Part 1: 25 April 1986

Photo credit: chidori@emptytriangle.com

April 25th began much the same way as would April 26th: Shift Supervisor Alexander Akimov took command in control room #4 at midnight and began reducing reactor power in preparation for maintenance-related tests. Turbogenerator No. 7 was shut down after 4:00. By the end of the eight-hour night shift, the operators dropped from the 'nominal' power of 3,200 MW (thermal) down to 1500 MW. Again, similar to April 26th, reducing power led to difficulties. 

 

At the start of the shift, the Operating Reactivity Margin (ORM) had been 31 manual control rods, which was typical for steady-state operation at nominal power. Owing to the reduction to half-power, the reactor underwent xenon poisoning and by shift's end the ORM was measured at 13.2 rods, below the 15-rod limit where an immediate shutdown was required by the regulations.

 

Technical explanation (Xenon): Reactors constantly produce xenon, a potent neutron 'poison' that dampens reactivity. The higher the reactor power, the more xenon is produced, but the reactor is also best able to 'burn off' the poison via fission. During steady-state operation an equilibrium is reached, but following a power reduction, the fuel retains a high concentration of xenon even as the lower reactor power is less able to overcome the poison. The result is a drop in reactivity, requiring that control rods be raised to compensate.

Technical explanation (Operating Reactivity Margin): Very roughly speaking, ORM refers to the number of control rods inserted into the active zone of the reactor. However, ORM in an RBMK is an abstract value, calculated by 'weighting' the various rod positions in accordance with neutron flux measurements. While inserting rods always increases ORM and removing rods always reduces it, the value can shift on its own as power distribution and coolant parameters change. In practice, ORM was calculated with mediocre accuracy by the Skala computer at regular intervals of up to two hours. The process took 10-15 minutes and was therefore completely unreliable except during steady-state operation. The control room personnel would also refer to a simple graph to anticipate how power level changes would affect ORM (power reduction = more xenon poisoning = removal of control rods). The core function of ORM was to ensure that the RBMK could undergo an automated power reduction to 1600 MW (via emergency protection signal), but still regain its former power level expeditiously. If restoring power required the removal of so many rods that ORM fell below 15, the reactor control engineer (SIUR) would have limited ability to manipulate the distribution of neutron flux in the core. More importantly, a low ORM value indicated excessive xenon poisoning. This meant that a full shutdown (via an emergency protection signal such as AZ-5) would require waiting an extended period for the xenon to decay, incurring finanical losses to the plant.

Human Factors: While formally ORM was below 15 rods, the operators noted in the log that the PRIZMA code used to calculate the value had encountered an error, failing to take into account the positions of the 12 automatic regulators (control rods) (INSAG-7, 71). Therefore the true ORM value was likely around 18 rods. While these may seem like reasonable extenuating circumstances, the episode can nonetheless serve as a hint that the operators were accustomed to treating ORM as a less-than-critical parameter. Their training presented it as a primarily economic metric, as Station Shift Supervisor Rogozhkin testified at his trial for criminal negligence in 1987. Furthermore, it is unclear whether Chief Engineer Fomin ever provided the necessary approval to operate below 26 rods ORM, on the way towards the ultimate limit of 15. Given that low ORM was a fact of life for power reductions, said approval may have been taken for granted. The seemingly-strict requirement to "immediately" shut down the reactor at 15 rods ORM was also undercut by other clauses of the regulations. Most importantly, nowhere in the regulations or in actual practice was ORM regarded as relevant to safety.

The Skala computer, which on the night of the accident was scheduled to calculate Operating Reactivity Margin every two hours. The calculation took 10-15 minutes, at which point the Skala operators printed off a report and walked down the 'golden corridor' to deliver it to the control room. By this point the value would be hopelessly out of date unless the reactor was in steady-state operation.

At 8:00 the day shift took over, led by Igor Kazachkov. ORM was evidently still reading below 15 rods at this point, because he later made the following comments to Yuri Scherbak:

 

Why did my colleagues and I not shut down the reactor, when the number of protection rods fell? Because none of us imagined that this heralded a nuclear accident. We knew that it was forbidden, but did not think... No one believed in the threat of a nuclear accident, and no one told us about this. I have worked at the plant since 1974 and have seen much more difficult regimes. And if I shut down the reactor they will tan my hide. After all we are always chasing the plan... And for such a reason as the number of control rods we never had a shutdown, not once... I think they would have chased me out of my job. They definitely would have had me dismissed. Not for this, of course, but they would have found some pretext. We simply didn't regard this parameter [ORM] as something serious. I say again: we frequently had fewer rods inserted than required, but nothing happened. Nothing blew up, everything was normal.

Graph copied from accidont.ru (Author Viktor Dmitriev)

Towards the end of Kazachkov's shift came the infamous delay in the test:

We had been preparing for the turbine rundown experiment all morning, and had completed almost the entire program... At 14:00, 15 minutes before we planned to begin the experiment, shift supervisor Baranov* called and said that the test was postponed because a unit at some other plant had gone offline and there was a deficit of power. And our unit—at that point it was at 1500 MW, that is 50% power—had to keep working. A normal situation, overall, it happened all the time. We were in the MinEnergo system, after all. We prayed to the Plan, for kilowatt-hours and all the rest.

 * The Baranov referred to here is one of the famous "divers".

