Part 2: Shift Change

By 23:00 on April 25th, much of the xenon in the reactor had decayed, allowing more control rods to be re-inserted. ORM at this point was 26 rods. By midnight power would be reduced from 1600 to 760 MW (thermal), which (as expected) began a new process of xenon poisoning. The rundown test program required 700-1000 MW, while the turbine vibration test required substantially less than that. In the meantime, Deputy Chief Engineer Anatoliy Dyatlov walked the four kilometers to work, as he always did.

 

Photo credit: chidori@emptytriangle.com

Officially, Yuri Tregub's work was done for the day:

 

I could have left. But I decided that I should stay. I very much wanted to see how the turbine would behave itself, what the rundown would be like. It was midnight, and I was losing what leisure time remained before the next shift. I acted a bit egotistically, perhaps like a station shift supervisor. I couldn't order Sergei Gazin—an engineer from my shift—to stay. I asked him, saying: "It's your turbine they're going to test. How can't you stay?" He said: "Fine, I'll stay." If only I had known how it would end...

 

The shift was Sasha Akimov, shift supervisor, Lenya Toptunov, SIUR, Stolyarchuk, SIUB, Kirshenbaum, SIUT and the fifth was the shift supervisor of the Turbine Division. That is the usual composition of a shift: five people. Plus Gazin and I stayed, so that was seven people. There were at least two people there to measure the vibrations, and at least two more from Dontekhenergo. Also there was Orlenko, shift supervisor of the Electrical Division, there was the late Lelechenko, Dyatlov was there... Also some guys from the start-up division. In short, a fair number of people. That was a normal situation for all such tests.  

 

Technical Explanations: The shift supervisor had formal responsibility for the reactor, and above him in the operational hierarchy was station shift supervisor (Boris Rogozhkin, during that shift). The SIUB (Senior Unit Control Engineer) controlled the pumps, while the SIUT (Senior Turbine Control Engineer) was responsible for the turbine. Dyatlov's official role was advisory only, despite his overarching authority at the plant.

Yuri Tregub:

 

Around 11:00 PM Unit 3 called. They said: "We've got Dyatlov here, he's working someone over." He had stopped by Unit 3 on the way and evidently found some sort of deficiency in terms of discipline. He set them straight. And that's why he was delayed. He appeared sometime around midnight.

 

Also present in the control room was Razim Davletbaev of the Turbine Division:

 

Razim Davletbaev

Sasha [Akimov] took over the shift in a difficult situation, which is often the case during unstable transitory or start-up regimes: there are a lot of people in the control room, the regime is unstable, the operators are overwhelmed, meanwhile one has to find the time to review the operator's manual, take complete control of the situation, read through the shift's assignments and programs. Immediately after the start of shift Dyatlov began to demand the continuation of the test program. When Akimov sat down to read the program, he began to reproach him for taking too long, for not paying attention to the complexity of the situation unfolding with the reactor. With a cry, Dyatlov shooed Akimov out of his chair and started to rush him along.

 

Human Factors: What you have just read is the sum total of Dyatlov's aggressive behavior that night, as recorded by eyewitnesses. Naturally, he had a reputation as a tough and demanding manager in general. Reducing power during a shift change was one of the more ill-advised decisions made that night.

Soon after the shift change, there was a curious development. After being stabilized at 760 MW in accordance with the rundown test program, the reduction in reactor power resumed. Around this time, Yuri Tregub observed an interaction which he relayed at Dyatlov's trial in 1987:

I chose the control panel of the Senior Turbine Control Engineer (SIUT) as a place to observe from, near the panel of the 8th turbogenerator. Around 0:05-0:15 I heard an exchange between Akimov and Dyatlov. Its nature was that Dyatlov wanted the reactor to work at 200 MW. Akimov—he was holding the test program in his hands—disagreed, he was clearly objecting. This was judging by his facial expressions and body language. It makes me suppose that the power reduction was performed at Dyatlov's instructions. However I did not hear a direct command from him.

 

Controversies: A key aspect of the criminal charges against Dyatlov was his decision to run the test at 200 MW instead of 760 MW. The prosecutor was therefore keen to prove that this was a premeditated decision, rather than a reactive one. Dyatlov stated at the trial that Akimov's own testimony did not include any mention of this disagreement over power levels. Tregub's account strikes me as highly suspicious, given that he did not actually hear what was spoken. Indeed, he seems to draw attention to the fact that his testimony is mere surmise. The prosecutor was not impressed by this performance and had to lead the witness to make a less equivocal statement, drawing on a transcript of Tregub's pre-trial written testimony. Of course, this testimony may have been extracted under pressure, since Tregub was extremely fortunate to avoid prosecution himself, for reasons we shall soon see.

Human Factors: Suspicions aside, Tregub's testimony was not at all implausible. Why might Dyatlov have wanted a power level of 200 MW? One obvious potential motivation would be the need to run the turbine vibration test, since 760 MW was excessive for that purpose. Dyatlov may also have shared Tregub's concerns (which Tregub had aired to Akimov several minutes previously) about where to vent the excess steam when the stop-valves of the turbine were closed at the start of the test.

 

At the trial, the prosecution would introduce another piece of evidence that Dyatlov had wanted 200 MW from the start. Witness A. Kryat (head of the ChNPP's Nuclear Physics Laboratory):

 

I was familiar with the schedule of tests during the power reduction, especially the reduction from 1660 to 200-300 MW (that was the initial draft version). I expressed an opinion that I did not agree with 200-300 MW. 700-1000 were necessary. The thing is, power levels less than 700 MW lead to a loss of operating reactivity margin [ORM]. In this regime the 'Prizma' program also works poorly. That is, the system that allows operators to control the reactor's physical state. I made objections to Dyatlov at the meeting. I said that at 200 MW the reactor loses control...

 

Controversies: For his part, Dyatlov insisted that he had been unaware of the power reduction from 760 MW, stating that he had only learned of it after his arrest. At the trial, he indicated that the reduction was due a lack of communication between Akimov and (fellow defendant) station shift supervisor Boris Rogozhkin, the latter of whom supposedly gave instructions to continue reducing power. His version is not terribly convincing, but Dyatlov nevertheless did have a valid reason not to share Kryat's objections. Dropping power to 200 MW would ordinarily initiate significant xenon poisoning and require the removal of control rods, reducing ORM. However the reactor had been working at 1600 MW for such an unexpectedly long period that xenon concentrations were lower than they would have been in Kryat's assumed scenario. Therefore the 200 MW level could have appeared more reasonable to Dyatlov, in his desire to salvage the vibration test.

 

Possible drama aside, Razim Davletbaev continued his narrative of the night's events:

 

Akimov, holding a sheaf of papers in his hand (evidently it was the test program), began to visit each of the control room operators and explain the necessary configuration of the equipment. Because working at low power is difficult for the senior unit control engineer (SIUB), Akimov assisted Stolyarchuk with some of the procedures... 

 

Yuri Tregub:

 

At first everything proceeded normally. There was no reason for concern. To feel uneasy you need to have a reason for it. But then... then the SRV signal sounded: reduction of water flow. Most often this signal is a false positive, related to a defective sensor... The SIUR's job is to immediately send the electrician on duty to verify whether the signal is true or false. And at the same time send operators into the chamber where they can open the regulator valve and increase water flow. And this step Toptunov either forgot about, or else he was just very busy with the reactor... In short I grabbed the phone and gave instructions, sent them to check on it. I did that. And ended up next to the control panel. There on this part of the panel is a display you can toggle to find out the water flow rate. And precisely this rate was not good. If there is a complete zero on the screen, that makes sense, it means that the signal is gone. But here I saw that the flow rate had fallen. Not zero, but low numbers... they bothered me. 

 

This episode observed by Tregub played no role in the accident but goes to show how the operators were still struggling to keep up with events, following their complex shift change with a reactor in a transitional regime.

 

Continue to Part 3: The Iodine Pit

 

Sources Cited: 

• Nikolai Karpan, Revenge of the Peaceful Atom.
• Yuri Scherbak, Chernobyl.
• Razim Davletbaev, The Last Shift.