核反应堆的运行原理和故障原因

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地震以及接踵而至的海啸让日本的几大核电站受损严重。以下信息将帮助你解读关于日本核泄漏的新闻

Fukushima Dai-ichi Nuclear Plant, March 14, 2011 DigitalGlobe via Getty Images

2011年3月14日，福岛第一核电厂/DigitalGlobe/盖蒂图片社

Several of Japan's nuclear power plants, especially the Fukushima Naiishi plant in northeastern Japan, are experiencing serious problems in the wake of Friday's earthquake and tsunami. If you've been following the news, you've seen some pretty alarming stuff going on at this plant--terms like "explosion," "partial meltdown," "evacuation," and "radiation exposure." Scary stuff. Here's what you need to know to understand and keep up with the news unfolding in Japan.

继周五的地震和海啸之后，日本的几大核电站尤其是日本东北部的福岛（Fukushima）的第一核电厂正在经历严重危机。如果你这几天一直在关注新闻，你肯定会注意到这处核电站遭遇了类似“爆炸”、“部分熔化”、“泄露”以及“辐射曝露”这些相当紧急的状况。为了理解新闻背后的事实真相，《大众科学》将为您普及一下关于核电站的相关常识。

What Is a Nuclear Reaction?

什么是核反应？

A nuclear reaction is at its most basic nothing more than a reaction process that occurs in an atomic nucleus. They typically take place when a nucleus of an atom gets smacked by either a subatomic particle (usually a "free neutron," a short-lived neutron not bound to an existing nucleus) or another nucleus. That reaction produces atomic and subatomic products different from either of the original two particles. To make the kind of nuclear reaction we want, a fission reaction (in which the nucleus splits apart), those two original particles have to be of a certain type: One has to be a very heavy elemental isotope, typically some form of uranium or plutonium, and the other has to be a very light "free neutron." The uranium or plutonium isotopes are referred to as "fissile," which means we can use them to induce fission by bombarding them with free neutrons.

从最通俗的角度讲，核反应实质上就是发生在原子核中的一种反应过程。当原子的原子核被亚原子微粒（通常是“自由中子”，一种原子核所束缚的短命中子）或另一个原子核碰撞时，核反应就会爆发。这种反应会产生与参与反应的两种粒子完全不同的原子和亚原子产物。为了制造人类所需的核反应类型即裂变反应，对参与反应的两种初始微粒有特定要求：一种必须是超重元素如铀或钚的同位素，另一种微粒被限定为很轻的“自由中子”。铀或钚的同位素被称为裂变材料，即我们可以利用自由中子去轰击它们来产生裂变反应。

In a fission reaction, the light particle (the free neutron) collides with the heavy particle (the uranium or plutonium isotope) which splits into two or three pieces. That fission produces a ton of energy in the form of both kinetic energy and electromagnetic radiation. Those new pieces include two new nuclei (byproducts), some photons (gamma rays), but also some more free neutrons, which is the key that makes nuclear fission a good candidate to generate energy. Those newly produced free neutrons zoom around and smack into more uranium or plutonium isotopes, which in turn produces more energy and more free neutrons, and the whole thing keeps going that way--a nuclear fission chain reaction.

在裂变反应过程中，较轻的微粒（自由中子）碰撞较重的微粒（铀或钚的同位素），并使后者分裂为两至三个组分。裂变过程产生巨大的能量，能量会以动能和电磁辐射能的形式释放出来。产生的新组成包括两个新原子核（副产物）、一些光子（伽马射线）以及更多的自由中子，它们是参与核裂变反应并产生能量的关键。新产生的自由中子四处游动，与更多的铀或钚同位素碰撞，释放出更多的能量和更多的自由中子，如此反复即形成了原子核裂变连锁反应。

Nuclear fission produces insane amounts of energy--we're talking several million times more energy than you'd get from a similar mass of a more everyday fuel like gasoline.

核裂变产生的能量极其巨大——相比同等质量的普通燃料如汽油等，核裂变材料释放出的能量要多出几百万倍。

Getting Usable Energy From Fission

从裂变中获取可利用的能量

There are several types of nuclear fission reactors in Japan, but we're going to focus on the Fukushima Naiishi plant, probably the most hard-hit facility in the country. Fukushima, run by the Tokyo Electric Power Company (TEPCO), has six separate reactor units, although numbers 4, 5, and 6 were shut down for maintenance at the time of the earthquake (and more importantly, the subsequent tsunami). Numbers 1, 2, and 3 are all "Boiling Water Reactors," made by General Electric in the early- to mid-1970s. A Boiling Water Reactor, or BWR, is the second-most-common reactor type in the world.

