ITER解決最棘手難題：用鎢與鈹涂裝反應(yīng)堆內(nèi)層

一直以來，籠罩在ITER核聚變反應(yīng)堆項(xiàng)目——目前在法國建造的一個(gè)大型國際合作項(xiàng)目——頭頂?shù)囊粋€(gè)最大問號(hào)便是用什么材料來涂裝反應(yīng)堆的內(nèi)壁。要知道，它必須要能夠抵擋10萬攝氏度的高溫，以及猛烈的粒子轟擊。

如今，研究人員終于從用一個(gè)類似于計(jì)劃在ITER中所使用的內(nèi)層改裝的目前世界上最大的核聚變裝置中找到了答案。研究人員報(bào)告說，位于英國牛津附近的歐洲聯(lián)合環(huán)形加速器（JET）的新的“像ITER一樣的墻壁”是一種鎢與鈹?shù)慕Y(jié)合產(chǎn)物，與較早的核聚變反應(yīng)堆所使用的內(nèi)層相比，它被侵蝕的速度更為緩慢，并且吸收的燃料也更少。物理學(xué)家Peter de Vries表示：“這是一個(gè)非常棒的消息，因?yàn)樗馕吨覀優(yōu)镮TER所選擇的材料是正確的。”

核聚變是為太陽和恒星提供能量的過程，或者說，它是最完美的能量來源。它所需的燃料（氘和氚）非常容易獲取，并且?guī)缀跏怯貌煌甑模@一過程也并不會(huì)產(chǎn)生任何溫室氣體或長期存在的核廢料。

在早期核聚變反應(yīng)堆中，最常見的反應(yīng)堆內(nèi)層是由碳構(gòu)成的，這是因?yàn)樗鼧O耐高溫與腐蝕，并且不會(huì)污染燃料被加熱后所形成的等離子體。然而碳有一個(gè)最大的缺陷便是它非?！皹酚凇蔽针碗?。對(duì)ITER來說，其反應(yīng)堆會(huì)經(jīng)常使用氚，因此對(duì)氚的吸收必須保持在最低限度，碳顯然是不合適的。

由于并不存在一種完美的材料，因此研究人員不得不向使用兩種材料的方案妥協(xié)。如今，大多數(shù)的內(nèi)壁會(huì)涂裝鈹——這種金屬對(duì)等離子體的污染最低，但其熔點(diǎn)卻很低，無法耐受等離子體的高溫。另一方面，在JET的底部有一個(gè)被稱為偏濾器的裝置，它類似于反應(yīng)堆的排氣管。這種裝置會(huì)與等離子體接觸，因此需要一種更為耐用的涂層。為此，研究人員使用了鎢，并取得了很好的效果。

JET從2010年5月至2011年5月進(jìn)行了一次內(nèi)部改造，其間，研究人員用打算在ITER中使用的鈹和鎢替代了原有的碳內(nèi)層。結(jié)果顯示，在升級(jí)后的JET中，與之前的碳內(nèi)壁相比，鈹內(nèi)壁在等離子體的影響下腐蝕速度要慢得多。研究人員在日前召開的一次會(huì)議上報(bào)告了這一結(jié)果。ITER是目前全球規(guī)模最大、影響最深遠(yuǎn)的國際科研合作項(xiàng)目之一，它的建造大約需要10年，耗資數(shù)十億美元。ITER裝置是一個(gè)能產(chǎn)生大規(guī)模核聚變反應(yīng)的超導(dǎo)托克馬克，俗稱“人造太陽”。

How to Line a Thermonuclear Reactor

One of the biggest question marks hanging over the ITER fusion reactor project—a giant international collaboration currently under construction in France—is over what material to use for coating its interior wall.After all, the reactor has to withstand temperatures of 100,000°C and an intense particle bombardment.

Researchers have now answered that question by refitting the current world's largest fusion device, the Joint European Torus (JET) near Oxford, U.K., with a lining akin to the one planned for ITER.JET's new "ITER-like wall," a combination of tungsten and beryllium, is eroding more slowly and retaining less of the fuel than the lining used on earlier fusion reactors, the team reports."This was very good news, because it means that our choice of materials for ITER was the right one," says physicist Peter de Vries, task force and session leader at JET.

Fusion is the process that powers the sun and stars, and, potentially, it's the perfect energy source.The necessary fuels are easily accessible and virtually inexhaustible, and the process doesn't produce any greenhouse gases or long-lived nuclear waste.For fuel, it requires deuterium and tritium (forms of hydrogen with one and two extra neutrons, respectively, in their nuclei).These have to be heated so that they form plasma—an ionized gas—and when they reach about 150 million°C, the nuclei collide with such force that they overcome their mutual repulsion and fuse into a new, larger nucleus.The products of the reaction are a helium nucleus and a very energetic neutron, whose energy is later harvested in the form of heat.

But the harsh truth is it's not at all easy to run this fusion process in a controlled way.The current favored technique is to use a reactor called a tokamak, which employs powerful electromagnets to confine the plasma inside a doughnutshaped reactor vessel.The magnets aim to hold the plasma away from the walls of the vessel long enough for the nuclei to fuse but plasma can often shift around in unpredictable ways.If the plasma touches the wall, this can cool it to below reaction temperature and also scour off atoms of the lining material that poison the fusion reaction.And tritium is a radioactive isotope that reactor operators have to account for very carefully.Any tritium that embeds itself in the reactor wall has to be painstakingly extracted.

No fusion reactor has yet produced more energy than was put in to heat the plasma in the first place.But researchers have high hopes for ITER, the massive reactor with an estimated price tag of as much as $20 billion that is now being built in the south of France by China, the European Union, India, Japan, Russia, South Korea, and the United States.

The most common reactor lining, known as the first wall, in earlier fusion reactors was carbon because it is extremely resistant to high temperatures and erosion and doesn't pollute the plasma if atoms do get into it.Carbon's big drawback is that it's very happy to absorb deuterium and tritium.For ITER, the first reactor to use tritium on a regular basis, absorption of tritium has to be kept to a minimum, so carbon is out.