Dropped rods are an incident but one that occurs because of pressurized water reactors being very default safe. Controls rods are one way the criticality of a reactor is controlled and US reactors (in general) will go sub critical if even one rod is fully inserted into the core. You will likely have heard of a reactor scram (which goes back to the safety control rod ax man) where in an emergency, all the rods are dropped back into the core, greatly reducing its criticality. In some cases, an interruption of electrical power will cause a rod (or three) to drop accidentally. This is a "dropped rod" incident and will force a reactor shut down because it is now sub critical.
Lots of knock on effects -- sub critical, let heat in the primary loop, less steam and electricity generated in the secondary loop, etc -- but generally a non event that you practice for.
There's no reason this would lead to a radiological event or more significant casualty.
OG article makes it seem the control rods detached/broke off from an actuator mechanism right into the core.
As someone who’s had to fish out things from awkward places (and sometimes to lose them again right after), I started wondering how they’d be getting it out.
Maybe a non-event for safety but what about getting the plant operating again?
I don't know this plant. In navy submarine reactors like the S5W, you have 440V/3-phase power holding the clamp. If you dropped a rod due to a power or clamp malfunction, usually you'd simply shut down the reactor, drop all the rods and mechanically re-engage the clamp. It could take a few hours which isn't fun because no power which probably means submarine/snorkel depth to run the incredibly noisy diesel, but no step was particularly hard or dangerous.
Now I’m curious. What if it occurs when you’re like under the enemy and you can’t surface? Do you have to just chill at depth until you can get the reactor running again?
Almost certainly a better problem to have.
In case you weren’t aware, the “safety control rod ax man” thing is aprocryphal and almost certainly not reflective of the origins of the term.
It's one of the more truthy backronyms. There was an actual ax man with an ax whose job was to cut a rope to let loose the control rods, and they did use the (existing, apt) word, scram, for the emergency procedure: https://en.wikipedia.org/wiki/Scram
Exactly what I would have answered :)
None of this contradicts the point I was making, which is that the OP implied that’s what the term meant, and it simply isn’t.
Simpson's?
Well, we don't know exactly why the reactor was manually shut down. Did the person actually know that the control rods had dropped? Or was it reported by sensors? I suspect the procedure us to initiate shutdown in response to any reports of uncommanded rod behavior. I doubt they would wait around to see if this was a sensor or reporting system error.
The dropped rods already shut down the reactor in the sense that it is sub critical. It still has enough leftover heat that it will continue to produce steam for a while, but it would have been almost instantly clear to the operators what had happened.
[Correction] as noted below, this type of reactor -- unlike US Navy ones -- can remain critical even with dropped rods. So a better statement here would be that the operators would immediately notice a reduction in criticality. thus power and heat. Then it's a process/policy question about shutdown timing.
Navy reactors are physically tiny. A rod in a reactor the size of a garbage can means much more than a rod in a reactor the size of a studio apartment.
Thats a releaf, nothing like an uncontrolled saftey event to take a bit of pressure off
I think you said it: it's a non-event. It's the reactor reactor-ing within the design basis. But, I think "single rod-drop" will not shut down a reactor. That's a crazy amount of rod-worth. My opinion is that reactor safe-shutdown margin entails "single rod-stuck" scram to be feasible.
> But, I think "single rod-drop" will not shut down a reactor
Why do you think that? When rods are used to control the reaction rate I thought it was by raising and lowering them small amounts usually.
Being able to have a single fully inserted rod stop things would certainly make it more likely dropping rods into the core would stop an even than if you needed (say) all 3 to be fully inserted.
3x the chance of successfully stopping a serious problem, 1/3rd the chance of a blockage prevent the emergency shutdown.
I'm already getting downvoted to oblivion for my gp comment, but whatever: what I was trying to convey based on my limited exposure is that a single rod will have a zone restricted shadow or local effect and multiple uniformly distributed rod banks are therefore needed for a an effective control or emergency shutoff. Additionally, on the flip side, I feel a single rod capable of shutoff will insert excessive positive reactivity in case of an ejection (could be a local steam overpressure, idk, don't hunt me down with hounds).
Why don't you just look it up instead of actively arguing an assumption?
This reactor type has 73 assemblies of 5 rods each.
Nuclear safety regulations require that the control rod system must be powerful enough to bring the reactor completely subcritical with all Control Element Assemblies (CEAs) inserted minus one assembly stuck in the fully withdrawn position.
One assembly (or 3) inserted would not bring the reactor subcritical.
Yup, this is me focusing on US Navy reactors, which is my experience. For them, a rod would already take you sub critical.
>generally a non event
depends on what your priors are. Is a dropped rod a fukashima or chernoble international emergency? No.
However, depending on the design characteristics of the core, dropping rods lowers the heat distribution proximal to the dropped rod(s), but may result in higher peaks distal to the rods as physics characteristics attempt to maintain the average, which may exceed design limits.
Furthermore, rods aren't supposed to drop. How that came to pass is a matter that requires honest investigation - manufacturing/quality control, maintenance practices, personnel error? Much to learn to ensure safety.
Saying "we just had to rely on a critical reactor safety feature and scrammed, this is a nonevent" doesnt quite hit the mark.
Of course, but that's not what happened here. For a pressurized water reactor, there are maintenance and procedural questions about why the rod was dropped. People will likely get fired over this. But from a standpoint of reactor and radiological safety, this is largely the design operating correctly and prioritizing radiological containment.
and for the curious who didn't what Chernobyl on HBO, other types of reactors have brief criticality increases on rod insertion, which is somewhere between bad, BAD, and OMFGWTFBBQVERYBAD. In the Chernobyl's case, they entered a criticality region known as "prompt criticality" which is where power doubling can happen at prompt neutron rates (e.g. milliseconds or faster). Very very bad. In the case of pressurized water reactors, this isn't what happens. Every moment of control rod descent is reducing criticality.
> Of course, but that's not what happened here
They dropped rods and had to scram, unclear what part of that you think is not relying on a safety feature. Why do you think they scrammed?
>this is largely the design operating correctly and prioritizing radiological containment.
Is that largely what is was, or entirely what it was? Im not sure what your point is.
> there are maintenance and procedural questions about why the rod was dropped.
I agree, that's why I said it.
> People will likely get fired over this.
I dont know why youre telling me this, you're the one that called this a nonevent
The entire point of critical safety features is to turn events into non events.
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Except no bot would write "chernoble", unless it's manually edited after the fact
Or "fukashima", which I think also rules out an AI editing pass at the end. That said, the smell of Claude specifically is pervasive in that comment. Perhaps someone who's ingested a bit too much Anthropic output?
"I was in the US nuclear navy, now I work in IT."