6 ms·
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.
by adammunich 6y ago
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.
- phkahler 6y agoAnd so a pool of water is not enough because it will boil away. A continuous flow must be present that can not be interrupted.
- sitharus 6y agoYou 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.
- phkahler 6y agoGreat, and ideally gravity can be used to move it. Remember, at Fukushima power to the pumps was lost.
- tpxl 6y agoAfaik some reactor designs drop the core in a tank below in case of emergency.
- asdfman123 6y agoThe article states the whole reactor will be submerged in a pool of water, making it passively safe.
- meepmorp 6y agoReactor 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.
- throwaway0a5e 6y agoAnd if you size the pool right it will by cool before the pool boils off. It's simple math and engineering.
- ixfo 6y agoIn 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.
- garmaine 6y agoThe reactor is submerged in the pool. It’s a passive design.
- AngryData 6y agoIt also helps to not be running a reactor based on designs from the 1950s and then decades later ignore all the known potential problems.
- _jal 6y agoIt 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.
- cameldrv 6y agoFukishima 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.
- Gravityloss 6y agohttps://en.wikipedia.org/wiki/Isolation_condensor https://en.wikipedia.org/wiki/Isolation_condensor Sounds like a quite simple design indeed, and only one valve opening away from use?
- URSpider94 6y agoNothing 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.
- rkagerer 6y agoAnd if the pool gets a leak?
- microcolonel 6y agoPatch 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.
- pdonis 6y agoLeak to where? It's below grade and it's not under any pressure greater than atmospheric.
- pvaldes 6y agoThe elephants foot in Chernobyl was so hot that melted the concrete and dug into the soil. Is this pool bottom impossible to melt?
- piannucci 6y ago1. 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
- pdonis 6y agoChernobyl 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.
- URSpider94 6y agoThe 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.
- tgsovlerkhgsel 6y agoAround 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 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...
- Cthulhu_ 6y agoMakes 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?
- smilespray 6y agoThat was a hydrogen explosion.
- evgen 6y agoSeems 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.
- sesutton 6y agoAccording 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 https://www.youtube.com/watch?v=h--FAVoAQvk
- Lineup 6y agoMaking 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
- rkangel 6y agoIt'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.
- jjoonathan 6y agoHave people really found no way to dump it in an emergency with t^4 transfer?
- et2o 6y agoWhat is t here?
- tominous 6y agoI assume they meant T^4 radiation (Stefan-Boltzmann law): https://en.m.wikipedia.org/wiki/Stefan–Boltzmann_law https://en.m.wikipedia.org/wiki/Stefan–Boltzmann_law
- Gravityloss 6y agoNuclear 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.
- jjoonathan 6y agoRight, 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.