It's very high, but not as bad as it sounds. There is less than 1 gram of fuel in the reactor at any given time, so it's extremely diffuse, and mostly contained by magnetic fields. Much of the energy can also be radiated away by facilitating ion-electron recombination before the plasma reaches any materials.

The current design for ITER, operating at 150 million Kelvin, focuses the exhaust plasma onto a dedicated divertor surface with an estimated peak heat flux of 20 MW/m2. Experiments show that tungsten handles this fairly well, although with some cracks appearing after many cycles, but this is an active area of research.