16 ms·
Here is the NRC website about this reactor: https://www.nrc.gov/reactors/new-reactors/smr/nuscale.html https://www.nrc.gov/reactors/new-reactors/smr/nuscale.ht
by jhallenworld 6y ago
Here is the NRC website about this reactor:
https://www.nrc.gov/reactors/new-reactors/smr/nuscale.html https://www.nrc.gov/reactors/new-reactors/smr/nuscale.html
Here is an interesting sub-report:
https://www.nrc.gov/docs/ML2022/ML20224A525.pdf https://www.nrc.gov/docs/ML2022/ML20224A525.pdf
Information withheld for security reasons. One item concerns the "ultimate heat sink". What happens when the ultimate heat sink is lost?
Well a design assumption is that it is not lost:
https://www.nrc.gov/docs/ML2020/ML20205L410.pdf https://www.nrc.gov/docs/ML2020/ML20205L410.pdf
"A key assumption of the PRA is the availability of the UHS to provide an adequate heat sink. To support passive heat removal with the DHRS or ECCS, the reactor modules are housed and partially submerged in the UHS such that most of the outer surface of the CNV directly contacts the UHS, which is a large pool of water in the reactor building (RXB). "
DHRC is decay heat. CNV is reactor containment vessel. So drain the pool and the reactor is in trouble.
- adammunich 6y agoIt'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 ago
- 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.
- natcombs 6y ago>> What happens when the ultimate heat sink is lost? What's an example event where the ultimate heat sink might be lost?
- ColanR 6y agoNo expert, but if it's a pool of water then an earthquake might cause a leak?
- newacct583 6y agoOr an attack, of course. Or some other event (social unreset, invasion, coup, etc...) causes an evacuation of staff and it boils off during the resulting excursion. People tend to have poor mental models for the long tail of external failures that happen in real life. It's easy to imagine that things that have never happened in the last century would Never Happen. But... they will, somewhere.
- godelski 6y agoNuclear sites are designed to withstand a strike from a commercial airliner (747). Like you, the designers imagined many of the events you mentioned and more. A good rule of thumb is that if you, a non-expert, can think of a scenario within 10 minutes, an expert has probably already thought of this scenario. Nuclear power plants and weapons sites have always been considered targets and thus considered extra scrutiny in their design.
- thekyle 6y agoWow. How do they design them to withstand a strike from a 747? That seems really difficult.
- selectodude 6y agoA whole lot of concrete, mostly.
- 6y ago
- kbenson 6y agoWasn't the big push (or maybe big PR push) for more research and development of Thorium reactors a few years ago because they fail closed/safely? That seems like the kind of thing you'd want for smaller reactors (which I assume means more and more geographically diverse, but don't really know).
- ses1984 6y agoThere's no research being done on materials to safely contain molten radioactive salt, that research could be dangerous, pretty much only superpowers would have the resources to do the research.
- bbojan 6y agoThis was being done three years ago: https://www.powermag.com/thorium-molten-salt-reactor-experiment-underway-in-the-netherlands/ https://www.powermag.com/thorium-molten-salt-reactor-experim... Also, I believe Copenhagen Atomics is doing this at the moment, but couldn't (quickly) find a reference.
- ses1984 6y agoThat's cool, thanks for the link.
- dheera 6y ago> materials to safely contain molten radioactive salt Wouldn't a giant steel or stone vat work, in theory?
- ClumsyPilot 6y agoThe problem is that the nuclear reactions produce many random materials which in turn might be corrowive to the reactor veasel. Dont forget that the vessel has to last decades, so even minor corrosion is a big deal.
- patmorgan23 6y ago
- wbl 6y agoThe pool is big. It is easy to fill holes in the ground with water. In some parts of the country very hard to keep them dry.
- birdyrooster 6y ago>What happens when the ultimate heat sink is lost? The reactor ceases to transmit power and is shutdown for maintenance?
- Pfhreak 6y agoReactor's aren't boolean. If a reactor has no heat sink and attempts to shut down, there's still going to be a catastrophic amount of heat to disperse. That said, rapidly losing the UHS should be incredibly rare/difficult (as several other posters have mentioned).
