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Comment by adammunich | Hacker News Reader

It's really hard to comprehend just how much heat a reactor can make from decay alone. Like... boiling a hot tub in only a minute.

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And so a pool of water is not enough because it will boil away. A continuous flow must be present that can not be interrupted.

You need a sufficiently large reserve to allow the reactor to cool, not an infinite supply. Reactors can be shut down and in this case the pool is sized to absorb all decay heat from the shutdown, plus a significant safety margin.

Great, and ideally gravity can be used to move it. Remember, at Fukushima power to the pumps was lost.

Afaik some reactor designs drop the core in a tank below in case of emergency.

The article states the whole reactor will be submerged in a pool of water, making it passively safe.

Reactor cores continue to produce waste heat when shut down, and water evaporates. It’s passively safe till you run out of coolant, then it’s actively dangerous.

In this case the pool boils off slowly enough from thermal load to let decay heat reduce to the point where air cooling is adequate. So it remains passively safe unless there is a sudden loss of water from the pool.

Engineering a very resilient pond does not feel like as complex a problem as engineering highly complex cooling systems to be resilient.

And if you size the pool right it will by cool before the pool boils off. It's simple math and engineering.

The reactor is submerged in the pool. It’s a passive design.

It also helps to not be running a reactor based on designs from the 1950s and then decades later ignore all the known potential problems.

It does, but it makes sense to explore fail-safe designs.

Depending on a lack of incompetence in dangerous systems works until it doesn't. To the extent that these things can simply halt when incompetently managed, they should.

If anyone disagrees, I'd like to know why they think the next hundred years are going to be so much freer of political shortsightedness and corruption than the last hundred.

Fukishima is a perfect example. Unit 1 had been retrofitted with an isolation condenser, which should have been able to prevent a meltdown even with no power, but it wasn't activated, for reasons that remain murky.

https://en.wikipedia.org/wiki/Isolation_condensor

Sounds like a quite simple design indeed, and only one valve opening away from use?

Nothing has to move it. It’s in a pool and always there. The pool is big enough to sink all of the decay heat of the reactors it hosts, without needing a refill or heat exchange.

And if the pool gets a leak?

Patch it? Keep adding water? There's lots you can do with a (non-catastrophic) leak, and building water vessels that don't leak in your lifetime is honestly not that hard.

Leak to where? It's below grade and it's not under any pressure greater than atmospheric.

The elephants foot in Chernobyl was so hot that melted the concrete and dug into the soil. Is this pool bottom impossible to melt?

1. Don’t let that happen.

2. There have been proposals before for core dilution buckets: a wide shallow dish under the reactor full of something like gallium for the hot core to dissolve into. As it spreads out into the dish, the heat and radiation fluxes become less unmanageable, and the core material becomes less critical.

3. Don’t let that happen. :-P

Chernobyl is not a good comparison, because that reactor design had a number of flaws that nobody in their right mind would have designed into it even then, let alone now. (The Soviet Union was not in its right mind.) And then on top of that, the operators were running an experiment with the reactor without having thought through the consequences.

Chernobyl was an ancient design that didn't have many safeguards. This one had safety built in from the start, and it's smol and can be contained easily.

It was a cheaper design, and better for enriching uranium. At the time it was built there were already many passively-safe reactors in operation.

The elephants foot was generated because of the failure to cool the core when the pumps failed. This design puts the core in thermal contact with a giant water reservoir to keep it from ever getting hot enough to melt.

Around 600-700 kWh per cubic meter depending on temperature. The reactor outputs around 200 MW thermal.

So if you have one of them in an olympic size swimming pool 50x25x2 meters, 2500 m^3, it'd need ~8 hours to evaporate the whole pool at full output.

If you assume decay heat as 1% of regular output (https://en.wikipedia.org/wiki/Decay_heat), you'd need to add (or have stored) ~3 m^3 of water per hour, or slightly less than a liter per second, to keep it from melting down.

If you assume an average of 2% for the first two hours, that'd be 8 MWh -> 12-13 m^3 for the first two hours, so a 5x5x5 = 125 m^3 pool (only considering the part above the "must always stay submerged" level) should be able to cool it for days.

I think _as long as the containment pool is intact_ (and you manage to SCRAM the reactor), this isn't going to be a major issue. But if e.g. an earthquake breaks the pool...

Makes me think; the water will not boil if the pressure goes up, was the Fukushima explosion caused by a hidden mechanism where if the water starts to evaporate, the steam can't go anywhere as to push the pressure up to keep it liquid?

That was a hydrogen explosion.

Seems to me that you are also assuming that the water is not dumping any heat on its own. I would bet that most of that decay heat is going to conduct from the water to the pool containment vessel and from there to the rest of the environment faster than the reactor is putting more heat into the pool.

According one of their videos[1] by the time the water has boiled away the reactor will be cool enough that it can be air cooled.

[1]https://www.youtube.com/watch?v=h--FAVoAQvk

Making statements as fact does not help to clarify the discussion. Try posing a question instead. You are not adequately informed on reactor design or operation

It's not necessary as discussed elsewhere, but even if it was a concern simply build it near a river, below the water level and dig a connecting canal (that is normally closed off). In the very worst case just lift a sluice gate and the tank will remain topped up.

Have people really found no way to dump it in an emergency with t^4 transfer?

What is t here?

I assume they meant T^4 radiation (Stefan-Boltzmann law): https://en.m.wikipedia.org/wiki/Stefan–Boltzmann_law

Nuclear reactors run at cool temperatures compared to say gas turbines. And they are powerful. So the surface area for radiative cooling is proportional to the power divided by temperature to the fourth power. So the cooling would need to be very big.

Right, but there is no shortage of metals and ceramics that can maintain cohesion (and strength!) at high temperature -- I'm thinking about those videos the machinists post of tools slicing through metal at an obscene rate with incandescent tooling. You don't even need ceramics to do that, there are steel alloys that stay hard and strong enough to slice through (soft) steel while incandescent, although for wear optimizaton they typically only actually do it with ceramics. In any case, it seems like someone should be able to figure out a "retract rods, let them glow" mode that dumps the energy into the sky like a lightbulb.

I'm sure there's a reason why it hasn't been done. Maybe you really do need high enough temperatures that you can't engineer compatible cladding, or it's hard to make IR windows low-loss enough to pass the energy, or something. Still... fourth power! The temperature you need the "lightbulb mechanism" to withstand is the fourth root of power/area! That's a powerful wind at one's back! It's easy to think of reasons why it might be impossible but if it's "just" a hard engineering problem then that's where things get interesting.

My non-expert guess is that given the large amount of power you have in a small area in a nuclear reactor, it would be hard to reach equilibrium while keeping the fuel solid.

If the fuel is in a liquid state (as in a molten salt reactor), then you could more easily since you could have it pour into a wide container, increasing surface area. (Basically the freeze plug approach).

It would be more possible for a high temperature reactor that can stand the high heat. A regular low temperature reactor would require a huge radiator for the same power.

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