Hacker News Reader

Comment by kbenson | Hacker News Reader

Wasn't the big push (or maybe big PR push) for more research and development of Thorium reactors a few years ago because they fail closed/safely? That seems like the kind of thing you'd want for smaller reactors (which I assume means more and more geographically diverse, but don't really know).

View on HN

Yes. Molten Salt Reactors (MSR), while operating very hot, are not operating under a lot of pressure. A pressurized water reactor (PWR) does, and if there is a leak or other problem, it can turn into an explosion.

you are confusing two different characteristics of reactor design.

one is whether or not the coolant is highly pressurised relative to atmosphere, the other is what the thread was originally about, which is how reactivity changes with respect to temperature. this is the temperature coefficient of reactivity.

https://www.nrc.gov/reading-rm/basic-ref/glossary/moderator-...

the people you were responding to were referring to the latter. specifically, if a reactor becoming more reactive results in more or less total reactivity. this characteristic is related to, but independent of, how pressurized the coolant and/or moderator are.

While not under pressure, molten fluoride is still molten fluoride. It needs to be contained well. You wouldn't want water getting anywhere near it.

There's no research being done on materials to safely contain molten radioactive salt, that research could be dangerous, pretty much only superpowers would have the resources to do the research.

This was being done three years ago: https://www.powermag.com/thorium-molten-salt-reactor-experim...

Also, I believe Copenhagen Atomics is doing this at the moment, but couldn't (quickly) find a reference.

That's cool, thanks for the link.

Bill Gates is funding a company called Terrapower that is planning to bring compact MSR to market in the next 10 years. They already have working prototypes but are still a few years behind NuScale.

Wasn't Terrapower trying to develop a travelling wave reactor? Or is that a MSR design as well?

Terrapower is (was?) developing multiple technologies. They got approval for their traveling wave reactor (which uses molten sodium, not molten salt, but they are easily confused for obvious reasons) last week. Not sure if they're going to keep developing other techs.

Really? I literally read dozens of papers about it a decade ago, at least... I guess I don't work adjacent to there so maybe no one else does either. Odd, I never have heard a reason why they were all that much worse than alternatives other than for corrosion and licensing costs, I am surprised that such a well defined problem just stopped being studied.

I suppose at some point it's not competitive with solar so what's the point. Maybe we reached that threshold recently.

Ok, there probably is actually lots of research going on, what little I know of it is several years out of date by now, at the time, advocates for the tech complained about the lack of research being done and I took them at their word.

> materials to safely contain molten radioactive salt

Wouldn't a giant steel or stone vat work, in theory?

The problem is that the nuclear reactions produce many random materials which in turn might be corrowive to the reactor veasel. Dont forget that the vessel has to last decades, so even minor corrosion is a big deal.

No, steel doesn't work, the salt is too corrosive. It's a very challenging problem, imagine you have half the periodic table floating around in a very radioactive salt at high temperature.

Early molten salt reactor experiments used Hastelloy N (nickel: 71 wt%, molybdenum:16 wt%, chromium: 7 wt%, iron: 5 wt%, others:1 wt%), which apparently works. But as others have noted, there is ongoing research to further improve it.

Metals melt at lower temperatures than salt.

Also think of how saltwater effects metal. That's just a bit of salt dissolved in water. Molten salt is like that, but way more concentrated, and hotter. Corrosive reactions go faster when you add heat. Molten salt is like metal's worst nightmare.

That's before you even add the radioactivity into the mix, I believe that makes this problem an order of magnitude harder than just trying to contain molten salt, which is already kinda pretty hard.

Stone, maybe, something like a specialized ceramic could work?

You'd still need a heat exchanger at some point, and you want a lot of surface area and thin walls for that.

Random thought experiment -- why not just drop it in the middle of a lifeless desert, forget about heat exchangers, and just let it slowly melt its way down through the crust of the earth? Assuming it went straight down and into the mantle, it would probably not impact the ecosystem.

Even in a desert, you'll cross the water table before reaching the mantle. And people prefer to consume electricity in hospitable locations.

"probably" being the key word here; Chernobyl was in a fairly remote area but the fallout spewed over a big part of Europe.

If you go to the desert to make use of the space, might as well build a solar array instead.

Wouldn’t be particularly helpful for power generation since the salt needs to be cooled by water to create the steam to make the power.

That's more or less what terrapower plans to build: dig a hole fill it with nuclear stuff and let it burn for decades.

Like an uncontained nuclear reaction spewing radioactive materials over a large area?

brb inventing teleportation

Reply on news.ycombinator.com