The idea is to put multiple reactors at the same site, not to spread out mini reactors all over the landscape.

So the end result would be a powerplant that produces about as much power as a "normal" nuclear power plant, just that it contains many small reactors instead of a few big ones.

This undermines some of the claimed benefits. A bunch of little Fukushima Daiichi reactors in the same location wouldn't have fared any better than the actual Fukushima Daiichi reactor.

In this particular case, a power plant with these nuscale reactors would probably have survived a Fukushima type accident. One effect of being small is that the reactors are designed to be passively cooled after shutdown, using convection instead of pumped flow. So there is no need for emergency diesel generators to keep the coolant pumps running.

What's the radius around a reactor where a critical core can trigger fission in the next reactor over?

You'd have to place both cores within the same reactor pressure vessel, which isn't possible.

So as long as the reactor vessel is intact it absorbs 100% of the neutrons? Going out as well as coming in?

Because in a failure you’ll have only one pressure vessel between the two cores. If everything else goes right.

> So as long as the reactor vessel is intact it absorbs 100% of the neutrons? Going out as well as coming in?

Neutron diffusion is a statistical process, so yeah, a small amount of neutrons will escape through the reactor wall. But not enough that the neutron flux from one reactor would influence the next one in any measurable way.

> Because in a failure you’ll have only one pressure vessel between the two cores. If everything else goes right.

Yes, also in this case.

In practice, 0.

Sure, it's safer for reactors to tolerate flooding, if indeed these reactors actually do tolerate flooding. Definitely that would be a better design in flood/tsunami-prone areas. No design (from history, especially no reactor design) is perfect. A set of smaller reactors that were not co-located would be more tolerant of site-specific vulnerabilities in their design.

> ... if indeed these reactors actually do tolerate flooding

The reactor modules are partially immersed in a pond, the ultimate heat sink. Cooling is passive, i.e., no cooling pumps, and does not require electrical power.

>A set of smaller reactors that were not co-located would be more tolerant of site-specific vulnerabilities in their design

That depends on what risks you are trying to mitigate. IF you are talking about site specific natural disasters and freak occurrences damaging a site. Then spreading them out increases risk of failure.

If you are worried about power loss after a site destroys a site, then colocation is worse. Of the two, damage to a site is the more pressing concern.

I think you're screwed either way.

The odds that a concentrated site has an event are much higher with colocation. But the changes of having an event at multiple locations are higher if they're spread out.

Some things that I believe would matter for colocation would be:

- chance of cascading failures

- economies of scale/safety in numbers (1 large team vs many small)

- plant-to-home efficiency (n fewer reactors due to smaller transmission losses)

Uh, there are a bunch of reactors at Fukushima. It’s in the name: “daiichi” means “complex-1”, there is also a “daini”, etc.