What would be the real dangers of putting water on a nuclear meltdown?

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So I watched Chernobyl and they had some nice moments in the show but what confused me was the part where the water tanks were full and they said that if the fuel hit the water tank then it would explode again. So I tried Googling it and found some articles that says it wouldn’t have exploded and the show got it wrong. So what I’m wondering is if the scientists weren’t worried about it exploding then why was it such a huge deal to drain the water tanks? Especially since they state several times that water cools the fuel.

In: Physics

Very basically, explosions aren’t always fire, simply, they’re rapid expansions of matter, so in the show, they were worried about the extreme heat of the melting core touching a tank full of water. Now water when liquid is quite dense sitting in its container, but when it boils, it expands and becomes a gas (see where this is going?) so the scientists were worried that the massive amounts of water being touched by extremely high temperatures would instantly vaporise the water, creating a very rapid expansion of a great volume of water, creating a very large explosion. This explosion would cause even more damage to the reactor, totally damaging the other reactors and spreading radioactive material even further around. Hence they drained the water tanks 🙂

in the show they mention another explosion. While this isn’t physically possible to the degree the show expresses, in 1986 they didn’t know that.

Superheated fuel hitting water can cause a lot of rapid steam and hydrogen generation and can cause a steam release. However no explosion.

You have to put water on the fuel to stop a core melt in most cases. The question is usually, how much. If you put too little water on you just make more steam and hydrogen without actually cooling the fuel and risk a hydrogen explosion (see Fukushima). If you put too much on you make more heavily contaminated water and risk making a big mess of things.

The goal we have for boiling water reactors is to use around 3000-5000 gallons per minute to quench the fuel. This is high enough to cool it down even though it’s going to have chemical reactions to this, and low enough to not flood out your containment vent paths and create more of a challenge than necessary. Once the core mass is quenched you drop down to only injecting the minimum debris retention injection rate, which is the lowest amount of water required to prevent melting through the reactor or containment (typically by the time you get here it’s 200 gpm or less).

The unknown fears the engineers and scientists had in 1986 were that you might cause the reactor to become critical again and explode, or you might have a large hydrogen explosion, which not only would be a big mess on site but might also impact the other 3 reactors on site and possibly turn it into a multi reactor accident. The reality is that a melted core mass does not have the geometry and proper moderation to become critical again, and the hydrogen would be a challenge but would not be megaton level.

They drained the water but installed a heat exchanger. Water exchanges heat very well but what you don’t want is the water vapor to take tiny radioactive particles out to the atmosphere where it can be deposited somewhere else. There was a book about the Rocky Flats nuclear trigger facility where they had a fire and attempted to put it out with fire because the proper firefighting equipment wasn’t readily available. They essentially let a bunch of radiation leak out into the atmosphere as a result. That was a small fire compared to the molten mass of nuclear fuel going through meltdown. The water would be let off as steam, dry up, then the pile of hot nuclear material would melt down into the water table. Solution, install a massive radiator below the pile of nuclear material so you can relieve it of heat while not emitting radiation into the atmosphere.