ther ‧ mo ‧ nu ‧ cle ‧ ar /ˌθɜːməʊˈnjuːkliə◂ $ ˌθɜːrmoʊˈnuːkliər◂/ BrE AmE adjective
thermonuclear weapons use a ↑ nuclear reaction, involving the splitting of atoms, to produce very high temperatures and a very powerful explosion:
a thermonuclear device