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Fusion's false dawn 聚变的假曙光(2)

(2010-04-24 23:30:06)
标签:

杂谈

分类: 自然科学

(字数超过限制,不得不分为两篇)

反应堆的困境

 

    不管你用什么样的方法产生聚变反应——无论使用兆焦耳级的激光器或强磁场的约束——能量都是以中子为载体的。由于这些粒子是中性的,它们不会受电场或磁场的影响。此外,它们可以沿直线通过大部分固体材料。

 

 

     唯一能使中子停下来的方法,是让中子直接轰击一个原子核。这些碰撞往往是破坏性的碰撞。从氘和氚聚变反应出来的中子的能量是如此的高,它们可以将一个坚硬的金属,比方说钢铁,的原子从它原来的位置上打下来。长此以往,这些重击将会使反应堆结实的组件变得脆弱。

   Over time these whacks weaken a reactor, turning structural components brittle. Other times the neutrons will turn benign

material radioactive. When a neutron hits an atomic nucleus, the nucleus can absorb the neutron and become unstable. A steady stream of neutrons—even if they come from a “clean” reaction such as fusion—would make any ordinary container dangerously radioactive, Baker says. “If someone wants to sell you any kind of nuclear system and says there is no radioactivity, hang onto your wallet.”

    A fusion-based power plant must also convert energy from the neutrons into heat that drives a turbine. Future reactor designs make the conversion in a region surrounding the fusion core called the blanket.  Although the chance is small that a given neutron will hit any single atomic nucleus in a blanket, a blanket thick enough and made from the right material—a few meters 'worth of steel, perhaps—will capture nearly all the neutrons passing through. These collisions heat the blanket, and a liquid coolant such as molten salt draws that heat out of the reactor. The hot salt is then used to boil water, and as in any other generator, this steam spins a turbine to generate electricity.

     Except it is not so simple. The blanket has another job, one just as critical to the ultimate success of the reactor as extracting energy. The blanket has to make the fuel that will eventually go back into the reactor.

     Although deuterium is cheap and abundant, tritium is exceptionally rare and must be harvested from nuclear reactions. An ordinary nuclear power plant can make between two to three kilograms of it in a year, at an estimated cost of between $80 million and $120 million a kilogram. Unfortunately, a magnetic fusion plant will consume about a kilogram of tritium a week. “The fusion needs are way, way beyond what fission can

 

   有些时候,这些中子也会使一些物质产生放射性。当一个中子轰击一个原子核的时候,这个核可能会俘获这个中子,从而变的极不稳定。一束稳定的中子流——即使它们是从像聚变这样“洁净”的反应中产生的——都有可能使普通的容器产生危险的放射性。Baker 说到,“如果有人想卖给你一个核反应的系统装置并且声称不会产生放射性,那么你就要小心你的钱袋子了。”

 

     一个聚变反应的核电站必须把中子的动能转换成热能以驱动一个涡轮机。未来反应堆设计使这种转换发生在包围聚变反应核心区域的一个被称为“再生区”的地方。虽然某一个中子击中再生区的单个原子核的可能性很小,但是如果再生区足够的厚,并用合适的材料——可能是数米长的钢,来做的话,几乎所有穿过的中子都会被俘获。这些碰撞加热这个再生区,其中的液体冷却剂,如熔盐,会把热从反应堆中导出。高温的熔盐用来加热水,就像在任何其他发电机上一样,这些产生的蒸汽用来旋转涡轮发电。

 

     但是它的目的不仅在于此。这个再生区还有另外一个任务,这个任务和从反应堆中提取能量对反应堆的最终成功一样重要。再生区必须把核燃料再送回反应堆中去。

 

     尽管氘很便宜,储量也很丰富,但是氚却出奇的稀有,只能从核反应中得到。一个普通的核电站一年可以产生两三千克的氚,每公斤的代价大概在8000万到1亿2000万美元之间。不幸的是,一个磁约束的聚变核电站每周要消耗大概一公斤的氚。“聚变反应所需的原料需求裂变反应是无法满足的”,穆罕默德阿卜杜说。他是加州大学洛杉矶分校聚变科学技术中心的主任。

 

supply," says Mohamed Abdou, director of the Fusion Science and Technology Center at the University of California, Los Angeles.

