Can a fusion reactor experience a Chernobyl-like situation?

[GTOM]

With any realistic fusion reactor designs, can you imagine a Chernobyl like situation, where they can't stop a critical reaction? Or no, since no unstable isotopes, reaction can be stopped anytime, the worst thing can happen is no power?

 

[.Scott]

With the current (and only) fusion reactor we are using for power, large amounts of H and He were assembled to a critical mass - with no hope of shutting the thing down in the near future.
I am referring to our Sun.

But short of creating a star, fusion requires a lot of stuff to go right - and anything that goes wrong would stop it.
This doesn't mean that there would be no hazards. There can be hazardous materials involved and lots of waste heat and electric power - but nothing unique to Fusion that I can thing of.

 

[rootone]

If the plasma containment failed nuclear reactions would halt instantly .
There could be blast and heat damage in the immediate vacinity of the failed reactor but not a nuclear explosion as such and very little residual radioactive contamination other than perhaps in debris of the reactor walls,

 

[GTOM]

And what if fusion is ignited by fission, a controlled chain reaction? (not a nuke) Doesnt it sound to be realistic possibility?

 

[rootone]

And what if fusion is ignited by fission, a controlled chain reaction? (not a nuke) Doesnt it sound to be realistic possibility?

No because a fusion reactor does not require any fissionable heavy elements in order to work.

 

[mfb]

If the plasma containment failed nuclear reactions would halt instantly .
There could be blast and heat damage in the immediate vacinity of the failed reactor but not a nuclear explosion as such and very little residual radioactive contamination other than perhaps in debris of the reactor walls,

The reactor itself is more than sufficient to contain the energy of the plasma. ITER will store a few hundred MJ in its plasma, or the planned fusion power of about a second. A power plant would store a bit more energy, but we are still taking about seconds, much shorter than the timescale of the cooling system. In the worst case you damage the surface of the reactor walls.

And what if fusion is ignited by fission, a controlled chain reaction? (not a nuke)

Please provide a reference for such a proposed scheme. I don't see how that would work. The opposite has been proposed, neutrons from fusion driving a subcritical nuclear reactor. In this case you would have some residual heat from radioactive fission products.

 

[GTOM]

The reactor itself is more than sufficient to contain the energy of the plasma. ITER will store a few hundred MJ in its plasma, or the planned fusion power of about a second. A power plant would store a bit more energy, but we are still taking about seconds, much shorter than the timescale of the cooling system. In the worst case you damage the surface of the reactor walls.Please provide a reference for such a proposed scheme. I don't see how that would work. The opposite has been proposed, neutrons from fusion driving a subcritical nuclear reactor. In this case you would have some residual heat from radioactive fission products.

Most articles i could found proposes what you say, use fusion for fission, and not the opposite.
I just wondered if a hydrogen bomb uses fission to ignite fusion, isn't there any reasonable plan to do it in a controlled way.

Well i found that one.
https://www.sciencedirect.com/science/article/pii/S0375960104018043

 

[rootone]

A hydrogen bomb does use one or more fission explosions as detonators to start the hydrogen fusion.
However this is far from being uncontrolled, the fission explosions have to be exactly right, and if not then the fusion reaction doesn't happen

 

[Xforce]

With the current (and only) fusion reactor we are using for power, large amounts of H and He were assembled to a critical mass - with no hope of shutting the thing down in the near future.
I am referring to our Sun.

But short of creating a star, fusion requires a lot of stuff to go right - and anything that goes wrong would stop it.
This doesn't mean that there would be no hazards. There can be hazardous materials involved and lots of waste heat and electric power - but nothing unique to Fusion that I can thing of.

But even the reactor is going normal, it releases huge amount of neutron radiation as if it runs on DT fusion..,

 

[phyzguy]

I think the worst case accident would be the release of the on-site tritium inventory. This is a lot of radioactive material. Estimates are 1-10 kg, which if I did the math correctly is on the order of 10 Giga Curies.

 

[Xforce]

I think the worst case accident would be the release of the on-site tritium inventory. This is a lot of radioactive material. Estimates are 1-10 kg, which if I did the math correctly is on the order of 10 Giga Curies.

