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.
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.
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
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.
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.
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.
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?
Can someone check my understanding: Control rods are suspended above the reactor as a sort of a "deadman's switch" - if a loss of power occurs, they will drop into the reactor core to limit reactivity. As a result, sometimes control rods will drop into the reactor accidentally. In these cases, the automatic system may withdraw other rods in order to maintain power output. However, this may overshoot the target power output or create a localized, dangerous "hot spot" elsewhere in the reactor core. In order to avoid this, in this case, the operators chose to insert other control rods instead of trying to stabilize the reactor under load.
Yup. Often one rod fully in is no big deal -- it has been analyzed and you keep operating. But no one has certified to run the reactor with a few rods fully in.
The control systems are designed to err on the side of dropping the rods -- a failure to insert a rod is a lot worse than inserting one accidentally. So now they figure out why this happened while the reactor is in standby.
> sometimes control rods will drop into the reactor accidentally. In these cases, the automatic system may withdraw other rods in order to maintain power output
So this suggests that the dead man's switch has an automated anti-dead-man's switch that may result in more danger?
It'd be an interesting design choice, but defendable, if the "accidental dropping" rate is too high.
Safety wise, it would seem more sensible to try to keep the automatic drops unchallenged and try to prevent the conditions that make it happen.
Fission reactors are ran by pulling rods out just enough to keep the system at a state of equilibrium. If one of rods accidentally goes in, for any reason, the nuclear chain reaction therefore heat generation will slowly reduce. If there was also an automatic system that wasn't aware of that rod going in, it wouldn't be so weird that it will try to fight the fire suddenly dying down.
I actually thought they were talking about the island in the Caribbean since Florida is not mentioned in the article. Anyway, in case you did not know, the reactor in question is in Florida: https://en.wikipedia.org/wiki/St._Lucie_Nuclear_Power_Plant
> The name "St. Lucie" is originally derived from the name of a settlement near Jupiter Inlet, which was founded on St. Lucia's Day in 1566. Due to numerous errors, the name later came to be associated with the present-day town of St. Lucie Village, Florida, north of present-day Port St. Lucie. After La Florida and St. Augustine, it is the oldest still-in-use European place name in the United States.
I came here looking for that information, as I was incredibly surprised to learn of nuclear energy existing in the Caribbean.
It would be a dream to no longer have to rely on fossil fuels, but my native island is at the peak possible cross-section of hurricane, earthquakes, and volcano eruption risks, so I guess we're only missing tsunamis and dry forest fires to complete our bingo for "best place to build nuclear"
A challenge with news like this is there isn't an easy risk reference to put it into perspective. People only think of Chernobyl, Fukushima, 3-mile island, etc etc. We don't have something personal to relate it to to help us determine 'NBD, why is this a story?' or 'Wow! That was close!'. In everyday life I generally use driving to help me determine how risky something is. Driving is something most people do somewhat regularly and that we recognize as risky so we put controls on like training, regulations, special services, etc etc. Anything more risky than driving makes me really take notice. Anything less and I don't shrug it off, but I also don't get too worried.
I appreciate the factual nature of this story but I think an important aspect of journalism is giving people relatable references that allow them to better understand the information presented. It would be helpful if a story like this gave that context because without it it is hard to understand if this is 'Wow! What a close one!' or 'I guess this is a little interesting?'.
I have mixed feelings. It seems like nothing serious happened, no one was hurt, and all the systems worked, but on the other hand, this is still an emergency. Why did those rods fall in the first place?
The rod holding mechanisms require active power to hold the rods out of the reactor. It sounds like once in a while, a rod accidentally drops, which is infinitely preferable to the alternative, of once in a while a rod failing to drop. But that unplanned adjustment to reactivity means, you don't want to simply use other means to boost the reactivity back up (like reducing boric acid in the water), because once you get the rod back out, it'll be too reactive. So they insert a couple more rods and call it a shutdown to take a few days or weeks to investigate and perform maintenance. Sounds like this is a highly planned for and fairly routine procedure, if not ideal. I'm sure they'll find a couple reasons why the rod unexpectedly dropped, and make changes to prevent an identical incident in the future.
The rods prevent an emergency. They’re a fail-safe.
It’s possible nothing was wrong and some sort of “blip” triggered them when they weren’t needed. And if so that’s annoying for the operators, but nothing bad was happening. And it confirmed the failsafe works.
If anything was starting to happen and the rods dropped and immediately stopped it that’s a failsafe working perfectly.
Either way this is fine. I know nuclear + unexpected = scary for many (maybe not you), but I don’t think that’s necessary here.