    For a fusion plant to generate its own tritium, it has to borrow some of the neutrons that would otherwise be used for energy. Inside the blanket channels of lithium, a soft, highly reactive metal would capture energetic neutrons to make helium and tritium. The tritium would escape out through the channels, get captured by the reactor and be reinjected into the plasma.

When you get to the fine print, though, the accounting becomes precarious. Every fusion reaction devours exactly one tritium ion and produces exactly one neutron. So every neutron coming out of the reactor must make at least one tritiumion, or else the reactor will soon run a tritium deficit—consuming more than it creates. Avoiding this obstacle is possible only if scientists manage to induce a complicated cascade of reactions. First, a neutron hits a lithium 7 isotope, which, although it consumes energy, produces both a tritium ion and a neutron. Then this second neutron goes on to hit a lithium 6 isotope and produce a second tritiumion.

     Moreover, all this tritium has to be collected and reintroduced to the plasma with near 100 percent efficiency. “In this chain reaction you cannot lose a single neutron, otherwise the reaction stops,” says Michael Dittmar, a particle physicist at the Swiss Federal Institute for Technology in Zurich. “The first thing one should do [before building a reactor] is to show that the tritium production can function. It is pretty obvious that this is completely out of the question.”

    "This is a very fancy gadget, this fusion blanket," Hazeltine says. "It is accepting a lot of heat and taking care of that heat without overheating itself. It is accepting neutrons, and it is made out of very sophisticated materials

 

 

    对聚变反应核电站来说,要想产生它所需的氚,它必须利用部分高能中子。在再生区的管道里,一种柔软的、高度活泼的金属锂会俘获高能中子从而产生氚和氦。氚会从管道里逃逸出去,被反应堆收集后再注入到plasma里面去。

 

 

   当你看到旁边的小字(注:见pdf文档),上面的计算就靠不住了。每一个聚变反应会耗一个氚核并只产生一个中子。因此从反应堆中出来的每一个中子都必须至少产生一个氚,要不然反应堆里面的氚很快就会减少——产生的比消耗的少。为了避免这种情况,科学家们只好引入了一个复杂的链式反应。首先,一个中子轰击一个 , 尽管会消耗能量,但是它会产生一个氚和一个中子。这个中子继续轰击 ,产生第二个氚。

 

 

   此外,所有这些氚都要被收集起来并几乎100%的重新注入到plasma里面。“在这个链式反应里面,不能漏掉一个中子,否则反应就会中止。”Michael Dittmar说到。他是苏黎世瑞士联邦技术研究所的一个粒子物理学家。“在建造核反应堆之前,首先要做的是,确保氚的产出可以满足要求。很明显可以看出,这是完全不可能的。”

 

    “这真是一个好玩意儿,我是说这个再生区”,Hazeltine 说。“它接受了大量的热自己却不被加热。它要吸收中子,就必须是由一种很复杂的物质来构造,这样在这些中子的轰击下它不会那么短命。它吸收这些中

 

so it doesn't have a short lifetime in the face of those neutrons. And it is taking those neutrons and using them to turn lithium into tritium.”

    ITER, unfortunately, will not test blanket designs. That is why many scientists—especially those in the U.S., which is not playing a large role in the design, construction or operation of ITER—argue that a separate facility is needed to design and build a blanket. “You must show that you can do this in a practical system,” Abdou says, “and we have never built or tested a blanket. Never.” If such a test facility received funding tomorrow, Abdou estimates that it would take between 30 and 75 years to understand the issues sufficiently well to begin construction on an operational power plant. “I believe it’s do-

able," he says, “but it’s a lot of work.”

The Big Lie

    Let's say it happens. The year is 2050. Both the NIF and ITER were unqualified successes, hitting their targets for energy gain on time and under budget. Mother Nature held no surprises as physicists ramped up the energy in each system; the ever unruly plasmas behaved as expected. A separate materials facility demonstrated how to build a blanket that could generate tritium and convert neutrons to electricity, as well as stand up to the subatomic stresses of daily use in a fusion plant. And let's assume that the estimated cost for a working fusion plant is only $10 billion. Will it be a useful option?

    Even for those who have spent their lives pursuing the dream of fusion energy, the question is a difficult one to answer. The problem is that fusion-based power plants—like ordinary fission plants—would be used to generate baseloadpower. That is, to recoup their high initial costs, they would need to always be on. "Whenever you have any system that is capital-intensive, you want to run it around the clock because you are not paying for the fuel," Baker says.

