Visions of a nuclear future
How do you turn a Green bright red? Just mention nuclear power, judging by the response of environmentalists to the growing use of the N-word in debates about future energy sources.
Last week, Tony Blair, the UK prime minister, told a committee of senior parliamentarians that America was pressing Britain to re-examine the case for building a new generation of nuclear power stations and that nuclear power could not be removed from the agenda.
Phumzile Mlambo-Ngcuka, South Africa's minerals and energy minister, last month incurred the wrath of environmental campaigners for suggesting that nuclear power might give her nation greater energy diversity and security of supply. A few weeks earlier, Professor James Lovelock, the British environmental guru, had provoked outrage with his apparent apostasy in calling for huge investment in nuclear power to help combat global warming.
Sir Crispin Tickell, a leading British environmentalist, has even managed to annoy both eco-activists and ministers by accusing the UK government of failing to make the case for nuclear power in tackling global warming.
At first sight, it is hard not to share the frustration of environmentalists at being presented with a bleak choice between climatic disaster or reliance on the technology that gave us Three Mile Island, the Chernobyl disaster and thousands of tonnes of radioactive waste.
Yet it is a false dichotomy, based on a view of nuclear technology as outdated as the cold war. Whisper it quietly, but experiments are about to begin on a new form of nuclear power that even eco-warriors might tolerate, if not welcome. It is radically different from the traditional reactor designs that prompt fear and loathing: for one thing, its safety is underwritten by the laws of physics, rather than human ingenuity.
Better still, it can use a range of different fuels - including some that would minimise the risk of weapons production by "rogue" nations. But best of all, this new form of reactor can incinerate waste from other reactors, turning today's noxious stockpiles into energy.
Such are the prospects held out by the new reactor, the Accelerator-Driven Subcritical (ADS) system. First proposed in the 1990s by scientists at the Los Alamos National Laboratory, New Mexico, and Carlo Rubbia, the Italian Nobel prize- winning physicist, the idea behind ADS is more elegant than its name. Conventional nuclear power exploits the energy released by the splitting - "fission" - of uranium atoms. Along with energy, this fission process releases neutrons capable of splitting further uranium atoms, triggering a chain reaction.
If the numbers of neutrons are just sufficient to keep the reaction in balance, the result is a "critical" reactor - and a steady flow of power that today provides about 17 per cent of the world's electricity demand. But if the chain reaction runs out of control, the reactor could explode as an atomic bomb does. This has compelled designers to devise measures to prevent a disaster - yet these are no guarantee of safety.
The ADS system adopts a different approach to nuclear safety - one that even Homer Simpson could not undermine. As its name implies, the ADS reactor is "sub-critical" - that is, its fuel simply does not generate enough neutrons to sustain a chain reaction. Instead, the reactor is fed with neutrons created by a particle accelerator. Cut off this supply of neutrons - deliberately or accidentally - and the reactor reverts to its natural, somnolent state. An explosive chain reaction is not just unlikely: it is prevented by the laws of physics.
Moreover, as chain reaction stability no longer depends on the type of fuel used, the ADS system is a nuclear omnivore, able to work with fuels that are wholly unsuited to weapons production. To cap it all, an ADS reactor can even consume radioactive waste from conventional reactors.
This holy trinity of advantages has made the ADS the subject of intense theoretical research for more than a decade. Now the theory is to be tested in experiments by an international team of scientists at Italy's Casaccia Research Centre, near Rome.
A small research reactor at the centre has been modified to make it sub-critical, and a particle accelerator is to be built to feed the reactor with neutrons knocked out of a tantalum target. The first experiments, expected to start within two years, will focus simply on gaining experience in the art of feeding neutrons to a reactor. Once these are completed, the team plans to use a more powerful reactor and accelerator system to create an ADS system capable of waste incineration. A pilot plant could be completed within five years.
That is the plan. In reality, the ADS approach may still harbour an unexpected problem that stops it realising its potential. Most concern surrounds the creation of the neutrons needed to feed the reactor. A commercial power station consuming substantial amounts of waste will require an accelerator substantially more powerful than any now available.
However, the biggest cloud hanging over the project is the same one that has dogged nuclear power for decades: economics. No matter what its benefits, if the electricity ADS generates costs too much, it will have no role to play in future energy policy.
With the public rightly chary of the return of old-style nuclear power, but a growing belief even among some environmentalists that renewables alone are not enough, a lot is riding on the success of those experiments near Rome.
The time has come to wake the Dorset Dragon, Says MIT
In 'The Future of Nuclear Power', a review of the future of nuclear power published last year, a team of experts at the Massachusetts Institute of Technology focused attention on a little-known product of the heyday of nuclear reactor design: the High Temperature Gas-cooled Reactor (HTGR).
While the designs of most of the world's 400-plus power reactors reflect their cold war origin and its demand for plutonium, the HTGR reactor was designed simply to create a safe, energy-efficient source of power.
Using tiny pellets of uranium wrapped in graphite, the HTGR can operate safely at far higher temperatures than conventional reactors, whose metal fuel rods would buckle in such heat.
As well as being more resistant to melt-down, the HTGR is more efficient than today's pressurised water reactors (PWRs). Its helium coolant also emerges from the reactor at more than 1,000°C and packs enough thermal energy for other uses, from desalination to hydrogen generation.
