This is actually backwards. Fusion weapons are substantially higher yield because they result in more fission, partly by preventing the fission primary from blowing itself up before it has finished.
Wikipedia: "Fast fission of the tamper and radiation case is the main contribution to the total yield and is the dominant process that produces radioactive fission product fallout."
Most of the fission energy in an H-bomb comes from the massive amounts of neutrons created by the fusion bomb initiating a fission reaction in the U-238 tamper of the secondary. The primary is used for its X-Rays, which cause the incredible pressures within the secondary by ablating the surface of the cylinder. When you look at this experiment and see it uses x-rays to ablate case containing the hydrogen, causing an implosion, the purpose of the experiment is clear.
It varies quite a bit by design, apparently the USSR’s initial design was only 15-20% fusion while US designs where closer to 50% which is still apparently the most efficient option in terms of warhead size.
However it’s possible to have higher fusion ratios at the expense of a larger device for the same yield. Most notably in the case of the Tsar Bomba’s which reduced the contribution of fission and too massively reduce the amount of fallout produced.
Almost all nuclear weapons rely heavily on fission of the tamper for yield.
Suggest "Ripple: An Investigation of the World’s Most Advanced
High-Yield Thermonuclear Weapon Design" from the Journal of Cold War studies to read about a predominantly fusion device family.
This seems to say to me that D-T reactions produce neutrons, and that the kinetic energy of the neutrons is smaller than what you get by hitting U with that neutron. You already have the energy from the neutron (which will land somewhere in the system eventually), and you might as well get a multiplier by putting a blanket of U-238 in front of it.
That could be carbon-copied to a fusion power plant, and indeed, there are many proposals of hybrid fusion-fission plants in the literature that only require Q values marginally greater than 1. But if you go that route, you have radiation just like a fission plant, and one starts to question why you don't just build a fission plant (indeed, why don't we?).
My personal pet theory of the future is that, one day, we'll progress so far in fusion research that we get economic energy. But at the same time, the line blurs between both fission and weapons technology, so people are unhappy with the result. This doesn't feel particularly contrarian but no one ever seems to bring it up.
Since you asked: We don't build fission plants because they cost more than every other energy source. Fusion plants, if they could ever be made to work at all, would cost a lot more. So, there won't be any.
