I went to a talk by one of the materials scientists at JET a few years ago, and he stressed that in terms of that part, the biggest challenge is the neutron bom

I went to a talk by one of the materials scientists at JET a few years ago, and he stressed that in terms of that part, the biggest challenge is the neutron bombardment of the steel reactor vessel. He was saying that the biggest problem is that current calculations mean you'd need to replace the entire highly irradiated reactor vessel after 3/4/5 years or so because of structural defects and helium bubbles forming in the cracks.