子然后用它们把锂变成氚。”

 

    很不幸的是,ITER项目不会去检测下这个再生区的设计。这也是很多科学家——尤其是美国的,争论道应该单独设计一个设备来建造这个再生区。由于这个原因,美国科学家在ITER的设计、建造、和运行中没有扮演重要的角色。“你必须证明你可以在一个实际的系统中做到这些”,Abdou 说,“但是我们从来没有建造过一个或着测试过一个再生区,从来就没有过。”如果这样一个待检测的设备明天就得到资助,他估计要花30到75年的时间来充分的了解上面的那个问题,然后才能开始建造一个可以运行的核电站。“我相信这是可行的,但是要做很多工作才行。”他补充道。

 

 

弥天大谎

 

    我们先假设它成功了。时间是2050年。NIF和ITER都完全成功了,在预算内按时完成了能量增益的目标。随着物理学家们不断增大这两个系统的能量,大自然母亲对我们变得不那么神秘了。一直以来极难控制的plasma也按照预期的方式运作着。由某些物质构造的一个单独的设备可以用来做再生区,它可以产生氚并且将中子的能量转换为电能,也能承受亚原子范围内终日不息的碰撞。我们假设一个运转着的聚变核电站的花费大概是100亿美元。这个项目听上去有用吗?

 

    即便对于那些穷其毕生精力追求聚变能实现的人,这也是一个很难回答的问题。问题在于一个聚变核电站——和普通的裂变核电站一样——会被超负荷运转。也就是说,为了收回早期的巨额投资,它们必须一刻不停地运转着。“不论什么时候,对于任何一个资本密集型的大项目,你都希望它一直运转着因为你不想白白地为花钱,”Baker说。

 

     Unfortunately, it is extremely difficult to keep a plasma going for any appreciable length of time. So far reactors have been able to maintain a fusing plasma for less than a second. The goal of ITER is to maintain a burning plasma for tens of seconds. Going from that duration to around-the-clock operation is yet another huge leap. "Fusion will need to hit 90 percent availability," says Baker, a figure that includes the downtime required for regular maintenance. “This is by far the greatest uncertainty in projecting the economic reliability of fusion systems.”

    NIF director Moses thinks he has the answer. He has introduced a proposed design for a hybrid fusion-fission reactor—one that uses the neutrons from laser-driven fusion reactions

to drive fission reactions in a blanket of ordinary nuclear waste. He calls his system LIFE—for laser inertial fusion engine—and says he can have one connected to the grid in 20 years.

The system relies on the fact that only 5 percent of the uranium that goes into power plants gets used before it is pulled out and put into long-term storage. LIFE would bombard this spent fuel with neutrons, thus accelerating its decay into lighter and less radioactive elements, all the while producing heat that could be used for electricity. “Our studies show that we would be competitive with all the energy sources that are available today,” Moses says. “Or even cheaper than them.”

     Of course, LIFE is not without its pitfalls.

    "You want to look at the big lie in each program," says Edward C. Morse, a professor of nuclear engineering at the University of California, Berkeley. "The big lie in [laser-based] fusion is that we can make these target capsules for a nickel a piece.” The target capsules, the peppercorn-size balls of deuterium-tritium fuel, have to be exquisitely machined and precisely round to ensure that

  

   糟糕的是,要想让plasma约束一个可观的时间是极其困难的。到目前为止,反应堆能够约束聚变反应用的plasma的时间不到一秒钟。ITER的目标是能够约束几十秒。从几十秒到一刻不停地转是另外一个巨大的挑战。“聚变堆要达到90%的时间利用率,”Baker说,这个数字考虑了日常维修引起的停工期。“这也是目前投资聚变项目经济上最大的不确定因素。”

 

    NIF的主任Moses认为他已经找到了解决之道。他提出的一个方案是设计一个裂变—聚变混合反应堆。这个反应堆利用激光约束聚变反应堆产生的中子来诱发一个有普通核废料构成的再生区发生裂变反应。他把自己的系统命名为“LIFE”—是laser inertial fusion engine 的简称—并声称他可以在20年内使其并网发电。

 

    这个系统是基于这一事实:裂变反应堆中只有5%的铀被利用来发电,其余部分都被作为核废料被永久掩埋了。LIFE会用中子来轰击这些核废料,从而加速它的衰变,使其变成更轻和放射性更低的元素。在这一过程中产生的热能用来发电。“我们的研究表明,这个项目比当今所有的能源都有可竞争性,”Moses说,“甚至比它们更便宜。”

