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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 r

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.

joebig 2mo ago
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.
- MBCook 2mo ago
  > 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.
  - s1artibartfast 2mo ago
    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completely subcritical with all Control Element Assemblies (CEAs) inserted minus one assembly stuck in the fully withdrawn position.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.
    - CoryOndrejka 2mo ago
      Yup, this is me focusing on US Navy reactors, which is my experience. For them, a rod would already take you sub critical.
  - joebig 2mo ago
    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).
jwkpiano1 2mo ago
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.
- Onavo 2mo ago
  Simpson's?
- wahern 2mo ago
  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 https://en.wikipedia.org/wiki/Scram
  - CoryOndrejka 2mo ago
    Exactly what I would have answered :)
  - jwkpiano1 2mo ago
    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.
  - euroderf 2mo ago
    At the U Chicago squash court reactor there were also people with buckets of boron solution at the ready.
metalman 2mo ago
Thats a releaf, nothing like an uncontrolled saftey event to take a bit of pressure off
kryogen1c 2mo ago
>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.
- iLoveOncall 2mo ago
  [flagged]
  - gambiting 2mo ago
    Except no bot would write "chernoble", unless it's manually edited after the fact
    - jdiff 2mo ago
      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?
  - js2 2mo ago
    "I was in the US nuclear navy, now I work in IT."
- CoryOndrejka 2mo ago
  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.
thinkcontext 2mo ago
Maybe a non-event for safety but what about getting the plant operating again?
- roughly 2mo ago
  Almost certainly a better problem to have.
- CoryOndrejka 2mo ago
  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.
  - mlsu 2mo ago
    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?
    - 486sx33 2mo ago
      [dead]
    - johnecheck 2mo ago
      I'd imagine the operational capabilities of nuclear submarines are somewhat confidential.
    - wat10000 2mo ago
      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 https://en.wikipedia.org/wiki/Battleshort
sandworm101 2mo ago
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.
- CoryOndrejka 2mo ago
  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.
  - CoryOndrejka 2mo ago
    [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.
    - sandworm101 2mo ago
      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.
xattt 2mo ago
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.
- recursivecaveat 2mo ago
  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 https://youtu.be/5QcN3KDexcU?t=329
- enquirewithin 2mo ago
  Big flexible pearl catch, a very scratch telescopic inspection mirror, and a flat torch. Same way every technician fails to retrieve that dropped part.
anonymousiam 2mo ago
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.
- pfdietz 2mo ago
  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.
Aeolun 2mo ago
> 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.
- enquirewithin 2mo ago
  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?
- sitharus 2mo ago
  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.

joebig 2mo ago

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.

MBCook 2mo ago
> 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.
- s1artibartfast 2mo ago
  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.
  - CoryOndrejka 2mo ago
    Yup, this is me focusing on US Navy reactors, which is my experience. For them, a rod would already take you sub critical.
- joebig 2mo ago
  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).

jwkpiano1 2mo ago

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.

Onavo 2mo ago
Simpson's?
wahern 2mo ago
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 https://en.wikipedia.org/wiki/Scram
- CoryOndrejka 2mo ago
  Exactly what I would have answered :)
- jwkpiano1 2mo ago
  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.
- euroderf 2mo ago
  At the U Chicago squash court reactor there were also people with buckets of boron solution at the ready.

metalman 2mo ago

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

kryogen1c 2mo ago

>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.

iLoveOncall 2mo ago
[flagged]
- gambiting 2mo ago
  Except no bot would write "chernoble", unless it's manually edited after the fact
  - jdiff 2mo ago
    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?
- js2 2mo ago
  "I was in the US nuclear navy, now I work in IT."
CoryOndrejka 2mo ago
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.

thinkcontext 2mo ago

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

roughly 2mo ago
Almost certainly a better problem to have.
CoryOndrejka 2mo ago
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.
- mlsu 2mo ago
  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?
  - 486sx33 2mo ago
    [dead]
  - johnecheck 2mo ago
    I'd imagine the operational capabilities of nuclear submarines are somewhat confidential.
  - wat10000 2mo ago
    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 https://en.wikipedia.org/wiki/Battleshort

sandworm101 2mo ago

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.

CoryOndrejka 2mo ago
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.
- CoryOndrejka 2mo ago
  [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.
  - sandworm101 2mo ago
    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.

xattt 2mo ago

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.

recursivecaveat 2mo ago
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 https://youtu.be/5QcN3KDexcU?t=329
enquirewithin 2mo ago
Big flexible pearl catch, a very scratch telescopic inspection mirror, and a flat torch. Same way every technician fails to retrieve that dropped part.

anonymousiam 2mo ago

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.

pfdietz 2mo ago
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.

Aeolun 2mo ago

> 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.

enquirewithin 2mo ago
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?
sitharus 2mo ago
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.