On the outside, the 1975 accident at the LNPP looks very similar to the Chernobyl accident in 1986. It also happened at night, there was 1 TG in use prior to the accident, and, similarly, the TG capacity was reduced (after shift turnover) to 500 MW. Also similarly, capacity dropped down to zero before the accident (due to the operator’s mistake), and, they also started increasing it right after the drop.
But there are certain differences. At the ChNPP the accident occurred while shutting down the reactor for regular maintenance, while at the LNPP, on the contrary, the reactor was resuming its rated capacity after regular maintenance. At the ChNPP, the accident process started with stationary capacity of 200 MW and, lasting only a few seconds, destroyed the reactor completely. At the LNPP this process lasted dozens of minutes (if not hours) during capacity increase from zero to 1,700 MW; it destroyed (or damaged) about 30 FA’s, and one TC only was destroyed. The Chernobyl accident was to a large extent attributed to unstable thermohydraulic processes in the outer reactor cooling loop (MFCC), and to a much lower extent – to neutronic instability within the core proper. During the LNPP accident it was just the other way round.
Another difference between these two accidents was that they were investigated under absolutely different conditions. The Chernobyl NPP was under responsibility of the [Minenergo] (the Ministry of Power Industry) that was in charge of its operation, and the investigation was supposed to be carried out at least on the interdepartmental level. And since the accident took place at the outset of the ‘glasnost’ era and was too big an event to hide it, all the investigation materials, no matter how confidential, became public knowledge. And almost everything, down to the last detail, is known about this accident.
The Leningrad NPP was under responsibility of the [Minsredmash] (the Soviet ministry responsible for nuclear industry), and the accident occurred at the time of total secrecy. It was investigated as an accident that was a purely internal matter of that Ministry. No staff from the [Minenergo] that was at that time preparing to start operating similar reactors at the Kursk and Chernobyl NPP’s were allowed even to read the investigation materials, let alone any involvement in it. Therefore, there is currently no unbiased information available about the 1975 accident at the LNPP. All we have is what the Chief Designer of the RBMK has written in his book [Е2] and memories of eyewitnesses (that prefer keeping silence).
Nevertheless, based on these data and on 20 years old personal evidence gathered during investigation of the Chernobyl accident, it is possible to reconstruct this accident to a certain extent.
During rise to high power of the 1st power generating unit of the LNPP (after regular maintenance) on November 30’75, at the time when the reactor power reached 800 MW, one of the TG’s was shut down due to some problems in the control system, and the power was reduced to 500 MW. In this condition the unit was handed over to the next (night) shift to finish eliminating the problem and to continue the rise to rated power. Elimination of the problem lasted until 2:00 a.m., when the only TG in use was shut down by mistake. The reactor emergency protection system triggered, and it was damped.
Then reactor super-poisoning started, and within 3 hours operating reactivity margin (ORM) decreased from 35 to 3.5 control rods. I do not know what regulations said about it then, but the operators did not wait for poison override; instead, they started rising the reactor to power immediately after recovery of errors and failures. There is evidence of how it looked on the outside provided by an outsider V.I.Boretz, a trainee from the Chernobyl NPP that happened to be in this shift:
"During rising to power after shutdown, without any operator’s actions to change reactivity (without lifting any rods) the reactor would suddenly reduce acceleration time by itself, i.e., inadvertently accelerate; in other words, it would try to explode. The reactor acceleration was stopped twice by the emergency protection system {in fact, the emergency protection was triggered more than twice both on excess of power, and on speed of its growth}. Attempts of the operator to reduce capacity growth velocity by standard methods, lowering at the same time a group of manually controlled rods + 4 automatically controlled ones, failed, and rising to power was increasing. It was only stopped by triggering the emergency protection system."
You may have an idea what was happening at that time with neutron fields inside the reactor. Subsequent analysis of readings of neutron sensors outside and inside the reactor showed (according to the Chief Designer) that the maximum neutron field by height went down dramatically, and that there was a big variation (2 – 3 times by height, and 2.5 times by radius). It was in that condition when, at 6:15 a.m. the reactor capacity eventually was raised to 1,000 MW. And at 6:33 a.m., with the power of 1,720 MW, it was damped automatically by the emergency protection – this time due to certain process-related reasons (there were a number of alarms at once indicating that fuel channels had been damaged).

As a result of the accident investigation, the operating staff was found not guilty, as would be expected (the same investigators handled the accident at the ChNPP 11 years later). No one so much as mentioned that the Chief Designer might be at fault. The whole investigation was focused upon the issue of neutron fields’ instability. This problem actually exists in the RBMK, and the less absorbers there are in the core (including the less ORM), the more acute it becomes. In that case, however, this problem was secondary, while the primary one was the end effect of control rods, i.e., under certain conditions the sign (+/-) of reactivity inserted by the rods would be reversed.
No one, however, would acknowledge this end effect – officially, anyway (no one wanted to look into the abyss). And almost all the measures taken as a result of the 1975 accident were aimed at increasing durability and improving control of energy release fields in the reactor. Here’s a quotation from the Chief Designer [Е2] (may accuracy of these data and emphasis laid rest on his head):
" In order to avoid in the future any accident causing burnout of fuel elements and channel pipes due to local capacity increase, the following measures were taken at the RBMK reactors:
– implementation of a 7-12-zome system intended for local automatic control of power capacity and local emergency protection; the system is activated by inter-zonal neutron sensors;
– the number of control rods in the second generation reactors was increased from 179 to 211; the rods were installed instead of fuel channel in peripheral part of the core;
– the minimum admissible reactivity margin was set to be 15 control rods; it was prohibited to operate a reactor with less reactivity margin;
– an automatic reactor emergency protection was introduced that was activated by pressure increase alarm in the reactor space.."
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