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?

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    $\begingroup$ @Lubos, I agree that that part of the question was redundant, but GW is not at all an absurd. $\endgroup$
    – user68
    Mar 28, 2011 at 8:18
  • $\begingroup$ This question is about the risks to real people from real situations in the real world. And questions on nuclear reactions and their byproducts are firmly within the domain of inquiry of physics. The question itself seems a little ill-posed but to attack it for something which it didn't even mention ("climate change") is ridiculous. $\endgroup$
    – user346
    Mar 29, 2011 at 8:47
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    $\begingroup$ @deepak please be aware that the first two comments were made prior to an edit to the question. @all I don't believe that we need an off topic discussion such as this on physics. Consider creating a chat room of you must hash this issue out. $\endgroup$
    – Adam Davis
    Mar 29, 2011 at 16:50
  • $\begingroup$ @Adam thanks for pointing that out. The parts you edited out, however, make perfect sense and did not represent any sort of proselytizing any more than saying that slippery surfaces are slippery. So as @mbq noted that part of the question was redundant at best but certainly not incorrect in any form. $\endgroup$
    – user346
    Mar 29, 2011 at 20:45
  • $\begingroup$ @Deepak If you feel that the removed portion of the post is necessary in order to discuss the inherent dangers in operating a nuclear reactor, you are free to roll it back if your reputation allows, suggest an edit if not, or even bring it to meta and have the community hash it out. I offered the edit in order to make certain that off topic discussions such as those forming in the comments above are kept to a minimum. Even if such topics were allowed on Physics, they should be contained in a separate post, and not mixed in this one. $\endgroup$
    – Adam Davis
    Mar 29, 2011 at 22:34

6 Answers 6


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 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.

  • $\begingroup$ According to ITER the temperature needed to start and sustain thermonuclear reaction on earth is 10 times that of the temperature required to do the same at the core of the sun. This temperature is 150 million degrees celsius and while it is clear that this temperature will be localized to an extremely small area it is still a huge temperature and the energy required to achieve and sustain this temperature must be just enormous... $\endgroup$
    – Dean Kuga
    Mar 24, 2011 at 22:22
  • $\begingroup$ @kzen: The energy is high, but it is supported in a lot of mass. Look at the opperating parameter of various testbed reactors: en.wikipedia.org/wiki/Tokamak_Fusion_Test_Reactor en.wikipedia.org/wiki/National_Spherical_Torus_Experiment. Explosive release of that energy not in and of itself a widespread threat. $\endgroup$ Mar 24, 2011 at 22:41
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    $\begingroup$ @kzen the mass of plasma at that temperature is very small and would cool very quickly as soon as it hit air or the walls. Think of it like a sparkler - the sparks are at 1500C but contain so little mass they don't do any damage. $\endgroup$ Mar 25, 2011 at 2:44
  • $\begingroup$ @MartinBeckett: Think also of aluminium foil, which you can touch immediately when it comes out of the oven (provided that it is dry and not touching other materials close to where you touch it). $\endgroup$
    – Cerberus
    Mar 28, 2011 at 13:28
  • $\begingroup$ @Cerberus - that's a little different, the Al foil doesn't heat up. If you heated the same mass of Al to 10^6K it wouldn't be recommended to touch it. $\endgroup$ Mar 28, 2011 at 15:42

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 .

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    $\begingroup$ It's not free. Investment costs are huge, which makes this kind of electricity not cheap at all. $\endgroup$
    – gerrit
    Feb 25, 2013 at 14:11
  • $\begingroup$ "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" A car may crash and burn its passengers to death. Why isn't it possible for the fusion to spread to available fuel, or for the amount of injected fuel to be much too large and cause an unexpectedly fierce reaction? The latter I could imagine may be due to energy constraints to "heat" all the fuel to critical, but h-bombs/fires do maintain a fusion/ordinary chain reaction. They're infinitely scalable by adding hydrogen/fuel. $\endgroup$
    – Arc
    Jul 9, 2015 at 3:36
  • $\begingroup$ @Archimedix because the reactors are designed that way. As with the car combustion and gas container are well separated and there is a small feeding tube. In contrast to gas, which can inflame at relatively low temperatures to the fusion temperatures, the fuel for the reactor is very much colder than the temperatures of the plasma, and if the plasma region would explode ( by a meteorite falling on the reactor) the whole would cool immediately because there is not much mass in that plasma, and the plasma would never reach the fuel containers. H2bombs start by an implosion within the hydrogen $\endgroup$
    – anna v
    Jul 9, 2015 at 3:42
  • $\begingroup$ container. If an infinite container could be made, yes there is no limit, but even there, when the hydrogen in the container is fused , thats it. $\endgroup$
    – anna v
    Jul 9, 2015 at 3:44

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.


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.


Website with some information and pictures of working prototypes of "Electric fusion" reactors.


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.

  • $\begingroup$ Be of good cheer. By the time that happens everybody will be in the upper middle class range, having 1 child per woman on average, and in a few generations maybe a few hundred thousand will be left to pollute with their excesses in energy. $\endgroup$
    – anna v
    Mar 25, 2011 at 15:00
  • $\begingroup$ Yes, in countries whre the regulation of room temperature is done by windows, energy is wasted as produced. This is very similar to the American squatter thinking. Next valley, new trees to burn, new neighbors, new cattle to steal. $\endgroup$
    – Georg
    Mar 25, 2011 at 18:13
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    $\begingroup$ @BarsMonster And do you think your prophecies are more than dreams? Nightmares are dreams also. The belief that people can kill a planet is actually the same belief as a small baby has, who shuts its eyes and believes the world has disappeared. Very egocentric projected to all of humanity. People can kill people, and it can happen that some madmen will start a nuclear holocaust. The biosphere is a gnat on the back of an elephant. The planet will recover fast, in geological times. Even the biosphere. $\endgroup$
    – anna v
    Mar 28, 2011 at 6:02
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    $\begingroup$ I cannot agree with your comparison to a child who shuts it's eyes. Look at the world today - coal electricity generation plants are not prohibited, while being the most dirty way of producing energy. In fact, we build them as many as we want & as many as we can. Same will happen to thermonuclear energy. Noone would be able to limit their usage. Surely, earth will recover, in geological times, but I cannot say that thermonuclear energy promises any additional safety to the human kind. $\endgroup$ Mar 28, 2011 at 10:43
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    $\begingroup$ @annav Although humans can not destroy the planet, they can and presently do effect another mass extinction event, and it's not impossible humanity will be among the species affected. The Earth is our home, we need to keep it liveable. $\endgroup$
    – gerrit
    Feb 25, 2013 at 14:14

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