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

譯者: Sophia Hsu 審譯者: 瑞文Eleven 林Lim

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

在二戰期間芝加哥 十二月的一天下午

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

科學家打破了鈾原子中心的原子核

並一再地將核質量轉化為能源

and turned nuclear mass into energy over and over again.

They did this by creating for the first time

他們頭一次創造了

a chain reaction inside a new engineering marvel:

一個連鎖反應的新工程奇蹟：

the nuclear reactor.

核反應爐

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

自此，人們從鈾原子核中 提取大量能源

has led some to bill nuclear power

導致一些人認為核能

as a plentiful utopian source of electricity.

是非常充足的烏托邦式電力來源

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

現代核反應爐耗費一公斤的燃料

to power an average American household for nearly 34 years.

可以供給一個美國家庭 近 34 年的電力

But rather than dominate the global electricity market,

但核能卻沒能主導全球電力

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

核電供電率從 1996 年 18％ 的歷史高點

to 11% today.

下降到現在的 11％

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

且預計在未來的幾十年 還將進一步下滑

What happened to the great promise of this technology?

這項技術的美好前景去哪了？

It turns out nuclear power faces many hurdles,

事實證明，核電還面臨著很多障礙

including high construction costs

包括昂貴的建設成本

and public opposition.

和公眾的反對

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

而且這些問題背後還有 一系列罕見的工程挑戰

Nuclear power relies on the fission of uranium nuclei

核電廠的能量來自鈾核分裂

and a controlled chain reaction

和受控的連鎖反應

that reproduces this splitting in many more nuclei.

連鎖反應讓核分裂一再發生

The atomic nucleus is densely packed with protons and neutrons

原子核由質子和中子

bound by a powerful nuclear force.

受到強大的核力量集中而成

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

大多數鈾原子中 質子和中子共 283 個

but roughly one in every 140 lacks three neutrons,

但有 140 分之 1 的機率 會出現少 3 個中子的鈾原子

and this lighter isotope is less tightly bound.

這種較輕的鈾同位素較不穩定

Compared to its more abundant cousin,

和它較穩定的親戚相比

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

中子較少的鈾-235 較容易 分裂成更輕的放射性元素

into lighter, radioactive elements called fission products,

也就是分裂產物

in addition to two to three neutrons,

分裂產物包含二到三個中子

gamma rays,

伽馬射線

and a few neutrinos.

和一些中微子

During fission, some nuclear mass transforms into energy.

在分裂期間

有一些質量會轉化為能量

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

一部分產生的能源為中子提供動力

and if some of them strike uranium nuclei,

而如果一些中子再撞擊鈾核

fission results in a second larger generation of neutrons.

再發生分裂就會產生更多中子

If this second generation of neutrons strike more uranium nuclei,

如果產生的中子又撞擊到更多鈾核

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

則產生更多的鈾元素，如此一直重複

But inside a nuclear reactor,

但在核反應爐裡

this spiraling chain reaction is tamed using control rods

這種循環的連鎖反應受到控制棒操控

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

控制棒由可吸收中子的元素製成 可控制中子的數量

With a controlled chain reaction,

達到控制連鎖反應的功能

a reactor draws power steadily and stably for years.

反應爐能穩定維持運作多年

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

中子引發的連鎖反應 是核能發電的關鍵

but there's a catch that can result

但還有一項隱藏的危機

in unique demands on the production of its fuel.

就是核燃料的供給

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

事實證明，再分裂時 發射出的大多數中子

to be captured by uranium nuclei.

因動能太大而未能被鈾核吸收

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

分裂速率太低，導致連鎖反應停滯

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

在芝加哥的第一個核反應爐 以石墨為緩和劑

to scatter and slow down neutrons just enough

用於分散和減緩中子速率

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

到足以增加撞擊鈾核的機率 並提高分裂速率

Modern reactors commonly use purified water as a moderator,

現代反應爐通常使用 純化水作為緩和劑

but the scattered neutrons are still a little too fast.

但分散的中子仍然有點太快了

To compensate and keep up the chain reaction,

為了降低中子速率並保持連鎖反應

the concentration of U-235 is enriched

鈾-235 的濃度必須提高到

to four to seven times its natural abundance.

原本的四到七倍

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

現在，濃縮方法通常是

使氣態鈾化合物通過離心機

through centrifuges

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

分離較輕的鈾-235 與較重的鈾-238

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

但同樣的過程可以 持續提高鈾-235 的濃度

up to 130 times its natural abundance

達到天然濃度的 130 倍

and create an explosive chain reaction in a bomb.