This "dropping" activity is not defined well in the context. Was it e.g. a controlled but uncommanded insertion? Or did something or some three things slip, snap, or something else? Can the rods be retrieved using BAU procedures, or is reactor in an annoying state now?
The details divulged seems to have been driven by event reporting regulations, not any specific news gathering or investigation, right?
This article is just an over length summary of a very short preliminary notice published by NRC. Since it is non-emergency there will not necessarily be much follow up either, plants are just required to report any abnormality to NRC which publishes the reports online. Most of these reports are, like, procedural problems in employee drug testing.
Control rods are designed as fail safe so lots of scenarios can cause them to drop, could be a mechanical failure but probably more likely it was triggered by an electrical fault or intentionally by a safety system due to some condition they haven't quite worked out yet. Since having control rods in puts the reactor out of it's approved operating envelope it will lead to a trip out of an excess of caution. The fact that the trip wasn't triggered automatically suggests to me that the rods probably weren't inserted by a safety system but due to some fault.
Perhaps an interruption of power to the field windings of the affected control rods. Could be as simple as a control cabinet fried unexpectedly or some other electrical gremlin.
3 rods inserted into the core out of the many available would have disrupted the neutron flux profile away from what the reactor is intended to operate against in steady-state, so manual shutdown is the right answer until the cause of the control rod being dropped can be figured out, repaired, tested and then the reactor restarted.
But while not typical, this kind of event has happened many times before and will again. Pretty boring IMO, which is why I agree with the non-emergency declaration by NRC.
This could be an intentional design feature regarding power loss to critical reactor control systems.
Rig N+2 control rods with an actively powered electromechanical retaining mechanism. In any kind of serious electrical issue occurs on the protected circuit, all 3 drop automatically.
Yeah, on one of the reactor designs I was familiar with, control rods were bundled into groups which were managed as a set and were intended to be operated in different sequences over core life.
So when I read "3 control rods hit the bottom" what I hear is "there was an unexpected group scram", but I don't know enough about this reactor design to know if that's actually what happened, or whether it was 3 unrelated control rods instead.
This is not a paper. This is local TV news: it says WPTV West Palm Beach in the top left. It also says it is converted from an on-air report. My hunch is their audience knows what they mean when they say St. Lucie. Still, it seams like it wouldn't hurt in cases like this where the story gains a wider audience.
If you've never seen the Smarter Every Day series on nuclear reactors[0], it's must-watch. It completely reshaped how I think about reactors. Tremendous respect for the people who work on those plants.
Which indicates that there’s so much understanding of how it works and the power that it uses that compensatory systems have been created to increase its reliability
what precisely is your point of describing how quality production engineering for infrastructure works?
I doubt AGI will be used for life-or-death systems like that if it resembles anything like the LLMs of today. Systems at that scale are about risk management, and everything we know about AI suggests that it cannot mitigate risk at all. All AI is seemingly a net-positive risk factor.
Really, the "big AI story" will be when the next Snowden defects from the NRO and tells us what SENTIENT is doing. Until then it's just going to be a bunch of pie-in-the-sky wishcasting about what we want the future to look like. It's getting a little tiring hearing people insist that abundance is around the corner while surveillance is here-and-now.
The way I read it was that he expects large industrial systems with AI to have safeguards to "fail closed."
Which: it seems like we're already getting to the point of capability vs. unexpected behavior that having the ability to "trip" a complex system may be an important design criterion.
I mean, there's plenty of harm these systems can do besides trying to turn us all into paperclips.
A) Let AI override safety mechanisms (increases uptime while endangering humans)
B) Let AI impose additional safeties (decreases uptime with questionable safety uplift)
If you intend to go with option B) then there's basically zero benefit to investing in that kind of automation. AGI will be fallible, and even with all the safeties and safeguards it's hard to see what tangible benefit AGI would add to the system. It'll be a hard sell for critical infrastructure of any kind.
I understand that it's hard to get a man to understand something when his AGI Society executive credentials depend on it, but it feels glaringly obvious why AI isn't masterminding these sorts of systems. Even accounting for unprecedented advancement in AI, it's hard to see what AI could contribute to managing these things.
> It'll be a hard sell for critical infrastructure of any kind
Remember, critical infrastructure attached SCADA systems to the internet and has had trouble removing them all despite 30 years of warnings and audits.
There's definite advantages to having a LLM in the loop. The best we can hope for is that it's architected in a way that it fails safe and damages are limited, so we get some of the advantages but less of the downside.
That assumes people mistrust LLM output, which is a perspective inside the HN bubble.
