How do nuclear power plants work? - M. V. Ramana and Sajan Saini

翻译人员: Hongji Zhang 校对人员: Yolanda Zhang

On a December afternoon in Chicago during the middle of World War II,

二战期间，在芝加哥 一个十二月的下午，

scientists cracked open the nucleus at the center of the uranium atom

科学家们破开了铀原子中心的原子核，

并反复地将核物质转化为能量。

and turned nuclear mass into energy over and over again.

他们第一次在一个新的工程奇迹里

They did this by creating for the first time

a chain reaction inside a new engineering marvel:

创造出了一个连锁反应：

the nuclear reactor.

核反应堆。

Since then, the ability to mine great amounts of energy from uranium nuclei

从那时起，从铀原子核中 开采大量能量的能力

has led some to bill nuclear power

使核电被一些人列为

as a plentiful utopian source of electricity.

一种理想，丰富的电力来源。

A modern nuclear reactor generates enough electricity from one kilogram of fuel

一个现代核反应堆可以从 一公斤的燃料里产生足够的电力，

to power an average American household for nearly 34 years.

供给一个美国普通家庭近34年。

But rather than dominate the global electricity market,

但与主宰全球电力市场相反的是，

nuclear power has declined from an all-time high of 18% in 1996

核电从1996年18%的历史最高点

to 11% today.

下降到现在的11％。

And it's expected to drop further in the coming decades.

而且预计在未来的 几十年里它将进一步下滑。

What happened to the great promise of this technology?

这项技术的巨大前景去哪儿了？

It turns out nuclear power faces many hurdles,

事实证明，核电还面临着很多问题，

including high construction costs

包括昂贵的建设成本

and public opposition.

和公众的反对。

And behind these problems lie a series of unique engineering challenges.

而且这些问题背后还有 一系列独特的工程挑战。

Nuclear power relies on the fission of uranium nuclei

核能依赖于铀的核裂变

and a controlled chain reaction

和一个可控的

复制性核裂变的链式反应。

that reproduces this splitting in many more nuclei.

原子核密集地包裹着由强大核力

The atomic nucleus is densely packed with protons and neutrons

bound by a powerful nuclear force.

所束缚的质子和中子。

Most uranium atoms have a total of 238 protons and neutrons,

大多数铀原子总共有 238个质子和中子，

but roughly one in every 140 lacks three neutrons,

但是每140个原子中大约 有一个缺少三个中子，

and this lighter isotope is less tightly bound.

而且这种较轻的同位素结构并不紧密。

Compared to its more abundant cousin,

与它的近亲相比，

a strike by a neutron easily splits the U-235 nuclei

中子的撞击很容易 将铀 - 235的原子核

into lighter, radioactive elements called fission products,

分成更轻的，叫做 裂变产物的放射性元素，

in addition to two to three neutrons,

两到三个中子，

gamma rays,

伽马射线，

and a few neutrinos.

和一些中微子。

During fission, some nuclear mass transforms into energy.

在裂变过程中，一些核物质被转化为能量。

A fraction of the newfound energy powers the fast-moving neutrons,

新发现的能量中的一小部分 为速移的中子提供了能量，

and if some of them strike uranium nuclei,

如果它们中的一些撞到了铀原子核，

fission results in a second larger generation of neutrons.

那么裂变将产生第二代更多的中子。

If this second generation of neutrons strike more uranium nuclei,

如果这第二代中子撞到更多的铀原子核，

more fission results in an even larger third generation, and so on.

更多的裂变会导致 更庞大的第三代，以此类推。

但在核反应堆里，

But inside a nuclear reactor,

this spiraling chain reaction is tamed using control rods

这种螺旋链式反应使用元素控制棒来

made of elements that capture excess neutrons and keep their number in check.

捕捉多余的中子，并控制它们的数量。

采用可控链式反应，

With a controlled chain reaction,

a reactor draws power steadily and stably for years.

反应器可稳定供电数年。

The neutron-led chain reaction is a potent process driving nuclear power,

以中子领导的链式反应 是驱动核电的重要过程，

but there's a catch that can result

但有一个隐情会导致

in unique demands on the production of its fuel.

一些其燃料生产的独特要求。

It turns out, most of the neutrons emitted from fission have too much kinetic energy

事实证明，裂变产生的 大部分中子有太多的动能，

to be captured by uranium nuclei.

无法被铀核捕捉到。

The fission rate is too low and the chain reaction fizzles out.

这导致裂变率太低， 链式反应会逐渐消失。

The first nuclear reactor built in Chicago used graphite as a moderator

芝加哥的第一个核反应堆 用石墨作为慢化剂

to scatter and slow down neutrons just enough

来分散，减缓中子直到它们被铀捕抓到，

to increase their capture by uranium and raise the rate of fission.

从而提高裂变的速度。

Modern reactors commonly use purified water as a moderator,

现代反应堆通常使用纯化水作为慢化剂，

但散射的中子还是很快。

but the scattered neutrons are still a little too fast.

To compensate and keep up the chain reaction,

为了维持链式反应，

the concentration of U-235 is enriched

铀-235浓度会被浓缩到

to four to seven times its natural abundance.

天然浓度的4至7倍。

Today, enrichment is often done by passing a gaseous uranium compound

现今，浓缩通常是由气态铀化合物

through centrifuges

通过离心机

to separate lighter U-235 from heavier U-238.

