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Causes

There were two official explanations of the accident: the first, subsequently acknowledged as erroneous, was published in August 1986 and effectively placed the blame on the power plant operators. To investigate the causes of the accident the IAEA created the advisory group known as International Nuclear Safety Advisory Group (INSAG) which as a whole also supported this view, on the basis of the materials given by the Soviet side and the oral statements of specialists in its report of 1986 INSAG-1 . It was alleged that the accident which had such catastrophic consequences was caused by the gross violation of operating rules and regulations. "During preparation and testing of the turbine generator under run-down conditions using the auxiliary load, personnel disconnected a series of technical protection systems and breached the most important operational safety provisions for conducting a technical exercise". This was probably due to their lack of knowledge of reactor physics and engineering, as well as lack of experience and training. According to these allegations, at the time of the accident the reactor was being operated with many key safety systems shut off, most notably the Emergency Core Cooling System (ECCS). Personnel had an insufficiently detailed understanding of the technical procedures involved with the nuclear reactor and knowingly infringed regulations in order to speed up completion of the test.

"The developers of the reactor plant considered this combination of events to be impossible and therefore did not allow for the creation of emergency protection systems capable of preventing the combination of events that led to the crisis, namely the intentional disabling of emergency protection equipment plus the violation of operating procedures. Thus the primary cause of the accident was the extremely improbable combination of rule infringement plus the operational routine allowed by the power station staff."

By this account, deficiencies in the reactor design and in the operating regulations that made this accident possible were cast to one side and mentioned only casually. Serious critical observations covered only general questions and did not address the specific reasons for the accident. The following general picture arose from these observations. Several procedural irregularities also helped to make the accident possible. One was insufficient communication between the safety officers and the operators in charge of the experiment being run that night. The reactor operators disabled safety systems down to the generators, which the test was really about. The main process computer, SKALA, was running in such a way that the main control computer could not shut down the reactor or even reduce power. Normally the reactor would have started to insert all of the control rods. The computer would have also started the "Emergency Core Protection System" that introduces 24 control rods into the active zone within 2.5 seconds, which is still slow by 1986 standards. All control was transferred from the process computer to the human operators.

This view is reflected in the numerous publications on the theme of the Chernobyl accident, in the artistic works that appeared immediately after the accident and in the long time that it reigned in the public consciousness and in popular publications. In 1993 the IAEA Nuclear Safety Advisory Group (INSAG) published an additional report INSAG-7 which reviewed “that part of the INSAG-1 report in which primary attention is given to the reasons for the accident”. Most of the accusations against staff for breach of regulations were acknowledged to be erroneous, based on incorrect information obtained in August 1986. This report reflected another view of the reasons for the accident, presented in appendix I.
According to this account, turning off the ECCS, and also interfering with the settings on the protection equipment and blocking the level and pressure in the separator drum, did not contribute to the original cause of the accident and its magnitude, though they were possibly a breach of regulations. Turning off the emergency system designed to protect against the stopping of the two turbine generators was not a breach of regulations. In fact, pushing the button on the AZ-5 emergency protection system was a normal initial emergency action for damping down the reactor.
Human factors contributed to the conditions that led to the disaster, namely working with a small operational reactivity margin (ORM) and working at a low level of power in the reactor, less than 700 MW - the level documented in the run-down test program. Nevertheless, working at this low level of power was not forbidden in the regulations, despite what Soviet experts asserted in 1986.
Regulations forbade work with a small margin of reactivity. However, “the real role of the reactor ORM, as post-accident studies showed, is reflected, extremely controversially, in the technological regulations and in the design of the RBMK-1000 reactor.”, “the ORM did not deal with the limits of safe operation, the disturbance of which could lead to the accident” ,(see also ).

According to this report, the chief reasons for the accident lie in the peculiarities of physics and in the construction of the reactor. There are two such reasons:

• The reactor had a dangerously large positive void coefficient. The void coefficient is a measurement of how the reactor responds to increased steam formation in the water coolant. Most other reactor designs have a negative coefficient, i.e. they attempt to decrease the heat output in the presence of an increase of the vapor phase in the reactor, because if the coolant contains steam bubbles, fewer neutrons are slowed down. Faster neutrons are less likely to split uranium atoms, so the reactor produces less power (a negative feed-back). Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, and the water in it, on the contrary, acts like a harmful neutron absorber. Thus neutrons are slowed down even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing in the intensity of vaporization means that more neutrons are able to split uranium atoms, increasing the reactor's power output. This makes the RBMK design very unstable at low power levels, and prone to suddenly increasing energy production to a dangerous level. This behavior is counter-intuitive, and this property of the reactor was unknown to the crew.
• A more significant flaw was in the design of the control rods that are inserted into the reactor to slow down the reaction. In the RBMK reactor design, the lower part of the control rods was made of graphite and was 1.3 meters shorter than necessary and in the space beneath them were hollow channels filled with water. The upper part of the rod—the truly functional part which absorbs the neutrons and thereby halts the reaction—was made of boron carbide. With this design, when the rods are inserted into the reactor from the uppermost position, initially the graphite parts displace some coolant. This greatly increases the rate of the fission reaction, since graphite (in the RBMK) is a more potent neutron moderator (absorbs far fewer neutrons than the boiling light water). Thus for the first few seconds of control rod activation, reactor power output is increased, rather than reduced as desired. This behavior is counter-intuitive and was not known to the reactor operators.
• Other deficiencies besides these were noted in the RBMK-1000 reactor design, as were its non-compliance with accepted standards and with the requirements of nuclear reactor safety.

Both views were heavily lobbied by different groups, including the reactor's designers, power plant personnel, and by the Soviet and Ukrainian governments. The IAEA's 1986 analysis attributed the main cause of the accident to the operators' actions. But Report 1993 of the IAEA, a revised analysis, attributed the main cause to the reactor's design. The simultaneous existence of two such opposing viewpoints as to the reasons for the Chernobyl accident and the nonstop debate they engendered were made possible additionally because the primary data covering the disaster, as registered by the instruments and sensors, were not completely published in the official sources.

Once again, the human factor had to be considered as a major element in causing the accident. INSAG notes that both the operating regulations and staff handled the disabling of the reactor protection easily enough: witness the length of time for which the ECCS was out of service while the reactor was operated at half power. INSAG’s view is that it was the deviation from the test program taken by the operating crew that was mostly to blame. “Most reprehensibly, unapproved changes in the test procedure were deliberately made on the spot, although the plant was known to be in a very different condition from that intended for the test.”

As in the previously released report INSAG-1, close attention is paid in report INSAG-7 to the inadequate (at the moment of the accident) “culture of safety” at all levels. Deficiency in the safety culture was inherent not only at the operational stage but also, and to no lesser extent, during activities at other stages in the lifetime of nuclear power plants (including design, engineering, construction, manufacture and regulation). The poor quality of operating procedures and instructions, and their conflicting character, put a heavy burden on the operating crew, including the Chief Engineer. “The accident can be said to have flowed from a deficient safety culture, not only at the Chernobyl plant, but throughout the Soviet design, operating and regulatory organizations for nuclear power that existed at that time.”