Part 7: Bitter Wormwood

 

Image Credit: chidori@emptytriangle.com

When AZ-5 was pressed, 164 manual control rods were located at their upper limits. Each rod's neutron-absorbing boron section was outside the confines of the core, while the 4.5 meter graphite section (displacer) hung beneath it, in the central region of the core. Beneath each displacer was a column of water, filling the channel to a height of 1.25 meters. When the displacers moved down, replacing water with graphite, reactivity at the bottom of the core surged. 

 

The bottom-insert USP rods did not move, in accordance with their design, and could not counterbalance the destructive increase in localized neutron flux. The automatic control rods did not have graphite displacers and did not contribute to the 'tip effect'. The partially-inserted manual control rods also did not contribute, but it would take over 10 seconds for their boron sections to reach the bottom section of core, at which point the reactor would no longer exist.

Myths/Misinformation: Some very poor (and thus popular) sources describe the graphite displacers as 'tips' which 'entered the core first.' A 4.5 meter length of graphite is far from just a 'tip', of course, and the graphite could only 'enter' the core when a rod was being raised, not inserted. The entirety of the problem lay in that 1.25 meter water-filled bottom section of each technological channel. If no water was removed, the graphite itself could do no harm.

Technical Explanations: Why did the graphite displacer exist? To prevent the channel from filling with water when a control rod was raised, which would otherwise absorb neutrons. The displacer augmented the rod's ability to both increase and (more importantly) decrease reactivity. Therefore the graphite displacer was essential to both economics and safety. Why did the 1.25 meter water column exist underneath the displacer? In the original draft design of the RBMK it did not, but the designers could not find enough space in the already-massive core for the longer 6-7 meter displacer that would be required. The alternative was to use film-cooling for the gap beneath the displacer, but in the early 1970s the designers regarded this technology as immature or too expensive. Sometime in the later 1970s NIKIET actually made the water column taller (increasing from 1 to 1.25 meters) as a side effect of safety improvements implemented following the 1975 Leningrad accident (Kuzmin, 165). This likely exacerbated the tip effect. Today, Russian RBMKs use film-cooling to eliminate the tip effect, along with a host of other safety improvements (faster scram rods, lower void coefficient, etc).

Human Factors: One of the hardest questions to answer about the accident is 'how could this have not happened before?' My commentary has hopefully demonstrated that the operators made fairly logical decisions grounded in typical assumptions about how the reactor worked. The operating regime was not typical but also not outlandish, and could be easily recreated following any unexpected power drop (such as an AZ-2 signal). In retrospect the presence of the tip effect was obvious, as operators sometimes saw warning signals for power increase when scramming an RBMK. These were written off as yet another example of unreliable sensors, which themselves forced operators to fly by the seat of their pants rather than dutifully following the lead of the Skala computer. Ultimately, the tip effect likely required a very precise set of conditions in order to destroy a reactor: not only low ORM but also the shape of the neutron field and the specific pattern of boiling in the core. The RBMK's designers, meanwhile, had clearly underestimated the threat, given their lackadaisical response after the tip effect was 'discovered' several times in the late 1970s and early 1980s. Many specialists "were terribly incredulous and refused to believe that 'such nonsense' could blow up a reactor" (Dmitriev).

 

What happened after the graphite displacers pushed the water out of the bottom of the core? For the first few seconds we have a vague idea, but after that there are only theories. The sudden loss of neutron absorption inserted around 1 β of reactivity into this region of the active zone, triggering an uncontrollable chain reaction fueled by fast (rather than thermal) neutrons. This surge of energy caused several fuel assemblies to shatter, destroying their channels as well. Coolant reaching the superheated uranium oxide pellets continued to flash to steam and escape into the reactor space, creating enough overpressure to dislodge the 2000-ton upper biological shield. This severed the rest of the technological channels where they exited the core. With no water to absorb neutrons, reactivity and reactor power adopted a vertical line, as briefly captured on the graph below:

 

Reactor power diagram from April 25th and 26th. Power had increased to 216 MW when AZ-5 was pressed, thereafter surging to an unknown value that was recorded only as 2,648 MW. Image Credit: Dyatlov, Chernobyl: How it Was.

 

 

At 1:23:49 the systems recorded overpressure in the reactor space, suggesting that the rupture of technological channels had occurred. Simultaneously, the reactor building lost power, making this timestamp the clearest point marking the final destruction of the reactor.

Myths/Misinformation: Some sources claim that the descending control rods became stuck when core damage deformed the channels. However, this could only have occurred as the result of the power surge, not served as its cause. There is no evidence for the rods getting stuck, as opposed to being physically destroyed along with the core while only halfway to the bottom. Likewise, a reactor working at 200 MW with no emergency signals could not possibly have caused the metal channel caps in the reactor hall to bounce up and down, as often depicted in film. This is perhaps the most flagrant tall tale of all, without even an alleged source for the information. Supposed eyewitnesss Perevozchenko was nowhere near the reactor hall and never relayed such a thing to anyone (which I confirmed by asking one of his former colleagues).

 

Controversy: The topic of pump failure has taken on an almost religious significance in discussions of the accident sequence. Researchers from NIKIET and the Kurchatov Institute in particular have used the 6.5 second delay in the 'MPA' button to move the moment of pump failure backwards in time, from 1:23:46 and onwards to the moment of AZ-5 itself. In an apparent error, Nikolai Karpan described the four 'running down' MCPs failing at 1:23:41 as their voltage declined. INSAG-7 draws on the time-corrected data to show that all pumps suffered a near-simultaneous collapse in flow rates around 1:23:45 (as the power surge was presumably wreaking havoc on the primary circuit), shutting down shortly thereafter.

 

The question of the precise nature of the explosion(s) is far beyond the scope of this site. If any readers thought that the primary source materials for the lead-up to the accident were hopelessly muddled, the competing theories for the destruction sequence of the reactor put all that to shame. In lieu of a detailed discussion, below is a cursory list of the main explanations for the explosion(s).

• Steam explosion, followed by hydrogen explosion and (some degree of) graphite fire;
• This is the 'dominant' theory, although many have cast doubt on the ability of the graphite to burn (as opposed to oxidize/vaporize) without a constant source of heat from nearby fuel rods; 
• Steam explosion, followed by or combined with thermal explosion (as observed in some small research reactors);
• Singular, mid-air, nuclear 'fizzle' explosion vaporizing 50-90% of the fuel;
• AKA, the 'Kostya Checherov Flying Nuclear Reactor' theory, also supported by Nikolai Karpan; as suggested by the lack of evidence for high temperatures in the reactor pit;
  
• Some combination of options #1 and #2, but with a 'nuclear jet' of plasma released by several channels in the opening stages;
• Evidence includes the presence of short-lived isotopes at high altitudes and a singular seismic signature of the explosion. 

 

Epilogue

Photo Credit

 

The experiences, actions and sacrifices of our eyewitnesses following the explosion would require another project of similar scope. In the future, if there is popular demand I can add a section translating additional eyewitness accounts from the post-explosion period. In the meantime I will conclude with a brief summary of their fates below:

Alexander Fyodorovich Akimov: Remained on duty until morning, attempting to contain the accident. Took part in manually opening coolant valves, receiving a bone marrow dose of >10 Grays. Died May 10, 1986 in Moscow Hospital No. 6. Criminal charges dropped posthumously.

 

Razim Ilgamovich Davletbaev: Survived acute radiation sickness following treatment in Moscow Hospital No. 6. Died in Kazan of leukemia in 2017.

Anatoliy Stepanovich Dyatlov: Remained on duty until morning, surveying the damage and attempting to locate pump operator Valery Khodemchuk. Survived acute radiation sickness (6 Grays) and severe leg burns. Sentenced to 10 years in prison but released on medical grounds in 1990. Died of heart failure in 1995 before the publication of his book.

 

Mikhail Anisimovich Elshin: Survived the accident. Later biographical details unknown.

 

Sergei Nikolaevich Gazin: Unit 4 SIUT during the accident, survived. Later biographical details unknown.

 

Igor Ivanovich Kazachkov: Unit 4 shift supervisor during the accident, reportedly died in 2014.

 

Nikolai Vasilievich Karpan: Arrived at the scene the morning of the accident, took part in liquidation efforts and promoted to Deputy Chief Engineer for Science. Wrote a second autobiographical book (From Chernobyl to Fukushima), died in 2016.

 

Igor Kirshenbaum: Unit 4 SIUT during the accident. Reportedly dismissed from duty by Akimov, as of 2021 lives in Israel. 

 

Anatoliy V. Kryat: Head of the nuclear physics laboratory during the accident. Visited Dyatlov in prison. As of 1995 serving as Government Nuclear Safety Inspector of Ukraine.

Alexander Gennadievich Kudryavtsev: SIUR trainee, attempted to enter the reactor hall to manually lower control rods. Died May 14, 1986 in Moscow Hospital No. 6.

 

Grigorii Vasilievich Lisyuk: Electrician during the accident, later biographical details unknown, but apparently alive as of 2003.

 

Gennadii Petrovich Metlenko: Rundown test organizer from Dontekhenergo. Survived the accident. Later biographical details unknown.

 

Viktor Vasilievich Proskuryakov: SIUR trainee, attempted to enter the reactor hall to manually lower control rods and received bone marrow dose of 5-6 Grays. Died May 17, 1986 in Moscow Hospital No. 6.

 

Boris Vasilievich Stolyarchuk: Survived acute radiation sickness (ARS). As of 2021 works for the State Nuclear Regulatory Inspectorate of Ukraine.

 

Leonid Fyodorovich Toptunov: Unit 4 SIUR, dismissed from duty by Dyatlov but returned to assist Akimov. Received bone marrow dose of 10-12 Grays while attempting to supply coolant to the reactor. Died May 14, 1986 in Moscow Hospital No. 6. Criminal charges dropped posthumously.

 

Yuri Yurievich Tregub: Survived acute radiation sickness (ARS) after opening ECCS valves and surveying the exterior of the reactor building with Dyatlov. As of 2021 works as a researcher at the Scientific and Engineering Center for Nuclear and Radiation Safety (Ukraine).

 Sources Cited: 

• INSAG-7
• A. N. Kuzmin, Forty Years with the RBMK, History of Atomic Energy in the USSR and Russia, Part 3, History of the RBMK, 2003. 
• Viktor Dmitriev, The Causes of the Chernobyl Accident Are Known, 2013.