- 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.
From reading the NRC submission, the support structure for the modules is Seismic Category 1 (rated for design basis earthquake). This includes the entire Reactor Building, its foundation, and requirements for the site on which it is built. The reactor building is also designed to withstand an impact from a large commercial aircraft. Note that conjecture that the modules are top-heavy appears to be unfounded given the configuration described in the NRC submission.
Elsewhere in the probabilistic risk assessment, they address a module tipping over if it is dropped in the operating area-- the most probable scenario for a horizontal module.
In the case where the drop causes the containment to be breached and does not permit inflow of water from the pool (ostensibly due to pressure difference) the result is core damage and a release of radioactive contamination to the pool.
I encourage interested readers to look at Chapter 19 of the NRC submission. Much of the document is at a level where a lay engineer can understand it. NB: I am not affiliated with NuScale or the NRC.
As far as I'm aware we've never had a nuclear reactor go sideways, and it's kinda ludicrous to even ask. It's one of the most massive structures, built on bedrock because it's too heavy to be built on anything else, and even these "small" reactors will be incredibly heavy.
From a planning perspective, if verticality is a requirement then that verticality has to be protected as strongly as any other aspect. Any attachment or mounting points that maintain verticality must therefore be earthquake proof. If, as in the pictures, these reactors are held in place from the top, horizontal shaking from even a small earthquake would put massive strain on that joint. It isn't just the long/thin/heavy reactor but all the water sloshing around too. A super-strong joint and maybe something important bends/breaks under load? A flexible joint to allow the reactor to sway? Sway how much? What happens at the travel limit of the joint? Can we risk the reactor contracting its neighbours? These are not trivial engineering questions.
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.
https://www.sciencedirect.com/science/article/pii/B978184569...
Ahh i see, a flat earth believer ;)
Basically: What's so dramatically difficult about doing the same for a terrestrial compact reactor to make something like the NuScale take so long to get approval?
"NuScale spent over $500 million, with the backing of Fluor, and over 2 million labor hours to develop the information needed to prepare its DCA application"
Military reactors don't have to go through this process and those designs (modified to work on land, etc) would.
Thank you for that answer and link. checking further now.
https://www.neimagazine.com/features/featurethe-world-s-larg...
Anyway, what if the valves to the backup containment open while it's running at full power? I mean the electronics fail so the valves open. I suppose the steam generator is still running, but even so lots of heat would be dumped into the pool. Maybe there is an interlock so that the reactor scrams in this case.
one is whether or not the coolant is highly pressurised relative to atmosphere, the other is what the thread was originally about, which is how reactivity changes with respect to temperature. this is the temperature coefficient of reactivity.
https://www.nrc.gov/reading-rm/basic-ref/glossary/moderator-...
the people you were responding to were referring to the latter. specifically, if a reactor becoming more reactive results in more or less total reactivity. this characteristic is related to, but independent of, how pressurized the coolant and/or moderator are.
Also, I believe Copenhagen Atomics is doing this at the moment, but couldn't (quickly) find a reference.
I suppose at some point it's not competitive with solar so what's the point. Maybe we reached that threshold recently.
Wouldn't a giant steel or stone vat work, in theory?
Early molten salt reactor experiments used Hastelloy N (nickel: 71 wt%, molybdenum:16 wt%, chromium: 7 wt%, iron: 5 wt%, others:1 wt%), which apparently works. But as others have noted, there is ongoing research to further improve it.
Also think of how saltwater effects metal. That's just a bit of salt dissolved in water. Molten salt is like that, but way more concentrated, and hotter. Corrosive reactions go faster when you add heat. Molten salt is like metal's worst nightmare.
That's before you even add the radioactivity into the mix, I believe that makes this problem an order of magnitude harder than just trying to contain molten salt, which is already kinda pretty hard.
Stone, maybe, something like a specialized ceramic could work?
If you go to the desert to make use of the space, might as well build a solar array instead.
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.
This reactor is a much smaller PWR with a double containment and natural circulation, without these failure modes.
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...
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.
Sounds like a quite simple design indeed, and only one valve opening away from use?
Engineering a very resilient pond does not feel like as complex a problem as engineering highly complex cooling systems to be resilient.
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
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.
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).
What's an example event where the ultimate heat sink might be lost?
People are fallible on the best days, assuming everyone did their very best from nuclear physicists to construction workers, mistakes are made. You take steps to reduce the risk. Research gets review. Engineering schematics get review. Construction gets inspection. Still some mistakes will get through.
And people always act their very best all the time right?
You can even have a perfect design, perfect construction, that is mismanaged years after it's built, after the original engineers and bureaucrats lose control.
