So it requires gravity. It may seem trivial, but how would this thing react if suddenly not mounted vertically, say if it got knocked onto its side while running? Would coolant still flow as planned?

This is a great question!

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.

Safety on the ground is one thing, the other is whether this sort of design might be used in less-than-earth gravity. We need passive/fail-safe reactors for space and this thing looks like it could fit inside a rocket.

One module weighs 762 tons, which is more than 10X the capacity of Falcon Heavy.

Anything that knocks a fully functional multiple thousands of tonnes of concrete, steel, water and fissile material onto it's side was already going to fuck up your day.

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.

They are vertical cylinders to be suspended vertically in a pool of water, not massive squat objects sitting on bedrock. They are thin and relatively top-heavy. It is not ludicrous to ask what could happen should they not remain as vertical as designed.

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.

I think they are not trivial but relatively easy parts of the design. I’m sure they can design support structure to withstand whatever arbitrarily large earthquake you throw at it. The structure doesn’t need to be particularly light or compact so there shouldn’t be any design constraints getting in the way of strength. The heat transfer problem is much much harder and they’ve already solved that apparently.

I;m not saying it is impossible, just interesting. One option could be to suspend these reactors on cables from above. That might let them swing around all they want during even a huge quake. The water would act as the dampener. Not much extra engineering required.

I was assuming they'd bury these.

As other Nuke submariners in this thread can attest, there are definitely PWRs designed to tolerate massive rolls and “angles and dangles.” I guess it depends on your definition of sideways.

> 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.

I'd be more concerned about possible coolant loss and/or valve failures because of corrosion.

https://www.sciencedirect.com/science/article/pii/B978184569...

>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?

Ahh i see, a flat earth believer ;)

I suppose earthquakes and other disasters happen (and possibly tilt the reactor) however Earth is shaped?

Sure, you are right but i see HN needs the ⸮ ....

Layman's question: I don't see how this small reactor is so unique or why it took so long to be developed for regulatory approval. The article claims it produces about 50MW of power with a planned 60MW version. Well, there are already very compact reactors in use on today's aircraft carriers (the A1B and A4W) that produce a much larger 500 to 700MW per module and are also relatively small (after all they fit on individual ships). And obviously deemed quite safe for the exact same reason -they're on ships that travel to all major ports and contain thousands of crew members right inside the same metal hull that the reactor is in. Even more compact and fit into an even smaller vessel quite safely for the crew: submarine reactors like the S8G, which produces over 220MW and measures only 42 x 55 feet in size, with a weight that's not unreasonably greater than that of the NuScale

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?

The $1/2 billion they had to spend due to paperwork related to certification of the design. From an earlier comment[1] from a different thread about this approval quoting NuScale's press release:

"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.

[1]https://news.ycombinator.com/item?id=24346808

In other words, the "impracticality" of compact, safe terrestrial reactor designs is almost entirely created by an immense mountain of bureaucratic hurdle that the military is excepted from? I can see the need for safety and careful documentation of contingencies but the scale of what you describe is simply dismaying.

Thank you for that answer and link. checking further now.

Also, I can't even begin to imagine how much paperwork would be necessary (in literal visual, procedural terms) for the cost of fulfilling it to reach half a billion fucking dollars and 2 million man hours to fulfill. What would this even contain? Multiple 200+ page technical laboratory test breakdowns for every single nut and bolt and washer in the reactor set-up?

Naval reactors use fuel that is enriched to weapons grade. I'm pretty sure we don't want that sitting around in towns all over America.

I'm amazed that convection alone provides enough flow. I mean this is compared to the massive pumps that previous reactors needed. An innovation of the AP1000 was that the massive pump was fully inside the containment (actually attached to the steam generator) and they had to show that it would work without service forever.

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.

This is very interesting, can reactor like that be used on a ship or submarine(probably stupid question but I am curious). I remember reading some Tom Clancy novel that had a reactor with cooling that used no pumps as a major technical story point.

On submarines that might be a liquid metal coolant, as used on a few soviet submarine reactors. Lead-bismuth eutectic is most likely, it would be sensible to avoid sodium/potassium coolant on a submarine.

What is the maximum external temperature the design can operate in? Can you build it in hot places to power air conditioning and things like that during the worst days?

I think that the only impact the outside temp has on this is how it relates to the cooling efficiency of the steam loop. External temp will have almost no effect at all on the reactor itself or its operation.

Those reactors are small compare to usual one, but they still operate in a power plant like a gas or coal one. It's not a home or neighbourhood nuclear reactor.