It depends where on the "fundamental science <-> engineering" spectrum you think materials science sits. Some of the thorniest problems in reactor design are "

It depends where on the "fundamental science <-> engineering" spectrum you think materials science sits. Some of the thorniest problems in reactor design are "wtf do you make it out of and can such a thing exist". There's some science going on confirming material theoretical properties under reactor conditions. Mind you, insufficient compute is also a problem. Although "insufficient" is probably underselling the scale of the issue. Simulating plasma is horrid. It's all the worst bits of both fluid dynamics and electromagnetics all rolled into one: you've got a potentially chaotic fluid where all the particles aren't just interacting with the things they bounce off, they're also interacting with other particles all throughout the body of the fluid through electromagnetic effects. Everything affects everything else, and (because chaos) the details matter.