A fusion reactor tries to harness the energy that was explosive in the Hydrogen bomb.
The question of danger then goes as follows:
Is it possible for ITER to turn into a hydrogen bomb?
The answer for radiation catastrophies in a large area is no, it cannot even remain highly radiative in the sense that the Japanese reactors are now and cannot be controlled except by time and cooling.
The same reason that makes building a fusion engine so hard,it is over 50 years when the stellatron was being discussed, is the reason that makes it safe for the larger environment to have a fusion reactor in the vicinity. They needed an atomic bomb to trigger the hydrogen bomb. ITER is generating a plasma in a tokamak and a plasma is something that has to be nursed and is destroyed if disrupted. In addition the feeding of new fuel is done on the same principles as feeding gas to a car engine, incrementally. There is no way the unburned fuel will become critical.
Locally, as others have said, walls and metals will become radioactive and if an explosion happens for some unforeseen reason, war, terrorism etc, the debris will be local. No iodine and cesium etc byproducts in bulk to be sent to the atmosphere since the plasma has very little mass.
That is why countries are spending resources to support ITER. It is the ultimate free clean energy .