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Notes -
Mahaffey's Atomic Accidents is a little outdated (2014), but has a small chapter on NuScale, Gen4 Energy (then Hyperion), mPower, and Toshiba's 4S reactors, along with a handful of other also-runs. Most other summaries I've seen tend to be little more than stats breakdowns.
TerraPower is scheduled to be one of the first actually running (2027-2030, if you believe it, which you shouldn't), but it also straddles the line between small reactor and conventionally sized plant, neither modular, and then throws in sodium testbed on top. And I'm really skeptical of molten sodium -- I get the benefits, but the engineering and political problems are vast. Maybe if it's 'really' more of a research/production reactor, with the power a pleasant side effect?
NuScale's VOYGR is certified (kinda, only for the biggest config), and it's the most 'conventional, but smaller' reactor: make a reactor that can handle 100% of decay heat with a complete coolant loop failure, and put a pin it that design. On the downside, they got hit with skyrocketing costs and high uncertainty for demand, and their planned CFPB got 'indefinitely delayed' at the end of last year as a result. Also not a huge fan of the short fuel replacement cycle, at 18 months, both for non-proliferation reasons and because it even with individual modules offset in time, that cuts into the 'proven, reliable baseload' framework that nuclear plants excel at.
eVinci is theoretically promising and I like the combination of a very-small-reactor and some actual manufacturing expertise of previous nuclear plants (uh, forty years ago; they literally had to give up the last couple they tried), along with actually considering a use for all the 'waste' heat, but it's so early in the process that it's hard to say much at all -- history is filled with excellent technical briefings that didn't survive first contact with the NRC. I'd love the idea of a 5MW-scale microreactor that's completely passive, but even if the whole heat pipe system works, I just can't see the NRC letting it live without years of test operations, and even then probably still requiring some daily human oversight.
X-Energy's Xe-100s are planned for a test site in Texas and a (not hugely plausible) Washington State one, no real timeline yet. Abbot is pushing Texas hard, and the manufacturing demand could absolutely benefit from it. They're another one that's clever -- helium-gas-cooled pebble-bed reactor with live refueling capabilities -- but I'm not sure if they're too clever, and in particular the cost (and losses) of helium leave me a little nervous about how financially viable they might be. (Also, absolutely awful name.)
Oklo's Aurora is a tiny (2-15MW) sodium-cooled fast microreactor. They've not been laughed out of the building by the NRC yet and even have multiple sites planned, but they've had license applications denied using "novel, two-step process" (though things have been going better since). I like the idea of a fully passive sodium reactor more than a pumped-coolant one, but the tiny size and Chemistry Experiment refueling leaves a lot of potential reliability problems.
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