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New Advances Bring Fusion Closer to Reality
An anonymous reader writes "The Christian Science Monitor reports on new advances in nuclear fusion research. For years we've been waiting for the technical breakthroughs that would make cost-effective fusion energy a reality. Are we getting close, or are the problems insurmountable?"
"What is the failure mode for a collapsed fusuion capable magnetic field?"
The plasma disperses and the fusion stops. What do you think happens when they shut the field down now after their tests?
"Wow, these are bad, very very very bad also."
Really? Why?
"The folks that came to our little burg for a 'rah rah' meeting claimed that power would be so cheap, it wouldn't be metered."
And it would have been had the anti-nuclear nutters who stopped the whole thing in its tracks. Yes 3 mile island happened and then chernobyl. So what? When an airliner crashes 400 people die. Do we stop all flight? Tens of thousands of people die in car crashes every year. Do we ban cars? No.
"The situation with nuclear power has not changed just becuase we are looking at 'new and improved' fusion"
If the halfwitted political loudmouths of society can be convinced this new form is "better" than the old form (whether it is or not) then we may get somewhere with it. If it ever works that is.
The Joint European Torus (JET) fusion lab in Culham, Oxfordshire, UK 'jumped' a few years ago. The plasma touched the wall of the reactor vessel and dissipated. The entire reactor 'jumped', and the event is visible on seismograph traces. (The reactor, in total, was quite heavy)
This is not good for the retaining magnets - the magnetic field quenches, and the energy goes into heating up the magnets. Even the superconducting (and therefore cooled) ones warm up - boiling off a lot of refrigerant, and possibly/prbably distorting/damaging the coils..
Afer this event, th reactor was shut down for a long period (I think months), while the coils were checked for damage and realigned.
As for the amount of energy in the plasma itself - it's relatively small. Although the temperature is high, the particle density is actually quite low, so the total energy contained is (relatively) small. It *won't* go up like a hydrogen bomb.
The core lining in JET was lithium. It gets mildly radioactive due to being bombarded by neutrons all the time, but this is not a big deal. The neutron activation of the concrete and steel rebar used in the construction of the core (it has to withstand high mechanical forces from the magnetic fields) is more of an issue.
The plasma isn't meant to touch the tokamak wall, as it causes long and expensive downtime, but it's not as catastophic as (say) setting light to an oil well.
I am a Nuclear Enginneer,and work in Britain on the Joint European Torus Fusion Device. Check it out... http://www.fusion.org.uk When we fuse together the hydrogen, the helium formed is more stable and highly energetic. The thing to consider here is potential energy too. Just as there is chemical potential energy in the gun powder of a bullet, which allows the weapon to be fully automatic, so there too is nuclear potential energy. For large enough plasmas it is possible to use the highly energetic helium to sustain the fusion reaction, in a process known as ignition, so more energy can be retrieved than was put in. If all energies are considered, no laws are violated. You are right about the electricity generating process. The use of steam pressure and turbines is limited by the laws of thermodynamics, namely the Carnot cycle, so can only ever be approximately 40% efficient. The next step is the International Thermonuclear Experimental Reactor (ITER). As the politicians couldn't decide whether to build it in Japan or France, Europe has declared its going to build it anyway, and we're now just waiting for people to take sides :)
What are the civilian applications?
Nuclear fission does split nucleii into fragments. U-235 fission absorbs thermal neutrons (room-temperature kinetic energy) and splits in half, P-239 fission absorbs fast (high-energy) neutrons and splits in half. The resultant atoms form an assymetric distribution called the 'Mae West' curve because it forms two big peaks (mapped # vs Z) that look like mammaries to lonely nuclear engineers that don't see nekkid women that often.
While Uranium/Plutonium do decay naturally (stability of a nucleus is determined by the Nuclear Shell empirical formula, which is a rough analog of the electron shell theory - everybody wants to be Iron Fe/26, the most stable nucleus), there's another form of decay that's an outcome of genuine nucleus splitting. That's is the decay of of these usually-radioactive fragments. This decay is important to the operation of a fission reactor, but only in determining the criticality of a nuclear pile. 'Critical' == exactly as many neutrons are released in any time period as are absorbed, meaning steady power output. Basically, over 99% of the neutrons necessary to keep a steady level of fission events come from 'prompt' neutrons - neutrons that are freed in the splitting of an atomic nucleus. You get one small chunk (which could very well be gold), one big chunk, and a couple free/fast neutrons.
If these 'prompt' neutrons were enough to sustain criticality, then the number of fission events would increase geometrically. Since the time between generations is about a millionth of a section, this means that a reactor core that's 'prompt-critical' would quickly escalate in temperature until the structural integrity of the core failed, and you have a molten slag of Uranium - which is exactly what happened at Chernobyl.
So the way to avoid this, you have to put in neutron-absorbing control rods to keep the number of 'prompt' neutrons below the number necessary to sustain the next generation of fission events. If 'prompt' neutrons were the only neutron source, your nuclear reactor would quickly cool down. But the decay of the fragments (which are ususally radioactive isotopes of stable elements) release additional neutrons. The 'art' of tuning a nuclear reactor is to insert the control rods just enough so that the reactor isn't prompt-critical, but the decay neutrons are just barely enough to make the pile critical.
One of the biggest problems with fusion in general is fuel. The easiest fusion reaction is deuterium-tritium. Deuterium is plentiful - the ocean is full of 'heavy water' where one of the hydrogen atoms in a water molecule has a proton and a neutron. Tritium, however, is radioactive with a pretty short halflife. You have to make tritium by getting Lithium to absorb a neutron, then decay.
Last time I was up-to-date on fusion research, there was only an estimated 300 years of Lithium to sustain the predicted energy needs of the world. However, with fission fast-breeder reactors like they use in France, there would be 5000 estimated years of power. Fission fast-breeder reactors can be built today - it's just that to make them passively safe, you need to use a liquid metal coolant like sodium, and any disaster like Chernobyl (from terrorists, for example) would be catastrophic. Liquid sodium will explode if it gets wet, so it's a huge engineering challenge. Argonne Nat'l Labs has reactor designs like this, but the US population is scared of nuclear power plants (plus, the cost overruns at plants made them economically unfeasible).
[I am a published principal author and presentor of a fusion reactor design (presented at the 8th Topical Meeting on the Topic of Fusion Energy in Salt Lake City), so I have a tiny bit of credibility. I got out of the field specifically because of the 15-year carrot-on-a-stick paradox.]
it produces very long lasting nuclear waste;
The longevity of some waste is important to consider, but there are three important mitigating factors that are usually overlooked in discussion.
First, longevity and intensity of nuclear waste are inversely related. The really nasty stuff has halflives of hours to days. The mid level stuff on the order of months. It's actually the low level waste that can last such a long time.
Secondly, reprocessing of the low level waste would extract useful plutonium that can be used as fuel again and will further reduce the volume of waste to store. It may even be possible to reduce the volume yet again by irradiating the low level waste to force it to decay faster.
Finally, coal burning releases a great deal of thorium and other radioactives. If coal plants were held to the same standards for release of radioactive waste products as nuclear plants are, each one would produce many tons of low level radioactive waste a year. That waste would also have to be stored for thousands of years.
Perhaps we should measure low-level waste in the unit "hours of coal", that is, in terms of the released radioactive waste per hour from an average coal fired power plant.
The real problem with nuclear power in the U.S. is lack of standardization in plant design and waste management. With standard design, we could build a body of practical operational and engineering knowledge that would apply to every plant. That in turn would allow increased safety.