26.07.2020

The accident at the Leningrad nuclear power plant

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"Leningrad Chernobyl"


We realized the scale and devastating consequences of the Chernobyl disaster only with time.  Even the conclusions reached by the creator of the RBMK, the Academy of the USSR Academy of Sciences, Anatoly Petrovich Aleksandrov, turned out to be deadly.  He, who had previously argued that a nuclear reactor is so safe that it could even be placed on Red Square, was deeply shocked by what happened.


However, a similar accident almost happened 11 years earlier near Leningrad.  In those years, it was classified, not a single media outlet of the USSR told about it. Even the residents of Sosnovy Bor, where the plant is located, were unaware of what had happened.  Although the background radiation level on the streets of the city was exceeded thousands, or even more ...


Today we publish information about what happened 45 years ago at the Leningrad NPP.  The material is based on the memoirs of contemporaries and documents that are freely available.  For ethical reasons, we do not disclose the names of the station shift personnel.


On the morning of November 30, 1975, the Leningrad NPP duty officer received a call from a nearby research and development institute: “Are you all right?  Our dosimeters are off scale.  But everything is clean on the territory of the institute.  Most likely, it’s something you have ”… So in NITI (Research Institute named after A.P. Aleksandrov), located three kilometers from the first unit of the Leningrad NPP, they reacted to the aerosol release carried by air currents from the station.  This was the first signal of an accident recorded outside its zone.


According to Vitaly Abakumov, a participant in the events, who worked in this shift as a reactor control engineer, on November 30 at 6:33 am, at the reactor control room (MCR), “several alarms appeared at once, indicating a violation of the integrity of the process channels”.  This is the time of the accident.


But the information about the accident was immediately classified.  Neither the country, nor the city, nor even the station staff knew about it.
“By that time, I was working as a senior engineer for managing turbine plants,” says Valery Koptyaev, a former LNPP employee.  - On November 30, my shift was at the weekend.  When I came to the control room on December 1, I saw my replacement, Mikhail Khudyakov, in a respirator.  I already knew that the unit was stopped, but I had no idea why.  Usually, management from the director and chief engineer to the heads of shops and their deputies in those years came to us for the morning planning meeting in suits, ties and ordinary shoes.  On this day, I saw the leadership in white overalls and special boots.  I ask Mikhail: "Why in a respirator, what is the level of aerosols in the air?"  “I don’t know for sure, but more than 200 norms, the dosimetrists said,” he replied.  Then we found out how much "dirt" was spread not only around the station, but also in the city. "
So, what happened back in 1975?  Vitaly Abakumov talks about this in detail.  On the night of November 30, one of the two operating turbine generators (TG) was to be unloaded and taken out for repair.  The operators unloaded the required generator.  But by mistake, instead of the unloaded, the operating TG was disconnected from the network.  Which led to the activation of the emergency protection and the shutdown of the reactor.  “Realizing that the staff had made a mistake, the station shift supervisor gave the command to start the mistakenly disconnected TG as soon as possible,” recalls Abakumov.  - All preparation for switching on and accepting the load on the TG took place in a nervous atmosphere, against the background of a real threat of unacceptable poisoning of the reactor, falling into the iodine pit and subsequent long downtime of the unit.
To accelerate the reactor, the operators had to remove practically all control rods from it.  And bringing the reactor to the minimum controllable power level has become a dangerous and difficult task for the senior reactor control engineer (SIUR), prohibited by the technological regulations.  However, the shift supervisor and the SIUR went to the violation without hesitation.  They sought to compensate for the consequences of operator error, since the main indicator at the time was the plan for power generation.  A reactor downtime is a loss of the accumulated megawatt-hours!


Violations of technological regulations have never been welcomed.  But at the same time, they were not perceived as dangerous in those days.  “Therefore, violations of the lower regulatory limit of the operational reactivity margin (ORM) were a common practice at LNPP and were secretly perceived as evidence of the special skill of the SIUR”.
 “The RBMK reactor is large not only in terms of its design parameters, but also from the point of view of reactor physics, which means the possibility of achieving criticality not only for the reactor as a whole, but also in local areas of the reactor core,” continues Abakumov.  - With the total "poisoning" of the reactor core and the virtual absence of means of influencing the reactivity (all control rods were removed), the senior engineer managed to bring the reactor to the minimum controllable level not "in general", but only in a limited area adjacent to the fuel channel 13-33...  Outside this area, the core remained “poisoned”.


Further rapid energy loading of this local area led to overheating and massive destruction of the cladding of the fuel elements (fuel elements).  The destruction of fuel assemblies due to their melting is called a "goat" in professional nuclear slang.  As Abakumov recalls, when the alarm was triggered, “the reaction of the senior engineer was immediate: “I’m shutting down the reactor!  "And the reactor was shut down with the AZ button [emergency protection button], without hesitation or doubt."


 “Leningrad Chernobyl” could well have taken place at the 1st power unit of the Leningrad NPP after pressing the AZ button, which drops all control rods into the core to shut down the reactor,” Vitaly Abakumov believes.  - Just like it happened at the Chernobyl nuclear power plant, the operating personnel of which made a similar decision.  The situation was saved not by the actions of the plant operators, but by the physics of the reactor.  The fact is that the Laes reactor was significantly "fresher" than the Chernobyl reactor in terms of the average fuel burnup in the core. "
 

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