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BrightCharlie t1_jc1xudx wrote

Reply to comment by Hiddencamper in Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama

>Why there weren’t mechanical limits on the control rods equipped with followers or other system interlocks is beyond me

To be fair to the designers, they did have to override a bunch of automatic and safety features that existed precisely to avoid accidents like that.

I'd argue that what happened in Chernobyl wasn't exactly an accident, because they deliberately put the reactor in a state where bad things would definitely happen -- as they did.

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BullockHouse t1_jc23dm0 wrote

It's definitely a mix of poor design and operator error. In general, if the reactor is beginning to show behavior you don't understand, that's an emergency and you need to execute a safe shutdown immediately. When they got into the xenon pit behavior and didn't know what was going on, trying to power through it without actually understanding what was happening was idiotic.

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Shadeauxmarie t1_jc2rlc1 wrote

“Back away from the reactor. It’ll save itself.” Navy nuclear power program.

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BullockHouse t1_jc3ltwc wrote

The naval reactors are rad. Some very neat architectures on both the US and Russian side, and extremely good service records.

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Stillwater215 t1_jc5caks wrote

Passively safe reactors should be the future of electricity generation. Modern reactors are designed so that the job of the operators is to “fight” the reactor to make it more reactive. If they walk away or are incapacitated, the reactor brings itself into a steady, low-power state. But whenever people think of nuclear power, they only think of Chernobyl…

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Shadeauxmarie t1_jc5hki7 wrote

Common question for Navy nuclear watchstanders: “Everyone dies on the ship. How long will the reactor run and what will cause the shutdown?”

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ThatOtherGuy_CA t1_jc2aq85 wrote

The biggest issue with Chernobyl, which was also showcase in the show, wasn’t that the reactors had a potentially dangerous design, it’s that the Soviet Government hid the flaw from the reactor operators. So to their understanding an RMBK reactor couldn’t possibly blow up. Because the boron control rods would kill any reaction. And they either weren’t aware of the carbon tips, or at least the risks they posed.

So yes, the operators intentionally cooked the Chernobyl reactor to a point where it was a bomb, but they felt safe doing it because they had complete confidence that AZ-5 would kill the reaction. Not act as a detonator.

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Somnif t1_jc2td2d wrote

Yeah, something the show didn't really mention was that Chernobyl Unit 1 had actually suffered a similar (but much less severe) incident a few years earlier. It was not just a known issue, it was a known issue on site! And still, hushed up and hidden.

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Hiddencamper t1_jc2mhcl wrote

Exactly.

Today if you say the words “reactor safety limit”, that’s an inviolable parameter. If a reactor safety limit is exceeded the plant cannot restart without approval (10cfr50.36). And if there is a potential to exceed one, you are in a reportable event (for example is a safety system was found degraded such that it would actuate too late to protect the safety limit).

As reactor operators we are required to know them from memory.

The same level of deliberate caution around those limits likely did not exist with the USSR and the RBMK design, as evidenced by them withdrawing rods as much as they did. When rods are they far out in the RBMK, you not only get a positive reactivity spike on a scram, but you also magnify your positive void coefficient.

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RexStardust t1_jc27u7l wrote

Yes but they did so with the understanding that they had the ability to pull the plug with AZ-5. The designers and/or the overall operating authority knew that you needed to use AZ-5 earlier because of the initial reactivity of those graphite tips, but plant operators had not been informed.

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Y34rZer0 t1_jc4muxv wrote

That sounds fairly Soviet… “Let’s keep the reactor operators as in the dark as possible”

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Sythix6 t1_jc3eo36 wrote

I'd agree with that argument, everyone was warned, everyone still followed bad orders, all because Russia cannot look weak at any time, Soviet era Russia was rampant with these types of accidents, not as globally impactful though, most stemming from lack of quality materials caused by embezzlement from generals and other higher ups.

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pzerr t1_jc4ut9j wrote

Every time I read the events that led up to this I want to yell 'don't do it'. Even though I know the outcome I just feels if I yell loud enough they will hear me.

There were so many steps that led up to this. Had they stopped at any of them, this could have been averted. That design was just a disaster to happen all the same.

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