[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.