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Hiddencamper t1_jc2ms2l wrote

Reply to comment by PHATsakk43 in Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama

That’s true. They wouldn’t have attempted to do what they did if they weren’t flooded with xenon.

To be fair though, all light water reactors can overcome xenon except for the very end of the operating cycle. So you avoid issues related to xenon in most reactors out there which eliminates risk of potential power spikes. And the CANDU design simply doesn’t have enough reactivity to pull through a xenon peak (which is why their reactor protection systems will try to stabilize the reactor at 60% or 2% power when it is safe to do so, to allow the operators an opportunity to keep the unit running).

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PHATsakk43 t1_jc33vnn wrote

You’re answering your own question.

The low reactivity of the RBMK is why the moderator tipped control rods existed. The xenon-precluded startup is inherent in any reactor, regardless of enrichment as the xenon is from fission of U-235 and is a function of time at power (equilibrium xenon) or for a startup, time after shutdown (peak shutdown xenon.)

Repetitive startup/shutdowns that were being performed at Chernobyl would create the same xenon problems with any reactor, at any point in operating cycle if done excessively.

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Hiddencamper t1_jc35h9s wrote

BWRs never are xenon precluded. They always, at all parts of the operating cycle, have sufficient hot excess reactivity to have xenon override capability. They also naturally stabilize spatial and axial xenon tilt based on their design and the boiling boundary effect.

Pwr plants have total xenon override until the last 5-8% of cycle, when they are essentially at max dilution. They do not have natural flux tilt stabilization so the operator has to manually make adjustments to control tilt within limits.

I have personally started up a commercial BWR in peak xenon. It was very weird to have the reactor go critical moving a corner rod from 00 to 04, not see the criticality (power actually appeared to be going down at the time we notched it out), then as xenon burnout started happening we saw only one SRM period on scale. The PPC displays, when you have period in trend mode, you can see an inflection when critical occurs, and we saw the signature only on one instrument which didn’t make much sense. So we stopped pulling rods to watch, as a minute or two later the second SRM started to come on scale, then the third and fourth, as xenon burnout reduced shielding around the SRMs and allowed the core to finally couple. Then reactor period advanced over the next 12-15 minutes to about 82 seconds, when we finally hit point of adding heat and everything stabilized. Not a common evolution and I can see where other operators pulled too far and tripped their units, because you don’t see the core go critical on peripherals for quite a while.

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PHATsakk43 t1_jc3bqm4 wrote

I've worked around BWRs but never been an operator at one (my fleet had a two unit station BWR, while the rest were PWRs.)

I stand corrected in that case. Does make perfect sense when you game it out, as you can basically put a shitload of positive reactivity into a BWR.

I have heard of situations similar to Chernobyl at naval plants (all rods out, waiting on xenon decay, below POAH), but again, that was all stories as I only operated new naval plants as well.

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Hiddencamper t1_jc3czqr wrote

A full power BWR has a void defect around 40% of your total reactivity. When you scram, those voids go away, and you recover all of that reactivity. Voids are dominant in a BWR. The rule of thumb is Doppler 10^-5, moderator temp 10^-4, void coefficient 10^-3. So you always have enough to start back up in a BWR. And actually, especially if it’s a fast restart, more xenon helps a lot with getting to target rod pattern as it’s one of the things that impacts thermal limits and PCIOMR.

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