alt Hacker NewsI guess you ask why they are using water at ambient temperature (20°C; 68°F) instead of very cold water (0°C; 32°F). Some reasons I can think now:
They are using a lot of water, as most as possible, from pipes at whatever temperature it is. There are no enough mobile refrigerators, not enough electricity to make them work, and it's very hard to transport cold water or ice if you don't use the pipes.
Also, the center of the tank is hot and reacting, but the external part is a nasty block if plastic that acts like a shield and isolate it from the cold water outside.
This is a common problems in big chemical plants when you have exothermic reactions. It's not enough to cold it down, you need to ensure all parts are cold down.
For comparison, there is a nice video by NileRed https://www.youtube.com/watch?v=phNLecfyWS8 He is making Bakelite that is a type of plastic. It's a tiny amount, in a lab, on purpose and he may make a few attempts. Anyway it overheat and instead of a nice piece of plastic he got a nasty block of foam with burned plastic. No imagine a huge tank of a similar chemistry reaction.
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Ah, that makes sense. It's too bad they can't drill into it to relieve pressure without destroying the integrity of the tank (not that I'd want to be anywhere close to it either).
If they didn't have to worry about it imminently exploding I wonder if they could somehow wrap it with reinforcement (e.g., wrap some high strength metal around the tank to prevent it from deforming when drilled into) and then drill into it to extract the liquid?
One of my other less serious ideas was to helilift a Chernobyl style containment structure around it, but I imagine they don't have one of those just sitting around waiting to be used.
The difference in cooling potential between cold water and water at ambient temperature is minimal. Cooling with water primarily comes from phase change or heat exchange; both can move vastly more heat than a small difference in temperature.
Chilling the water would massively complicate the logistics with a very marginal improvement in heat removal.