到“縱深防禦”中更進一層。這一切，無論對我們看起來多麼不可思議，但卻是反應堆操作員的培訓的一部分——從日常運營到控制一個要融化的核心。

於是在這個時候外界開始談論可能發生的核心熔融。因為到了最後，如果冷卻系統無法恢復，核心就一定會融化（在幾個小時或是幾天內），然後最後一層防線——第三層護罩及護罩內的大碗，就將經受考驗。

但是此時最重要的任務是在核心持續升溫時控制住，並且確保第一層護罩（燃料棒的鋯錫合金外殼），及第二層護罩（壓力容器）能夠保持完整並盡可能多工作一段時間，從而讓工程師們能夠有足夠的時間修好冷卻系統。

既然讓核心冷卻是那麼重要的事情，因此反應堆內實際上有多個冷卻系統（反應堆給水清潔系統，衰變降溫系統，反應堆核心隔離冷卻系統，備用水冷系統，及緊急核心冷卻系統）。而究竟哪一個失效了或是沒有失效在此時無法得知。

所以想像一下，一個在爐子上的壓力鍋，持續地，慢慢地在進行加熱。操作員在採取各種手段去消除其中的熱量，但是鍋內的壓力在持續上升。於是當務之急是保住第一層護罩（熔點為 2200 攝氏度的鋯錫合金），及第二層護罩——壓力容器。而為了保住第二層護罩，其中的壓力就需要時不時進行釋放。因為在緊急時刻進行壓力釋放是一件重要的事，所以反應堆共有 11 個用於釋放壓力的閥門。操作員開始通過時不時地旋松閥門來釋放壓力容器內的壓力。此時壓力容器內的溫度是 550 攝氏度。

這就是關於“輻射泄露”的報道開始的時刻。我在上文中解釋了為什麼釋放壓力的同時實際上會釋放第二類放射性物質（主要是 N-16 和氬），及為什麼這樣做其實毫無危險。放射性氮元素和氬對於人類健康沒有威脅。

而就在旋松閥門的過程中，發生了爆炸。爆炸發生在第三層護罩外部，反應堆廠房內。反應堆廠房不具有隔絕放射性物質的功能。雖然目前並不清楚到底發生了什麼，但是這是一個很有可能的場景：操作員決定讓壓力容器內的蒸汽釋放到廠房內，而不是直接到廠房外部（這樣可以讓放射性元素有更長的時間用於衰變）。而問題在於，由於核心內的高溫，水分子會分解為氧和氫——一種易爆混合氣體，於是也確實在第三層護罩外爆炸了。歷史上也曾發生過一次類似的爆炸，不過是在壓力容器內（因為壓力容器沒有設計好並且操作失誤），進而導致了切爾諾貝利事件。而福島核電站不會有這樣的問題。氫氧混合氣體是在設計核電站時需要考慮的一個巨大問題，因此反應堆在建造時就考慮到了不能讓這樣的爆炸發生護罩內部。如果在護罩外部爆炸了，雖然也不是設想中的狀況但是可以接受，因為即使爆炸了也不會對護罩產生影響。

因此在閥門旋送時，壓力得以控制。而現在的問題時，如果水在一直沸騰的話，那麼水位就會持續下降。核心大概被幾米深的水覆蓋，使得其能夠在空氣中暴露前堅持幾個小時或幾天。而一旦沒有水覆蓋，那麼暴露的燃料棒就會在 45 分鐘後達到其 2200 攝氏度的熔點。而這樣就會導致第一層護罩，燃料棒的鋯錫合金外殼融化。

而這樣的事情正在開始發生。冷卻系統無法在燃料棒開始融化前恢復運轉，不過燃料棒中的核燃料此時依然是完好的，但是包裹燃料的鋯錫合金外殼已經開始融化。而目前正在發生的，就是一些銫和碘同位素開始隨著釋放出來的蒸汽，泄露到反應堆外。最嚴重的問題——鈾燃料，目前依然是受控的，因為氧化鈾的熔點在 3000 攝氏度。目前已經確認的是，檢測到有一部分銫和碘同位素隨著蒸汽泄露到了大氣中。

這似乎是一個啟動“B 計劃”的信號。通過在大氣中檢測到的銫和碘同位素，操作員可以確認某一根燃料棒的外殼（第一層護罩）已經存在破損。“A 計劃”在於恢復某個常規冷卻系統。為什麼這個計劃失敗目前並不清楚，而一種可能性是海嘯衝走或是污染了所有用於冷卻系統的純淨水。

用於冷卻系統的給水是非常純淨的，去除了所有礦物質的水。使用純淨水的原因在於：純淨水很大程度上不會被激活，因此可以保持相對無輻射。而如果是髒水，那麼更容易捕獲中子，進而變得更加具有放射性。這不會影響到核心——因為核心不會被冷卻水影響。但是會使得操作員更難處理這些具有輕度放射性的活化水。

