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Comment by fwipsy

10 hours ago

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.

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fwipsy

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mlyle  10 hours ago

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.

solarkraft  9 hours ago

> 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.

  • chippiewill  5 hours ago

    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.

  • numpad0  8 hours ago

    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.

  • amelius  4 hours ago

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