If your supply of Deuterium and Tritium was jammed wide open you, would stop the fusion (either by switching of the heating if you are in startup or by messing

If your supply of Deuterium and Tritium was jammed wide open you, would stop the fusion (either by switching of the heating if you are in startup or by messing with the magnetic confinement to make the plasma hit the diverter). At that point all you do is pumping low density, relatively low radioactivity gases into a large gas proof vacuum vessel. No explosion, no meltdown, no release of radioactivity to the containment building let along the environment. As far as worst case scenarios go it is pretty harmless. The entire ITER site will never store more than 4kg of tritium, most in the form of metal-hydrites.