为什么我不担心日本的核反应器（原题目）（全文）

原作者：
来源Modified version of original post written by Josef Oehmen
译者purehho

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.

这篇文章之前出现在Morgsatlarge，现在被迁移到这里。这里是由麻省理工学院原子能科学工程部门负责的。NSE小组的成员重新编辑了原作，并且会持续跟踪相关反馈，同时将不断更新最新消息。请访问这里获取更多信息。

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.

原文由Josef Oehmen撰写，题目为“为什么我不担心日本的核反应器”，已经用几种不同语言被重新发表。它们还未被核查/证实。

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

在我们进入正题之前，不得不先介绍一些工作原理。

Construction of the Fukushima nuclear power plants

福岛核电厂的建筑结构

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.

位于福岛的核电厂属于沸水反应器（简称为BWR，Boiling Water Reactors）。BWR是通过沸水来发电的，用它的蒸汽旋转涡轮实现这一功能。原子能使水加热，水沸腾形成水蒸气，然后水蒸气带动涡轮从而产生电力，之后水蒸气冷却，重新回到液态，液态水再被核燃料加热，如此反复。反应器的工作温度为285摄氏度左右。

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.

核燃料是铀的氧化物。它的熔点很高，接近2800摄氏度。燃料被制成子弹形状（1cm高截面直径1cm的小圆筒）。这些子弹状物体被放入锆锡合金（一种锆合金）制成的长管中，它的熔点在1200摄氏度左右（这一温度由自动催化氧化水来完成），并且被严格密封。这种长管被称为燃料棒。然后这些燃料棒被集装到一起，这样的几百个集装件共同形成反应堆的内核。

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.

固体的燃料弹（陶式氧化物母体）是第一道防护，隔开裂变过程中产生的裂变放射。锆锡合金形成第二道防护，把放射燃料跟反应堆其他的组成分隔开。

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.

内核被放置在一个巨大的压力管中。压力管由很厚的钢铁构成，可以在7MPa的压力下工作（大概是1000psi），它的设计涵盖了事故发生时产生高压的情况。高压管是放射材料的第三道防护。

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.

整个最初的核反应循环是这样的——高压管、管道、泵、冷却剂（水）——这些被安装在容器型建筑中。这个建筑就是放射性物质的第四道防护。它是高度密封，用钢铁和混凝土制成的非常厚的壁障。它的设计、建造和测试都为了一个目的：去承担，或许是，全部内核的熔毁。为了这一目的，在这个容器建筑的外围又浇灌了一层很厚的混凝土外壳，作为它的双重保障。

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).

主要容器建筑跟双重保障都被建在反应器建筑的内部。反应器建筑是整个核能源厂的外壳性建筑，以便保持厂内恒温，不受外界气候变化影响。（也正是这个部分在这次爆炸中受损，这些我们后面会更详细的讲到。）

Fundamentals of nuclear reactions

核反应的工作原理

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.

铀燃料是由中子导致的核裂变产生的能量加热的。铀原子裂变为更轻的原子（aka裂变产物）。这一过程产生热，以及更多的中子（原子的构成粒子之一）。当这些中子击中另外的铀原子时，原子又产生裂变，产生新的中子，依次类推。这被称为原子能的连锁反应。在通常的全动力应用下，内核里中子的产量是稳定的（保持一个数量不变），反应器处于临界状态。

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.

在这里值得一提的是，反应器内的核燃料永远不会产生原子弹那样的核爆炸。在切尔诺贝利, 乌克兰的城市 (1986 年核反应事件的发生地)，爆炸是由于过量的压力释放、氢爆炸以及建筑物开裂导致，致使溶解的内核材料流入外部环境中。需要注意的是，切尔诺贝利核电站并没有建造一个针对外部环境的容器型建筑。所以它所发生的事情没有也不会发生在日本，这会在接下来的篇幅中讲到。

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).

为了控制原子能的连锁反应，反应器的操作人员用到了控制棒。控制棒是由能够吸收中子的硼元素制成。BMR中的一个正常的运作里，控制棒用来保持连锁反应在临界点。它也用来关闭反应器，可以从100%的动力降到7%动力（残留的延迟的热量造成）。

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.