Outside, almost everyone I know - including highly educated people - takes even Google's search LLM output as fact, without a second thought, and even regarding serious issues (e.g., medical problems). Nobody raises a doubt or an eyebrow. A recent article says members of US Congress use it extensively for serious work. Lawyers use it to file cases. Doctors use it to summarize essential medical information. I even hear that software developers use it to create high-risk, high-value code.
In the public people think computers are superior to humans - more authoritative, objective, perfectly logical and error-proof. SV pushes that narrative, for example saying computer-driven cars are safer because computers don't have human flaws (that old sales technique - compare only the competition's unique flaws and don't mention your own; computers have their own flaws that humans lack). And of course, that one is popular on HN.
(IMHO the way to educate the public is to spread the narrative that every computer is created (designed, programmed) by normal, flawed, biased, etc. humans, and controlled by other flawed humans for their own agenda. And that computers have their own flaws.)
"Current Status: While "Difficult-to-Return Zones" still exist, decontamination and lifting of orders have reduced the designated area down to approximately 2.2% of the prefecture's total land area."
This graphic [1] may help others to understand this was normal behavior and what exactly took place. An actual engineer would have to confirm but from other comments this appears mostly accurate.
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.
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.
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.
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.
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
This link (second one down, first event is missing control rods in Texas...) has almost the exact same wording as the article.
https://www.nrc.gov/reading-rm/doc-collections/event-status/...
I found the root cause of the older issue on LinkedIn of all places.
https://www.linkedin.com/posts/robert-meyer-8672ab4a_st-luci...
Root cause of 2024 was apparently a procedural issue combined with electrical failure of the control rod grippers.
Can someone check my understanding: Control rods are suspended above the reactor as a sort of a "deadman's switch" - if a loss of power occurs, they will drop into the reactor core to limit reactivity. As a result, sometimes control rods will drop into the reactor accidentally. In these cases, the automatic system may withdraw other rods in order to maintain power output. However, this may overshoot the target power output or create a localized, dangerous "hot spot" elsewhere in the reactor core. In order to avoid this, in this case, the operators chose to insert other control rods instead of trying to stabilize the reactor under load.
The control systems are designed to err on the side of dropping the rods -- a failure to insert a rod is a lot worse than inserting one accidentally. So now they figure out why this happened while the reactor is in standby.
So this suggests that the dead man's switch has an automated anti-dead-man's switch that may result in more danger?
It'd be an interesting design choice, but defendable, if the "accidental dropping" rate is too high.
Safety wise, it would seem more sensible to try to keep the automatic drops unchallenged and try to prevent the conditions that make it happen.
> The name "St. Lucie" is originally derived from the name of a settlement near Jupiter Inlet, which was founded on St. Lucia's Day in 1566. Due to numerous errors, the name later came to be associated with the present-day town of St. Lucie Village, Florida, north of present-day Port St. Lucie. After La Florida and St. Augustine, it is the oldest still-in-use European place name in the United States.
[^1]: https://en.wikipedia.org/wiki/Saint_Lucia
[^2]: https://en.wikipedia.org/wiki/Port_St._Lucie,_Florida
It would be a dream to no longer have to rely on fossil fuels, but my native island is at the peak possible cross-section of hurricane, earthquakes, and volcano eruption risks, so I guess we're only missing tsunamis and dry forest fires to complete our bingo for "best place to build nuclear"
You may also find it interesting that South Africa had nuclear weapons:
https://en.wikipedia.org/wiki/South_Africa_and_weapons_of_ma...
Despite the appearance, is it uniquely prone to flooding or storm surges?
I appreciate the factual nature of this story but I think an important aspect of journalism is giving people relatable references that allow them to better understand the information presented. It would be helpful if a story like this gave that context because without it it is hard to understand if this is 'Wow! What a close one!' or 'I guess this is a little interesting?'.
It’s possible nothing was wrong and some sort of “blip” triggered them when they weren’t needed. And if so that’s annoying for the operators, but nothing bad was happening. And it confirmed the failsafe works.
If anything was starting to happen and the rods dropped and immediately stopped it that’s a failsafe working perfectly.
Either way this is fine. I know nuclear + unexpected = scary for many (maybe not you), but I don’t think that’s necessary here.
The details divulged seems to have been driven by event reporting regulations, not any specific news gathering or investigation, right?
Control rods are designed as fail safe so lots of scenarios can cause them to drop, could be a mechanical failure but probably more likely it was triggered by an electrical fault or intentionally by a safety system due to some condition they haven't quite worked out yet. Since having control rods in puts the reactor out of it's approved operating envelope it will lead to a trip out of an excess of caution. The fact that the trip wasn't triggered automatically suggests to me that the rods probably weren't inserted by a safety system but due to some fault.
