[evan_au (OP)]

Some people are promoting nuclear power using the reaction:
H¹ + B¹¹ → C¹² → 3He⁴ +8.6MeV

Would you call this "fission" or "fusion"?

See, for example: http://www.ialtenergy.com/lawrenceville-plasma-physics.html

[alancalverd]

Since the nuclide products are heavier than the inputs, it must be fusion.

[chiralSPO]

This equation doesn't sit right with me. Why would ¹²C decompose into 3 ⁴He? ¹²C is very stable, and stars release energy when fusing ⁴He together to form ¹²C.

Without looking up the numbers, it would appear then that the fusion of ¹H and ¹¹B would have to be the exothermic step, and the fission of ¹²C would be endothermic. Would it not make more sense to find a way to stop the second step from happening so that more energy can be harvested? If the second step were exothermic, wouldn't we already have developed ¹²C fission reactors?

EDIT: I suppose that the ¹²C formed could be in a very excited state that would facilitate decay...

[alancalverd]

Some confusion here. Promising reactions are, apparently,

Proton–boron fusion    ¹p + ¹¹B    →    3⁴He    + 8.7 MeV
Proton–nitrogen fusion    ¹p + ¹⁵N    →     ¹²C + ⁴He    + 5.0 MeV[2]

so it is possble that there is an intermediate metastable ^12C in the first reaction.

[chiralSPO]

As far as the question of fission of fusion goes, I think it would be most accurate to call it a nuclear reaction that is neither (or both). I don't know if there are agreed upon technical terms for other types of reactions, but I would imagine there are.

By analogy to chemical reaction classifications, fusion ≈ synthesis, fission ≈ decomposition, and the reaction discussed here seems more akin to substitution (really like an addition-fragmentation, which is not very common)

[evan_au (OP)]

the fusion of 1H and 11B would have to be the exothermic step

This reaction has a phenomenally high activation energy, requiring energies around 300keV (temperatures well over a billion degrees absolute), an order of magnitude more than some Hydrogen fusion options.

Temperature is a property of an ensemble of particles, so you can't really measure the temperature of a single C¹² particle, even one that is mashed together out of two incoming energetic particles.

But three He⁴ nuclei emerging at high velocity from the reaction is a clear way to deliver thermal energy released by the reaction.

It is the need for at least two reaction products that causes the problems with most candidates for nuclear fusion, because one of these products is often a high-energy neutron that is difficult to shield, and makes the surrounding structures radioactive.

[alancalverd]

Generating 300 keV protons is a doddle - you can do it with an off-the-shelf power supply.