- birdyrooster 6y agoIf it's big enough and makes enough noise, it will be harder to lose.
- pdonis 6y ago> So drain the pool As I understand it, the pool has no drain. There's nowhere the water can go. Which would make sense for a passive safety feature.
- hwillis 6y agothose fools making reactors with drains in them. So many cooling accidents could have been avoided!
- wahern 6y ago> DHRC is decay heat. CNV is reactor containment vessel. So drain the pool and the reactor is in trouble. In some reactor designs overheating slows and eventually stops the reaction in a controlled/deliberate manner. The reactor system may still fail irreversibly, but it wouldn't necessarily meltdown in a way that risks widespread contamination or excessively expensive site remediation, such as by exposing unapproachable material. I have no idea if this design has that quality.
- URSpider94 6y agoI haven't had time to read all of the info, but I get the impression that this is still a pressurized water reactor, just on a smaller scale. It will fail in pretty much the same way as current reactors like Fukushima and Chernobyl. I believe that the point is that the pool provides an extra level of failsafe against coolant loss, and additionally that the substantially smaller size of the core limits the amount of heat build-up in a meltdown.
- petre 6y agoFukushima is a BWR design using water moderator, closed with a lid bolted on a flange. It uses two pumps. Chernobyl is a RBMK design with graphite moderator and a highly positive void coefficient. This reactor is a much smaller PWR with a double containment and natural circulation, without these failure modes.
- hansthehorse 6y agoIn a standard commercial pwr the containment building is a steel shell surrounding the entire system, then there is a void of varying width then that large cement building that you can see when driving by. That building is called the shield building.
- URSpider94 6y agoYes. This is not an inherently safe design. However, it adds a second level of safety, in that each reactor has its own coolant loop that would have to fail first, followed by a second failure of the large pool. It looks like the large pool is also passive, in that it does not rely on circulator pumps to provide cooling.
- i_am_proteus 6y agoNuclear engineer here. I've read through a good portion of the regulatory submission to the NRC and have a few takeaways that oppose some of the less-well-informed takes in this thread: - The reactor differs substantially from existing PWRs by encasing the primary in double containment, using natural circulation flow for both normal operation and emergency cooling. There are no pumps needed (or installed) to move coolant through the reactor. - During normal operation, the primary is entirely contained within the primary containment, and circulates naturally, using the differential temperature and gravity. The steam system can is used to remove heat. - During an emergency, valves open to admit the reactor coolant into the backup containment. These valves are normally held shut by hydraulics with positive control from electronics-- their failure mode is to open with no other operator action in case of a loss of power. No additional operator action is needed to initiate the emergency cooling flow, which is also a natural circulation loop to the backup containment shell, which then conducts heat to the pool. - The backup containment shell is designed to withstand hydrogen explosions such as those that occurred at Fukushima, and which was possibly prevented at Three Mile Island by venting (whether there was a hydrogen explosion at TMI-2 is not fully understood). - There is no mechanism for positive reactivity addition via graphite moderator rod such as in the Chernobyl design. The specific failure mode at Chernobyl is not possible with this reactor. - The pool is specified as a stainless-steel-lined reinforced concrete and designed to withstand earthquakes. The safety systems are such that no reactor electricity supply is needed to remove heat-- the pool could be filled e.g. from a fire truck, and the immediate decay heat from shutdown does not require any additional heat removal or water addition. So the failure mode we saw at Fukushima (inability to remove decay heat due to loss of electricity) and hydrogen explosion breaching containment does not apply to this design.
- candiodari 6y ago> During normal operation, the primary is entirely contained within the primary containment, and circulates naturally, using the differential temperature and gravity. The steam system can is used to remove heat. If it's using gravity, what happens if this whole reactor gets tilted? What happens at 10 degrees? 45? 90? 180? Are there any critical angles where it would melt down? I'm curious how you would make such a design that would be both using gravity yet tolerate being moved around.