日本有几种不同类型的裂变反应堆，但我们将焦点主要集中在发生重大事故的福岛第一核电厂。这处核电站由东京电力公司（Tokyo Electric Power Company）负责运营，拥有六个彼此独立的反应堆单元，在地震发生时，第4、5、6号反应堆因为维护而关闭。而1、2、3号反应堆都是通用电气在上世纪七十年代早中期建起来的“沸水反应堆”，这种反应堆是核电站中第二常见的反应堆类型。

A BWR contains thousands of thin, straw-like tubes 12 feet in length, known as fuel rods, that in the case of Fukushima are made of a zirconium alloy. Inside those fuel rods is sealed the actual fuel, little ceramic pellets of uranium oxide. The fuel rods are bundled together in the core of the reactor. During a nuclear fission chain reaction, the tubes heat up to extremely high temperatures, and the way to keep them safe turns out to also be the way to extract useful energy from them. The rods are kept submerged in demineralized water, which serves as a coolant. The water is kept in a pressurized containment vessel so it has a boiling point of around 550 °F. The burning hot fuel rods turn the water to steam, which is actually what we want from this whole complicated arrangement--the high-pressure steam is used to turn the turbines on dynamos, producing electricity.

沸水反应堆中包含数千根细长的中空管即燃料棒，它长3.6米，福岛第一核电厂中的燃料棒是锆合金制成的。燃料棒中间密封着真正的燃料——氧化铀的陶瓷芯。燃料棒被束在一起放置于反应堆的核心。核裂变连锁反应发生时，导管被加热到极其高的温度，将热量安全导出则意味着可以提取裂变产生的可利用能量。燃料棒被浸没在用于冷却的软化水中，水被置于增压密封壳内，其沸点保持在摄氏290度。炙热的燃料棒将水变为蒸汽，蒸汽才是我们从这一系列复杂转换程序中真正想要的东西——蒸汽可以推动发电机上的涡轮旋转，产生电能。

Boiling Water Reactor Schematic: 1. Reactor pressure vessel (RPV) 2. Nuclear fuel element 3. Control rods 4. Circulation pumps 5. Engine control rods 6. Steam 7. Feedwater 8. High pressure turbine (HPT) 9. Low pressure turbine 10. Generator 11. Exciter 12. Condenser 13. Coolant 14. Pre-heater 15. Feedwater pump 16. Cold water pump 17. Concrete enclosure 18. Mains connection  Nicolas Lardot - Wikimedia Commons

沸水反应堆原理图：1.反应堆压力容器（RPV）；2.核燃料元素；3.控制棒；4.循环泵；5.引擎控制棒；6.蒸汽；7.供水；8.高压涡轮（HPT）；9.低压涡轮；10.发电机；11.励磁器；12.电容器；13.冷却剂；14.预热器；15.给水泵；16.冷水泵；17.混凝土外壳；18.主连接器

Safety

安全性

Since lots of heat is being produced, as well as the production and use of lots of pretty nasty radioactive materials, the engineers have implemented several safety efforts beyond simply the use of the cooling water. The plant's core, the fuel rods and the water, is encased in a steel reactor vessel. That reactor vessel is in turn encased in a giant reinforced concrete shell, which is designed to prevent any radioactive gases from escaping.

随着大量的热能被产生出来，危险的反射性原料也会随之出现，除了利用冷水冷却之外，工程师们已经实施了若干条安全保障措施。反应堆的核心，即燃料棒和软化水都被密封在钢制反应堆槽中，反应堆槽又被封存在巨大钢筋混凝土壳体中，以防止带有放射性的蒸汽从中逸出。1

There's typically a system of control rods in a functional fission plant, essentially structures that control the rate of fission by absorbing the roaming free neutrons, but those aren't of much use now. In Fukushima's reactors, the control rods were used to shut down the fission reaction, which they did correctly, but they've exhausted their use now.

功能化的裂变核电站中通常存在一个控制棒系统，它通过吸收游离的自由中子来控制裂变的速度，但现在这种控制方式已经较少应用了。在福岛核电厂的反应堆中，正常情况下控制棒本是用来停止裂变反应的，但现在它们已经失去这种效能了。

But there are still some vulnerabilities, both mechanical and chemical, that the earthquake and tsunami exploited in the Fukushima plant.

福岛核电厂还存在机械和化学上双重危险性，地震和海啸都有可能对它造成重大打击。

Is There an "Off" Switch?

还有关闭反应堆的“开关”吗？

Sure! But it's not as effective as unplugging a rogue kitchen appliance, mostly due to some chemical reactions inherent in the fission reaction--you need to keep certain certain precautions active, notably cooling, even after the plant has been "shut off."

