This Rolls Royce design isn't all that "small." A RR SMR design is a 470MWe PWR. About half the size of a typical PWR reactor. Fukushima Daiichi Unit 1 was 460MWe. Calling this an "SMR" is a stretch, likely for PR purposes.
It's a rather conventional design, low enriched fuel, no exotic coolants. There is a paper on it at NRC[1]. And they've never built one, so if they get it running by the 2030's they'll be doing pretty well for a Western company.
I downplayed there, because occasionally I try to moderate my extreme cynicism.
It would be miraculous, in the biblical sense of the word. Not only because it would be a technical and regulatory triumph for RR and the UK, but because it would mean this is something other than what it appears to be to me.
None of this will get built. It's all fake, and after the benefits are taken, and the subsidy budgets are drained, and the various political and academic and regulatory folks have populated the requisite non-profit no-show jobs, and the professional opposition leaders have collected all the anti-nook bucks, and RR et al. have wiggled out of whatever obligations they're pretending to pursue via the holes they've already carefully arranged for themselves, these papers and headlines will be forgotten.
Closer to a third for recent models (the French P4 reactors from the 80s were 1300, the later N4 1450~1500, the EPR is 1650). 500-ish is a relatively typical density for reactors from the mid to late 60s.
It's a rather conventional design, low enriched fuel, no exotic coolants. There is a paper on it at NRC[1]. And they've never built one, so if they get it running by the 2030's they'll be doing pretty well for a Western company.
[1] https://www.nrc.gov/docs/ML2521/ML25212A115.pdf