Fusion

If hydrogen fusion (including deuterium and tritium) was easy, we wouldn't be here. Very small masses of hydrogen would have "burned" to iron very early in the formation of our galaxy; not enough time for complex life to form.

"Cold Fusion" aka "Low Energy Nuclear Reactions" assumes that deuterium fusion is easy - that is, it can happen with electron-chemistry energy levels (electron-volts) and solid-matter atomic spacings (nanometers) rather than the 300 KeV, 300 femtometer deuteron spacings encountered (for tens of nanoseconds) in 100 million Kelvin temperature fission-fusion weapons, or the vastly slower (gigayear rates) proton-proton reactions that occur in the dense 15 million Kelvin temperature cores of gigameter-diameter stars like our Sun.

It isn't the "temperature" that really matters, what matters is the particle energy and velocity. Thermal velocity in this case, particle accelerators and lasers can also make high energy particles. For a hot gas, the thermal energy of the particle is 1.5kT.

• k is Boltzmann's constant, 8.62E-5 eV/Kelvin. eV electron-volts is the appropriate energy unit for atomsn-m

So, the energy per particle in a thermonuclear blast is 1.5*8.62E-5*1E8 eV = 13 KeV, and a head-on collision of two opposite-direction particles is 26 KeV.

For comparison, the temperature of a welding torch flame is around 3500 Kelvins, about 0.5 eV, and boiling water is 370 Kelvins, about 0.05 eV.

Some fusion designs assume a mixture of naturally occuring deuterium and artificially-created tritium. Tritium has a 12 year half-life; it does not occur naturally. Just as well; in terawatt fusion-fuel quantities it is a horrendous radiological health hazard. Tritium is currently a waste product of fission nuclear reactors, and while "burning it up" might be a worthwhile improvement in fission safety, separating and purifying reactor-sourced tritium will be hazardous and less than 100% efficient. Tritium decays into helium 3 and a beta particle (electron).

Fusion reactions (if and when they happen) leak neutrons, which can be captured in a thick lithium blanket to produce tritium. However, neutron capture will have "less than unity gain", so another source of tritium is needed.

The half-life of neutrons is around 15 minutes.

How much energy is needed to fuse two deuterons?

More later.