The MIT study highlighted another important advantage of the HTGR: its ability to run on a mix of uranium and thorium. Converting spent fuel from such a reactor into weapons-grade plutonium is extremely difficult - a vital issue in efforts to stop the spread of nuclear weapons around the world.
British scientists built an experimental HTGR called Dragon at the nuclear research station in Winfrith, Dorset in the 1960s. In spite of working successfully for more than a decade, the project was axed, leaving the question of the commercial viability of HTGRs unresolved. According to the MIT review, it may now be time to revive the Dragon, and see whether it flies.
展望核能新前景
你怎样让一个“绿色”环保分子变得“通红”？提一下核能就行了。在关于未来能源的争论中，人们越来越多地涉及核能一词。从环保分子对核能一词的反应，就可以得出这个结论。
上周，英国首相托尼?布莱尔对一个上议院议员委员会说，美国正对英国施压，要求重新评估建造新一代核电站的项目；并说核能问题不能排除在议程安排之外。
南非能源矿产部长恩格库卡（Phumzile Mlambo-Ngcuka）上月由于提出核能也许可以使南非能源更多样化、供应更安全，而激怒了环保运动者。几星期前，英国环境问题泰斗詹姆斯-拉夫克（James Lovelock）也呼吁大规模投资核能，以抵抗全球变暖。这一明显改变其原本立场的举动，令环保者感到愤怒。
英国主要的环保主义者克里斯平?迪克尔爵士（Sir Crispin Tickell），甚至因为指责英国政府没能为核电立项以解决全球变暖问题，把环保活动者和政府部长们都得罪了。
乍一看，对环保主义者提出在气候灾难与对核技术的依赖之间进行选择，难免令人感到沮丧。核技术带来了三英里岛事件、切尔诺贝利灾难和数千吨放射性废料。
但这是一种错误的二分法，是一种建立在像冷战一样过时的核技术观基础上的选择。尽管还是在悄悄流传，但新型核能的试验即将开始。对这种新型核能，虽然未必会受环保主义战士的欢迎，但至少可以忍受。它与传统上令人害怕憎恶的反应堆设计截然不同。首先一点，它的安全性是经得起物理学定律保证的，而不是靠人的智慧。
而且，它可以使用各种不同的燃料――包括一些能把“流氓”国家生产武器的危险降低到最低程度的燃料。最好的一点是，这种新型反应堆能烧掉其他反应堆产生的废料，把今天那些积压下来的有害物质转变成能量。
这就是新型反应堆――加速器驱动亚临界系统（ADS）――展现给我们的前景。它最初是由新墨西哥州洛斯阿拉莫斯国家实验室的科学家和诺贝尔奖得主、意大利物理学家卡洛-卢比亚（Carlo Rubbia,）于1990年代提出的，ADS背后的构思比它的名称更令人称道。常规的核能利用铀原子裂变释放的能量，伴随着能量释放，裂变过程中还释放出能使铀原子进一步裂变、引发连锁反应的中子。
如果中子的数量刚好足够是反应保持平衡，这个结果就是一种“临界”反应堆，今天世界用电需求量的17%就是由这种稳定的能量流供应的。但如果连锁反应失控的话，反应堆就会像原子弹一样爆炸。这已经驱使设计师必须采取措施防止灾难发生――但这些都不能保证安全。
ADS采用了另一种保证核能安全的方式――连辛普森爸爸都破坏不了（译者注：Homer Simpson，卡通连续剧《辛普森一家》中的爸爸，担任春田核电厂安全检查员）。正如名称所显示，ADS反应堆是“亚临界”的，这就是说，它的燃料不会产生足够引发连锁反应的中子。相反，这种反应堆是由一种粒子加速器产生的中子驱动的。不管是有意还是偶然，切断了中子的供应，反应堆就进入了它自然的、催眠的状态。爆炸性的连锁反应不再是可能不可能的问题――物理学定律保证爆炸不会发生。
而且，因为连锁反应的稳定性不再依靠所使用的燃料，ADS系统就成为一种“杂食”的核设备，用完全不适用于武器生产的燃料就能够运作。最绝的是，ADS反应堆甚至能够消耗掉常规反应堆产生的放射性废料。
这种集三种优点为一身的特性使得ADS成为十多年来热烈的理论研究主题。现在，一个国际科学家团队即将在意大利罗马附近的卡萨西亚研究中心将这一理论付诸试验。
这个中心已经有一个小型研究反应堆被改造成亚临界状态，一台撞击钽元素产生中子供应反应堆的粒子加速器也将安装起来。首批试验预计在两年内开始，主要的目只是积累用中子供应反应堆的技术经验。完成这些试验以后，这支团队计划用一个更强大的反应堆和加速器系统，来创建一个能烧毁废料的ADS系统。一套试验性装置能在在五年内完成。
那还是计划。在现实中，ADS技术可能还存在一个无法预料的问题，令ADS的潜力无法发挥。大多数担忧集中于用来供应反应堆的中子生成的问题上。一个使用大量废料的商业性发电站，将需要一个比现在所有变速器更强大的变速器。
但是，笼罩着这个项目的最大阴云也就是几十年来一直困扰核能的问题：造价。无论ADS会带来什么好处，如果它的造价太高昂，它在未来的能源政策中就没有立足之地。
公众对老式核能回归持有谨慎的态度，而即使是一些环保主义者也越来越相信，仅仅有可再生性是不够的，因此，那些在罗马附近进行的试验的成功，将事关重大。
作者系英国伯明翰阿斯顿大学客座高级讲师。
