Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What are we striving towards with fusion reactors?

  1. Apr 23, 2016 #1


    User Avatar
    Gold Member


    I have heard that fusion can generate power and potentially make breeder reactors obsolete; but to even bring atoms close enough to fuse, wouldn't take a significant amount of energy in the first place? I feel like we are allotting high voltage toward fusion and getting a lesser voltage from the output. I have also heard that fusion is a way to produce neutrons, but does not mixing beryllium and americium do the same?
  2. jcsd
  3. Apr 23, 2016 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    There's some misperceptions here.

    Firstly, the objective of controlled fission or fusion is to produce net energy. In the case of fission, the process involves a chain reaction in which neutrons are produced by fission, and at least one neutron from each reaction induces another fission. The fission process releases thermal energy in the fission process, and that energy heats water to produce steam (directly in a BWR, or indirectly in a PWR) or gas. The steam is used in a Rankine cycle, will gas is used in Brayton cycle, unless the gas is used to heat water to produce steam in a Rankine cycle, which then drives a turbine, which drives a generator, which provides AC electricity.

    In fusion, rather than fissioning nuclei, we fuse nuclei, which recombine to more stable nuclei and in the process release kinetic energy. Now, if the nuclei and electrons can be collected directly (direct conversion), we could generate DC electricity, or if we can use the thermal energy of the plasma to expand against the magnetic field, we develop AC electricity. Otherwise, we have to settle for a net energy production in the form of thermal energy, and use a Rankine or Brayton cycle to produce AC electricity.

    The objective with thermal energy is to use it in a thermodynamic cycle to drive a turbine to turn a generator to produce electricity, like most conventional thermal plants, and the voltage is stepped up for transmission. In stepping up the voltage, the transmission current drops.

    The easiest fusion reaction is d+t (deuteron + triton), which produces a 14.1 MeV neutron and a 3.5 MeV alpha particle. The fast neutron has to be absorbed in a blanket, which is heat, while the alpha particle heats the plasma. Another reaction is d+d, which produces t+p, or n+He3, and He3 might react with d, to produce p+He4. It turns out, fusion is not so easy, otherwise we'd have done it already.

    The 241AmBe combination produces a low level of neutrons, by virtue of an (α,n) reaction (the α from Am hits a 9Be nucleus, which forms 12C + n), which is a good starting source to initiate a fission reactor, when there is no spontaneous fission or delayed neutrons, as in a fresh core. The AmBe reaction rate depends on the decay of 241Am, and one cannot simply turn it off, so it does not do for a primary power source.

  4. Jun 25, 2016 #3
    Quantum tunnelling is one of the few friends of physics that fusion has. It allows to overcome the electrostatic repulsion of 2 atomic nuclei when they approach close enough. Therefore, the classical electrostatic law does not apply here and there is no need of exponentially increasing energies to get to atoms to fuse together, although the energies needed to have enough particle interactions so that the fusion rate is high enough is also not trivial.
  5. Jun 25, 2016 #4


    User Avatar
    Gold Member

    This is interesting. So how is quantum tunneling achieved; and what process must be undergone to make it possible?
  6. Jun 25, 2016 #5
    No additional actions are required for this. It is just due to the fundamental nature of the particles, which are described by quantum wave functions, that there is a finite probability that the particle-wave at the moment of the interaction just "leaks" thought the other particle-wave like a ghost through a wall. The probability that this happens is low, but not 0. Given the amount of such interactions in the core of the reactor, most of them are scatterings and absoprtions, but luckily some are quantum-tunnelings leading to the fusion of the atoms.

    Here's a picture explaining the effect:

    Source: http://abyss.uoregon.edu/~js/ast123/lectures/lec06.html
  7. Mar 21, 2017 #6
    This is not the principle used to build thermonuclear fusion reactors though. The idea is to simulate the phenomenon that happens inside the Sun. But since we cannot generate as much pressure as the Sun, engineers compensate by generating an extremely high temperature (100 million degrees Celsius).
  8. Mar 21, 2017 #7
    Hydrogen atoms are fused inside the Sun's core, but it happens because they're subjected not only to high temperature, but also a very high pressure. In a thermonuclear fusion reactor, we cannot reproduce the pressure, but we can compensate by creating an extremely high temperature, higher than in the Sun's core. Also, as explained above, we use hydrogen isotopes (tritium and deuterium).
  9. Mar 27, 2017 #8
    he didn't say it was. Your point about heat was mentioned already above. The Tunneling point was an addition to that.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted