OVERKILL
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Originally Posted by Wolf359
I think they will cover it in the final episode, but in the wiki, it mentions that the tips of the control rods were covered with graphite which instead of slowing down the reaction, actually increased it. So that's why the plant blew up when they hit AZ-5 instead of shutting it down. That was a design quirk that they weren't really familiar with because the Soviets kept so many things secret. It shouldn't really have mattered that much if it had been operating inside parameters, but it was too far outside the parameters they had originally envisioned.
Yes, the control rods were graphite tipped, but it was the fact that they were almost all completely retracted out of the unit and their holes were filled with nearly boiling water that aided in the power level shooting up dramatically (100-fold) to the point where the steam tubes ruptured and then things went sideways. If you haven't read the document I linked earlier in the thread, I suggest giving it a going through. It is far more thorough than the Wiki and it will give you a very concise account of the events leading up to the failure, and the failure itself.
Originally Posted by Wolf359
And yes, it was a strange test. There were 3 other reactors near the site. I think it would have been very unlikely for all of them to be shut down and the place to have no power but I suppose anything is possible, maybe an issue with the connection to the grid would have prevented it from getting power from the other reactors. At least with the gas plant I was at, it was the only plant in the area and if it was off the grid and both generators were offline, it would need backup power to continue to run the pumps etc for an orderly shutdown.
It wouldn't have mattered. The time gap between a full trip, including grid disconnect and back-up ramp up wasn't enough time for anything to go "horribly wrong". They were just looking for ways to minimize the time the circulation pumps were down, or prevent them from going down entirely, during an event like that, by harnessing the power produced while the turbine slowed.
IIRC, it's policy or protocol in most places/for most units for a shut down when a grid disconnect takes place. I believe the exception is some of our reactors in Canada, where they are allowed to idle, because they can, at massively reduced power output, reliably. So if Chernobyl were to experience a grid disconnect, the odds are that it would transition to backup power and all the units would have commenced shutdown protocol, with all the support systems running on backup (generator) power. There are usually batteries to run the control systems so that they don't experience an interruption. Things like pumps, which can tolerate a break in operation as a transition from one power source to another takes place, are not run on batteries. I don't know why they thought it was necessary to try and limp them along on the output from spin-down, since it was noted that it wasn't necessary, but for whatever reason, it was deemed to potentially be beneficial, so it was tested for. It wasn't of course the test that was the issue, it was the state of the unit, which falls entirely at the hands of the operator, that led to the disaster. Had the unit not been accidentally almost shut off, we wouldn't be talking about it.
Originally Posted by Wolf359
What's somewhat crazy is that even though the whole area was evacuated, they kept running the other 3 reactors for years afterwards, I think they finally shut down the last reactor there in the 2000's.
It was a 4,000MW site, it provided a significant amount of power, necessary power, to the area. It was 3,000MW (with the one unit gone) that couldn't be readily replaced, so yes, the decision was made to continue to operate the other three units, even after the disaster, with crews that rotated in and out. I'm not as surprised as to the fact that they kept operating them as I am to the fact that the three remaining units, which weren't any more inherently safe than the one that failed, as they all lacked containment, were allowed to remain that way. I would have expected that full containment for the remaining units would have been mandated, but as far as I know, that never happened
I think they will cover it in the final episode, but in the wiki, it mentions that the tips of the control rods were covered with graphite which instead of slowing down the reaction, actually increased it. So that's why the plant blew up when they hit AZ-5 instead of shutting it down. That was a design quirk that they weren't really familiar with because the Soviets kept so many things secret. It shouldn't really have mattered that much if it had been operating inside parameters, but it was too far outside the parameters they had originally envisioned.
Yes, the control rods were graphite tipped, but it was the fact that they were almost all completely retracted out of the unit and their holes were filled with nearly boiling water that aided in the power level shooting up dramatically (100-fold) to the point where the steam tubes ruptured and then things went sideways. If you haven't read the document I linked earlier in the thread, I suggest giving it a going through. It is far more thorough than the Wiki and it will give you a very concise account of the events leading up to the failure, and the failure itself.
Originally Posted by Wolf359
And yes, it was a strange test. There were 3 other reactors near the site. I think it would have been very unlikely for all of them to be shut down and the place to have no power but I suppose anything is possible, maybe an issue with the connection to the grid would have prevented it from getting power from the other reactors. At least with the gas plant I was at, it was the only plant in the area and if it was off the grid and both generators were offline, it would need backup power to continue to run the pumps etc for an orderly shutdown.
It wouldn't have mattered. The time gap between a full trip, including grid disconnect and back-up ramp up wasn't enough time for anything to go "horribly wrong". They were just looking for ways to minimize the time the circulation pumps were down, or prevent them from going down entirely, during an event like that, by harnessing the power produced while the turbine slowed.
IIRC, it's policy or protocol in most places/for most units for a shut down when a grid disconnect takes place. I believe the exception is some of our reactors in Canada, where they are allowed to idle, because they can, at massively reduced power output, reliably. So if Chernobyl were to experience a grid disconnect, the odds are that it would transition to backup power and all the units would have commenced shutdown protocol, with all the support systems running on backup (generator) power. There are usually batteries to run the control systems so that they don't experience an interruption. Things like pumps, which can tolerate a break in operation as a transition from one power source to another takes place, are not run on batteries. I don't know why they thought it was necessary to try and limp them along on the output from spin-down, since it was noted that it wasn't necessary, but for whatever reason, it was deemed to potentially be beneficial, so it was tested for. It wasn't of course the test that was the issue, it was the state of the unit, which falls entirely at the hands of the operator, that led to the disaster. Had the unit not been accidentally almost shut off, we wouldn't be talking about it.
Originally Posted by Wolf359
What's somewhat crazy is that even though the whole area was evacuated, they kept running the other 3 reactors for years afterwards, I think they finally shut down the last reactor there in the 2000's.
It was a 4,000MW site, it provided a significant amount of power, necessary power, to the area. It was 3,000MW (with the one unit gone) that couldn't be readily replaced, so yes, the decision was made to continue to operate the other three units, even after the disaster, with crews that rotated in and out. I'm not as surprised as to the fact that they kept operating them as I am to the fact that the three remaining units, which weren't any more inherently safe than the one that failed, as they all lacked containment, were allowed to remain that way. I would have expected that full containment for the remaining units would have been mandated, but as far as I know, that never happened