> During such a quench, the liquid helium in the magnet warms up and turns into a gas that is recovered by the cryogenic system to be re-liquified, ready to cool down the magnets again.
They're capturing the helium, so it isn't released usually. Not sure what they do with the nitrogen, I'd guess that is released as it is much cheaper. But in any case the amounts involved in such a quench are only a danger in the immediate vicinity, not outside. So if you're in the same room as a quenching magnet and the room isn't sufficiently sized or ventilated, there could be danger. But that's about the limit to it.
Took a tour in local university physics lab dealing with helium. My takeaway is that liquid nitrogen is essentially free. And used to protect or keep helium parts colder. So anyway in process you have so much of it that simply having it boil away or be discarded is not big deal.
A few years ago, while doing a physics lab for students in the university, we needed ice and liquid nitrogen. It was weird that is was easier to get some liquid nitrogen[1] than ice cubes[2]. IIRC we asked for the official price of liquid nitrogen, and it was similar to the price of soft drink like coke.
[1] Because it was used regularly in a few of the research labs. Just get a dewar flask that is not made of glass, and ask nicely.
[2] No refrigerator in the lab, the cafeteria didn't have any, most refrigerators had no ice cubes or a huge ice block where there use to be the ice cubes shelf.
It's comparatively free; the air is nearly 80% nitrogen. The equipment to produce liquid nitrogen has a cost and uses a fair amount of electricity. Labs that use a lot of liquid nitrogen may produce their own, but often it's produced industrially in large quantities and trucked to customer sites.
It has a lot of uses, from dermatology to machine shops.
They're capturing the helium, so it isn't released usually. Not sure what they do with the nitrogen, I'd guess that is released as it is much cheaper. But in any case the amounts involved in such a quench are only a danger in the immediate vicinity, not outside. So if you're in the same room as a quenching magnet and the room isn't sufficiently sized or ventilated, there could be danger. But that's about the limit to it.