 

 

    当然,LIFE并不是没有缺点的。“人们都想看到每个项目背后的巨大谎言,” Edward C. Morse说。他是加州大学伯克利分校的一位原子能工程方面的教授。“激光约束核聚变的大谎话是,我们制造一个靶丸只花一美分。”这个靶丸,胡椒粒大小的球状物里面装满了氘和氚,必须极其精致地加工出来,必须是绝对的球形,以保证它被压缩时各个部位受力均匀。靶丸上只要有凸起它就不会爆炸,由于这一点使得当前大批生

产它们贵的让人难以承受。尽管Livermore没有公布预期的成本(他们打算自己生产这

 

they compress evenly from all sides. Any bump on the pellet and the target won't blow, which makes current iterations of the pellets prohibitively expensive. Although Livermore, which plans to make its pellets on site, does not release anticipated costs, the Laboratory for Laser Energetics at the University of Rochester also makes similar deuterium -tritium balls. “The reality now is that the annual budget to make targets that are used at Rochester is several million dollars, and they make about six capsules a year,” Morse says. “So you might say those are $1 million apiece.”

     And unlike in the current iteration of the NIF, which is capable of blasting one pellet every few hours, targets will cycle through the chamber with the speed of a Gatling gun. “This is a 600-rpm machine,” Moses says. “It’s like a million-horsepower car engine—except no carbon.” A LIFE plant working around the clock will consume almost 90,000 targets a day.

Of course, it is impossible to predict what the worldwide energy situation will be 20 years out. Perhaps the need for fusion energy will be greater than ever. Or it could be that a breakthrough in solar, wind or some other as yet unforeseen alternative energy makes fusion appear expensive and unwieldy by comparison. “It is possible that people will say, ‘Yeah, it works, that's great, but we don’t need it anymore, because we've got a list of other things,' " Hazeltine says.

It used to be that fusion was held apart from these considerations. It was fundamentally different from dirty fossil fuels or dangerous uranium. It was beautiful and pure—a permanent fix, an end to our thirst for energy. It was as close to the perfection of the cosmos as humans were ever likely to get.

     Now those visions are receding. Fusion is just one more option and one that will take decades of work to bear fruit. Ignition may be close, but the age of unlimited energy is not.

 

些靶丸),罗彻斯特大学的激光热力学实验室也制造了类似的氘氚小球。“事实上罗彻斯特大学每年用于制造靶丸的预算是几百万美元,他们每年只造出来6个,”Morse说。“因此你可以说这些小球一百万美元一个。”

 

    并且和NIF几个小时轰击一个靶丸不同,靶丸在LIFE的反应室里循环的速度就像一架格林机关枪。“这是一个600转/min 的机器,”Moses说。“这就像一个百万马力的汽车发动机——只不过它不产生碳排放罢了。”一个LIFE电站一刻不停地运转,一天会花掉90,000个靶丸。

 

    当然,很难预测20年之后世界上的能源形势。也许对聚变能的需求比以往任何时候都要大的多。也有可能在太阳能、风能或者其他当前未知的可替代能源方面产生重大的突破,使得聚变能相比之下既昂贵又难以操作。“也许人们会说,‘是啊,它做到了,太棒了,但是我们再也不需要它了,因为我们有一长串其他的东西代替。’”Hazeltine 说。

 

   过去人们总不愿这样考虑聚变能。它和有污染的化石燃料以及铀都有极大的差别。它是美丽的,纯净的——一个一劳永逸的安排,是我们对能源追求的终结。它是人类对完美宇宙了解的极致。

现在这些构想开始消退了。聚变只是一个选项,人们必须花费数十年才能尝到甜头。点火的时间是快要到了,但是无限能源的时代还远着呢!

 

 

译后记:文中观点仅供参考,个人感觉过于悲观,想想曼哈顿工程和阿波罗登月计划,再想想石油在50年之内耗尽,科技的极限不容怀疑。

另:本文系课程作业,Word排版,直接复制到这里,未加整理。任何人可以用任何方式使用我博客里面的文章,但不要以盈利为目的(自恋了,但声明下,了胜于无),希望注明saladino.cn@gmail.com,谢谢。

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