With the progress in controlled thermonuclear fusion research, experiments having dimensions approaching those of future fusion reactors and discharge times of the order of 100 to 1000 s have become a necessity. Parameters close to those of ignited plasma are anticipated or achieved in today's four large tokamaks TFTR (Princeton), JET (Culham), JT-60 (Naka-machi) and T-l 5 (Moscow); the burning of a fusion plasma is to be investigated in the next generation of experiments. In these large experiments the load from the plasma on the surrounding vessel walls is considerably increased in duration and intensity compared to that of most previous plasma experiments. The wall load is approaching the values foreseen in a fusion reactor and the lifetime of the wall components represents a major physical and engineering problem.