Highest temperature fission reactors are the gas core (vapor core) and ion core reactors. They are fluid-fuel reactors, but not demonstrated. Currently solid-fu

Highest temperature fission reactors are the gas core (vapor core) and ion core reactors. They are fluid-fuel reactors, but not demonstrated. Currently solid-fuel reactors hold the record for highest temperatures. Fusion reactor runs hotter and again fuel is in fluid state.(Fluid: Liquid, gas & plasma.). But only runs for 10-100 minutes. Demonstrated fission reactors run for thousands of hours continuously.