St Lucie Nuclear Reactor Unit 1 manually shutdown, 3 control rods drop into core | Hacker News Reader

St Lucie Nuclear Reactor Unit 1 manually shutdown, 3 control rods drop into core

wptv.com

196 pointstoomuchtodo149 comments

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 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.

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completely subcritical with all Control Element Assemblies (CEAs) inserted minus one assembly stuck in the fully withdrawn position.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.

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).

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.

Simpson's?

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.

At the U Chicago squash court reactor there were also people with buckets of boron solution at the ready.

Thats a releaf, nothing like an uncontrolled saftey event to take a bit of pressure off

>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

People will be fired because a safety feature triggered?

Does the American nuclear power industry not do the equivalent of blameless postmortems? That seems... suboptimal.

The entire point of critical safety features is to turn events into non events.

Maybe a non-event for safety but what about getting the plant operating again?

Almost certainly a better problem to have.

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?

I'd imagine the operational capabilities of nuclear submarines are somewhat confidential.

Warships have a “battle short” on some critical equipment, which will bypass safety equipment like fuses. If your gun turret traversal motors pull too much current in the middle of a battle, you’d rather risk damage or fire to have them keep working rather than disable themselves to protect from a fault. Total speculation here, but it would make sense for military reactors to have something similar, to put them in a mode where the threat of a reactor accident due to a malfunction is less than the threat from the enemy if it goes down. https://en.wikipedia.org/wiki/Battleshort

Likewise commercial aircraft have bypasses on the fuel and oil filters in case they get clogged in flight. The engine will need a thorough inspection if this happens but it's better to damage the engine if it gets you to the nearest airfield.

I’ve never been in the Navy or on a submarine, but my impression is that SUBSAFE is about never ever having a reactor accident, at least partially for PR reasons. It wouldn’t surprise me if there simply are not procedures to deliberately put the reactor in a dangerous state.

The Navy may have done the math and determined the loss of a boat to the enemy is a less bad thing than the loss of a boat to a “nuclear accident”.

I desperately hope for and welcome anyone with experience in the space to correct me.

Wouldn't a nuclear powered ship sunk by enemy fire be a nuclear accident on its own? There would be a nuclear reactor on the seafloor, possibly a turned on one, surrounded by failed systems. I don't think it ever happened at war, but while I was looking for [1] I found a number of other incidents. I list them below

[1] https://en.wikipedia.org/wiki/Kursk_submarine_disaster

[2] https://www.sciencedirect.com/science/article/abs/pii/S00295...

[3] https://en.wikipedia.org/wiki/Soviet_submarine_K-19

[4] https://www.ebsco.com/research-starters/history/soviet-nucle...

[5] https://gizmodo.com/sunken-soviet-submarine-is-quietly-leaki...

[6] https://en.wikipedia.org/wiki/Submarine_incident_off_Kildin_...

[7] https://en.wikipedia.org/wiki/Submarine_incident_off_Kola_Pe...

If you find the story of the Kursk interesting, you should read "Darkness at Dawn" (Satter, 2003)

The nuclear incident in a sub would be less problematic than stealing the nuclear missiles out of the sub. Like a Titan out of the Kursk sub.

They initially didnt have quick restart procedures for that and it contributed to a nuclear sub sinking. After that they added those procedures and improved the technology

https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593)#Cause

> After the accident Rickover further reduced plant restart times, which had already been gradually improving with new technology and operating experience, in addition to limiting factors that could cause a shutdown.[26]

The ship can operate for a bit on decay heat. And they have battery banks to run major systems until conditions allow running at snorkel depth on the diesel engine.

And rumor has it some of the nuclear subs may work with a Stirling engine which can operate systems on heat alone.

The small sub world is going toward AIP. I wouldn't be surprised if the big ones have AIP for emergency power.

AIP?

Air-independent propulsion.

Wow, that’s a whole lot of Liquid Oxygen in most of those designs.

Not sure if that is better or worse than massive quantities of high test hydrogen peroxide.

Spicy.

AIP range is still limited though, especially at fast speeds. Thats why even the newest 212CDs still have have snorkels for their diesel and diesels are loud. SSNs are much better for global power projection.

Fine for running electric power for a time though. Which is probably what you'd use it for on a nuclear sub. Assuming you can spare the space/weight in your design.

out of curiosity, where's the heat for a Stirling engine coming from, or what would be the thermodynamic advantage if you were already collecting decay heat from the reactor? People breathing hotly in a tube?

An S5W scram probably has contributed to USS Tresher sinking because they couldn't get it running fast enough again

https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593)#Cause

The final paragraph of the article

> According to NextEra Energy, Unit 1 at St. Lucie is back online at 100% power after operators safely resolved an equipment issue.

