[morning_gelato]

Why is the runaway scenario unavoidable for nuclear fission? Pressurized water reactors for example have negative void and temperature coefficients, and from what I've read about potential future reactors designs (e.g. high temperature gas reactors and some of the molten salt reactors) passive/inherent safety is a major selling point.

[willis936]

All of those solutions involve better ways at keeping neutron multiplication factor below 1. Fission reactors need fuel that naturally have an eta above 1. Fission reactors are also designed to have large quantities of fuel inside of them. So if the control systems fail, even if those control systems are built-in chemically, then you have a disaster that lasts for timescales that humans would prefer not to be on the table.

[morning_gelato]

What's a scenario where passive safety systems, say those of a HTGR with TRISO fuel, fail and cause it to go prompt critical? I'm genuinely curious about this, as everything I've read suggests this is essentially impossible due to the design of the reactor, fuel, and coolant.

[willis936]

I don’t have an answer. Not knowing failure modes doesn’t mean they don’t exist. HBO’s Chernobyl series highlighted how dangerous surpressing information about fission reactors is. No one knew how the design could fail until it did and then it was painfully obvious. I’m not saying that HTGRs can meltdown nearly as readily as RBMKs, but the risk of the unknown needs to be given respect when the stakes are high.

It’s difficult to be sure of safety in complicated systems when the only people with enough technical expertise to fully vet the systems have an interest in their success. I’m not saying it can’t be done, but I think it slows policy down significantly.

[morning_gelato]

How could someone demonstrate the safety of these systems if their very association with those systems is a sufficient reason for you to doubt them? If the research and experiments of nuclear engineers, scientists, and regulators from around the world cannot be trusted to develop or assess the safety of fission reactors, why does this change with fusion? I also have not seen evidence that anyone is attempting to suppress information about nuclear safety. Overall nuclear power has an outstanding safety record and ranks among the lowest deaths per TWh of any energy source (and this includes Chernobyl)[1][2].

For the record the HBO series on Chernobyl, while a good show, greatly exaggerated parts of the story. There was no threat of a megaton-level thermonuclear explosion that would destroy Kiev or make huge parts of Europe uninhabitable from the melted core coming in contact with water. The soviets did know about the RBMK's propensity to have a runaway reaction, and the rest of the world never allowed those types of reactors to be built.

[1] https://www.statista.com/statistics/494425/death-rate-worldw...

[2] https://ourworldindata.org/grapher/death-rates-from-energy-p...

[tehjoker]

Assuming all that is true, reality is extremely unpredictable. Imagine a country is at war and they accidentally drop bombs on the reactor that crack the fuel and change the chemistry enough. A volcano erupts under the plant. Imagine a nuclear weapon going off nearby and causing a meltdown (for example, if the attacker was using a "low yield" neutron bomb). Imagine an astronomical phenomenon that happens to pass through the plant.

Low probabilities, but man they would suck.

[Ixio]

We keep finding new ways to make fission safer and reducing risk of runaway scenario but I'm pretty sure we'll never reach zero risk. Sure we might reach it on paper but human error can always happen. Chernobyl operators thought their reactor design had zero risk of exploding, current reactors are much safer but I'm pretty sure the risk isn't zero.

[morning_gelato]

With modern reactor designs the inherent safety mechanisms mean that humans are not in the loop to reduce reactivity or remove decay heat.

Here's an example from Argonne National Laboratory:

> In the first test, with the normal safety systems intentionally disabled and the reactor operating at full power, Planchon's team cut all electricity to the pumps that drive coolant through the core, the heart of the reactor where the nuclear chain reaction takes place. In the second test, they cut the power to the secondary coolant pump, so no heat was removed from the primary system.

"In both tests," Planchon says, "the temperature went up briefly, then the passive safety mechanisms kicked in, and it began to cool naturally. Within ten minutes, the temperature had stabilized near normal operating levels, and the reactor had shut itself down without intervention by human operators or emergency safety systems."

https://www.ne.anl.gov/About/hn/logos-winter02-psr.shtml

[stormbrew]

There is obviously a difference between:

- There are many passive systems that work in concert to prevent the fission material from having a runaway chain reaction that continues on its own,

and

- It is literally impossible within our understanding of physics for the reaction to continue without the continued application of power to the reaction chamber.

No matter how 'safe' the former gets, it's just asymptotically approaching the latter. There will always be more assumptions and caveats involved in preventing a self-sustaining reaction from continuing.

In particular, re. that article, a lot seems to be resting on the sodium cooling pool being present while there's something else going wrong. So what if an earthquake breaks it open and dumps it out. Or a bomb.

[nickik]

Their risk isn't zero but even the 60s the inventor of PWR said this was not very safe design and designing inherent safely systems are far better.

Of course you never have zero risk. That literally impossible and not a standard you would use for literally anything else in human existence.

The fact is, you can design nuclear power plants that are so safe that the chain of events you had to come up with to get any radiation outside of the reactor safety boundary is so ridiculous that the probability of them happening is barley measurable.

Sure if you have human error and 3 black swan events on the same day, the risk is not zero.

But even if you come up with these crazy events the damage from those events would be a far smaller then Chernobyl and Chernobyl was also far less damaging then in popular imagination.

The risk that somebody dies during the construction of the reactor confinement building is probably 100000x higher, but nobody seeks to prevent ever building large structures.

> Chernobyl operators thought their reactor design had zero risk of exploding, current reactors are much safer but I'm pretty sure the risk isn't zero.

This is where we are with nuclear. Any debate goes back to Chernobyl. Again, in no other area do we go and say 'well the soviet thought this in the 60s so therefore we can never moved past it'.

There is fundamental physics and chemistry involved and just because some soviet operators didn't know that does mean its unknowable.

Humanity should be living in the nuclear age. Climate change would not even be a thing if everybody had done what the French have done in the 70s. And we would be much better in terms of space exploration if the whole world were not so reluctant about using anything nuclear.