[PaulHoule]

The more detailed simulations have gotten the less bad a meltdown looks in a fast reactors. Usually some of the molten core flows away and no more critical mass. If it goes over critical there can be some energy release but over time it looks less and less and not a problem to contain.

Sodium has its problems (burns in carbon dioxide!) but the chemistry is favorable for a meltdown because the most dangerous fission products are iodine and cesium. The former reacts with the sodium to make a salt that dissolves in the sodium, the second alloys with the sodium. Either way they stay put and don’t go into the environment.

[pfdietz]

The problem is you need to ensure it's not bad in any possible configuration from an accident. This is hard to do. Will the energy release at criticality drive the material into an even more critical configuration? Such "autocatalytic" systems were considered for bomb design, but weren't chosen because of the large amounts of plutonium needed. But a fast reactor might have the plutonium of hundreds of atomic bombs.

Edward Teller famously warned about this is a nuclear industry trade publication in 1967.

The only fast reactors I'd trust would be ones with fuel dissolved in molten salt; it's hard to see how that could become concentrated in an accident that doesn't boil the salt. But such reactors have their own problems, in particular exposure of reactor structures to intense fast neutron fluxes (not as bad as in fusion reactors, but worse than LWRs.)

[ttfkam]

Increasing the heat past a certain threshold reduces the nuclear reactivity. Read up on "passive safety".

Teller may have warned about this in 1967, but nuclear technology hasn't been stagnant since 1967. Folks read his stuff and designed systems specifically to fail safe, not run away. Stop fear mongering based upon a 60-year-old supposition. Stop assuming everyone working in the nuclear industry is an idiot that hasn't thought about safety.

[pfdietz]

> Increasing the heat past a certain threshold reduces the nuclear reactivity. Read up on "passive safety".

The safety arguments for fast reactors are typically that a serious scenario will not occur, for example that fuel won't melt, not that if it does occur the results won't be bad. Do you trust that sort of argument? I don't.

[ttfkam]

Nice straw man you've constructed and burnt down.

Those are NOT the safety arguments used within the industry. For example in a molten salt reactor, the fuel is already melted! If it gets too hot, thermal expansion moves the radioactive isotopes further away from one another, reducing reactivity. If heat increases past a certain point, plugs at the bottom of the tanks will melt, allowing gravity to dump the fuel into multiple separated storage vessels sized to prevent further activity.

You do not know what you're talking about. You've read a bunch of fear mongering, and bought it. Do you really believe the entire industry of nuclear engineers and support staff are just blindly YOLOing their way through their jobs, damn the consequences?

I swear, you sound like the power production equivalent of antivaxers convinced the medical industry is trying to poison all of us.

"Passive safety" doesn't mean "stuff shouldn't go wrong."

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

It means "we're actively exploring everything that could go wrong and having the worst case scenarios fail to a safe state without requiring human intervention."

Those two positions couldn't be further apart.

[pfdietz]

So, how exactly do you claim conclusively that molten fuel (in a fast reactor with sold fuel elements) will not flow into a bad configuration? I don't see how one can possibly do that analysis. Teller didn't see how either.

I already said MSRs would be the one kind of fast reactor I could see the analysis work, so thank you for agreeing with me on that.

As a charitable act toward you I will ignore the rest of your comment.

[ttfkam]

> I don't see how one can possibly do that analysis.

Classic argument from ignorance fallacy. Because YOU cannot see how it could be done, no one has ever figured out how it could be done.