Editing Fusion power (section)

=== Deuterium ===
[[File:Deuterium Deuterium Fusion Cross Section.png|thumbnail|upright=1.7|Deuterium fusion cross section (in square meters) at different ion collision energies]]

Fusing two deuterium nuclei is the second easiest fusion reaction. The reaction has two branches that occur with nearly equal probability:

:{{nuclide|Deuterium}} + {{nuclide|Deuterium}} → {{nuclide|Tritium}} + {{nuclide|Hydrogen}}
:{{nuclide|Deuterium}} + {{nuclide|Deuterium}} → {{nuclide|Helium|3}} + {{SubatomicParticle|10neutron}}

This reaction is also common in research. The optimum energy to initiate this reaction is 15&nbsp;keV, only slightly higher than that for the D-T reaction. The first branch produces tritium, so that a D-D reactor is not tritium-free, even though it does not require an input of tritium or lithium. Unless the tritons are quickly removed, most of the tritium produced is burned in the reactor, which reduces the handling of tritium, with the disadvantage of producing more, and higher-energy, neutrons. The neutron from the second branch of the D-D reaction has an energy of only {{convert|2.45|MeV|abbr=on}}, while the neutron from the D-T reaction has an energy of {{convert|14.1|MeV|abbr=on}}, resulting in greater isotope production and material damage. When the tritons are removed quickly while allowing the <sup>3</sup>He to react, the fuel cycle is called "tritium suppressed fusion".<ref>{{Cite journal |last1=Sawan |first1=M. E. |last2=Zinkle |first2=S. J. |last3=Sheffield |first3=J. |date=2002 |title=Impact of tritium removal and He-3 recycling on structure damage parameters in a D–D fusion system |url=http://dx.doi.org/10.1016/s0920-3796(02)00104-7 |journal=Fusion Engineering and Design |volume=61–62 |pages=561–567 |doi=10.1016/s0920-3796(02)00104-7 |bibcode=2002FusED..61..561S |issn=0920-3796}}</ref> The removed tritium decays to <sup>3</sup>He with a 12.5 year half life. By recycling the <sup>3</sup>He decay product into the reactor, the fusion reactor does not require materials resistant to fast neutrons.

Assuming complete tritium burn-up, the reduction in the fraction of fusion energy carried by neutrons would be only about 18%, so that the primary advantage of the D-D fuel cycle is that tritium breeding is not required. Other advantages are independence from lithium resources and a somewhat softer neutron spectrum. The disadvantage of D-D compared to D-T is that the energy confinement time (at a given pressure) must be 30 times longer and the power produced (at a given pressure and volume) is 68 times less.{{Citation needed|date=November 2014}}

Assuming complete removal of tritium and <sup>3</sup>He recycling, only 6% of the fusion energy is carried by neutrons. The tritium-suppressed D-D fusion requires an energy confinement that is 10 times longer compared to D-T and double the plasma temperature.<ref>J. Kesner, D. Garnier, A. Hansen, M. Mauel, and L. Bromberg, ''Nucl Fusion'' 2004; 44, 193</ref>