I wish the article had said where on earth St Lucie is. Never heard of it

St Lucie: Island in the South of the Caribbean. Source: lunch games https://worldlegame.io https://flagle.org

This St. Lucie is in Florida USA, on the Atlantic coast north of Palm Beach and south of Orlando.

It's a local TV station, the target audience already knows where so the information is superfluous.

And the breadcrumbs at the top of the page give you enough information to find out if you're not in the target audience.

You can look it up.

I also didn't know "St. Lucie". I Googled for "St. Lucie Nuclear Power Plant" to find it.

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.

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.

What what I can tell they use "dropped" to refer to just falling to the bottom of their shaft, though it definitely does evoke the idea of them just rattling around down there in my head. I think that since water is such a good buffer, that once you turned off the pressure you could open a hatch and just kind of fish out whatever you wanted with a very long pair of tongs. This is obviously a research sized reactor, but you can see they're happily mucking about with it on the other end of long poles, and the scientist says they could hypothetically fish something back out: https://youtu.be/5QcN3KDexcU?t=329

Big flexible pearl catch, a very scratch telescopic inspection mirror, and a flat torch. Same way every technician fails to retrieve that dropped part.

There's a difference between control rods (which are used to reduce fission by absorbing neutrons) and fuel rods (which contain enriched fissile material). In this case, the control rods that fell into the core would pose no risk at all to anybody, but would inhibit the ability of the reactor to reach full capacity.

Yes. Note that fuel rods don't move, so they would not "drop into the core". They can later be lifted out when the reactor is refueled but are fixed in place when the reactor is in operation.

> There's no reason this would lead to a radiological event or more significant casualty.

Depends on whether you lose cooling at the same time right? If your rods drop because the power/cooling to your plant disappears (Fukushima) then all the safety in the world doesn’t help.

Is Hutchinson Island likely to be exposed to a tsunami, or other event, that might result in total loss of power and destroy emergency responders ability to cool the reactors?

If you lose cooling the reactor will certainly trip but it will take several hours until you start to get core damage - 4 hours in the case of Fukushima Daiichi according to the released timeline. This also requires multiple redundant power supplies and cooling systems fail.

This happened in 2024 too.

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.

Relevant 2024 excerpt from linkedin link:

  double hold was placed on Subgroup 4 resulting in the associated control rod lower grippers being engaged prior to removing the upper gripper drawer. After successful replacement of the UG coil CPMD the double hold was removed, per 2-NOP-99.07, and CEAs 43, 45, and 49 fell and fully inserted into the core.

CEA means Control Element Assembly. A CEA is a group of control rods that can be inserted into or withdrawn from a reactor core

https://www.nuclear-power.com/nuclear-power/reactor-physics/...

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.

You should write them an email suggesting they try to prevent unintended rod drops

Something, something... We cannot allow a rod drop gap!

You may even get a reply from Homer Simpson. I hear he's in charge of that sort of thing

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.

Nuclear reactors are in a delicate balance, they are in constant feedback loops that means an unexpected power drop or an unexpected rise can cascade. The withdrawal of the others rods is not specifically in response to one being dropped in accidentally - it will be in response to the drop in the power output.

There's not a safety risk from doing this because the control systems are doing this to specifically stay within the stable power output.

There are some significant negative consequences from the power dropping excessively and unexpectedly. One of them is an excessive build-up of Xenon in the reactor which is a feedback loop that continues to reduce fission (known as Xenon poisoning) and is one of the underlying causes of the Chernobyl incident. Now, as far as I'm aware there are no Nuclear reactors running today that are _unsafe_ if they encounter Xenon poisoning - but it's not something you want to stumble into.

If there were a genuine safety concern related to a control rod drop then the reactor would scram and the _all_ the rods would drop and the reactor would need to be restarted.

I think an occasional rod drop might be used as a test for the system, so there's no real need to prevent them.

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

I can't be the only one to see that location and think "Holy climate change risk, Batman!"

Despite the appearance, is it uniquely prone to flooding or storm surges?

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"

"... as I was incredibly surprised to learn of nuclear energy existing in the Caribbean ..."

You may also find it interesting that South Africa had nuclear weapons:

https://en.wikipedia.org/wiki/South_Africa_and_weapons_of_ma...

I knew about it, but your post is the first I've heard that SA no longer had those (I only clicked the link because you used the past tense!). I actually thought that no country had ever withdrawn their nuclear program.

This might be the most amazing and hope-inspiring I've read online this week, so I'm grateful for you sharing that link!

Ukraine also gave up its nuclear weapons, as it regained autonomy with the dissolution of the Soviet Union.

