Comment by urban_winter | Hacker News Reader

Comment by urban_winter | Hacker News Reader

Parent Full thread

Does anyone understand this? "Some of the main differences that the MMR has from conventional nuclear plants is that the reactor cannot meltdown as the heat is passively released into the environment..."

300MW is a lot of energy to passively release into the environment. Does this design really fail safe? And if so, how?

The Fukushima issues were caused (IIRC) by failure of the backup generators to run the cooling system after the reactors were shut down. Do these small reactors really not need any external power when in that state? And do they get into this state by default? I.e. do they shut themselves down without external power and if the control systems fail? That would seem to be a massive safety advantage if it's true.

I remember reading about UK magnox reactors where the graphite control rods were held above the core by electrically powered actuators and which were designed to fall into the core (and stop the reaction) if there was a power failure. Perhaps these reactors are similar.

This is all somewhat complicated.

Most modern designs claim that if you just switch everything off, the reactor is safe. This is either because having no machinery around the reactor working still makes it safe, or the few critical components have tons of redundancy, and everything is designed to be “broken”.

300MW is what the reactor produces with the moderators retracted. You put them in place and the reaction stops immediately. So you don’t need to cool 300MW passively. What you do have is the residual heat, and fission products decaying away and giving more heat. I’m not sure what power output that equates to, but far, far less than 300MW, and decreasing over time too as shorter-lived fission products go away.

Some major advantages of small reactors in this context:

- the ratio of volume (r^3) to surface area (r^2) is better so it’s easier to cool

- there is simply less bad stuff in any one reactor so the tail risks are smaller

For getting an intuition on it, I would recommend the clip from HBO’s Chornobyl which ELI5’s a nuclear meltdown. [1]

If the reactor is air cooled, that can be translated as “the heat can just burn off” as there isn’t a liquid cooling system, reducing this failure mode.

As mentioned in the video, reactions are about balance. If a ‘safe’ SMR gets shut off or has its systems fail, you would want the accelerators / enablers of the reaction to disappear, and roll out the reaction.

This is unlike most fission reactors in use today which are a ‘balanced dance’ where lots of the design is focused on reducing the speed of the burn (like a constant stream of water on a fire) rather than say controlling the oxygen of a fire with limited fuel. SMRs aim be more of the “control the oxygen” type of reaction rather than “keep the fire cool” kind of reaction. (Forgive my vast oversimplification, but this is my intuition on it).

[1] https://youtu.be/TmIEI4ky-Zc

TRISO fuel are encapsulated in very small balls with its own containment. Those balls are hardened and won't melt in operation.

If those balls are removed from the reactor, it won't have the critical mass to maintain the reaction.

We had a TRISO fuel nuclear reactor in Germany, it had a lot of problems and was an expensive failure. Among other things the balls broke.

See https://en.wikipedia.org/wiki/THTR-300 or if you can read german: https://de.wikipedia.org/wiki/Kernkraftwerk_THTR-300 with a lot of details.

The balls sound a lot tougher in the new design. From your wiki link:

> spherical fuel compacts each 6 centimetres (2.4 in) in diameter with particles of uranium-235 and thorium-232 fuel embedded in a graphite matrix

And from the article on Canada's reactor:

> The fuel inside of this vessel are solid kernels made from a mix of uranium, carbon, and oxygen, with each being roughly the size of a poppy seed. These kernels, which are covered in several ceramic coatings, are then encased in a diamond-like substance

That substance is silicon carbide: https://www.ans.org/news/article-2830/a-first-triso-fuel-mad...

Also, the wiki says "a fuel pebble became lodged in a fuel feed pipe to the reactor core." In the new design, there's "no on-site fuel storage, handling, or processing." If the fuel elements are just sitting there instead of getting moved around in pipes, there's little reason for them to break.

Agreed, they hopefully fixed that particular issue.

However, it is prudent to expect other problems in new technology. And nuclear reactors need a huge initial investment (they seem to be vastly over budget and time these days) and take a long time until they are available. It seems to be cheaper, safer and faster to go with renewables, doesn't it?

These are reactors small enough to build in factories. That's an entirely different situation from occasionally custom building huge reactors on site.

For now, we should certainly keep rolling out wind and solar as fast as we can. But whether they can run entire countries reliably through all seasons is unproven so far. They're backed up everywhere by hydro where available, otherwise fossil. It's entirely possible that mass-produced passively-safe nuclear will end up cheaper and faster than the vast amount of storage/transmission/etc we'd need for a 100% wind/solar grid.

Reply on news.ycombinator.com