In nuclear fusion the atoms are in a hot plasma and the temperature is >5000K for them to posses kinetic energy and overcome repulsion to fuse , this process generates some heat(which is equal to the mass defect).But how do we actually extract that given heat if the surroundings are already so hot ?
Most of the energy release in a fusion reaction is in the form of the kinetic energy of the reaction products plus a bit more in the form of radiative energy carried off by gamma rays. the kinetic energy of the reaction products is captured as heat in the part of the reactor known as the first wall, when the fast-moving products (helium nuclei) collide with the stuff from which the first wall is made (graphite, for example). The gamma rays also collide with the first wall and through a series of successive scattering events with nuclei inside the first wall they impart a bit more heat energy to the first wall as well. The heat from the first wall is then captured by heat transfer pipes that carry the heat out of the reactor and into a steam generator.
The plasma in which the fusion reactions occur must be confined in such a way that the plasma never comes into contact with the first wall, otherwise the fusion process will be immediately quenched and the reactor will shut itself down.
Since there is no material known which can stand up to fusion temperatures, the plasma confinement must be done either with magnetic fields or via inertial confinement.
Inertial confinement is the process in which the input energy required to trigger the fusion reaction is dumped into the fusion pellet so fast that the inertia of the pellet itself prevents it from escaping the reactor until the fusion process is finished. At that point, the input energy dump is over too which means the inertial confinement ceases and the energy (fast-moving helium nucleus plus gammas) streams freely out towards the first wall.
In a nuclear fusion reactor, the heat generated by the fusion reactions is typically extracted through a combination of convection and conduction. The plasma in the reactor is contained within a magnetic field, which acts as a barrier to prevent the plasma from coming into direct contact with the walls of the reactor. This magnetic barrier also helps to maintain the high temperatures required for fusion to occur.
The heat generated by the plasma is transferred to the walls of the reactor through conduction, which occurs as the energy is transferred from the hot plasma to the colder wall material. To extract this heat, the wall material is usually made of a heat-resistant material such as graphite, which can withstand high temperatures without melting or degrading. The heat absorbed by the wall material is then transferred to a coolant, typically helium or hydrogen, which circulates around the outside of the wall.
The coolant then carries the heat away from the reactor to a heat exchanger, where it is used to generate steam and drive a turbine, producing electricity. The coolant is then cooled and returned to the reactor to absorb more heat.
In summary, the heat generated by the fusion reactions in a nuclear fusion reactor is extracted through a combination of conduction and convection, using a heat-resistant material and a coolant to transfer the heat from the plasma to a heat exchanger where it can be converted into electricity.