Solving the fusion puzzle
15 Jan 2013 by Evoluted New Media
Completing a complex 4,000-piece ‘jigsaw puzzle’ inside the Joint European Torus (JET) fusion experiment in Oxfordshire has enabled European researchers to plot the next steps in developing the ultimate energy source…
Jigsaws can be fun but frustrating. You spend countless hours patiently waiting for the picture to emerge, and there always seems to be that elusive missing piece preventing you from finishing the puzzle. Imagine, then, working day and night on a massive jigsaw for over a year, with each piece taking up to 30 minutes to insert, using only remote-controlled tools – and if you damage a piece it will cost thousands to replace. This was the challenge facing engineers at Culham Centre for Fusion Energy near Oxford as they refitted the world’s largest fusion experiment, the Joint European Torus (JET). The wait has been worthwhile, however, because the newly-upgraded device has produced results that are giving researchers increasing confidence as they look to the first commercial-scale fusion reactor – the International Thermonuclear Experimental Reactor (ITER) project, now being built in southern France.
JET is a pan-European partnership to develop nuclear fusion, a highly promising but elusive technology that is often seen as the Holy Grail of energy. Over the last 30 years JET has taken European fusion research from an era of tabletop experiments to the stage where the construction of ITER, a giant power plant-sized machine capable of a 500MW output, is now going ahead at a cost of some €14 billion.
Now JET has a new role as one of the key test-beds for ITER. Until ITER starts up in 2020, JET will be the biggest present-day experiment, the closest to ITER conditions, and the only facility that can use the optimum fuel mix (deuterium and tritium) that is needed to unlock the vast amounts of energy that fusion can produce. It will be central to ITER’s prospects – in rehearsing operational scenarios, testing technology, and training a new generation of European physicists and engineers who can use their expertise to take fusion towards commercial fruition.
Fusion has always been central to life on earth, as the process that keeps the Sun burning. Each second in its core, 600 billion kilograms of hydrogen nuclei fuse together to form helium. The huge amount of excess energy that these reactions release – 10 million times more than standard chemical reactions – powers this, our nearest star, and indeed the entire universe. Ever since solar fusion was discovered by Arthur Eddington in the 1920s, the attractions for replicating it in our power stations have been obvious. However, nature has placed a number of obstacles in the way. The Sun, due to its size, is able to confine particles in its core for thousands of years using its own gravity, giving them plenty of chances to collide; we do not have that luxury. The Sun is able to achieve fusion at ‘only’ 15 million degrees Celsius; on earth, temperatures of over 100 million degrees are required. And fusion is by definition hard to achieve – involving the joining together of positive nuclei that are inclined to repel each other.
Research today focuses on two main ways of attacking these problems. One – the JET/ITER approach – is known as magnetic confinement fusion. This involves heating up hydrogen plasma inside a ring-shaped bottle, known as a ‘tokamak’, and controlling it with magnetic fields until the necessary conditions for fusion are achieved. At extremely high temperatures, the nuclei are moving so fast that they come close enough for the stronger, short-range, nuclear force to pull them together. The second method, inertial confinement fusion, trains high-powered lasers on a target containing a small pellet of fuel, imploding it to crunch nuclei into each other and bring about fusion. Both face considerable technical challenges, and overcoming these is a lengthy effort that can make progress in fusion appear glacial to the outside world. In fact, both research paths have made enormous advances in recent years. In particular, JET has performed controlled fusion reactions, produced 16 MW of power and indicated that scaling up to larger tokamaks can bring about the energy gains and continuous fusion reactions needed for power plants; in inertial confinement, the US National Ignition Facility has gone a long way towards its ambitious goal of achieving self-sustaining reactions to show the feasibility of this technique.
In tokamak research, despite ongoing challenges with the control of turbulent plasma inside the magnetic container, much of the physics is already well understood. Magnetic fusion R&D is now therefore largely an engineering task, to build the machines themselves and their associated systems: heating devices that can make plasma ten times hotter than the Sun; superconducting magnets that will facilitate very long-pulse or steady-state operation; and blankets surrounding the tokamak that will trap and extract the energy emerging from the fusion reactions.
