History of Chоrnobyl

Chornobyl: Causes and consequences

Chornobyl: Causes and consequences 

More than 35 years have passed since the Chornobyl accident, but its consequences remain the subject of discussion in the world scientific community today. According to UNSCEAR and WHO, the Chornobyl disaster is one of the highest-level nuclear accidents.

●      3/4 of Europe was contaminated with radioactive caesium.
●      For ten days - from April 26 to May 6 - the release of activity from the damaged reactor lasted at the level of tens of millions of curies per day.
●      According to independent experts, 500,000 people died from radiation.
●      In the coming days after the accident, 8.5 million people in Ukraine, Belarus and Russia received significant doses of radiation.
●      Ninety thousand seven hundred eighty-four people were evacuated from the 81st settlement of Ukraine by the end of the summer of 1986.
●      More than 600,000 people became liquidators of the accident - fighting the fire and clearing the debris.
●      Eleven tons of nuclear fuel were released into the atmosphere due to the accident at the 4th power unit of the Chornobyl NPP.

Due to imperfect construction, violation of construction technology, the use of low-quality building materials, numerous mini-catastrophes, it was almost impossible for the Chornobyl tragedy not to happen.

Historians emphasize the political responsibility of the communist regime, which endangered the lives and health of millions of citizens for ideological interests. No wonder Chornobyl is considered one of the main events that influenced the collapse of the Soviet Union.

Chronology of the accident
On April 25, 1986, the fourth power unit was to be shut down experimentally at the Chornobyl NPP to study the possibilities of using the inertia of the turbogenerator in the event of a power outage. Even though the technical circumstances did not correspond to the test plan, it was not cancelled.

The experiment began on April 26 at 01:23. The situation got out of control. At 01:25, two explosions erupted a few seconds apart. The reactor completely collapsed. More than 30 fires broke out. The main ones were extinguished in an hour, and the fire was eliminated by 5 am on April 26. However, later there was an intense fire in the central hall of the 4th block, which was fought using helicopter equipment until May 10.

At the time of the accident, 17 employees were on the premises of Unit 4. Valery Khodemchuk, a senior reactor shop operator, died under the rubble. On April 26, Volodymyr Shashenok, an adjuster, died from radiation. 11 workers received radiation doses. They all died of radiation sickness on May 20, 1986, in a Moscow hospital № 6. Another 14 people from the station's staff received doses that caused grade 3 and 4 radiation sickness.

The day after the accident, the government commission decided to immediately stop the 1st and 2nd power units and evacuate the population of Pripyat (the so-called 10-kilometre zone).

The KGB report states that as of 8:00 am on April 28, the radiation level at Units 3 and 4 was 1,000–2600 micro-X-rays per second, and at some parts of the city, it was 30–160. At this point in the document, Volodymyr Shcherbytsky made his now well-known note - "What does this mean?". This eloquently shows that even the highest officials were not fully aware of the danger.

Who is responsible?
Initially, the blame for the disaster was placed solely (or almost exclusively) on staff. This position was taken by the State Commission formed in the USSR to investigate the causes of the catastrophe, the court, and the KGB of the USSR, which conducted its investigation. The IAEA, in its 1986 report, also broadly supported this view. According to this version, the gross violations of the NPP operation rules committed by the Chernobyl personnel were:
●      conducting tests at any cost, despite changes in the state of the reactor;
●      decommissioning of working technological protection, which would stop the reactor before it would go into dangerous mode;
●      silence of the scale of the accident by the Chernobyl management in the first days.

However, this explanation of the causes of the accident was later revised, including by the IAEA.

The Nuclear Safety Advisory Committee published a new report in 1993, focusing on severe problems in reactor design. In this report, many of the conclusions made in 1986 were found to be erroneous.
In the current statement, the causes of the Chernobyl accident are as follows:
●      the reactor was improperly designed and dangerous;
●      the staff was not informed of the danger;
●      the staff made several mistakes and inadvertently violated the available instructions, partly due to the lack of information about the dangers of the reactor;
●      disabling protection would either not affect the development of the accident or would not contradict regulations.

Imperfect reactor
The RVPK-1000 reactor had several design flaws, which, according to IAEA experts, were the leading cause of the accident.

It is also believed that due to improper preparation for the experiment on the "run-out" of the generator and the operators' errors, there were conditions under which these shortcomings manifested themselves to the maximum. It is noted that the program was not correctly agreed upon and did not pay sufficient attention to nuclear safety issues. After the accident, measures were taken to eliminate these shortcomings in such reactors at other NPPs.

Design errors
Positive vapour reactivity coefficient.
During the reactor's operation, water is pumped through the core, which is used as a coolant. Inside the reactor, it boils, partially turning into steam. The reactor had a positive vapour reactivity coefficient, i.e. the more steam, the greater the power released by nuclear reactions. At low power, at which the power unit operated during the tests, the effect of the positive steam coefficient was not compensated by other phenomena affecting the reactivity. 

