the future of fusion

Monday, 01 February 2010

If you’ve been reading the science news sources, you may have noticed that there have been some exciting steps recently along the path toward sustainable fusion : the holy grail of nuclear power. I thought it would be an idea to run through my geek layman’s take on the concept, the various approaches and the implications.

Your common or garden nuclear reactor works on the concept of nuclear fission, where we quite literally spit atoms in order to release heat energy for boiling water and use the resultant steam to turn turbines in the manner of conventional power stations. You’re probably aware of the historical issues with this method with regard to waste disposal and the proliferation of weapons-grade material. Indeed, it is impossible to have a civilised discussion on the matter in some quarters, so politicised and emotional has the subject become.

So many in the field are looking to nuclear fusion as the answer to all our woes; it being theoretically clean, sustainable and with zero carbon emissions –if that's what floats your boat. Fusion is the process of joining together several atomic nuclei (the ‘centre’ of the atom) to form a larger nucleus. This produces or absorbs incredible amounts of energy -potentially creating super-heated plasma- and is the mechanism at the heart of every star. The idea is that we use this as our heat source for producing electricity. Yes, I too am always disappointed that all power stations essentially boil down (pun probably intended) to an overly elaborate kettle; 2010 is supposed to be the future damn it. But that’s what we’ve got to work with.

Now we have artificially produced atomic fusion before, *ahem* , just not in a controlled manner suitable for power generation. The problem is, any viable energy source needs to produce more energy than it took to run to initiate the process in the first place, and getting two positively-charged nuclei to share the same space is an astonishingly energetic enterprise. We, at present, haven’t managed to break-even on this score, so there is a fair way to go yet.

The two main approaches to fusion power are Inertial Confinement and Magnetic Containment facilities. although there have been some novel recent developments which merit mentioning.

Lasers and Inertial confinement

Image courtesy of Lawrence Livermore National Laboratory Laser insertion devices work on the principle of inertial confinement fusion (ICF): that if you can focus enough intense laser light on a small spherical target (containing fusion fuel like the hydrogen isotopes deuterium and tritium) you can induce an explosive reaction on the outer layer, causing a resulting implosion on the inside of the target, which forces the fuel to compress to exceptionally high densities and temperatures. This would initiate nuclear fusion at the very centre of the target and then, because the surrounding fuel can’t move away fast enough due to it’s inherent inertia, the expanding shockwave can induce fusion in that too- causing a chain reaction amongst the remaining fuel.

This line of research is primarily the domain of the Americans, and it was the NIF facility that recently overcame a major foreseen obstacle, namely the possible interference of the generated plasma with the delivery of the lasers on-target. In fact they're confident of full ignition before the end of the year. Something to watch out for.

Getting ahead of ourselves, if NIF is successful there is a future application being designed for which could actually use the existing long-lived nuclear waste -which causes so much consternation- as fuel. It is called LIFE and is an example of possible ‘Hybrid’ reactors, a mixture of fission and fusion. It would involve a ICF facility (as described above) to also provide a constant source of neutrons for burning up fissile material like spent nuclear fuel or weapons-grade plutonium.

Image courtesy of Lawrence Livermore National Laboratory There are many substantial steps in between that goal and today –not least achieving ignition with the standard ICF facility- but if and when this is achieved, the LIFE system could be revolutionary.

Magnetic Confinement

This is the oldest approach to nuclear fusion, and the mainstay of the European contribution to the field.

Image courtesy of the European Nuclear Society With this technology, our fusion fuel (deuterium and tritium again) are heated up to 100 million degrees kelvin in order to strip away the electrons and leave us with our plasma of bare nuclei. Now, if we achieve a high enough density and temperature, the nuclei will fuse and we will have achieved fusion.

And if that sounded too easy, that’s because it is. Once you achieve fusion conditions, you need to contain and maintain them. The magnetic containment method generally uses a Tokamak-type facility, consisting of a toroidal (i.e, Doughnut-shaped) arrangement of conductive coils which heat, manipulate and contain the plasma by passing a current through it and inducing magnetic fields to stop it drifting into the walls.

Facilities like JET in Culham have made huge advances advances in the the field of plasma physics, but no facility has managed to produce even break-even energy production. JET holds the record with 70% of input power. The energy requirements for generating the plasma, containing it, and cooling the cryogenic components and the other parts you just don’t want to get hot are just too large. At least so far.

The ITER facility to be built in France is designed to be the first tokamak facility to produce greater than input energy: 500MW for 50MW input. Something to watch for in 2012.

Levitating Dipole

I’ll give a brief mention to a very recent innovation which has been in the news: the Levitating Dipole or LDX. This involves the use of a large half-ton torus magnet which is suspended inside a magnetic field, which causes plasma to form around it by the mechanism of ‘Turbulent pinching’', a phenomena reverse-engineered from astronomical observations of the way plasma in space interacts with the planetary magnetic fields.

The lab has rather thoughtfully produced a video clip of the first experiment:

More on this one as soon as I have the first clue what is going on.

So there you go, two contenders and one hopeful for the title of ‘solution to all our future electricity needs’. To my mind, it’s been a long while coming (about 50 years and counting) but I think we’ll see real, useful progress inside the next few years.

Otherwise I want to speak to the manager-I should have a robot servant and rocket boots by now.

Blogger: Mr Salted Slug

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