My understanding is that they gave up the USSR's weapons (and handed them to Russia), rather than them having built, owned and dismantled the weapons outright through their own nuclear program.

They could have spent substantial effort to make those weapons "theirs" (I've heard people simplify this as "they didn't have the codes but could figure them out") but it would have cost them dearly in international sanctions, versus the assurances and aid they could negotiate by relinquishing them.

That’s true. But Ukraine wasn’t just babysitting weapons made totally elsewhere, either. There was substantial expertise, research and manufacturing built up in Ukraine during the Soviet era.

I think it’s true enough to say they gave up nuclear weapons, even if they didn’t wholly own the nuclear weapons.

Saint Lucia[^1] vs St. Lucie[^2]:

> 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

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?'.

You only accept the risks of driving because its normalised in your community. In many communities, public transport is the norm and it would be considered foolish to hand over car keys to a majority of the population.

Sure, but the news usually writes it in the relative terms of their viewership. It's alright it's not an absolute so as people can relate to where the news org would rank concerns relative to them.

I'm pretty sure the context here would require college level courses plus industry training on operational procedures to actually understand in any meaningful sense. Not much different than any other time some safety feature triggers at any other highly complex and hazardous industrial facility. Except that in all those other cases the media ignores it and leaves it to the technical experts who do understand the context to sort it out, and everybody goes on with their lives never knowing about it. But since this particular highly complex and hazardous industrial facility is a nuclear power plant what we will get is journalists who know nothing about the subject screaming about it to millions of other people who know nothing about it who will assume it was a near miss on killing half the population of Florida.

This is the equivalent of a Kubernetes pod restarting.

Following this metaphor, what was Chernobyl?

Crash of the monitoring/C&C process of a 48xxl AWS hosted spot priced EC2 instance, locked in an infinite loop?

Someone runnning Kubernetes on an old unpatched on-prem Windows box, conducting a "failover test" by pulling one of its power supplies, spilling coffee on the other one, catching their house on fire, and waiting to call the fire department due to being embarrassed.

One shouldn't make jokes about a situation where people died. Just to sum up Chernobyl 1. no containment building 2. flawed reactor design 3. operators turned off safety systems and improperly operated the reactor 4. delayed / improper emergency response

What's probably more interesting than the incident itself is that it elicited well over a hundred comments on this site. Explosions happen at natural gas plants semi-frequently, few make the news, and fewer still are likely to get much attention here.

It's standard human bias — a plane falling out of the sky with 200 people on it once a year is a major catastrophe we all know about within hours, but 40_000 people dying per year in road accidents is just a statistic. Besides, driving can't be all that unsafe — I do it every day and I'm fine!

Similarly here: something irrelevant happened at a nuclear power plant? PANIC! But 100_000 people died last year from coal plants dumping their waste products into the air we all breathe? Meh, who cares, how boring.

A more accurate one: Comanche Peak Unit 1 SCRAM'd because the 2021 snowpocolypse caused a feed water issue...

Good luck finding that as more than a footnote.

source: https://www.nrc.gov/reading-rm/doc-collections/event-status/...

I liked it better in the olden days when the paper would print the location of the story at the beginning of the first paragraph.

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.

These types of comments always come up when local TV news makes its way to a broader audience. Yes, the story was originally spoken out loud on a TV broadcast to an audience who knows what St. Lucie is.

It is a global journalistic standard to explain where something happened in context to geography that the intended audience would understand.

It would be really weird to watch local news if every story reiterated “this happened in West Palm Beach in the American state of Florida”. They don’t need to say that when everyone watching lives there.

It’s only weird because we’re expecting everything on the internet to be written for a global audience. But really this is like going to a Chinese newspaper website and getting pissed off that it’s not in English. If you don’t understand, you’re not the intended audience.

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.

Did anyone else see the footer for the article?

    > This story was reported on-air by a journalist and has been converted to this platform with the assistance of AI.

Very cool! It probably helps these local TV stations to better monetize their on-air content by making it available in text.

I wonder why so much repetition in the article. Do they have word limit to hit?

I wish it have provided some reasons on this

From the article: "This story was reported on-air by a journalist and has been converted to this platform with the assistance of AI. [...]"

Why they dropped?

I was unable to find the NRC filling, if someone finds the link to it, please drop a reply with it.

This is potentially interesting as this reactor recently had its operating license extended earlier in the year.

https://www.nrc.gov/reading-rm/doc-collections/event-status/...

Event 58408, 'manual reactor trip'.

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.

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 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.

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.

Sounds like the Nuclear Regulatory Commission explicitly categorized it as a "non-emergency"

This isn't even worthy of a news article. Nothing of consequence happened. It's only news because the media loves to scare people. Does a gas plant breaking or shutting down deserve a big media presence? No? Then this shouldn't either.

