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Comment by Accujack | Hacker News Reader

It uses TRISO fuel, which are fissionable materials enclosed in a carbon and ceramic shell that's extremely tough and can handle far higher temperatures than are present in a reactor without melting.

The shell prevents the release of radioactive materials, so TRISO fuel encapsulation would turn a melt down into a reactor damaging event rather than an emergency/crisis. A runaway reactor would get hot, melt its structure, and maybe drop parts inside its containment, but the fuel wouldn't melt and wouldn't travel far. Most likely the reaction would stop as soon as the structure of the core fell apart.

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> It uses TRISO fuel, which are fissionable materials enclosed in a carbon and ceramic shell that's extremely tough and can handle far higher temperatures than are present in a reactor without melting.

I was going to make a top level comment along the lines of:

"How thick/expensive a bunker would one need to build around one of these to prevent it from spreading radioactive materials about when hit by say, an airplane?

As I understand it, large legacy reactors are hardened for this. Would the same level of safety still allow these smaller reactors to be economically viable?"

But now that I have read your comment, would a kinetic event happening to a TRISO fueled reactor be more of a non-event? Follow up, would non-fuel components of the reactor become radioactive over time as well?

Sorry if dumb questions, not very educated in this space.

For small reactors, with minimal requirements for support equipment and ~no need for active cooling during emergency shutdown, the design trend I've seen is "put it underground". Which makes sense - bomb shelters, military bunkers, etc. have been put underground for centuries because that's a relatively quick & cheap way to make something that's very hard to destroy.

The small size is a serious advantage for this kind of thing.

I could be wrong, I'm not an expert on the matter by any means, but I think the reason traditional nuclear reactors are so heavily fortified against attack is specifically because of the threat of a catastrophic meltdown if their cooling and control systems are destroyed. If these nuclear batteries can't melt down and rely on passive safety measures then the need for fortification is drastically reduced. Placing them in a fairly standard concrete building either underground or bermed with earth on all sides would likely be sufficient. You'd probably still want security around to make sure no one tried to manually tamper with the equipment.

I wonder if that fuel is usable in a breeder after it's "spent". Usually "spent fuel" has spent a few percent of the usable fissionable material, but the products of decay (and partly synthesis) prevent it from working efficiently.

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