但是“計劃 A”失敗了——系統無法冷卻，並且也沒有額外的純淨水。因此“計劃 B”被啟動。而這就是目前正在發生的：

為了避免核心融化，操作員開始使用海水來冷卻核心。我不是十分清楚，他們是用海水浸泡住壓力容器（第二層護罩），還是淹住反應堆外殼（第三層護罩）。不過這個不是我們現在要討論的。

要點在於核燃料現在確實已經冷卻下來了。因為鏈式反應早就已經停止，所以目前只有非常少量的余熱在產生。已經使用了的大量冷卻水可以帶走這些余熱。因為是注入了大量的水，所以目前核心已經無法再產生足夠的熱量去大幅度提升壓力。並且，海水中加入了硼酸。硼酸是一種“液體控制棒”。無論在發生什麼樣的衰變，硼都可以捕獲產生的中子並進一步加速核心的冷卻。

福島核電站曾經十分接近核心融化。但是目前最壞的情況已經被避免：如果沒有將海水注入，那麼操作員就只能繼續旋松閥門以釋放壓力。第三層護罩必須完全密封，以避免其中發生的核心融化泄露出任何的放射性物質，然後會經過一段等待期，等待護罩內的裂變副產品完成衰變，所有的放射性粒子會附著在護罩內壁。冷卻系統最終會被恢復，融化的核心也會冷卻至一個可控的溫度。護罩內部會被清理。然後需要做一項棘手骯髒的事情——將融化了核心移出，將凝固了的燃料棒及燃料一塊一塊地裝入運輸裝置，然後運送到核廢料處理廠進行處理。根據損壞狀況，核電站的這塊區域需要進行修理或是徹底拆除。

immanuel

那麼，目前留給我們的是什麼呢？

我的總結：

• 核電站會回到安全狀態並始終安全

• 日本處於第 4 級別 INES 核緊急狀態：核電站內事故。這對於擁有電站的公司是件糟糕事情，對其他人來說沒什麼影響。

• 在釋放壓力時同時釋放了一些放射性物質。包括非常小劑量的銫和碘同位素。如果在釋放時你正好坐在出口上，那麼你可能需要考慮戒煙使得你的期望壽命值回歸從前。這些銫和碘同位素會被帶入海水，然後就不會再檢測得到。

• 第一層護罩出現了一些損壞，意味著一定數量的銫和碘同位素也被釋放到了冷卻水中，但是不會有鈾或是其他什麼髒東西（因為氧化鈾不溶於水）。在第三層護罩內有用於淨化水的裝置，這些具有放射性的銫和碘同位素會在那裡被去除並且存儲為核廢料。

• 用於冷卻的海水會在一定程度上被活化。但是因為控制棒已經完全插入，所以鏈式反應是不會發生的。這就意味著“主要的”核反應沒有發生，因此也就不會加劇海水的活化。鏈式反應過程的副產物（銫和碘同位素）在這個階段也基本上消失殆盡。這進一步減輕了海水的活化。因此最壞情況就是：用於冷卻的海水中會具有一定程度的放射性，但是這些海水也同樣會經由內部淨化裝置進行處理。

• 最終會用正常的冷卻水取代海水。

• 反應堆核心會需要進行拆除並運到處理廠，就像通常的燃料更換一樣。

• 燃料棒和整個核電站需要進行徹底安全檢查，以避免潛在的危險。這通常需要 4 到 5 年。

• 全日本的核電站的安全防護會進行升級，以確保他們可以抵抗住九級地震及隨之而來的海嘯（甚至更糟糕的情況）。

• 我認為更顯著的問題是隨後的全國供電。日本的 55 座反應堆中的 11 座已經全部關閉並等待進行檢查，這直接減少全國 20% 的核電電力，而全國 30% 的電力靠核電供應。我目前還沒有去考慮國內其他核電站可能發生的事故。短缺的電力會需要依靠天然氣發電站供應，而這些電站通常只是在供電高峰時用於應急。我不是十分清楚日本國內的石油，天然氣和煤礦的能源供應鏈，及港口，煉油廠，存儲及運輸網絡在此次地震中遭受了怎樣的損失。這些都會導致電費增加，及用電高峰和重建時的電力短缺。

• 而這一切只是更大的問題的一部分。災後應急需要解決避難所，飲用水，食物，醫療，運輸，通訊設施等一系列問題，當然也包括電力供應。在一個供應鏈傾斜的時代，所有的這些領域中我們都會遇到挑戰。

如果你希望持續了解事實，那麼就忽略那些膚淺的媒體並關注以下網站：

• http://www.world-nuclear-news.org/RS_Battle_to_stabilise_earthquake_reactors_1203111.html

• http://www.world-nuclear-news.org/RS_Venting_at_Fukushima_Daiichi_3_1303111.html

• http://bravenewclimate.com/2011/03/12/japan-nuclear-earthquake/

• http://ansnuclearcafe.org/2011/03/11/media-updates-on-nuclear-power-stations-in-japan/