从较重的 铀-238 分出 较轻的 铀-235来实现。

但同样的步骤也可以 继续来使铀-235高量浓缩

But the same process can be continued to highly enrich U-235

up to 130 times its natural abundance

致它的天然丰度的130倍，

and create an explosive chain reaction in a bomb.

并在炸弹中创造出爆破连锁反应。

Methods like centrifuge processing must be carefully regulated

像这种离心处理的步骤必须小心处理

to limit the spread of bomb-grade fuel.

来限制核燃料的扩散。

Remember, only a fraction of the released fission energy

记住，只有一丁点的核裂变能

goes into speeding up neutrons.

是从加速中子运动中得来的。

Most of the nuclear power goes into the kinetic energy of the fission products.

大部分核能是核裂变产生的动能

Those are captured inside the reactor as heat by a coolant,

这些能量获取于在反应堆内部加温冷却剂，

通常是纯净水。

usually purified water.

This heat is eventually used to drive an electric turbine generator by steam

这些热量最终会被用来在反应堆外部

just outside the reactor.

蒸汽驱动汽轮机。

Water flow is critical not only to create electricity,

水的流动不仅对制造电流十分关键，

but also to guard against the most dreaded type of reactor accident,

它还被用来防备最可怕的反应堆事故，

the meltdown.

溶解。

If water flow stops because a pipe carrying it breaks,

如果承载水流的管道破裂，

or the pumps that push it fail,

或者水泵失灵，

the uranium heats up very quickly and melts.

铀就会迅速升温并溶解。

During a nuclear meltdown,

在核溶解过程中，

放射性蒸汽会浸入反应堆，

radioactive vapors escape into the reactor,

and if the reactor fails to hold them,

如果反应堆无法承受它们，

a steel and concrete containment building is the last line of defense.

那么坚固的钢铁容器将成为最后的防线。

But if the radioactive gas pressure is too high,

但如果放射性气压太高，

containment fails and the gasses escape into the air,

容器无法阻拦它， 那么放射性气体就会渗入空气，

spreading as far and wide as the wind blows.

然后被风广泛散播。

The radioactive fission products in these vapors

这些蒸汽中的放射性裂变物质

eventually decay into stable elements.

会最终衰退成稳定元素。

While some decay in a few seconds,

有的物质会在几秒钟内定型，

others take hundreds of thousands of years.

但有的则需要上百上千年的时间。

The greatest challenge for a nuclear reactor

对于一个核反应堆的最大挑战就是

is to safely contain these products

安全的存储这些放射性元素

and keep them from harming humans or the environment.

并使它们不能伤害到人类与自然。

Containment doesn't stop mattering once the fuel is used up.

核燃料的耗尽不代表 放射性元素储存不再是个问题。

In fact, it becomes an even greater storage problem.

实际上，存储变成了一个更严重的问题。

每过一两年，

Every one to two years,

some spent fuel is removed from reactors

一些用完的燃料就会被从反应堆中提走，

and stored in pools of water that cool the waste

然后储存在废水池中来冷却这些残渣

and block its radioactive emissions.

和隔绝辐射。

The irradiated fuel is a mix of uranium that failed to fission,

辐射性燃料是由裂变失败的铀，

fission products,

裂变过的铀，

and plutonium, a radioactive material not found in nature.

和一种自然中不存在的 放射性钚混合而成的。

This mix must be isolated from the environment

这种混合物必须在它变得安全稳定化之前

until it has all safely decayed.

从自然环境中隔离出来。

许多国家都计划在深层地下管道中 长期储存这些混合物，

Many countries propose deep time storage in tunnels drilled far underground,

but none have been built,

但都没有付诸实施，

and there's great uncertainty about their long-term security.

并且这些项目的长期安全性 还存在着不确定性要素。

一个只存在了几百年的国家

How can a nation that has existed for only a few hundred years

plan to guard plutonium through its radioactive half-life

又怎么能计划去安存 半衰期为24000年的

of 24,000 years?

钚元素呢？

Today, many nuclear power plants sit on their waste, instead,

今天，核能设施都建设在核废料上面，

storing them indefinitely on site.

而这些废料并没有得到合理储存。

Apart from radioactivity, there's an even greater danger with spent fuel.

不单是核辐射， 用过的核燃料可能更加危险。

Plutonium can sustain a chain reaction

钚可以经受连锁核反应

and can be mined from the waste to make bombs.

并可以从残渣中被挖掘出来制造新的炸弹。

核废料的储存不仅对环境有着安全威胁，

Storing spent fuel is thus not only a safety risk for the environment,

but also a security risk for nations.

对于一个国家来说，它也有着安全隐患。

那谁又该是这些燃料的监管者呢？

Who should be the watchmen to guard it?

Visionary scientists from the early years of the nuclear age

科学家先驱在核年代伊始

pioneered how to reliably tap the tremendous amount of energy

率先开发出了可靠的能将大量能量

inside an atom -

从原子中提取

as an explosive bomb

并制成炸弹，

和拥有无限潜力的可控能源的方法。

and as a controlled power source with incredible potential.

But their successors have learned humbling insights

但后继者们却只惭愧的学习到了

about the technology's not-so-utopian industrial limits.

受工业限制的非理想科技。

Mining the subatomic realm makes for complex, expensive, and risky engineering.

次原子领域的开采成为了复杂， 昂贵，和有风险的工程。