10^10 curies of radiation does sound like a lot, and probably deadly... by the way how did you calculate it?

 

[DEvens]

With any realistic fusion reactor designs, can you imagine a Chernobyl like situation, where they can't stop a critical reaction? Or no, since no unstable isotopes, reaction can be stopped anytime, the worst thing can happen is no power?

The thing to keep in mind is that for any design currently being contemplated, the maximum amount of plasma working at any moment is less than a gram, or conceivably with a commercial scale reactor, a few grams. Say, in the less-than-10 grams sort of range. And most research reactors have a few 100 milligrams working at any given time.

There is no possibility of an uncontrolled chain reaction. It's difficult enough to get the sucker to function at all. It has no tendency to run away. If it goes out of range on just about any parameter, the first thing it tends to do is stop operating. The plasma destabilizes and touches a wall or screws up the magnetic field or some such, and it all just stops in a few milliseconds. Some designs with some accident sequences might produce some damage to the facility. But this is along the lines of scorched walls and fried transistors rather than structural damage.

The cleanup is annoying but compared to cleaning up an accident at a fission reactor, much easier. You might need to clean up a few grams of radioactive dust from the interior walls of the reactor, and maybe resurface those walls. And maybe you need to replace a circuit board here and there.

And the amount of decay heat from the fusion products is correspondingly small. A typical commercial fission reactor has tens of tons of fuel, with decay heat that starts at a few percent of full power, and slowly decreases. A fusion reactor has a few grams of fuel and the fusion products have really tiny decay heat.

So accidents involving the actual reactor are, comparatively speaking, not that dangerous due to isotopes. The dangers on-site due to things like large volumes of very low pressure, high voltage, high frequency, cryogenic temperature gasses at pressure, and similar things, are much more dangerous. And these things are engineering. Complicated and challenging but quite doable.

It was mentioned by phyzguy that the storage of Tritium is a challenge. Depending on the specific design, yes this can be an issue. Different designs will need to store a lot of fuel, or less and harvest it from the breeding blankets as needed. But yes, some designs might well have several kg of Tritium on hand, especially at commercial scale.

A fire affecting the Tritium storage facility is probably, from a radioactivity point of view, one of the worst accidents at a fusion reactor. It has the potential to be explosive, as well as releasing T2O, which then is easily absorbed by humans (and other organisms) and so can produce huge dose. However, there are very many engineering methods available for mitigating such possibilities. There are sights with several kg of Tritium stored now, and they manage to satisfy civilian regulations. So yes, it is something that needs to be dealt with, but it can be.

 

[anorlunda]

However, there are very many engineering methods available for mitigating such possibilities.

Therein lies the difficulty of convincing an untrusting public. We say, exactly the same thing about fission reactors.

One of the realities of modern life is that mistrust of authority is deep seated and widespread. The mere fact that a statement like that is authoritative in nature, makes it suspicious in the minds of many people regardless of the subject.

 

[mfb]

You can store tritium as part of various solid compounds, probably you'll find some that are fireproof as well.

 

[TeethWhitener]

I thought the plan (e.g., for ITER) was to breed tritium from lithium in situ so that there was very little tritium in the reactor at any given time.

 

[phyzguy]

I thought the plan (e.g., for ITER) was to breed tritium from lithium in situ so that there was very little tritium in the reactor at any given time.

In the reactor, yes. But there needs to be an inventory of tritium on site as it is going through the various stages of processing (extracting it from the blanket, preparing it to be injected, ...) Estimates I have seen of the on-site inventory are in the 1-10 kg range, but I don't think I can find a source for that.

 

[GTOM]

My main concern isn't the reactor core itself, but the coolant system. If the plasma only has enough power to damage it, wouldn't it result in a really bad (and more or less radioactive) steam outburst?

 

[mfb]

the coolant system. If the plasma only has enough power to damage it

It doesn't.
As magnet quenches are an issue in every system with strong magnets ITER will probably have some way to dissipate the stored energy there, too.