Descriptions of and references to fusion energy are almost always accompanied with something to the effect of “limitless clean energy!”
If we’ve never really gotten it to work in a manner that produces more power than it consumes, how do we know this is true?
We know the laws of physics. And they tell us it’s not precisely clean. The neutron radiation turns the reactor parts themselves radioactive.
Unless you’re talking about frauds. They know it’s clean because nothing is quite clean.
The reason fusion power has not been achieved yet is due to technical reasons. The physics and chemistry (and the expected byproducts) behind fusion are well understood.
Fusion is clean in that, other than producing neutrons, alphas and a small amount of other products, it doesn’t leave a significant amount of radioactive fuel residue or produce chemical pollution. On the other hand, as naita notes, most fusion chains will produce highly energetic neutrons which can both activate other elements (convert them from stable nuclear forms to unstable isotopes that will decay and release additional radiation) and will embrittle most common structural materials. The former isn’t necessarily a bad thing insofar as it can be used to both create or burn up other radioactive materials that can be used as fuel or for other purposes. The latter is a major challenge in terms of design of a commercially useable reactor, e.g. you don’t want to spend hundreds of millions of dollars building a reactor that you have to dispose of in a few weeks because it is now falling apart. However, compared to burring fossil fuels or dealing with the residual waste of the nuclear fission fuel cycle (which is not only the exhausted fuel but also all of the intermediate waste produced in processing raw ore and yellowcake into useable or enriched fuel) most hypothesized nuclear fusion cycles have a negligible amount of waste products to dispose of.
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There is such a thing as aneutronic fusion. It isn’t completely aneutronic, but it releases no more than 1% of its energy in the form of neutrons, so it would produce far less radioactive material.
the p-[SUP]11[/SUP]B reaction, which is generally held to be the preferred aneutronic fusion reaction, has a Lawson criterion of two and a half orders of magnitude higher than the baseline D-T fusion and an power density of over three orders of magnitude less. In other words, unless there is some way to produce this reaction with a massive reduction in effort, it is way harder to achieve just for the sake of eliminating neutron emission.
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