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Comment by koeng | Hacker News Reader

I'm wondering what kind of safety these would have. A 100 MW reactor in a single shipping container is crazy efficient - just curious what infrastructure around that would have to be built

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You mean crazy dangerous. At that power density, everything has to work for this thing to not meltdown.

So actually that’s the whole design of these things. You have to work to keep them running. In case of a meltdown, the fuel is passively drained by a plug.

In case anyone else was wondering how having the molten fuel pour out of the reactor is safe - it is designed to pour into a container with criticality safe geometry to stop fission:

"in an emergency situation [the fuel] can be quickly drained out of the reactor into a passively cooled dump tank. MSRs designs have a freeze plug at the bottom of the core—a plug of salt, cooled by a fan to keep it at a temperature below the freezing point of the salt. If temperature rises beyond a critical point, the plug melts, and the liquid fuel in the core is immediately evacuated, pouring into a sub-critical geometry in a catch basin. This formidable safety tactic is only possible if the fuel is a liquid." [1]

[1] https://www.sciencedirect.com/science/article/pii/S168785071...

> This formidable safety tactic is only possible if the fuel is a liquid." [1]

Isn't something similar possible with pebble reactors, at least in principle?

Possible, and implemented in recent reactors - for the "corium". Though in that case it's for the extreme situations, since after that the reactor is completely done for.

I would love to see what a sub-critical geometry looks like. I also wonder what kind of math and engineering they did to come up with the shape they ended up with.

http://thorconpower.com/wp-content/uploads/2019/03/ColdWallL...

This one is just a bunch of separate tanks spaced as far away from each other as possible.

I've also seen a "Bed of nails" design where the liquid salt would flow over a bunch of moderating spikes, seep down between them and freeze.

In some contexts, a subcritical geometry is simply a plastic jug that's too small to hold a critical mass. Obviously they're not using plastic jugs for this, but the principle of the thing is pretty straight forward. They could create a wide 'dish' under the reactor which, if the liquid fuel were drained into, would spread the fuel out into a shallow puddle using the self-leveling property of fluids.

> I also wonder what kind of math and engineering they did to come up with the shape they ended up with.

I assume this is why some Finite Element Analysis packages come with warnings along the lines of "You must not use this to help Kim Jong-Un do you know what"

Feynman wrote of this when he was working at Los Alamos, i.e. the labourers weren't informed as to what they were really working with, so they could come very close to criticality accidents.

A pyramid may work with a wide flat basin at edge, especially if core material is moderating itself (vs just separating).

a pancake ?

So instead of melting down by accident it melts down deliberately, controlled by a fan?

Somehow I'm not as reassured as I might be.

Well, no. A meltdown is when the stuff escapes containment and then continues undergoing fission, heating up and melting out of containment and into groundwater/the environment.

The drain tanks are shaped specifically so if the (liquid) fuel flows in, it will stop reacting. And presumably the whole apparatus (drain tanks included) is inside of a biological shield.

So a "meltdown" here just means a big hunk of solid nuclear salt inside the bio-shield. I think the idea being that you could just bury the whole thing in case of failure.

IIRC draining the fuel salt into the drain tanks was standard operating procedure for one of the early MSR experimental reactors.

Next morning when they came in to continue experiments, started the (electrical?) heaters to liquify the salt and pump it back into the reactor core.

Wouldn't surprise me if the modern designs are similar.

This would not be melting down. Melting down is melting out of the containment device, which this is not doing. True it is melting INTO a secondary containment device but unless it continues to melt through that it would not be a 'melt down'

Also, there is no pressure build up, which is what caused the big explosion in Fukushima. All the coolant starts boiling, turning into gas, and well the rest was in the news

Just for clarification, the big explosion at Fukushima was a hydrogen explosion. At high temperatures, fuel cladding breaks water into hydrogen and oxygen. Hydrogen collected at the highest point in the structure and later exploded.

My understanding is that these reactors are designed to have a lot of passive safety features (e.g. if all operators walk away the reactor will cool itself and go sub-critical), so quite the opposite of what you are claiming.

Those safety systems don’t exist in a shipping container sized nuclear reactor. One method that I think you’re talking about is when the temperature of the molten salts goes beyond a certain safety threshold then a heat sensitive plug is disintegrated and the Milton salts are drained into a safe underground reservoir for them to cool. (This is my recollection from that why thorium is the future video that went viral years ago) Can’t do that in a shipping container.

I was thinking of Copenhagen Atomics' Waste Burner design where they describe their passive walk-away safety features as "Prime minister safety" [1].

> The CA Waste Burner has a set of systems governed by the laws of physics that cannot be overruled by humans, and which will cause the reactor to shut down safely if something goes wrong...This means that operators are not required to watch for alarms and act in accordance. The CA Waste Burner must be able to automatically shut down before any human can react to an alarm and choose what to do. If human action were ever required for operation, other than during startup procedures, then we would consider it a design failure...

[1] https://doi.org/10.1016/B978-0-08-101126-3.00023-3

> The CA Waste Burner has a set of systems governed by the laws of physics that cannot be overruled by humans, and which will cause the reactor to shut down safely if something goes wrong.

This is really good, but humans have an amazing knack for messing stuff up and I really hope corners aren’t cut building and maintaining it.

What comes to mind is the situation in Japan where workers inadvertently had some material go critical, and while this was being investigated it was found that they were carrying waste uranium and nitric acid around by hand in buckets.

https://www.newscientist.com/article/dn20263-japans-record-o...

Reactor shipped with "some assembly required."

Isn't molten salt already molten?

The shipping container only has the reactor, it does not produce the actual power. You would still need turbines etc for that externally.

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