当然有！但它的效果并不像拔掉厨房电器插头那样立竿见影，主要原因在于裂变反应堆中的一些固有的化学反应——你需要采取主动防护措施，确保反应堆保持冷却，即便是它已经被“关闭”之后。

At the onset of the earthquake, the plant automatically shut down the fission process, which normally would leave the coolant system--both the main one and a backup generator--still operational. But the tsunami wiped out power to the plant, which took down the main coolant system, and a wave destroyed the diesel-based backup system. Even though the fission had stopped, coolant is still very much required to keep the plant safe.

地震发生时，核电站已经自动中止裂变反应过程，一般只剩下冷却系统——主发电机和备用发电机——仍处于运行状态。但海啸切断了核电站的电力供应，导致主冷却系统停机，以汽油为燃料的备用系统同时被毁。尽管裂变已经停止了，但仍然需要大量的冷却剂来保证反应堆的安全。

That's due to the heat that remains in the nuclear core, both from the recently-disabled but still-hot fuel rods and from the various byproducts of the fission process. Those byproducts include radioactive iodine and caesium, both of which produce what's called "decay heat"--essentially residual heat that very slowly dissipates. If the core isn't continuously cooled, there's still enough heat that can still cause a meltdown even after it's been "turned off."

燃料棒虽然停止工作，但仍然非常炙热，而且裂变反应生成的多种副产物也会产生很多热量，这些热量一直维持在反应堆的核心。副产物包括反射性的碘和铯，它们会产生“衰变热”——即需要缓慢释放的残余热。如果核心得不到持续冷却，热量累积依然会在反应堆关闭之后导致核心熔化。

In the case of the Fukushima plant, with both the main and backup coolant systems down for the count, TEPCO was forced to rig a method to flood the core with seawater laced with boric acid (the boric acid to stave off another fission reaction if one were to restart due to a meltdown--more on that below). That's a bad sign--it's a last-ditch effort to prevent a meltdown, as the salt in the seawater will corrode the machinery. It's also a temporary fix: TEPCO will need to pump thousands of gallons of seawater into the core every day, until they can get the coolant system back online. Without it, the seawater method might have to go on for weeks, even up to a year, as the decay heat subsides.

福岛核电厂的主副冷却系统都已停摆，东京电力公司被迫使用溶解有硼酸的海水来淹没反应堆核心（核心溶解会重启另一种裂变反应，硼酸正是用来抑制这种裂变反应的）。这是一个坏兆头——注入海水以及硼酸是防止熔化的最后防线，海水中的盐分会侵蚀机械。而且这也只是一直临时性的解决方案：东京电力公司需要每天想堆芯中泵入数千加仑的海水，直到冷却系统成功运作起来。如果冷却系统无法修复，则需要连续注水几个礼拜乃至一年时间，直到衰变热渐渐减弱下来

The Dreaded Meltdown

可怕的堆芯熔化

First of all, a "meltdown" is not a precisely defined term, which makes it fairly useless as an indicator of what's going on. Even the terms "full meltdown" and "partial meltdown" are pretty unhelpful, which is partly why we've written this guide--you'll be able to understand what's actually happening without relying on spurious terms that the experts themselves are often loathe to use.

首先需要说明的是，“熔化”并不是什么精确定义的专业术语，在用来衡量反应堆状况时，这个词显得相当无用。即便是“全熔化”和“部分熔化”这样的字眼也帮不上什么大忙，这也是为什么我们要撰写这份指南的原因之一——这些没什么营养的术语甚至连专家自己也懒得使用，我们需要在不依赖这些名词的前提下理解正在发生的实际情况。

Anyway, let's start at some of the less severe (though still unsettling) things that can happen when the coolant liquid is no longer present in the core. When the fuel rods are left uncovered by water, they'll get far too hot--we're talking thousands of degrees Celsius here--and begin to oxidize, or rust. That oxidation will react with the water that's left, producing highly explosive hydrogen gas. This has already happened in Unit 1 at Fukushima (see the video below). The hydrogen gas can be vented in smallish doses into the containment building, but if they can't vent it fast enough, it'll explode, which is exactly what happened at Unit 1. Keep in mind, this is not a nuclear reaction, but a simple chemical explosion that often (as in this case) results in little or no radioactive material being leaked into the outside world.

