[zkms]

The irreplaceability of the ice cores is why in my opinion, an independent backup refrigeration system (without common points of failure) is a necessity, along with an independent system that can isolate the primary chillers and activate the backup system should the primary chillers misbehave.

An emergency refrigeration system that works with consumable cryogenic material (say, dry ice or liquid nitrogen https://news.ycombinator.com/item?id=14127835) would be totally feasible and utterly inexpensive compared to the tragic costs of these cores melting.

I don't know the state of readiness of phase-change materials with appropriate transition temperatures (would need to be lower than -40 C) but if such a material exists, it'd work as well.

[zkms]

So I just learned my suggestion of using LN2/CO2 in an emergency refrigeration system isn't something I came up with -- LN2/CO2-powered backup refrigeration is in fact a standard option on ultra-low temperature freezers (the kind that get used to store biological samples and stuff):

https://www.thermofisher.com/order/catalog/product/6214 http://www.helmerinc.com/Assets/helmer/knowledge-center/manu...

Given that it's commercially-available on freezers that get used to store critical biological material, there is no reason that a sample vault that stores irreplaceable ice cores can't have a similar, scaled-up version of this as well.