system
Given the following post, respond as if you're commenting on Reddit:
human
Are the terms "nuclear" and "thermonuclear" considered interchangeable when talking about things like weapons or energy generating plants or the like?

If not, what are the differences?
gpt
No, they're not interchangeable. 

"Thermonuclear" refers to nuclear reactions occurring in an environment where the temperature is very high (think millions of Kelvin, at least). The term is particularly meaningful for certain kinds of reactions where both nuclei in the initial state are charged (as opposed to the case where you have at least one neutron in the initial state), because positively-charged nuclei repel each other. 

Because of that Coulomb repulsion, two charged nuclei need a fairly high relative kinetic energy in order to have any chance of reacting with each other. This can be done either by accelerating particles to these energies using an accelerator/making use of particles which are produced at high enough energies, *or* by creating extremely high temperatures such that the kinetic energies of the particles in their random thermal motion is high enough. The latter is what's referred to as "thermonuclear". 

So this term would apply to the reactions that happen in stars and other astrophysical processes, in fusion reactors, and to nuclear weapons which make use of light charged particle fusion reactions. In all of these cases, the temperatures are very high compared to what humans normally experience, corresponding to average kinetic energies at least on the order of around 1 keV, which allows some of the charged nuclei in the plasma to react with each other. (Even if they don't have enough energy to overcome the Coulomb barrier classically, they can still tunnel through, and the tunneling rate increases strongly with temperature.)

So when you're using a particle accelerator or radioactive source to initiate nuclear reactions, you wouldn't call that "thermonuclear". Or for neutron-induced reactions like the ones occurring in a fission reactor, would not be called "thermonuclear". But the high-temperature plasmas in stars and supernovae, in fusion reactors, and in modern nuclear weapon designs are all referred to as "thermonuclear".