達到核彈爆炸性連鎖反應的等級

Methods like centrifuge processing must be carefully regulated

所以過程中必須仔細調整離心機

to limit the spread of bomb-grade fuel.

以限制核彈級燃料的擴散

Remember, only a fraction of the released fission energy

記住，只要一小部分的分裂能量

goes into speeding up neutrons.

就能加速中子

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

大部分的能量成為分裂產物的動能

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

而純水通常被拿來當成 反應爐裡的冷卻劑

usually purified water.

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

熱能通常是為了要推動

just outside the reactor.

反應爐外的渦輪發電機來發電

Water flow is critical not only to create electricity,

水流不只是發電用

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

更是為了防止最令人擔憂的意外

the meltdown.

反應爐核心熔毀

If water flow stops because a pipe carrying it breaks,

如果水流因為運送的水管破裂

or the pumps that push it fail,

或推動的幫浦無法運作而停止

the uranium heats up very quickly and melts.

鈾會很快被加熱到熔化

During a nuclear meltdown,

當核心熔毀

radioactive vapors escape into the reactor,

具有放射性的蒸氣外洩到反應爐裡

and if the reactor fails to hold them,

而反應爐無法承受壓力而釋放蒸氣

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

鋼筋混泥土做的圍阻體 將會是最後一道防線

But if the radioactive gas pressure is too high,

但如果放射性氣體的壓力太高

containment fails and the gasses escape into the air,

氣體被釋放到空氣中

spreading as far and wide as the wind blows.

在風的幫助下散布越來越遠

The radioactive fission products in these vapors

氣體裡的放射性物質

eventually decay into stable elements.

最終會衰退成為穩定的元素

While some decay in a few seconds,

有些會在幾秒鐘內穩定

others take hundreds of thousands of years.

但有些需要數百甚至上千年的時間

The greatest challenge for a nuclear reactor

核能反應爐最大的挑戰

is to safely contain these products

是必須安全地容納這些放射性氣體

and keep them from harming humans or the environment.

並且保護人類及環境不受到危害

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

當核燃料使用完後 並沒讓我們停止煩惱

In fact, it becomes an even greater storage problem.

事實上將轉變成更大的儲存問題

Every one to two years,

每一到兩年

some spent fuel is removed from reactors

使用過的鈾會從反應爐裡

and stored in pools of water that cool the waste

被拿到冷水裡冷卻

and block its radioactive emissions.

並阻擋放射性物質被釋放

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

被輻射照過的燃料 包括分裂失敗的鈾

fission products,

分裂後的產物

and plutonium, a radioactive material not found in nature.

及不存在於自然界中的 放射性元素「鈽」

This mix must be isolated from the environment

這些東西必須與環境隔離

until it has all safely decayed.

直到這些元素全都安全的穩定下來

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

許多國家計劃要挖掘洞穴 將這些物質埋藏一段時間

but none have been built,

但沒有一個洞穴被挖掘出來

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

而且還有個疑問是 他們要如何長期管理這些地方

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

當一個國家才建立數百年

plan to guard plutonium through its radioactive half-life

他們要如何去計劃管理 一個有放射物質的地方

of 24,000 years?

兩萬四千年呢？

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

直到今日，許多核電廠仍有 核廢料未被安全的處理

storing them indefinitely on site.

廢料就永遠存在核電廠裡

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

除了放射性，核廢料 還有一個更嚴重的問題

Plutonium can sustain a chain reaction

鈽可以引發連鎖反應

and can be mined from the waste to make bombs.

所以可能來製作原子彈

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

核廢料的儲存不只有環境安全的顧慮

but also a security risk for nations.

也有國家安全的危機

Who should be the watchmen to guard it?

誰應該負起這個重責大任？

Visionary scientists from the early years of the nuclear age

早年的科學家在核能出現的年代

pioneered how to reliably tap the tremendous amount of energy

首創了將這個原子裡巨大的力量

inside an atom -

確實提煉出來的方法

as an explosive bomb

並把它製成威力強大的炸彈

and as a controlled power source with incredible potential.

讓它成為擁有絕大潛力的可控制能源

But their successors have learned humbling insights

但他們的後繼者已經學到

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

不怎麼光彩的見解： 核能不似烏托邦的美好

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

次原子領域是複雜、 昂貴且具風險的工程

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