The reactor had a positive reactivity power factor. This means positive feedback - the increase in power caused such processes in the core, which led to an even more significant increase in power. This made the reactor unstable and dangerous. In addition, operators were not informed that positive feedback could occur at a low capacity.

"Final effect"
Even more dangerous was the error in the design of the control rods. To control the power of the nuclear reaction, rods containing a neutron-absorbing substance are introduced into the core. When such a rod is removed from the core, water remains in the channel, absorbing neutrons. To eliminate the negative impact of this water, the RVPK under the rods were extruded from graphite. But when the rod was fully raised, a column of water 1.5 meters high remained under the extruder.

When the rod is lowered, the absorber enters the upper part of the zone and introduces negative reactivity. In the lower part of the channel, the graphite displacer replaces water and introduces positive reactivity. At the accident, the neutron field had a dip inside the core and two maxima at the top and bottom. With this field distribution, the total reactivity introduced by the rods during the first 3 seconds of movement was positive. This is the so-called "end effect". As a result, the operation of the emergency protection in the first seconds increased the power instead of stopping the reactor immediately.

Operator errors
It was initially claimed that the operators made many mistakes. In particular, the fault of the staff was considered to be the shutdown of the central protection systems of the reactor, the continuation of work after the power drop to 30 MW and the fact that the reactor was not stopped. However, they knew that the operational reactivity was less than allowed. It was alleged that these actions violated established instructions and procedures and were the leading cause of the accident.

The 1993 IAEA report revised these conclusions. It was acknowledged that most of the operators' actions, which were previously considered violations, actually complied with the rules adopted at the time or did not affect the course of the accident. In particular:
●      long-term operation of the reactor at a capacity below 700 MW was not prohibited, as previously claimed; any document did not prohibit simultaneous operation of all 8 pumps;
●      shutdown of the reactor emergency cooling system (ACR) was allowed subject to the necessary approvals. The approved test program locked the system, and therefore, the required permission from the station's chief engineer. This did not affect the development of the accident - by the time the CAOR could work, the core had already been destroyed;
●      blocking the protection that stops the reactor in case of shutdown of 2 turbogenerators was not only allowed but was mandatory when working at low power;
●      the fact that low water protection was not included in the separator tanks was technically a violation of the regulations. However, this violation is not directly related to the causes of the accident, and, in addition, other protection (at a lower level) has been included.

In the analysis of staff actions, the focus is not on specific violations but a low "safety culture". It should be noted that nuclear safety experts began to use this concept only after the Chernobyl accident.

The accusation concerns not only the operators but also the reactor designers and the NPP management. Experts point to the following examples of insufficient attention to safety issues:
●      After the reactor, the emergency cooling system (CAOP) was shut down on April 25.
●      Kyivenerho's dispatcher was instructed to postpone the shutdown of the unit.
●      The reactor worked with the CAOR off for several hours.

The staff could not turn on the SAOR again (this would require manually opening several valves, which would take several hours), but from a safety point of view, the reactor had to be shut down, despite Kyivenerho's request.

On April 25, the operational reactivity margin (RRP) was less than allowed (these measurements may have been an error that staff were aware of; the actual value was within the allowable range). On April 26, just before the accident, OZR was also briefly less than allowed. This was one of the leading causes of the accident.

IAEA experts note that reactor operators were unaware of the importance of this parameter. Before the accident, it was considered that the restrictions set in the operating regulations were related to the need to maintain a uniform energy release throughout the core. Although the reactor developers knew (from the analysis of data obtained at the Ignalina NPP) that with a small reactivity margin, the operation of the protection could lead to an increase in power, the corresponding changes were not made to the instructions. In addition, there were no means to control this parameter quickly. The values ​​violating the regulations were obtained from the calculations made after the accident based on the parameters recorded by the recording equipment.
After the power drop, the staff deviated from the approved program and decided not to increase the capacity to the specified 700 MW. According to Anatoly Dyatlov, this was done at the suggestion of the head of the Akimov bloc. Dyatlov, as the head of the tests, agreed because the regulations in force at the time did not prohibit work at such a capacity, and high capacity was not required for the tests.

IAEA experts believe that any deviation from the pre-established test program, even within the framework of the regulations, is unacceptable.

Despite the fact that the new report shifted the emphasis and named the leading causes of the accident as shortcomings of the reactor, the IAEA believes that insufficient staff skills, insufficient knowledge of the characteristics of the reactor that affect safety, and reckless actions were also important factors leading to the accident.

"Declassified" facts

Since independence, Ukraine has removed the "secret" stamp from many documents related to the Chernobyl accident. They showed the negligent attitude and strategy of "closing one's eyes" to not one but many design and information errors during the construction and subsequent operation of the Chernobyl NPP.

The last appeal about the unreliability of RVPK-1000 reactors and the warning about possible significant consequences of a potential accident is dated 14.08.1984 - 2 years before the accident.