Aneutronic fusion

Aneutronic fusion is any form of fusion power in which the majority of the energy released is carried by charged particles. While the lowest-threshold nuclear fusion reactions release up to 80% of their energy in the form of (uncharged) neutrons, there also exist reactions in which the energy is released in the form of charged particles, typically protons or alpha particles. Successful aneutronic fusion would greatly reduce problems associated with neutron radiation such as ionizing damage, neutron activation and requirements for biological shielding, remote handling and safety. Aneutronic fusion is any form of fusion power in which the majority of the energy released is carried by charged particles. While the lowest-threshold nuclear fusion reactions release up to 80% of their energy in the form of (uncharged) neutrons, there also exist reactions in which the energy is released in the form of charged particles, typically protons or alpha particles. Successful aneutronic fusion would greatly reduce problems associated with neutron radiation such as ionizing damage, neutron activation and requirements for biological shielding, remote handling and safety. Since it is simpler to convert the energy of charged particles into electrical power than it is to convert energy from uncharged particles, an aneutronic reaction would be attractive for power systems. Some proponents see a potential for dramatic cost reductions by converting energy directly to electricity, as well as in eliminating the radiation from neutrons, which are difficult to shield against. However, the conditions required to harness aneutronic fusion are much more extreme than those required for the conventional deuterium–tritium (D-T) nuclear fuel cycle. Fusion reactions can be categorized by the neutronicity of the reaction, the fraction of the fusion energy released as neutrons. This is an important indicator of the magnitude of the problems associated with neutrons like radiation damage, biological shielding, remote handling, and safety. The State of New Jersey has defined an aneutronic reaction as one in which neutrons carry no more than 1% of the total released energy, although many papers on aneutronic fusion include reactions that do not meet this criterion. Multiple fusion reactions have no neutrons as products on any of their branches. Those with the largest cross sections are these: The difficulty of a fusion reaction is characterized by the ignition barrier, the energy required for the nuclei to overcome their mutual Coulomb repulsion, while the rate of reaction is proportional to the nuclear cross section ('σ'). In a self-sustaining reaction, the rate of reaction is high enough to maintain the temperature above the ignition barrier. Any given fusion device has a maximum plasma pressure it can sustain, and an economical device would always operate near this maximum. Given this pressure, the largest fusion output is obtained when the temperature is chosen so that <σv>/T2 is a maximum. This is also the temperature at which the value of the triple product nTτ required for ignition is a minimum, since that required value is inversely proportional to <σv>/T2 (see Lawson criterion). (A plasma is 'ignited' if the fusion reactions produce enough power to maintain the temperature without external heating.) Because of the higher atomic number (and hence higher charge) of the reacting species, and the resulting higher Coulomb barrier, the aneutronic reactions are more difficult than conventional D-T fusion, and hence require higher temperatures. The table below shows the ignition temperature and cross section for three of the candidate aneutronic reactions, compared to the simple D-T reaction As can be seen, the easiest to ignite of the aneutronic reactions, D-3He, has an ignition temperature over four times as high as that of the D-T reaction, and correspondingly lower cross sections, while the p-11B reaction is nearly ten times more difficult to ignite. Although the deuterium reactions (deuterium + helium-3 and deuterium + lithium-6) do not in themselves release neutrons, in a fusion reactor the plasma would also produce D-D side reactions that result in reaction product of helium-3 plus a neutron. Although neutron production can be minimized by running a plasma reaction hot and deuterium-lean, the fraction of energy released as neutrons is probably several percent, so that these fuel cycles, although neutron-poor, do not meet the 1% threshold. See Helium-3. The D-3He reaction also suffers from the 3He fuel availability problem, as discussed below.