User:VicDim/Sandbox

Here is my correcting of the section "Causes". The text, isolated with italics, initially existed, before my interference, and it remained without any changes.

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 operational rules and regulations. “During preparation and testing of the turbogenerator under run-down conditions using the auxiliary load, some 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 operational 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 operational regulations that made this accident possible were played down and were mentioned only casually. Serious critical observations covered only general questions and did not address the specific reasons for the accident. These observations give the following general picture: 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 system 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 and also 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.

(edited as far as here)

Accordingly this view, turning off of ECCS even if this was the disturbance of regulations, did not influence the appearance of emergency and its scales just as both to manipulation with the settings of technological protection and blockings on the level and to pressure in Separator Drum. There was not by the disturbance of regulations turning off of emergency protection system on the stopping of two turbogenerators. And, on the contrary, the initial event of emergency was usual, in general, operational action on the damping of reactor -pushing of the knob of emergency protection system AZ -5.
By the actions of personnel those contributed to the creation of conditions for the appearance of emergency were work with the small operational reactivity margin and at the small level of the power of reactor less than 700 MW, i.e., of the level that recorded in the program of run-down test. Meanwhile the work at the small level of power was by regulations not forbidden in spite of the fact that the Soviet experts asserted in 1986.
Regulations forbade work with the small margin of reactivity (ORM). However, “the real role of ORM of reactor, as showed after-emergency studies, extremely contradictorily is reflected in the technological regulations and in the design of reactor RBMK-1000.”, “ORM did not treat as the limit of safe operation, whose disturbance could lead to the emergency” ,(see also ).

Accordingly that another point of view the main reasons for emergency lie at the special features of physics and construction of 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 has negative coefficient, i.e., they attempt to decrease the heat output with an increase in the reactor of vapor phase, 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 water in it vice versa plays the role of 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 contradicts the common sense, and this property of 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 were made of graphite and was shorter than necessary by 1,3 meters and lower than this there were hollow regions of 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 extreme upper position, initially the graphite parts displace some coolant. This greatly increases the rate of the fission reaction, since graphite (in 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. Besides those indicated were noted other deficiencies in the RBMK-1000 design, and also its nonconformity to the standards accepted and to requirements on the nuclear reactor safety.

Both points of view 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, i.e. a revised analysis, attributed the main cause to the reactor's design. Existence of the simultaneously two such opposite points of view to the reasons for Chernobyl' emergency and the continuously discussions around this they became possible also on the fact that the primary data about the emergencies, registered by instruments and sensors were not completely published in the official sources.

The human factor has stil to be considered as a major element in causing the accident. INSAG notes that, both the operating procedures and operators sufficiently lightly regarded to disabling of reactor protection: witness the length of time for which the ECCS was out of service while the reactor was operated at half power. In the opinion the INSAG of the greatest censure they deserve deviation from the test program, commited by personnel. “Most reprehensibly, unapproved changes in the test procedure were deliberately made on the spot, although the plant was known to be in condition very different from that intended for the test.”

As in the previously released report INSAG-1, close attention in the report INSAG-7 is paid to the insufficient (at the moment of emergency) “culture of safety” at all levels. Deficit in safety culture was inherent not only to the stage of operation, but also and to no lesser extent to 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 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.”