A molten salt reactor is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Molten salt reactors are considered safer than conventional reactors because they operate with fuel already in a molten state, and in event of an emergency, the fuel mixture is designed to drain from the core where it will solidify, preventing the type of nuclear meltdown and associated hydrogen explosions (like in Fukushima) that are at risk in conventional (solid-fuel) reactors.