总之，如果堆芯中的冷却剂已经不复存在，那么让我们先从一些不太严重（也仍然没有得到解决）的可能情况开始谈起。燃料棒暴露于水面之上就会变得过热，它们的温度会骤升至数千度，并开始氧化或腐蚀。氧化物和剩下的水发生反应，产生容易爆炸的氢气。福岛核电厂的1号机组已经发生了这样的爆炸（见下方的视频）。氢气本来应该慢慢进入安全壳体内，但如果它们泄出的速度过快就会发生爆炸，1号机组的爆炸正是因此而发生。注意，这并不涉及核反应，只是简单的化学爆炸，它基本不会导致放射性物质泄漏出来。

(TEPCO has announced that after the explosion, radiation levels in the area around the plant were still within "normal" parameters.) This is an important distinction--not to say that a hydrogen explosion at a nuclear plant is particularly fun news, but it is not nearly as panic-inducing as a nuclear explosion.

（东京电力公司已经宣布，在爆炸发生以后，核电站周围区域的辐射水平仍处于“正常”范围内）这是化学爆炸和核爆炸之间的重大区别——也不是说核电站的氢气爆炸就是什么特别搞笑的新闻，但它远非核爆炸那样恐怖。

What people mean when they say "meltdown" can refer to several different things, all likely coming after a hydrogen explosion. A "full meltdown" has a more generally accepted definition than, say, a "partial meltdown." A full meltdown is a worst-case scenario: The zirconium alloy fuel rods and the fuel itself, along with whatever machinery is left in the nuclear core, will melt into a lava-like material known as corium. Corium is deeply nasty stuff, capable of burning right through the concrete containment vessel thanks to its prodigious heat and chemical force, and when all that supercharged nuclear matter gets together, it can actually restart the fission process, except at a totally uncontrollable rate. A breach of the containment vessel could lead to the release of all the awful radioactive junk the containment vessel was built to contain in the first place, which could lead to your basic Chernobyl-style destruction.

“熔化”其实涉及几种不同的情况，这些情况一般都发生在氢气爆炸之后。相比“局部熔化”，“全熔化”是一个更易于接受的定义。全熔化是最糟糕的状况：锆合金燃料棒、燃料本身以及堆芯中剩下的机械设备都会熔化成岩浆一般的熔融物质。熔融物质非常可怕，它极高的温度和化学活性足以烧毁混凝土安全壳，当增压的核物质聚集到一起，就会重启裂变过程，而且反应速度完全失控。保护容器的缺口将导致本来需要包裹起来的可怕放射性物质释放出来，切尔诺贝利式的破坏局面将再度重演。

The problem with a full meltdown is that it's usually the end result of a whole boatload of other chaos--explosions, fires, general destruction. Even at Chernobyl, which (unbelievably, in retrospect) had no containment building at all, the damage was caused mostly by the destruction of the plant by explosion and a graphite fire which allowed the corium to escape to the outside world, not the physical melting of the nuclear core.

全熔化的难题在于它通常会以爆炸起火这种完全的破坏形式而终结。即便是在当时还没有修建安全容器的切尔诺贝利，大部分破坏还是由于爆炸和石墨火灾所引起的，爆炸和火灾导致熔融物质泄露出来，而堆芯并没有发生物理性的熔化。

Over the weekend, Chief Cabinet Secretary Yukio Edano somewhat hesitatingly confirmed a "partial" meltdown. What does that mean? Nobody knows! The New York Times notes that a "partial" meltdown doesn't actually need to have any melting involved to qualify it as such--it could simply mean the fuel rods have been un-cooled long enough to corrode and crack, which given the hydrogen explosion, we know has already happened. But we'd advise against putting too much stock in any term relating to "meltdown"--it'll be much more informative to find out what's actually going on, rather than relying on a vague blanket term.

日本内阁官房长官枝野幸男（Yukio Edano）于周末略带犹豫的证实了堆芯发生了“局部”熔化。这句话到底是什么意思，没人能知道！《纽约时报》上的一篇文章就认为“局部”熔化与熔化关联不大——它只是简单意味着燃料棒长时间处于过热状态已经被腐蚀到破裂，正是过热导致了氢气爆炸。我们奉劝救援小组不要把宝押在什么“熔化”上——查明实际情况则更有助于救援行动的开展，语焉不详的术语起不到作用。

As TEPCO grapples with the damage the earthquake and tsunami did to the nuclear system, there's going to be lots of news--there could be more explosions, mass evacuations, and more "meltdowns" of one kind or another. All we can do is learn about what's going on, think calmly about the situation, and hope that TEPCO can keep the damage done by the plants as low as it is now.

就在东京电力公司努力解决核电站危机的同时，还传来了很多新闻——爆炸可能还会发生，其他几个机组也有可能发生“熔化”，需要进行大规模疏散。我们现在能做的就是密切关注事态发展，冷静思考，希望东京电力公司能尽可能的将核电站造成的伤害降到最低点。