Is there any inherent danger in operating a Thermonuclear Reactor? Nuclear fusion research appears to promise sustainable, practically unlimited energy, and according to ITER it will be a perfectly safe, non-polluting source of energy.
My question is if there are any plausible scenarios under which operating a thermonuclear reactor could cause a major environmental disaster?
"Products of the fusion process are Helium, which is inert and harmless, and neutrons, which will lodge in the vessel walls and produce heat and activation of materials." This obviously assumes normal operating conditions, but what would happen if some man-made or natural catastrophe was to impact the reactor and create abnormal operating conditions?
 A: A heck of a lot of energy in one place always represents a local danger.
The question of a more wide spread danger depends on a lot of details. Does the core reaction produce neutrons? How bad is the activation rate in the plant? Would a disaster event spread activated material over a wide area?
In the absence of data I would guess "less dangerous than fission", but it is just a guess.
A: 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 .
A: As far as a catastrophic plasma disruption is concerned, it might be a problem for the first wall, and hence the ability to use the reactor. But despite the high energy per nucleon, the density of the plasma is extremely low, the total energy is dominated by the energy in the magnetic fields, and that's not tremendous.
Of course you do have the radioactivity produced by the absorption of high energy neutrons. In some hybrid schemes these neutrons are to be absorbed by fissile material, which multiplies the net energy per fusion by a factor of about 10. Once you actually produce a decent amount of fusion in a thermo reactor, then its environs become radioactive and are no longer easy to work in.
A: You need an aneutronic energy cycle to make fusion secure. Examples: p+B11, D+He3. TriAlpha is a running commercial project aimed on p+B11 fusion.
The main myth that the thermonuclear lobby spreads is that a thermonuclear reactor produces clean energy, that is, it does not create radioactive contamination. A deuterium-tritium mixture reactor will produce radioactive waste in the form of spent reactor structural elements, which will need to be replaced periodically. In addition, tritium itself is radioactive. Leaks are almost inevitable.
I am afraid that the development of solar and wind energy will put an end to thermonuclear energy even before its birth.
A: The main risk of thermonuclear energy is unlimited production.
If humanity is not limited by fuel supply or production capacity, energy production will rise exponencially untill this would change earth climate directly, instead of indirect change due to CO2.
We'll just start having 10kW CPUs, personal jets and oasises at north pole.
A: Should google go nuclear.
You should all watch that video. It is EXTREMELY informative. It completely trounces the tokamak style reactor. It also mentions that there are different combinations of hydrogen atoms that could be utilized to produce slightly less power output, but have zero radiation.
Yes, it is far less dangerous than fission. Yes it produces neutrons, but as I mentioned, using a different type of hydrogen drastically reduces the energy (and thus damage potential) of those neutrons. Neither fusion or fission reactors could ever "explode" like an atomic bomb.
The problem with fission is that they use a physical core of uranium or plutonium. Fusion is done with only atoms. The fissile material undergoes a chain reaction, and is submerged in water to generate heat. Once the chain reaction begins, you can only stop it by inserting graphite rods to absorb the excess neutrons. All of the major nuclear incidents, Three Mile Island, Chernobyl, and now Japa, have involved the rods not being inserted, and the core somehow gets exposed to the air, and radiation pours out. Chernobyl for example, was a explosion caused by steam that blew the reactor core apart, exposing the radioactive material inside. At this point it becomes far to hot and radioactive to get anywhere near it. The crisis in Japan is the first time it has been the result of some catastrophic event. The others were human error. This could obviously never happen with fusion. If there was a disaster, the reactor would just shut off.
http://emc2fusion.org/
Website with some information and pictures of working prototypes of "Electric fusion" reactors. 