The same people problems apply to basically every human endeavor, but nuclear's capability to cause accidents that have a lasting impact is pretty scary. You don't feel even a twinge of existential dread when you think about? If you don't, then I don't think I want you working on a reactor.
There are pools and pools and there are leaks and leaks. Anybody that has built an aquarium knows that a 80cm high design is much more complicated to made leak-proof than a 40cm high design holding the same water.
A shallow pool would not be enough to contain a small nuclear plant, so you need a non standard bigger pool. Higher the pool, higher the weight of water column, the force pressure against the walls will increase, and the leak will be much faster because the water weight in the upper level of the pool will force the water in the low levels to go out. If your leak is in the upper side of the pool will be a small self contained problem but if the bottom leaks is a different thing. As the bottom needs to support much more force against it and there is a weak area when walls meet bottom, is more probable to fail first.
If our smallest manmade lakes (swimming pools) can leak a lot, and our largest manmade lakes (dams) can also leak a lot the idea there's an in-between size that doesn't leak might need a bit of elaboration.
The safety requirement here is not "doesn't leak", it's "holds most of the water for 30 days (after which water is not required)". You would have to get an implausibly-large leak, during a situation where nobody can add more water for a month.
We should be orders of magnitude more worried about all the carbon dioxide we're dumping into the atmosphere, than the failure modes of an engineered hole in the ground.
Maybe a particular reactor design could use such a mechanism as one failsafe, but that alone is not enough, and no design is perfect, and the people operating it are not perfect.
I think some of the risks of nuclear are acceptable, I am actually very pro nuclear, but we should acknowledge them instead of pretending they don't exist. The only way risk can be properly managed is if it's acknowledged.
I would prefer to see inherently safe designs like LFTR gain traction, but NuScale has one of the few designs likely to be built in the near term, where you could SCRAM and take a vacation without causing a meltdown. That is a major advancement in safety; let's not let perfect be the enemy of good.
Unless we plan on getting the pope to build nuclear power plants, which does sound cool.
"Substantially larger" is not the same as "impossible". And, given substantially larger consequences if a reactor pool breaks (compared to a swimming pool breaking), I don't think the question is out of line.
We learned from Fukushima that natural disasters don't always follow the parameters that we expect them to.
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.
Everyone who remembers the post 9/11 "but what if the terrorists attack X" pandemonium knows that spent fuel pools are outside the containment building. That was one of the scenarios that got paraded around to maximize FUD.
https://www.researchgate.net/figure/Cut-away-view-of-reactor...
Designers are awesome. Sadly they were also unable to find some time in 50 years to raise a wall a few m so it can stand a Tsunami. It seems that the extra scrutiny, was not so extra in the real life when the company will need to allocate real money.
We should note that a lot of rare things happened all at once, more than just the freak earthquake and freak tsunami. There is no such thing as perfect. But consider that there were no lives lost due to the reactor accident. Yes, there is economic damage, but that is the worst. Lives were not lost and the environment was not irreparably damaged. Nature has actually started to take back the region and it more looks like a scene out of I Am Legend rather than The Road or The Book of Eli. I do not intend to dismiss the event, as it is concerning (and we've learned a lot since then), but however you measure it coal or oil or gas have had far greater environmental (or human/health) impacts than nuclear. The difference is that it is more in our mind. Despite the Fukushima cleanup estimate (2x Chernobyl's) costing about 9x Deep Water Horizon (2010) I'll let you decide what costs more[0], even if we ignore all the costs to health and atmosphere. There simply is no free lunch.
There is no doubt that the 2011 earthquake was an extreme event, but it's incorrect to say that it was not foreseeable or that the plant's safety systems could not have prevented the disaster.
Further up Japan's coast, the Onagawa nuclear plant was much closer to the earthquake's epicentre. It was subjected to extreme shaking, far more than any other nuclear plant in history, and like Fukushima was also flooded by the tsunami.
Yet it was able to shut down safely as designed in the hours that followed, and its structure was "remarkably undamaged" considering the extreme magnitude and duration of the shaking. 2 of its 3 reactors are expected to be restarted soon following structural repairs and seismic upgrades.
Repeating this again is infuriating. No lives lost? seriously?
It is trivial to design a system that powers off when unstaffed. Without power, this reactor will SCRAM and passively switch to air cooling over the course of a month.
Presumably, a skilled attacker could compromise the passive safety systems and force a meltdown, but wouldn't it be easier to steal some spent fuel and disperse it?
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.
The reactor ceases to transmit power and is shutdown for maintenance?
That said, rapidly losing the UHS should be incredibly rare/difficult (as several other posters have mentioned).
https://www.nrc.gov/reactors/new-reactors/smr/nuscale.html
Here is an interesting sub-report:
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
"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.