剩余热量是由裂变产品的放射延迟造成的。裂变产物通过在小粒子（alpha、beta、gamma、中子等）状态下释放能量得到稳定，这一过程就叫放射延迟。在反应器中，有各种各样的裂变产物，包括铯和碘。剩余热量要在反应器关闭后的很长时间内才能减少，而且它们需要通过冷却系统来转移，以防燃料棒被过度加热，不再能承担放射泄漏的屏蔽作用。为反应器中延迟的热量提供足够的冷却，是目前日本遭到破坏的反应器最主要的挑战。

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.

需要注意的是，大部分的裂变产物能够很快的衰退下来，在你拼写“R-A-D-I-O-N-U-C-L-I-D-E（放射性核种）”的时候，它们已经无害了。剩下的一些延迟消退的很慢，比如铯、碘、锶和氩。

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

福岛核电厂所发生的事故（截止到2011年3月12日）

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).

接下来的主要数据只是概述。日本这次所经历的地震强度系数比当初建立这个核电厂所考虑到的最强烈的振幅还大了几倍（克里特地震测量法用对数测量系数，也就是说8.2级地震跟8.9级地震之间的倍数为5倍，而不是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.

地震来到之际，所有的核反应器都自动关闭。在地震开始的几秒之内，控制棒被植入内核中，中子连锁反应马上停止。这时候，冷却系统必须带走剩余热量，大概是全动力正常运作时候的7%。

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.

地震破坏了核反应器的外部能源供应。对于一个核电厂来说，这是个很有挑战性的意外，被认为是“装置外能源缺失”。反应器和它的备份系统设计中包括，当这种事件发生的时候，备份系统需要保持冷却系统的运行。此外，既然能源厂已经关闭，它自身是无法产生任何电力的。

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.

在关闭后的第一个小时，第一套并联应急柴油机启动，提供必要的电力。但是，当海啸到来的时候（远大于预期的罕见的海啸），它淹没了柴油动力机，切断了电力。

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.

设计核电厂有一个基本准则叫“纵深防御”。这一准则要求工程师在设计核电厂的时候要使它能够抗过严厉的大灾难，甚至是几个系统都失效的情况下。一个大强度的海啸会同时破坏所有的柴油发电装置，更何况这次的海啸比任何预期都严重。为了抵御这次的大事件，工程师们画出了一条额外的警戒线，把所有的物体都放入这个容器型建筑中（之前介绍过），当初设计的时候它就能容纳所有的物体。

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.

当柴油发电机被海啸破坏，反应器操作员开启了应急电池动力。电池组是后备系统之一，用来为冷却工作提供8小时的电力支持，它们做到了。

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.

8小时之后，电池能源耗尽，剩余热量不再能够转移。这时候核电厂操作员开始进行接下来的程序，为了填补“冷却功能的缺失”。这是承接“纵深防御”的程序。所有的这些对我们而言都惊心动魄，但是作为操作员，却是每天都要训练的内容。

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).

这时候，人们开始讨论内核熔毁的可能性。因为如果冷却功能不能重新启动，内核最终会熔化（几天之后），很可能被容纳在容器之中。需要注意的是，“熔毁”这个词定义含糊不清。“燃料失效”是个更贴切的词汇来描述燃料棒这层屏障的失效（锆锡合金）。它会发生在燃料熔化之前，由机械的、化学的、热量失效造成（太多的压力、太多的氧化物，以及太多的热量）。

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.

但是，从一开始到熔化需要很长的时间，这时候，首要任务是当内核温度升高时怎么处理，以保证燃料电镀没受破坏，并且保持尽可能长时间的可操作性。

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.

冷却内核是重中之重，反应器有大量的独立冷却装置（反应器水净系统、剩余热量排放系统、反应器内核隔离冷却系统、液态冷却系统、还有一些其他的共同构成内核应急冷却系统）。只是这时候哪些能工作哪些不能，无法判断。

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.