This is not even in the same realm of reality as an emergency. There really was not even a reason this should've been reported in the news.

To rephrase it: nothing happened, and reactor is back online generating clean electricity.

This is the equivalent of a Kubernetes pod restarting.

Apparently people surf at the beach near this reactor. I never went there when I lived in Florida, but my dad told me about it.

I guess everyone in west palm beach is nuclear reactor specialist according to this tv station’s reporting.

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.

[1] https://chatgpt.com/s/m_6a82114f63a481919380c5f1e2b432e8

It’s great that nuclear power safety events are so well managed that nearly 100% of events go completely unnoticed by the public.

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.

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.

Correct interpretation

So, which do we choose?

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's a configuration issue, not people preferring a bad idea. SCADA itself is not a hard sell because it's ostensibly deterministic.

If AI cannot provide the same baseline promises while only bring right a fraction of the time, then it's not the right solution. I don't want my car's AC to ask Anthropic what the right temperature is; it's a waste of tokens that isn't optimizing jack shit 99.99% of the time. If a system isn't failing currently, then it's probably in the same boat as car ACs. Shoehorning AI into it won't solve anything.

There are more than two options

Okay, explain your third path.

I suggest you research organizations like IAEA and EAA which create communities around managing the problem of “scary new technology” of nuclear and aviaton respectively (two just for example).

There are ways to manage “scary new technology” that are well trodden,

It does require people to let go of whatever pitard they have decided to commit to.

I think the IAEA and EAA would both agree with everything I've written. Can you link specifically to statements that you think refute me?

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.)

The fact that you conflate LLMs and AI means you’re unqualified to be part of the discussion

The Elon buttlicker types praise llms as the new knowledge gods.

The general public distrusts them cuz they know that they're coming for their jobs. But they will use them out of convenience

I’m absolutely willing to believe there are some people that stupid.

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.

AI labs are already working on getting LLMs to control nuclear reactors, and hiring experienced reactor operators to help train them:

https://www.reddit.com/r/nuclear/comments/1qnzxvg/this_job_p...

This was a power safety event in the sense that safety was accidentally added. Like oops, we accidentally put the pin back in the grenade.

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?

> nearly 100% of events go completely unnoticed by the public

"Nearly" is doing a lot of heavy lifting there. Those "nearly" 0% of events which DO go noticed by the public, like you know, Fukushima, tend to be world news for weeks at a time.

Is this another post title that was mangled into ambiguity because of trying to hit the arbitrary HN word count restriction?

Florida. RIP 5Ys.

3.6 rods? Not great but not terrible either

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.

[0] https://www.youtube.com/watch?v=v0afQ6w3Bjw&t=4007s

And here is a reaction by an actual nuclear engineer. He has many good reactions to other videos as well, not limited to just nuclear stuff.

https://www.youtube.com/watch?v=Vncf70UBsr4

better article

* https://lakelandgazette.info/news/2026/08/16/st-lucie-nuclea...

archive because the website is overwhelmed and down most of day:

* What Happened:

At 9:47 a.m. EDT, Unit 1 was operating at 100% power when three control rods (part of the Control Element Assemblies) fell into the core due to an equipment/control fault.

* Operator Action:

Under standard safety protocols, dropping multiple control rods alters the reactor’s core dynamics. Plant operators with Florida Power & Light (FPL) executed a manual reactor trip (scram) to safely halt the fission reaction.

* Current Status:

The trip was uncomplicated, and all safety systems responded normally.

Operators stabilized the reactor in Mode 3 (Hot Standby), with decay heat safely being removed via steam discharge to the main condenser.

Unit 2 was completely unaffected and continued running normally.

The Nuclear Regulatory Commission (NRC) Resident Inspector was notified via standard 4-hour non-emergency reporting procedures.

* Why Control Rods Matter

Control rods absorb neutrons inside a reactor. Inserting them slows or stops the nuclear chain reaction.

While a single dropped rod can often be managed by reducing turbine output, multiple dropped rods require an immediate shutdown to prevent uneven power distribution and localized overheating in the fuel assemblies. Gravity-assisted dropping is a fail-safe mechanism designed into commercial pressurized water reactors so that if power to the drive mechanisms is lost or faulted, the rods default to falling safely into the core.

this is really interesting

I live about four miles from the plant. I have not grown a second head... yet.

for "reasons" that they carefully left out?

Florida should be added to title, it's not like the European shutdowns for overheating

No one knows the reasons yet.

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.

could have been something as minor as some sensor malfunctioning.

Overheating isn't exactly right, they got shut down because they couldn't be cooled, but they didn't overheat.

At least they didn't make 300 square km uninhabitable.

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."

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