Powerful Attraction

A new British-pioneered nuclear reactor can help cut our reliance on fossil fuels

In a world running out of oil and where the fear of climate change would tell us not to emit the CO2 even if we had the oil, you might think the public would be clamouring for a crash programme to increase our use of the one proven high-capacity low-carbon energy generation technology currently available to us — nuclear power. There are many perceived, and perhaps a few real, shortcomings in existing nuclear technology which explain that lack of enthusiasm. First, people think nuclear power stations are dangerous, with their fears dominated by a Chernobyl-style core runaway leading to an explosion. Second, nuclear fission produces long-lived radioactive waste, the disposal of which is perceived to be a dangerous, risky and expensive business. Third, the widespread use of nuclear power could lead to an increased risk of the proliferation of nuclear weapons because of an increased availability of enriched uranium and plutonium. And fourth, if we actually did try to generate a very large fraction of our worldwide energy usage from nuclear fission we would run out of easily recoverable uranium quite quickly unless we begin wide-scale use of some alternate to the current once-through method of burning nuclear fuel.

You can argue (the nuclear industry certainly would and I would agree) that most of these fears are based on wildly exaggerated public perceptions of the real risks. From a lifetime of building particle physics experiments that are as clean as possible of radioactive contaminants, I know very well just how ubiquitous and highly variable radioactivity is in our environment. Uranium, thorium and potassium are everywhere and all are radioactive. The odds are good that if you live in a brick house, your walls approach a large fraction of a part-per-million uranium and thorium. Despite this, people are terrified of getting a hypothetical enhanced radiation dose that might, at worst, amount to a tiny fraction of the difference in radiation exposure between one house on a block and the next. And this is in a country where almost a quarter of adults smoke cigarettes. 

Another article (or whole book) could be written on the causes and effects of this irrational fear, but for the purposes of this article it doesn’t really matter whether the fear is groundless. It matters only that it makes people very reluctant to use nuclear power even if it is currently the only realistically implementable alternative to the risk of catastrophic climate change. That is why I have become very interested in an alternative form of nuclear power that could greatly reduce or eliminate the problems listed above. Called the accelerator-driven sub-critical reactor, or ADSR, it could turn out to be one of the most important technologies of the next 50 years. A small group of researchers in the UK (of which, to declare an interest, I am a member) called ThorEA (www.thorea.org) is now exploring how the UK could leap into this technology and try to make it an important part of our energy mix (and if it does turn out to be important, make sure that UK industry gets a piece of the pie and we don’t have to buy it all from overseas). 

In order to explain the benefits of an ADSR, I will first have to say a bit about how nuclear reactors work. Reactors derive their energy from neutron-induced fission, where a neutron strikes a heavy nucleus and causes it to split into two lighter nuclei, releasing large amounts of energy and, crucially, a few more neutrons in the process. Only certain types of nuclei (called fissile nuclei) do this easily enough to be useful. Some examples are uranium-235 and plutonium-239. (The numbers indicate the total number of neutrons and protons in the nucleus; nuclei with the same number of protons but different numbers of neutrons are called isotopes — they have almost the same chemistry but very different nuclear properties, which is why it is both useful but extremely difficult to separate fissile uranium-235 from non-fissile uranium-238.)

The neutrons released in fission can then hit another nucleus and trigger another fission, and what happens next depends on the amount and arrangement of fissile material present and on any other materials that are around. If the arrangement is such that the neutrons from fission produce, on average, exactly one more neutron then the number of nuclei that undergo fission is constant in time and the reactor is described as critical. If, on average, each neutron produces less than one more neutron, the reactor is sub-critical and, in the absence of any external source of neutrons, the number of neutrons decreases exponentially and the reactor very quickly ceases to produce energy. If, on the other hand, each neutron leads to more than one more neutron, the reactor is supercritical and the amount of energy produced will increase exponentially until either some process or control system alters the neutron multiplication factor to bring it back under one, or until the reactor melts or explodes. Normal reactors are operated at criticality, on the knife-edge between sub-critical shutdown and supercritical runaway, with a number of inherent processes and control systems constantly adjusting the neutron multiplication factor to keep it precisely tuned to one. 

An ADSR differs from an ordinary reactor in two critical aspects — the first being that it isn’t critical, in fact it is intentionally designed to always be sub-critical. This means that left to itself it immediately shuts itself off. That would be very safe, but pretty much useless, without the other difference — an ADSR has an extra source of neutrons that keeps it running in the absence of enough neutrons from fission. These extra neutrons are supplied by having a particle accelerator that can collide a beam of energetic protons with a target in the centre of the reactor. These collisions produce neutrons by a process called spallation, and these neutrons are then multiplied up in number by fissions in the reactor core, resulting in the desired power level. 

