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Comment by docandrew | Hacker News Reader

We use them in nuclear submarines and aircraft carriers, so the tech seems feasible - not sure why commercial use for small reactors is such a difficult stretch.

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Nuclear submarines cost on the order of $6B a pop, so I'd hesitate to describe anything about them as "cheap". We use nuclear reactors in them because there's no other power source that's as compact and self-contained.

Plus worst case for a nuclear submarine is that you flood the reactor with sea water if you have a loss of cooling.

Surely there are worse cases for a sub than this?

The idea is that you are not going to have a meltdown even in a badly malfunctioning reactor.

Is it? It's not at all difficult to stock enough seawater to flood a terrestrial plant. What's distinguishing the submarine?

A submarine (when submerged) has a large body of water for heat to radiate out into.

A (previous generation) terrestrial plant already has systems that can dump ludicrous amounts of water onto the core, but they all rely on there being power to operate pumps. A nuclear submarine that can dump in sea water does not need this - the body of water it is in is cold enough (relative to the heat in the core) that it will naturally carry heat away.

Smaller size than commercial power station reactors, easier to keep cool.

This contradicts the comment I responded to, though. The scenario is that we've already failed to keep it cool.

There is less of it to cool.

Wikipedia says a 688 costs less than $2B in 2021 dollars. And that’s all in for the submarine on top of the reactor.

Sub and ship reactors are effectively sealed units that are not intended to be refueled on a regular basis. They can be serviced and refueled eventually, but it is a major overhaul to do so.

https://en.wikipedia.org/wiki/Refueling_and_overhaul

The problem with a small reactor is that there's a lot of per reactor cost. Things like security, communicating with the grid, and paperwork don't get cheaper with a smaller reactor so the percent of cost going to overhead goes up.

don't they normally run on highly enriched fuel?

I'm not sure there we currently have the supply chain, security and ability to deal with geopolitical fallout of using the same fuel for mass domestic use.

Naval reactors usually run on weapons grade nuclear fuel, but not always.

The French K15 reactor runs on 7% enriched fuel.

The exception proves the rule. If we have them already for subs, but still don’t have them on land then they must be bad ideas when cost matters.

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