The answer is one of the key advantages of fusion as a potential energy source over nuclear fission power stations – its inherent safety. Although the plasma in a tokamak is extremely hot, it is at low pressure, and so its total heat energy is not large – there is only a gram or so of fuel present.
Plasma is kept away from the vessel containing it by the use of magnetic fields. The magnetically confined fusion plasma can only operate under strict conditions. If any of the systems fail (such as the confining toroidal magnetic field) or if, by accident, too much fuel is put into the plasma, the plasma will naturally terminate (what we call “disrupt”) – losing its energy very quickly and extinguishing before any sustained damage is done to the structure. There is no concept of ‘meltdown’ in a fusion reactor.