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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 and turned nuclear mass into energy over and over again.
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。
They did this by creating for the first time a chain reaction inside a new engineering marvel: the nuclear reactor.
他们第一次在一个新的工程奇迹里创造出了一个连锁反应：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 as a plentiful utopian source of electricity.
使核电被一些人列为一种理想、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 to 11% today.
核电从1996年18%的历史最高点下降到现在的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, including high construction costs and public opposition.
事实证明，核电还面临着很多问题，包括昂贵的建设成本和公众的反对。
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 that reproduces this splitting in many more nuclei.
和一个可控的复制性核裂变的链式反应。
The atomic nucleus is densely packed with protons and neutrons bound by a powerful nuclear force.
原子核密集地包裹着由强大核力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, and this lighter isotope is less tightly bound.
但是每140个原子中大约有一个缺少三个中子，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 into lighter, radioactive elements called fission products,
中子的撞击很容易将铀-235的原子核分成更轻的、叫做裂变产物的放射性元素，
in addition to two to three neutrons, gamma rays, and a few neutrinos.
两到三个中子，伽马射线和一些中微子。
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, fission results in a second larger generation of neutrons.
如果它们中的一些撞到了铀原子核，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。
But inside a nuclear reactor,
但在核反应堆里，
this spiraling chain reaction is tamed using control rods made of elements that capture excess neutrons and keep their number in check.
这种螺旋链式反应使用元素控制棒来捕捉多余的中子，并控制它们的数量。
With a controlled chain reaction, a reactor draws power steadily and stably for years.
采用可控链式反应，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 in unique demands on the production of its fuel.
但有一个隐情会导致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 to be captured by uranium nuclei.
事实证明，裂变产生的大部分中子有太多的动能，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 to scatter
芝加哥的第一个核反应堆用石墨作为慢化剂来分散，
and slow down neutrons just enough to increase their capture by uranium and raise the rate of fission.
减缓中子直到它们被铀捕抓到，从而提高裂变的速度。
Modern reactors commonly use purified water as a moderator, but the scattered neutrons are still a little too fast.
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。
To compensate and keep up the chain reaction, the concentration of U-235 is enriched to four to seven times its natural abundance.
为了维持链式反应，铀-235浓度会被浓缩到天然浓度的4至7倍。
Today, enrichment is often done by passing a gaseous uranium compound through centrifuges to separate lighter U-235 from heavier U-238.
现今，浓缩通常是由气态铀化合物通过离心机从较重的铀-238分出较轻的铀-235来实现。
But the same process can be continued to highly enrich U-235 up to 130 times its natural abundance
但同样的步骤也可以继续来使铀-235高量浓缩至它的天然丰度的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 to limit the spread of bomb-grade fuel.
像这种离心处理的步骤必须小心处理来限制核燃料的扩散。
Remember, only a fraction of the released fission energy goes into speeding up neutrons.
记住，只有一丁点的核裂变能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, usually purified water.
这些能量获取于在反应堆内部加温冷却剂，通常是纯净水。
This heat is eventually used to drive an electric turbine generator by steam just outside the reactor.
这些热量最终会被用来在反应堆外部蒸汽驱动汽轮机。
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, the meltdown.
它还被用来防备最可怕的反应堆事故，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, the uranium heats up very quickly and melts.
或者水泵失灵，铀就会迅速升温并溶解。
During a nuclear meltdown, radioactive vapors escape into the reactor,
在核溶解过程中，放射性蒸汽会浸入反应堆，
and if the reactor fails to hold them, a steel and concrete containment building is the last line of defense.
如果反应堆无法承受它们，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, spreading as far and wide as the wind blows.
容器无法阻拦它，那么放射性气体就会渗入空气，然后被风广泛散播。
The radioactive fission products in these vapors eventually decay into stable elements.
这些蒸汽中的放射性裂变物质会最终衰退成稳定元素。
While some decay in a few seconds, others take hundreds of thousands of years.
有的物质会在几秒钟内定型，但有的则需要上百上千年的时间。
The greatest challenge for a nuclear reactor is to safely contain these products
对于一个核反应堆的最大挑战就是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。
Every one to two years, some spent fuel is removed from reactors
每过一两年，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 and block its radioactive emissions.
然后储存在废水池中来冷却这些残渣和隔绝辐射。
The irradiated fuel is a mix of uranium that failed to fission, fission products,
辐射性燃料是由裂变失败的铀，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 until it has all safely decayed.
这种混合物必须在它变得安全稳定化之前从自然环境中隔离出来。
Many countries propose deep time storage in tunnels drilled far underground,
许多国家都计划在深层地下管道中长期储存这些混合物，
but none have been built, and there's great uncertainty about their long-term security.
但都没有付诸实施，并且这些项目的长期安全性还存在着不确定性要素。
How can a nation that has existed for only a few hundred years
一个只存在了几百年的国家，
plan to guard plutonium through its radioactive half-life of 24,000 years?
又怎么能计划去安存半衰期为24000年的钚元素呢？
Today, many nuclear power plants sit on their waste, instead, storing them indefinitely on site.
今天，核能设施都建设在核废料上面，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 and can be mined from the waste to make bombs.
钚可以经受连锁核反应，并可以从残渣中被挖掘出来制造新的炸弹。
Storing spent fuel is thus not only a safety risk for the environment, but also a security risk for nations.
核废料的储存不仅对环境有着安全威胁，对于一个国家来说，它也有着安全隐患。
Who should be the watchmen to guard it?
那谁又该是这些燃料的监管者呢？
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 inside an atom as an explosive bomb
率先开发出了可靠的能将大量能量从原子中提取并制成炸弹，
and as a controlled power source with incredible potential.
并拥有无限潜力的可控能源的方法。
But their successors have learned humbling insights about the technology's not-so-utopian industrial limits.
但后继者们却只惭愧的学习到了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。