One scientific stumbling block remains, however – materials research. The inner walls of the tokamak, although not exposed to the hottest parts of the plasma, are still subjected to temperatures of around 1,000oC. Not only that, they are exposed to a severe bombardment by the fast-moving 14 MeV neutrons that are produced during fusion. It is these neutrons – typically travelling at 50,000 km per second, about one-sixth of the speed of light – that hold the energy that will one day be harnessed for electricity. But finding wall materials that can withstand their onslaught for prolonged periods in power plants is problematic.
Which brings us back to the jigsaw puzzle inside JET. The objective of current research is to validate materials for the next-step ITER device. The materials of choice for ITER’s plasma-facing components are beryllium and tungsten, whereas JET had previously operated with a carbon-fibre composite inner wall. During the course of 2010 and 2011, therefore, remote handling engineers stripped out the old tiles and replaced them with an ‘ITER-like’ wall to allow physicists to test the new materials mix. The selection of beryllium and tungsten was governed by a number of key requirements. Firstly, the wall components must be robust and resistant to erosion and melting in the intense conditions within the tokamak. Secondly, when small amounts of material do erode and flake off into the plasma, they should have a minimal impact (for fusion performance to be optimised, the plasma must be kept as clean as possible). Lastly, it is important to find materials that do not suck valuable fusion fuel out of the plasma and trap it in the wall. Beryllium, which is being used for the main part of the wall, is a light metal that absorbs less fuel than carbon and does not pollute the plasma as much. For the lower reaches of the fusion chamber, where plasma is exhausted from the machine and heat loads on wall components are greater, tungsten – with its melting point of 3,422oC (almost three times higher than beryllium) – is being employed.
JET is now in effect a ‘mini ITER’ – ideally equipped to simulate plasma experiments that can be scaled to the parameters of its larger successor. Since the upgrade, a year of tests have been carried out using the new wall. Right from the restart of operations in August 2011, the beryllium and tungsten lining has enabled more reliable plasmas to be produced. And crucially, researchers from the 28 European countries which participate in JET have found that the amount of fuel being retained in the wall is at least ten times less than in the previous carbon configuration.
The results achieved so far are very promising for ITER and should lead to a significant reduction and time and cost for the project. The original plan for ITER’s commissioning phase was to play it safe and operate with a partly carbon wall before transferring to the all-metal lining. However, the recent work at JET is aimed at validating ITER’s new proposal to adopt beryllium and tungsten from the outset, which would save as much as €400 million. Physicists at ITER have been following progress at JET closely, and future collaborations are likely to open up the exploitation of Europe’s flagship fusion facility to ITER partners from around the globe, which include China, India, Japan, Russia, South Korea and the United States.
The next run of tests at JET in early 2013 will aim to demonstrate continued improvements in plasma performance. Looking further ahead, the European Fusion Development Agreement, which co-ordinates the scientific programme, is already planning a full ‘dress rehearsal’ for ITER in 2015 – an experimental campaign at JET using the optimum deuterium-tritium fuel mix that is needed for high-power fusion operation.
Success at ITER will pave the way for the commercialisation of fusion. The results from ITER will be combined with those from a parallel experiment, the International Fusion Materials Irradiation Facility (an accelerator that will simulate the action of fusion neutrons on full-scale power reactor components, currently being designed in Japan). Scientists and engineers will then be in a position to construct a prototype power plant to put the first fusion electricity on the grid in the 2040s. By the end of this century it is estimated that fusion could be providing over one-third of the world’s electricity. With almost limitless supplies of fuel (from seawater and the crust of the earth), no greenhouse gas emissions, inherent safety advantages over nuclear fission and no long-lived radioactive waste, fusion looks to be one of our best long-term energy bets. So the puzzle being solved in a corner of rural Oxfordshire today should benefit the entire human race for centuries to come.
