Part 4: Those 200 Megawatts

Amidst all the confusion and competing claims, we have neglected the processes taking place inside the reactor. Between 0:30 and 1:00 the reactor was in a precarious state, forcing the personnel to take immediate action to stabilize it.

Photo Credit

 Razim Davletbaev:

...the test was again delayed, because there was a power drop (this was apparent to me due to the megawatt-meter of Turbine-8). Senior turbine control engineer Igor Kirshenbaum had to reduce turbine power to a level similar to idling (around 3-5 MW). Since this occurred in my presence, I advised him to carefully control two parameters: 1) in case of a pressure reduction in the drum-separators reduce the load on Turbine-8, but not allow a motor regime; 2) if pressure in the drum-separators nevertheless fell, and a motor regime persisted for more than 2 minutes, to disconnect the the turbine. Myself, I went up to Anatoliy Stepanovich Dyatlov and told him that if steam production fell down to motor regime level, we would disconnect the turbine. Dyatlov nodded at the group of people around the panel of senior reactor control engineer (SIUR) Leonid Toptunov (who died from acute radiation sickness in May of 1986) and said that they would raise reactor power shortly. I returned to the turbine control panel. Indeed, pressure in the drum separators did not collapse, and turbine power hovered for a while at 50-60 MW.

 

Photo credit: chidori@emptytriangle.com

Technical Explanations: The 'turbinists' were concerned that the reactor would stop supplying sufficient steam to the drum separators (which sent water back to the pumps and steam to the turbine), leading to a lack of pressure and instability in the turbine itself. Since reactor power was so low, less water was supplied to the drum separators, which caused emergency signals to appear in the control room at 00:34:03. Pressure in the drum separators began dropping towards 55 kg/cm^2, at which point the reactor would shut itself down automatically. To prevent this, at 00:36:24 the operators changed the 'reactor trip' level to 50 kg/cm^2. This was not against the rules, and part of the operator's normal purview. That fact did not stop the Soviets from claiming it as a reckless violation of the regulations. Simultaneously, the water level in the drum separators continued dropping, below the -600mm level. There was an automated AZ-5 signal (reactor trip) that would shut down the reactor if water levels fell to -1100mm. Because reactor power was now lower, the operators were supposed to change from the -1100mm trigger to the -600mm trigger. Due to the fact that this would mean shooting themselves in the foot, they continued with the -1100mm setting, and worked on getting the water level back above -600mm instead. This was a violation of the letter—but not necessarily the spirit—of the rules. It is difficult to imagine any contemporary Soviet operator deliberately scramming their reactor for such a reason. SIUB Boris Stolyarchuk would pump feedwater into the drum separators at a high rate to restore water levels, and soon stabilized the separators on the right side of the reactor. However, levels in the left side drum separators would remain depressed almost up until the explosion.

Meanwhile at the SIUR panel, reactor parameters started improving by 0:42. The first automatic regulator (AR-1) was put back in operation, the second automatic regulator (AR-2) was on hot standby, and reactor power had reached 160 MW. 

 

Myths/Misinformation: In some Western sources Toptunov's manual manipulation of the automatic regulators is presented as if it was a violation of the rules. Likewise it is sometimes claimed that Toptunov needed to override fail-safes in order to remove control rods, or that control rods were not supposed to be raised to their upper limits. At other times it is intimated that the automatic control systems would have prevented the accident if left alone. These claims are all false. The SIUR had complete freedom to move rods as he saw fit, and the RBMK was simply not provided with automatic systems of sufficient reliability and accuracy to prevent the accident that did occur. Operators were always expected to block or transfer emergency protection signals as needed. 

 

Technical Explanations: AR-1, AR-2 and AR-3 refer to different parts of a group of 12 automatic control rods, which assist 12 more automatic control rods in the Local Automatic Regulator group (LAR). In addition to the 115 manual control rods (RR), these were the SIUR's primary tools for controlling the reactor. Other categories of rod include the 24 Local Emergency Protection rods, which are supposed to remain extracted, as well as the 24 shortened USP rods which inserted from the bottom of the core. Fatefully, the USP rods were unique in that they were not inserted into the core during an AZ-5 emergency shutdown signal. 

 

After the accident, investigators extracted data concerning control rod positions as recorded at 0:39, shown below in terms of number of centimeters inserted (maximum insertion would equal 655 centimeters for the 7-meter core). 

Diagram copied from Karpan, Revenge of the Peaceful Atom. Plain numbers indicate manual control rods. Underlined numbers indicate automatic control rods. Boxed numbers are bottom-insert (shortened) USP rods.