I wish it have provided some reasons on this
3 rods inserted into the core out of the many available would have disrupted the neutron flux profile away from what the reactor is intended to operate against in steady-state, so manual shutdown is the right answer until the cause of the control rod being dropped can be figured out, repaired, tested and then the reactor restarted.
But while not typical, this kind of event has happened many times before and will again. Pretty boring IMO, which is why I agree with the non-emergency declaration by NRC.
Rig N+2 control rods with an actively powered electromechanical retaining mechanism. In any kind of serious electrical issue occurs on the protected circuit, all 3 drop automatically.
So when I read "3 control rods hit the bottom" what I hear is "there was an unexpected group scram", but I don't know enough about this reactor design to know if that's actually what happened, or whether it was 3 unrelated control rods instead.
This is potentially interesting as this reactor recently had its operating license extended earlier in the year.
Event 58408, 'manual reactor trip'.
[0] https://www.youtube.com/watch?v=v0afQ6w3Bjw&t=4007s
Florida should be added to title, it's not like the European shutdowns for overheating
Some rods dropped. Something went wrong with the rod control system. Maybe an errant trip, maybe something mechanically wore, etc.
When a few rods dropped "accidentally" into the reactor, humans pushed the button to shut it down.
Now they figure that out and then turn the reactor back on.
Unironically this is the kind of headline I’m expecting in 2040 about one or more major AGI systems running an electrical grid, factory or similar.
what precisely is your point of describing how quality production engineering for infrastructure works?
Really, the "big AI story" will be when the next Snowden defects from the NRO and tells us what SENTIENT is doing. Until then it's just going to be a bunch of pie-in-the-sky wishcasting about what we want the future to look like. It's getting a little tiring hearing people insist that abundance is around the corner while surveillance is here-and-now.
I doubt anyone who works on reactors is that stupid.
But some idiot somewhere is going to put some industrial stuff under AI control pretty soon and murder a few people at minimum.
Which: it seems like we're already getting to the point of capability vs. unexpected behavior that having the ability to "trip" a complex system may be an important design criterion.
I mean, there's plenty of harm these systems can do besides trying to turn us all into paperclips.
A) Let AI override safety mechanisms (increases uptime while endangering humans)
B) Let AI impose additional safeties (decreases uptime with questionable safety uplift)
If you intend to go with option B) then there's basically zero benefit to investing in that kind of automation. AGI will be fallible, and even with all the safeties and safeguards it's hard to see what tangible benefit AGI would add to the system. It'll be a hard sell for critical infrastructure of any kind.
I understand that it's hard to get a man to understand something when his AGI Society executive credentials depend on it, but it feels glaringly obvious why AI isn't masterminding these sorts of systems. Even accounting for unprecedented advancement in AI, it's hard to see what AI could contribute to managing these things.
Remember, critical infrastructure attached SCADA systems to the internet and has had trouble removing them all despite 30 years of warnings and audits.
There's definite advantages to having a LLM in the loop. The best we can hope for is that it's architected in a way that it fails safe and damages are limited, so we get some of the advantages but less of the downside.
Outside, almost everyone I know - including highly educated people - takes even Google's search LLM output as fact, without a second thought, and even regarding serious issues (e.g., medical problems). Nobody raises a doubt or an eyebrow. A recent article says members of US Congress use it extensively for serious work. Lawyers use it to file cases. Doctors use it to summarize essential medical information. I even hear that software developers use it to create high-risk, high-value code.
In the public people think computers are superior to humans - more authoritative, objective, perfectly logical and error-proof. SV pushes that narrative, for example saying computer-driven cars are safer because computers don't have human flaws (that old sales technique - compare only the competition's unique flaws and don't mention your own; computers have their own flaws that humans lack). And of course, that one is popular on HN.
(IMHO the way to educate the public is to spread the narrative that every computer is created (designed, programmed) by normal, flawed, biased, etc. humans, and controlled by other flawed humans for their own agenda. And that computers have their own flaws.)
The general public distrusts them cuz they know that they're coming for their jobs. But they will use them out of convenience
https://www.pref.fukushima.lg.jp/site/portal/kyoten-kuiki.ht...
"Current Status: While "Difficult-to-Return Zones" still exist, decontamination and lifting of orders have reduced the designated area down to approximately 2.2% of the prefecture's total land area."
[1] https://chatgpt.com/s/m_6a82114f63a481919380c5f1e2b432e8