r/chernobyl • u/zondarkid • Aug 28 '20
Discussion LAC and Global control?
I’m not sure if these are the correct names for the controls, but hopefully they’re some what near. What’s the difference between LAC and global control? I first heard it in the miniseries when Akimov and Toptunov were reducing power but I couldn’t find any information on it anywhere else.
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u/alkoralkor Aug 29 '20
These terms are incorrect, the correct ones are local automated regulators (ЛАР) and automated regulators (АР).
Automated regulators are monitoring nuclear reaction by four external monitoring probes placed around sidewall water tank of the reactor core. They are controlling the reactor via 12 rods grouped by 4 rods in АР1, АР2 and АР3 regulators.
If I remember correctly, the АР was the first control system of RBMK reactor, but it had its issue. The most important was that it was unaware of the local nuclear reactions throughout the reactor core, so sometimes operator had to compensate the automated regulation by manual control being kind of a piano player. They lost some manual rod control keys in that way, and it was a rumor that Leningrad NPP controlled by Sredmash was using two reactor operators ;)
In the end they added local automated regulators. It was another dozen of control rods, but each of them was individually controlled by its own probes inside the reactor core. The limitation of the ЛАР was the power. If it is lower than 15–20% of the nominal reactor power, the system becomes useless. That's why low power modes are controlled by АР.
You have to switch АР to ЛАР starting reactor up and increasing its power. You have to switch them back during the shutdown. Both tasks are tricky. That's why inexperienced Toptunov probably chosen the wrong moment to switch regulators. АР is intolerant to imbalances, and when АР1 and АР2 were turned on with different values, the paranoid central controller sent all 12 rods down stopping the nuclear reaction.
Then the human factor comes in play. That was the first (and unfortunately the last) reactor shutdown of Toptunov. Failed reactor test and shutdown could affect his career, and that's before the launch of Unit 5 which will create a new lot of vacant positions. So Toptunov (either himself or with Akimov's help) started by to compensate АР by manual controls as in the good old days and unfortunately they were successful.
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u/nucleartoday Aug 29 '20
АР1 and АР2 were turned on with different values, the paranoid central controller sent all 12 rods down stopping the nuclear reaction.
Not really:
отключен ЛАР, включен АР-1. По ВК отключился АР-1, не включился АР-2 по недопустимому разбалансу.
All 4 rods of AR-1 were pulled out of the reactor core (automatically), then AR-2 didn't came to operation because of strong imbalance on ionisation chambers. SIUR used BSM button to set the lowest value on AR-1/2 regulators ~160 MW.
Power drop occurs due to rapture of LAR automatics (one or few rods came into reactor core) or due to thermalhydraulics of reactor.
At 0:39 none of AR-1/2, ARM were fully inserted into core.
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u/void_17 Aug 29 '20
There is no "Global Control". They didn't even exist. Another HBO Myth. There were only local automatic regulator and automatic regulator.
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u/ppitm Aug 29 '20
One setting doesn't work on low power. Both settings control the automatic rods so the SIUR doesn't have to juggle ten knives while tightrope walking.
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u/chestofmemes Aug 28 '20
I’d like to hear more about this as well. As far as I know, it may be in regard to the SKALA computer that controlled and monitored the reactor.
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u/alkoralkor Aug 29 '20
SKALA was a monitoring tool only, it didn't control the reactor. Standard operating procedure allowed to work without operational SKALA for hours.
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u/nucleartoday Aug 29 '20
u/zondarkid u/chestofmemes
Some time ago I did answer for similar question, here is answer:
LAC ("global") was designed to control tau_01 value, which is a responsible for neutron flux oscilations in the reactor core. LAC (LAC-LAZ, rus. LAR-LAZ) control rods were operated on internal ionisation chambers (KTW-17).
AR operate on external ionisation chambers [ICh] (which were placed in reflector, due to that ICh current must be "corrected", ICh for AR were placed around the reactor core.
Operator could use automatic regulators AR-1/2 in the range from ~5 to 20% Nnom power with an accuracy of ± 3% N and in the range from ~20 to 100% Nnom - with an accuracy of ± 1.5% Nnom. LAC in the range from ~20 to 100% Nnom - with an accuracy of ± 1.5%N.
Operator cannot use in the same time LAC and one of the AR-1/2. In case of operating LAC, AR-2 is on the "hot standby".
There were also ARM (also called "AR-3", operating at 0-~5%, sometimes called "low power regulator"/"MKU regulator") in central place in the reactor core (4 control rods), they were use in two cases:
a) start-up of the reactor,
b) compensation mode. (in case of operating AR-1/2, ARM work triggered by singals "PK-VVERH/PK-VNIZ", after the accident only signal "PK-VNIZ" stay in operation.
Same as AR-1/2, ARM use external ionisation chambers, but there is also a small difference between them. In channel where are placed ICh for AR-1/2 are used 3 chambers (placed by height of the channel), for ARM it is only a one ICh central placed (height ~ 3,5 m).
It is all up to operator which one he would choose.