An ADSR is obviously more complicated than a reactor because of the need also to build the high-current, high-reliability accelerator. However, this added complexity buys a number of potential major advantages. First, the sort of runaway criticality excursion that blew up the Chernobyl reactor is essentially impossible for an ADSR, since they run very far from criticality. Second, the relaxation of the need to balance the neutron economy so finely means that an ADSR could run with a wider range of potential nuclear fuels. In particular, the extra neutrons produced in an ADSR could be used to convert non-fissile thorium-232, which has such a large natural abundance that we would have fuel for many centuries, into fissile uranium-233, which could then fuel the reactor. The extra neutrons can also potentially clean up another problem, which is the long-lived nuclear waste produced by current reactors (and which would be produced, although in much smaller quantities, by a thorium-fuelled ADSR). The long-lived component of the waste is mostly the plutonium and other similar elements called minor actinides. These minor actinides can also be made to undergo fission, so they can be separated and burned up in the ADSR, greatly reducing the long-term radiation hazard from the waste. Another major (and underappreciated) advantage of an ADSR is that the power output is controllable over a wide range just by changing the current from the particle accelerator. A conventional reactor usually runs efficiently over only within a small range of output power, which makes it hard to match power production to consumption if most of your power comes from such reactors — a set of ADSRs would provide the perfect source for tunable power to deal with fluctuations in demand. 

What about proliferation? Here the answer is a bit subtle. There are basically two materials used to build fission bombs today. First, there is highly-enriched uranium, or HEU, where the uranium-235 has been separated from the far more abundant uranium-238. This is the extremely tricky process that Iran is trying to master. However, if you can do the separation and make HEU, converting that into a nuclear weapon is actually quite easy. The other option is to use the uranium in a reactor, which will transmute some fraction of it into plutonium-239. This can be chemically separated from the parent uranium, so it is rather easier to get your hands on plutonium-239 than HEU. However, the compensating disadvantage is that it is much more technically challenging to make an effective nuclear weapon out of plutonium-239 (which is no doubt why the North Koreans were able only to produce a damp squib of an explosion with their first try). A thorium-cycle ADSR nicely avoids both of these weapons materials. First, it doesn’t require any HEU, so there is no danger of leakage of uranium-235 from a power programme into a weapons programme. Second, it does not produce significant quantities of plutonium (in fact it would probably be used to consume plutonium, not produce it), so it does not lead to the ability to build that type of bomb either. The one fly in the ointment is that it does produce large quantities of uranium-233, which is the fissile material that actually produces the power from the reactor. Now there are various reasons why this uranium-233 would be less desirable for use in any weapons programme (for one thing, if made in an ADSR it would inevitably contain very intensely radioactive uranium-232, making any resulting bomb hard to build, hard to store, and extremely hard to conceal). However, you could not say from first principles that you could not possibly build a bomb out of it, so I don’t think you can say that you cannot build a bomb based on an ADSR programme — only that it would hugely complicate the task of making a nuclear weapon starting from an energy programme. 

All of these advantages come with a price. The resulting system will be more complicated than a conventional reactor and therefore probably more expensive. Even if the price were higher, however, it is still likely to be competitive with any alternatives once cheap fossil fuels are taken out of the equation, unless somebody comes up with a 100 per cent efficient way to store energy for free. The big challenge, however, is that no proton accelerator of sufficiently high power and reliability to drive such a device currently exists. Partly that is because there has never been a need for such a machine, but it is certainly true that producing a reliable enough accelerator would require substantial improvements to currently available technology. These improvements would probably be embodied in a new kind of accelerator called an FFAG, which is also being developed for medical and other applications. If ADSRs are ever to contribute to our energy future, we urgently need research on higher-power, more reliable accelerators. 

Is the lack of an accelerator the only reason that ADSRs are not already in use? I don’t think so. The idea itself is rather old, having been first proposed by the Nobel Prize-winning physicist Ernest Lawrence in the 1950s, and revived by Charlie Bowman at Los Alamos and especially by another Nobel Laureate, Carlo Rubbia, when he was Director-General of Cern in the 1990s. There is current active work on this idea throughout the world, especially on the continent and in Japan. However, this has not yet led to any working device, and there has been little work within the UK until recently. Partly I think this arises from a Catch-22 — if people think the technology is useful they probably don’t think it is necessary. That is because for some years people in the UK have usually fallen into two camps on nuclear power. The larger camp thinks it vile, nasty and dangerous and it doesn’t want any part of it, accelerator driven or otherwise. The other camp thinks nuclear power is just fine and doesn’t see why you need something as complex as an ADSR when normal reactors work so well. I hope that we are going to come to a more considered view of both the strengths and weaknesses of nuclear power, and a more rational evaluation of any dangers, because otherwise we risk ignoring the one workable low-carbon energy generation technology we may actually get in time to avert catastrophic climate change. 

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