GTOM said:
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.