 

Myths/Misinformation: This episode is perhaps the best acid test for determining whether a Chernobyl commentator knows what they are talking about. Numerous sources claim that delaying the test and requiring the reactor to continue running at half power caused greater xenon poisoning than was otherwise expected. In fact the delay in the test meant that xenon concentrations leading up to the test were lower, rather than higher. This is due to the fact the xenon poisoning increased following the power reduction, reaching a peak concentration around 8:00. From then on levels would only fall, in accordance with the 9-hour half-life of Xe135. The neutron poison had largely decayed away by midnight of April 26th, enabling the operators to gradually insert control rods, as reflected by the ORM value of 26 rods at that point in time. Indeed, if not for the grid dispatcher's delay, Kazachkov's shift would have likely struggled to complete the test before having to remove most control rods from the reactor in an attempt to counteract much more rapid xenon poisoning.

 

Also around 14:00, Kazachkov's shift manually closed the valves of the Emergency Core Cooling system. The test program's authors were concerned that the ECCS could be triggered needlessly, with the sudden flood of cold water damaging the core.

In 12 hours the ECCS' pressurized water tanks would be left sprawling in the rubble, as seen in this photo from April 27th by plant photographer Anatoliy Rasskazov.

Igor Kazachkov:

 

The only deviation from the operating instructions in this test program was the deactivation of a safety system. During my shift I disabled this safety system. All of that was written in the program. I looked at every section—do this, do that. And in all these steps I didn't see that they required anything that was forbidden by the instructions. I repeat: the only thing was deactivating the ECCS...

 

Again, why did I do it? This system is there in case a large-diameter coolant pipe bursts. And that is, naturally, a very low probability. About the same as a plane crashing on top of you, I think. Yes, I supposed that in an hour or two the reactor would be shut down.

 

I disabled the safety system. And then all the newspapers said—even overseas I read that the Americans said the explosion happened because the Russians disabled their safety systems. But I can affirm that there is no connection whatsoever between the explosion and the deactivation of this reserve cooling system. None at all. And I said as much at the trial, when I testified as a witness... The same question was posed to the experts, and the experts gave the same answer.

 

Myths/Misinformation: When the Soviet delegation—led by Valeriy Legasov—presented to the IAEA at Vienna, they infamously claimed that the ECCS would have saved the reactor. In reality, not only did the electrical signals to trigger the system never fire, the ECCS was wholly incapable of halting a sudden power surge simply by inserting cold water. Whenever someone claims that the operators disabled safety systems, they are referring exclusively to this system, which could have played no role in the accident.

 

Human Factors: Kazachkov also could have pointed out that 'first generation' RBMKs such as Chernobyl Units 1 and 2 lacked an ECCS entirely. Indeed, in the over 100 'reactor years' of operation, an ECCS system has never been activated 'in anger'. At the same time INSAG-7 rightly pointed out that leaving the system disabled for twelve hours (even after the delay in the test) was indicative of a lax safety culture.

At left: Yuri Tregub

At 16:00 shift supervisor Yuri Tregub took over, and explained in his own words why the ECCS wasn't re-enabled following the delay in the test:

The shift change itself was very difficult, because there were several test programs on the table: the turbine rundown program, the program for air-cooling the reactor, a vibration measuring program and a fourth... I forget which... Igor Kazachkov handed the shift over to me. The tests were supposed to be performed during his shift, but they were pushed back to mine...

 

During the shift change they said that safety systems were disabled. Well naturally I asked Kazachkov: "What do you mean, disabled?" He said: "According to the test program, although I objected."

 

The ECCS (an emergency reactor cooling system) was deactivated during Kazachkov's shift. That was a ton of work, since we have hand-cranked equipment. Imagine, a single valve takes forty minutes. The valve itself is like the wheel of a sailing ship, just a bit smaller and mounted horizontally. Closing it requires the efforts of two people, ideally three. Kazachkov spent almost the whole shift disabling this emergency system. It is seriously hard work.

 

And how long would it have taken me to enable it again? I wouldn't have done so. And then if we have to disable it again to carry out the test? By the way, as the accident sequence showed, the ECCS did practically nothing, all the connections blew off, everything flew away, all the valves at once. 

 

Tregub then spent several hours acquainting himself with the various programs, which he could only assume would be carried out on his shift, once the grid dispatcher gave the go-ahead:

Only after carefully studying the test program, only then did I end up with a ton of questions regarding the procedure... I did not like the program's vagueness. It was clear that it was written by an electrician: Metlenko or someone else from DonTekhEnergo.

 

Somewhere around 8:00 PM I asked [station shift supervisor] Dik again, worrying that he had gotten distracted, and maybe the dispatcher had given permission to begin the experiment? Dik said: "No permission given yet. But you need to call Dyatlov to attend the test. 

 

I call Dyatlov at home... he says: "Don't start without me." I tell him: "I have questions. A lot of questions." "This isn't a conversation for the phone, don't start without me," he said. Only after 9:00 did it become clear: at 10:00 PM the tests would start. The Kievenergo dispatcher allowed the plant to reduce power.

Continue to Part 2: Shift Change

 

Sources Cited: 

• INSAG-7
• INSAG-1 
• Nikolai Karpan, Revenge of the Peaceful Atom.
• Yuri Scherbak, Chernobyl.