Nuclear Fusion Reactor

Rabies

news.bbc.co.uk

I don't know much (anything) about nuclear power but this sounds cool. It will either get built in Japan or France

Fusion powers stars and is seen as a cleaner approach to energy production than nuclear fission and fossil fuels.
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To use fusion reactions as an energy source, it is necessary to heat a gas to temperatures exceeding 100 million Celsius - many times hotter than the centre of the Sun.
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One kilogram of fusion fuel would produce the same amount of energy as 10,000,000 kg of fossil fuel.

dudara

Research projects into nuclear fusion have been ongoing in Europe for ages now. JET in England is well known, as is the ASDEX tokamak in Garching, Germany. There is also the RFX reverse field pinch in Padua, Italy. In America, they have the stellarator. These are just some illustrative examples of projects.

We're still under the break-through barrier however. At the moment, we're not getting back the energy that we put into the generators. Though that's steadily getting better and looks likely to be achieved soon.

One of the main problem lies in steady confinement of the plasma. At operating temperatures the plasma, an ionised gas, is so incredibly hot, that it cannot be held by conventional containers. It would burn through steel walls. But, as it is an ionised gas, it can be contained by a suitably shaped magnetic field. However, maintaining an adequate magnetic field requires constant monitoring of the plasma, which is often done with laser diagnostics. This data must be then fed back into the confinement systems as quickly as possible to allow suitable readjustment of the magnetic field.

There is no doubt that nuclear fusion is a smart way to proceed in power generation. It is "clean", with few nasty by-products, and fuel will be readily available. It's just a matter of time, before there will be significant success. Have a look here for more information



JET

Woden

the most viable fusion reactor at the moment is the one be used by jet, the two fuels consist of deutrium which is basically hydrogen with an extra neutron and then tritium which is hydrogen with two extra neutrons.

the reaction is D + T = alpha particle + neutron

the reason this reaction is used is that it has a high cross section i.e the D and T are more likely to interact with each other or fuse. Also there is a substantial amount of energy in this reaction as an alpha particle is released which has low binding energy as its stable helium nuclei (alpha particle is just a doubly ionised helium atom).

there are however issues with this reation also, just looking at kinematics from the conservation of energy and momentum the neutron which is only about 25% of the mass of the alpha particle will carry away about 80% of the energy from this reaction. this is a bit of a bitch as the neutron is uncharged and thus can't be confined in a magnetic field.

other reactions have been looked at such as D-D reactions but these have a low cross section and also some deuterium and helium reactions, but due to the extra proton in helium and as electrostatic repulsion (the coulomb barrier) goes up as the square of the charge so an atom with 2 protons has a coulomb barrier 4 times higher then an atom with 1 proton this reaction isn't used either.

another issue with the DT reaction is that T = tritium is radioactive with a short enough half life. it has to be produced from lithium with in the reactor. this is not as bad as it sounds though as the neutrons from the reaction about are used. the reactor core has a lithium blanket around it. the neutrons emitted are moderated i think (i.e. slowed down) and they react with the lithium to form more tritium

the alpha particle produced then deposits its heat to maintaining the required temperature of the plasma.

another important aspect of fusion reactors is bremstrahlung (spelling? from the german for breaking radiation). any charge which accelerates produces bremstrahlung and this is an energy loss mechanism in fusion reactors so even if you think you are about to break even this energy loss mechanism means that you won't. Again like the coulomb barrier this energy loss mechanism rises as the square of the charge on an atom so once again high proton number impurities are not good and this another reason for why hydrogen isotopes are prefarred in fusion reactions.

I swear btw i didn't have an exams on stuff like this recently really i swear, didn't bloody come up though did it, but now that i have vomited the information out onto the page like i do in exams it can safely leave my brain and be forgetten about

data

dudara

It's all about the confinement. If we can't confine the plasma adequately, it will not be hot enough, and we will not achieve fusion. The american stellerator, which is a twisted torus, as compared to a normal torus acheives good comfinement. The tomaks are pulsed, being that they are on half the time and off the other half.

The physics of of confinment of the magnetic fields is horrendous, and can often only be donw by number crunching. However, it has to be done if we're going to make fusion a viable source.

seamus

Iter would be more than double the size of the facility at Jet, and would aim to generate 500 megawatts of fusion power for 500 seconds or longer.

Heh, a way off, but miles ahead of where we were.

Anyone care to estimate? Stable, commercial fusion reactor within 20 years?

Woden

Originally posted by dudara
It's all about the confinement. If we can't confine the plasma adequately, it will not be hot enough, and we will not achieve fusion. The american stellerator, which is a twisted torus, as compared to a normal torus acheives good comfinement. The tomaks are pulsed, being that they are on half the time and off the other half.

The physics of of confinment of the magnetic fields is horrendous, and can often only be donw by number crunching. However, it has to be done if we're going to make fusion a viable source.

indeed there is a triple product that defines basically how hard it is to get a fusion reaction going and plasma density and thus confinement is one of the components. its about 500 times more difficult for a D-D reaction than a D-T one.

yeah weird magnetic fields are needed for confinemnent as i think the magnetic fields that are typically obtained aren't uniform and weaken as you approach the out side of the torus. so in the tokamak they have a colloidial and pollodial magnetic fields. i think the pollodial one is generated by a current running through the plasma. this gives the mag field a weird shape.

dudara

they're actually constantly varying magnetic fields, which are neccessary in order to contain the plasma. The stellarator achieves this easily through it's weird twisting shape. But more conventional toroids create the required varying magnetic field through methods such as pulsing.

The plasma shape is indeed odd, and looks like a kind of squished teardrop.