4
$\begingroup$

I understand that combining deuterium and tritium will form helium and a neutron. There are three methods to do this (1) tokamak (2) lasers and (3) cold fusion. I would like to know after helium is formed. How is that energy extracted from tokamak and stored?

$\endgroup$
5
$\begingroup$

If you search the ITER site, ITER being the international prototype fusion reactor which will demonstrated the possibility of getting megawat useful energy from fusion, one sees that their main aim is to demonstrate this feasibility:

The main carrier of energy out of the plasma is the neutron, and methods to efficiently use this energy have not been developed yet, but wait for the commercial prototype.

The helium nucleus carries an electric charge which will respond to the magnetic fields of the tokamak and remain confined within the plasma. However, some 80 percent of the energy produced is carried away from the plasma by the neutron which has no electrical charge and is therefore unaffected by magnetic fields. The neutrons will be absorbed by the surrounding walls of the tokamak, transferring their energy to the walls as heat.

In ITER, this heat will be dispersed through cooling towers. In the subsequent fusion plant prototype DEMO and in future industrial fusion installations, the heat will be used to produce steam and—by way of turbines and alternators—electricity.

They have developed methods for cooling the system and dissipating the energy to the environment.

$\endgroup$
3
$\begingroup$

Another, non mainstream thought is the use of Helium-3 instead Helium-2 of as a fuel:

3He + 3He → 2 1H + 4He + 12.9MeV

or:

2H + 3He → 1H + 4He + 18.4MeV

Here the reaction products are all charged, which means that they could work directly on an electrostatic field, thus transferring their kinetic energy directly to a current. Depending on the reactor's design, there would also be bremstrahlung from the decelerating charges - this could be absorbed by water and maybe used as heat in co-generation.

The main technological point of these reactions is that there are no neutrons. There is some concern that conventional fusion may be "dirty", even as dirty as current fission power, because the reactor case will need to be replaced often owing to the huge damage it sustains from absorbing so many neutrons (indeed, as discussed in Anna V's answer, the neutron is the main carrier of kinetic energy, so it would be the case that would be directly converting this kinetic energy to the useable heat). These neutrons cause transmutations in the case, which means that there could be substantial radioactive waste problem.

There is active fusion research underway into the idea of helium-3 fusion. Indeed I understand that Harrison Schmidt is even seeking investment for moon mining operations to win helium-3 as a "clean" fuel. Clearly, once commericial interests and the need to convince investors are afoot, one has to look at touted "advantages" of this approach with due skepticism, especially when touted by a conflicted party, even when they do show up at "physics" conferences to spruik their idea! Nonetheless, I find this interesting as it does show that mainstream fusion would not be without its waste problems.

$\endgroup$
  • 1
    $\begingroup$ It really stretches the imagination to believe that a lunar He-3 mining operation could be economically feasible, even if it is technically possible, since you must process tens to hundreds of millions of kg of regolith to obtain one kg of He-3. Solar power is a much cheaper alternative. $\endgroup$ – Michael Brown Jul 8 '13 at 2:54
  • 1
    $\begingroup$ @MichaelBrown I totally agree with you, Michael. But it is a process different enough from "conventional" fusion that I thought the OP might be interested. What I find most interesting about this work is mainly that it made me aware that "conventional" fusion is likely far from altogether clean - which is the opinion that many laypeople have: the realisation that it may not be might tend to support your view of solar energy (I'm trying hard not to be too political on this site!) $\endgroup$ – WetSavannaAnimal Jul 8 '13 at 3:14
  • 1
    $\begingroup$ It is clean in the sense that it does not spread to the environment, only to the acre or so of the reactor. I think the idea is that when the structures become non sustainable because of transmutations ( all, the reactor etc) they will be sealed for centuries and new ones built. The safety consists in that there are no gases to blow away, not material to explode and spread radiation far, no melt down possible. $\endgroup$ – anna v Jul 8 '13 at 14:45
  • 1
    $\begingroup$ @annav True. As I hoped I made quite clear, one must take due heed that the people touting the advantages are not only scientists, but also seeking investment - so they cannot be taken to be impartial. Nevertheless, there is some waste problem with conventional fusion. I think a talk I went to by Harrison Schmitt cited calculations to show that reactor cases might only last two years (clearly in his favour) - what would your take on this be? $\endgroup$ – WetSavannaAnimal Jul 8 '13 at 23:00
  • $\begingroup$ you can peruse the iter.org/safety , and the safety answers in the FAQ iter.org/faq . They foresee replacing walls etc. $\endgroup$ – anna v Jul 10 '13 at 12:30
2
$\begingroup$

The basic idea in all three cases is that the energy becomes heat, and is extracted using turbines, just like in a fossil fuel burning power station. On a microscopic level the energy is released in the form of kinetic energy of the helium nucleus and the neutrons that are given off. These then collide with other particles, so that this kinetic energy rapidly becomes heat.

$\endgroup$
0
$\begingroup$

As others mentioned it depends on what the product is of the reaction. It's hard enough to get deuterium-tritium reactions to happen efficiently, but if we get the science down it usually gets more efficient as we scale up. If we can get p + B11 -> 3 He4 to work it yields 3 highly energetic He4 ions (alpha particles), which have a +2 eV charge and here is where I am fuzzy:

They have high energy, but are also highly affected by charge, so if the fuel was electrostatically confined, a bit of the potential energy will be given up as it climbs out of the well. Assuming it does escape with most of it's energy still, they easily interact, so by using a funnel that decelerates those alpha particles and induces current, it can directly capture this energy as electrical current, without steam turbines. That said isolating them from the fuel could be tricky.

I mean you can still do the steam turbines but p 11B is aneutronic, so the neutrons are less than 1% of the energy produced.

$\endgroup$
  • $\begingroup$ Alpha particles have a typical kinetic energy of 5 MeV (or ≈ 0.13% of their total energy, 110 TJ/kg) and have a speed of about 15,000,000 m/s, or 5% of the speed of light. There is surprisingly small variation around this energy, due to the heavy dependence of the half-life of this process on the energy produced (see equations in the Geiger–Nuttall law). Because of their relatively large mass, +2 electric charge and relatively low velocity, alpha particles are very likely to interact with other atoms and lose their energy, and their forward motion can be stopped by a few centimeters of air. $\endgroup$ – John Butterfield Oct 21 '17 at 6:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.