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Comment by blake1 | Hacker News Reader

Increasing temperatures increase steam pressure until a rupture occurs, and the water boils away. The steel reactor vessel and structural elements will then inevitably melt, and the reaction will continue until the molten fuel can spread out enough to lose criticality. This is called a meltdown.

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This is why many new reactor designs do not use water as coolant but instead molten salt[0] or inert gas[1], to avoid the risk of steam explosion. This alone does not make those reactors inherently safe but it can help keep radioactive material from being violently spread outside the reactor in the event of catastrophic failure.

[0] https://en.wikipedia.org/wiki/Molten_salt_reactor

[1] https://en.wikipedia.org/wiki/Gas-cooled_reactor

> Increasing temperatures increase steam pressure until a rupture occurs, and the water boils away.

Generally, no. Chernobyl is the biggest example of this happening, but it only happened because the reactor itself was basically exploding anyway.

Steam pressure doesn't build up gradually, generally. Relief valves -even just burst points- are just too simple and robust. The much more common issue is that hot reactor fuel causes water to break down and release hydrogen which accumulates at the top of the reactor and eventually explodes, and then the water boils off.

> Relief valves -even just burst points- are just too simple and robust

I'm not sure you've heard about this[0] accident:

"The accident began with failures in the non-nuclear secondary system, followed by a stuck-open pilot-operated relief valve in the primary system, which allowed large amounts of nuclear reactor coolant to escape."

[0] https://en.wikipedia.org/wiki/Three_Mile_Island_accident

The relief valve worked perfectly, as they said, there was no steam explosion.

Unfortunately too much of the steam was released and the reactor melted.

Uhhh, the valve worked perfectly but released too much steam resulting in a meltdown? Think about this for just a wee bit more.

okay not perfectly as it was stuck open, but it did release the pressure as was said.

But isn’t the water that is being cycled from the reactor to the turbine reducing the temperature? Isn’t cooling the reactor throwing energy away?

Similarily to bikebreaks, it's often better to be safe than most efficient

As one of the other sibling comments noted, a turbine can extract maybe 33% of the thermal energy if it’s operating at the theoretical maximum Carnot efficiency. The other 66% must be dumped somewhere to avoid accidents.

So what is being cooled is really the backend of the turbine, not the reactor.

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