I 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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jandrewrogers 21 hours ago | parent | next [-]

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.

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foota 16 hours ago | parent [-]

Heat exchange is proportional to the difference in temperature though (in reference to your "or heat exchange"). Colder water would cool faster. The tank isn't at boiling either, so it's not like you'll be able to phase change away a bunch of energy. I guess you'll still get some evaporative cooling, but there's a limit to how much you'll get just from the ambient temperature (the exterior of the tank is relatively cool, presumably because the "gummed up" interior is inhibiting heat transfer)

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jandrewrogers 7 hours ago | parent [-]

The increase in heat removal rate for a 20K temperature difference is quite small unless the source and sink have a similar temperature. This emergency exists because they are not even close to being similar temperatures. Evaporative (phase change) cooling does most of the heavy lifting and is very efficient.

It is the same reason sweating cools the body much more effectively in low humidity than high humidity environments regardless of the temperature when you drink it. Attributing how drinking water cools the body via various effects is a classic introductory thermodynamics exercise.

	▲	foota 3 hours ago \| parent [-]
		I think you're misunderstanding the tank's temperature. The internal temperature is 32C, the external temperature is only 16C (they used thermal cameras to measure the external temp, but when they got closer they read the internal gauge which showed 32C). Groundwater in LA is apparently right around 16C, so the difference in temperature between the tank and the water would be basically 0. So while I do agree that the evaporative cooling is probably doing most of the heavy lifting (in fact, necessarily so if the water temp ~= the tank's exterior temp), it's not unreasonable to suggest that using colder water would be more effective. In fact, the wet bulb temperature there is apparently right around 16C, so they've basically cooled the outside of the tank as much as they can using evaporative cooling alone. They can certainly use it to _keep_ it there, but without something else they wouldn't be able to cool it further. Presumably if there's an exothermic reaction happening internally then the core will continue to rise in temperature based on the temperature gradient through the material forming in the tank. I would assume (since my understanding is that it's some kind of plastic) that it has a fairly low thermal conductivity, so the core temperature will continue to rise as more of it turns to plastic even as the outside is cooled to the same ~16C. In the limit if they were able to immerse it completely in very cold water (~0C) then the exterior of the tank would also be ~0C (supposing they were able to access sufficient quantities of water). I don't think that's practical of course, but again I don't think it's unreasonable to suggest that cold water would make a meaningful difference.

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foota a day ago | parent | prev [-]

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.

	▲	tolciho 17 hours ago \| parent [-]
		Wrapping metal around it sounds like one of those mythbusters episodes where they did not get as much shrapnel as they wanted.