JET is a 1980s research project into how to contain a plasma and squish it enough to achieve fusion at all, not a practical generator. It doesn't have superconducting magnets, it runs on big lumps of copper, with limited cooling and no way of recovering output power.
In the latest run, they achieved fusion and kept it running until JET ran out of power for its magnets. The plasma didn't touch the reactor walls, didn't destabilize, didn't crap out in any of the numerous other ways. They had to shut it down before it overheated and burned out.
(I've been reading reports for a while about the use of massive amounts of computer power -- and deep learning -- being necessarily to stabilize plasma in a reactor. That seems to be what this demonstrates.)
This is probably JET's swan song. They've pushed it as far as it can go; also, the D-T reaction produces surplus neutrons so tends to generate radioactive waste via secondary irradiation of the reactor core -- which makes it hard to get inside and fiddle with it.