Nuclear fusion reactor. An engineer welds compon- ents on the experimental tokamak nuclear fusion reactor at JET (Joint European Torus). The JET tokamak is the world's biggest nuclear fusion machine. It consists of a large, doughnut-shaped vacuum vessel, in which a minute quantity of hy- drogen is super-heated to over 100 million degrees (10 times hotter than the centre of the Sun). In the resulting plasma", electrons are torn apart from atomic nuclei, and some of the nuclei collide and fuse, releasing fusion energy. Problems with impurities in the plasma mean that nuclear fusion technology is yet to get beyond the experimental stage. JET is based in Culham, UK."