因为大部分的冷却能力都因为缺乏电力而丧失，我们不得不运用现有的冷却能力来尽可能多的带走热量。但是，当产生的热量渐渐超过目前的冷却能力，会有大量的水变成水蒸气的状态，使内部的压力增大。现在首要的是保持内部温度在1200摄氏度以下，以确保燃料棒的安全，亦即是内部压力处于可管理的水平。为了这个目的，水蒸气（还有其他反应器内的气体）不得不定期排放。因此，反应器在当初设计的时候，在压力管和容器建筑上预留了几个气体排放口。为了保证压力管和容器建筑的安全，操作员定时的排放气体以控制内部压力。

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.

之前提到过，水蒸气和其他气体被排放。其中的一些气体是放射性裂变产物，但是数量很小。因此，当操作员打开排放系统的时候，一些这样的气体被释放到外部环境中，但它们的数量很有限（经过过滤和冲洗数量非常小）。因为这些放射性物质的数量很小，对于公众安全甚至是工作人员都不会有重大的风险。相对于不排放气体保证不了容器安全的风险而言，这一程序所带来的后果是非常轻微的。

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.

这期间，移动发电机被运送到事发地点，一些电力已经恢复。但是，被蒸发并排出的水远多于注入反应器的水，这就减慢了仍然在工作的冷却系统的工作能力。同时，排放过程中，水平线有可能低于燃料棒的顶端。不管怎样，如果一些燃料棒的电镀温度超过了1200摄氏度，锆锡合金跟水之间就会马上产生化学反应。这种反应会产生大量的氢气，跟其他的气体一起被释放出来。这个过程是被预见的，但是大量的氢气是否会产生无法预测，因为操作员并不知道燃料棒的确切温度，也不知道水位线到底多高。因为氢气是高可燃气体，当足够多的氢气进入空气中，它就会跟其中的氧气反应。如果排放过程排出了大量的这种氢气（容器建筑内部并没有空气），当它跟空气接触，就会发生爆炸。这个爆炸会发生在容器建筑的外面，但却是反应器建筑的里面和周围（反应器建筑没有安全功能）。注意接下来在3号反应器发生的相似的爆炸。这一爆炸破坏了反应器建筑的顶部以及周边，但是没有破坏容器建筑或者压力管。虽然这没有预料到的，但它发生在容器的外面并没有给工厂的安全设施带来风险。

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.

因为一些燃料棒的电镀点超过1200摄氏度，发生了一些燃料破坏。核材料自身还完好无缺，但是它们周围的锆锡合金已经熔化。这时候，一些放射性产物（铯、碘等）开始混入水和水蒸气中。报道称小数量的铯和碘已经在周围大气中被检测到。

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.

因为反应器的冷却能力有限，而且反应器中的水量减少，工程师们觉得把海水引入其中（溶解了一些硼酸用来吸收中子），以确保燃料棒仍然被水覆盖。既然反应器已经关闭，注入其中的硼酸数量有限以确保反应器能保持关闭的状态。硼酸还可以吸引一些残留在水中的碘，让它们无处可逃，当然套牢碘并不是硼酸的首要任务。

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.

用在冷却系统中的水是经过蒸馏处理的，去除了矿物质的水。用纯净水的目的是为了限制日常操作中对冷却剂潜在的腐蚀。引入海水，需要在事件过后给予设备彻底的净化，但是目前却能够提供降温服务。

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.

这一过程把燃料棒的温度降到了一个无破坏的安全温度之下。因为反应器已经关闭了很长时间，残余热量下降到了一个显著的低水平，因此工厂内部的压力已经稳定，不再需要继续排气。

***UPDATE – 3/14 8:15 pm EST***

***3月14日晚8:15更新

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).

据东京电力公司报报道，1号和三号反应器状态已经稳定，但是燃料的破坏程度未明。报道称，截止3月14号下午2:30，福岛核电厂的辐射水平已经降到231微西府茨（23.1毫雷姆）。

添加新评论

相关文章：

  看看中国人刚刚收到的新闻警报：“如果下雨，请24小时不要出门，辐射可能会到达菲律宾......”

  爆炸、冷却系统失灵，引发民众对核辐射的恐惧

  『经济学人』日本地震：岛国的千钧一发

  里氏9.0级地震——需要救援、水、食品和药物

  为什么我不担心日本的核反应堆（一）