 

Technical Explanations: The lengths of control rods inserted totaled 102 linear meters, which in the current core conditions translated to an operating reactivity margin (ORM) value of 16.5 rods. In other words, the reactor was still within normal limits despite restoring power after the drop. From this point xenon poisoning would continue to require the removal of control rods, but ORM would fall by no more than 2 rods for this particular reason. In other words, the operators likely felt that it was reasonable to continue with the tests. At the same time, it is clear that there were very few control rods in the central region of the core, and an ORM value of anything less than 30 rods would soon be regarded as nuclear-hazardous in the post-accident period. Indeed, the dangerous tip effect of the descending control rods could manifest with an ORM value as high as 18 rods (Kopchinsky). In short, the RBMK could explode without warning even when operating in a completely authorized regime.

 

Once power reached 160 MW, the remaining 40 MW came easier. Indeed at 00:49:19 the signal "1 overcompensation downwards" was recorded, which meant that automatic control rods were inserting themselves to reduce the rate of increase. With its positive void coefficient, the RBMK was generally only too happy to ramp up.

As soon as it appeared that Toptunov/Tregub had brought the situation back under control, the operators disconnected Turbogenerator #8 from the reactor to carry out the turbine vibration test. This vibration-measuring process would continue from 00:41 until 1:16, no doubt a relief to Dyatlov and others. Since the 7th turbine was already shut down, the operators needed to block the AZ-5 signal that would otherwise be triggered by the disconnection of the 8th turbine. This action was not only allowed but required by the regulations at the current power level. Legasov's report to the IAEA would falsely report that blocking the reactor trip was a violation of the rules. However, blocking the reactor trip did have an impact on the turbine rundown test, because the trip was not re-enabled after 1:16.

 

Perhaps due to the partial re-insertion of automatic control rods, the ORM value remained within normal limits as power climbed towards 200 MW. As Yuri Tregub testified at the trial:

 

The last value which I saw was about 19.5 rods in the active zone...

 

Anatoliy Dyatlov:

 

Sometime around 1:00 I asked Toptunov what the reactivity margin was, and received answer of 19 or 18 rods, I don't remember precisely. On the numerical display Tregub saw 17 or 18, that is, Toptunov was checking periodically. But a SIUR cannot pay attention to one parameter, which requires him to enter a code and wait some time for it to appear on the display. A SIUR has over 4,000 parameters, besides controlling the reactor, and can't devote all his attention to one parameter. Toptunov was handling reactor control well; this was visible from the shape of the neutron field and the power printouts on teletype. 

Yuri Tregub:

  

That moment with maintaining the power had been a bit stressful, but in general as soon as we reached 200 MW and went on automatic mode, everyone calmed down. True, I didn't like those 200 megawatts; after all I was once a SIUR myself and consider that this is not the best regime for an RBMK. But here it wasn't up to me. Two hundred means two hundred. So as soon as we entered automatic mode I stepped away from Toptunov. Went back to the SIUT's station. There was no kind of pre-accident fuss. It was just the usual working fuss: conversations all the time, discussions.

 

(Here Tregub's description of the atmosphere is closely echoed by fellow survivors Stolyarchuk and Kirshenbaum, not to mention Dyatlov.) 

 

Tregub's objection to the 200 MW was presumably due to the fact that at this power level it is very difficult for the SIUB to control the feedwater levels (water added to the drum separators to make up for coolant lost to evaporation). Furthermore, at low power the in-core neutron sensors are overwhelmed by gamma radiation, and the SIUR can only rely on readings from the side ionization chambers. In the words of INSAG-7:

 

At low power levels a reactor operator has to operate 'blindly,' relying more on experience and intuition than on the readings of the control instruments. Although 'blind' control of an RBMK is to some extent acceptable during startup of a poison free reactor when its power density is controlled in accordance with preliminary calculations, the situation is different when a non-uniformly poisoned reactor is being shut down. In this case there is a large risk of large field distortions and critically high non-uniformities of both the axial and radial power density distributions. These facts were not taken into account before the incident and, unfortunately, no limitations were imposed on reactor operation at low power levels. 

 

Myths/Misinformation: In their report to the IAEA in 1986, the Soviets would try to claim that operating the reactor at 200 MW was forbidden. Setting the record straight took several years. It has also been claimed in various sources that 200 MW was too low for the turbine rundown test, rendering the results meaningless. In fact the low power had no effect whatsoever on the turbine's available inertia, and Section 3.5 of the test program even instructed the personal to reduce reactor power to a very low level ("level of electrical self-sufficiency") immediately preceding the rundown procedure.

 

The test program was sorely lacking in detail.

 

Continue to Part 5: The Test Begins 

 

Sources Cited:

• Yuri Scherbak, Chernobyl.
• Razim Davletbaev, The Last Shift. 
• Anatoliy Dyatlov, Chernobyl: How It Was. 
• INSAG-7.
• G. Kopchinsky, N. Shteynberg, Chernobyl: Causes, ProAtom, 2020.