page_title: Chernobyl disaster
section_title: INSAG-7 report 1992
hierarchical_section_title: Chernobyl disaster / Investigations and the evolution of identified causes / INSAG-7 report 1992
caption_attribution_description: Deutsch: Kernkraftwerk Tschernobyl (Ukraine), Reaktorgebäude von Block 1, Innenaufnahme English: Chernobyl nuclear power plant, Reactor hall #1, Pripyat, Ukraine. Magyar: Csernobili Atomerőmű (Pripjaty, Ukrajna), I. kiépítés, "A"-blokk, +20,20 m szint, 612/1 helyiség Հայերեն: Չեռնոբիլի ատոմակայան, 1-ին էներգաբլոկի կենտրոնական սրահ: Русский: центральный зал 1-ого энергоблока ЧАЭС-ы (ЦЗ-1)
context_page_description: The Chernobyl disaster was caused by a nuclear accident that occurred on Saturday 26 April 1986, at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR. It is considered the worst nuclear disaster in history and was caused by one of only two nuclear energy accidents rated at seven—the maximum severity—on the International Nuclear Event Scale, the other being the 2011 Fukushima Daiichi nuclear disaster in Japan.
  The accident started during a safety test on an RBMK-type nuclear reactor, which was commonly used throughout the Soviet Union. The test was a simulation of an electrical power outage to aid the development of a safety procedure for maintaining reactor cooling water circulation until the back-up electrical generators could provide power. This gap was about one minute and had been identified as a potential safety problem that could cause the nuclear reactor core to overheat. It was hoped to prove that the residual rotational energy in a turbine generator could provide enough power to cover the gap. Three such tests had been conducted since 1982, but they had failed to provide a solution.
context_section_description: In 1991 a Commission of the USSR State Committee for the Supervision of Safety in Industry and Nuclear Power reassessed the causes and circumstances of the Chernobyl accident and came to new insights and conclusions. Based on that, 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," and this included the text of the 1991 USSR State Commission report translated into English by the IAEA as Annex I.
  By the time of this report, Ukraine had declassified a number of KGB documents from the period between 1971 and 1988 related to the Chernobyl plant. It mentioned, for example, previous reports of structural damage caused by negligence during construction of the plant (such as splitting of concrete layers) that were never acted upon. They documented more than 29 emergency situations in the plant during this period, eight of which were caused by negligence or poor competence on the part of personnel.
  In the INSAG-7 report, most of the earlier accusations against staff for breach of regulations were acknowledged to be either erroneous, being based on incorrect information obtained in August 1986, or less relevant. The INSAG-7 report also reflected the view of the 1991 USSR State Commission account which held that the operators' actions in turning off the Emergency Core Cooling System, 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, although they may have been a breach of regulations. In fact, turning off the emergency system designed to prevent the two turbine generators from stopping was not a violation of regulations.  Soviet authorities had identified a multitude of operator actions as regulation violations in the original 1986 report while no such regulations were in fact in place.
  The primary design cause of the accident, as determined by INSAG-7, was a major deficiency in safety features, in particular the "positive scram" effect due to the control rods' graphite tips that actually initially increased reactivity when control rods entered the core to reduce reactivity. There was also an overly positive void coefficient of the reactor, whereby steam generated voids in the fuel cooling channels would increase reactivity because neutron absorption was reduced, resulting in more steam generation, and thereby more voids; a regenerative process. To avoid such conditions, it was necessary for the operators to track the value of the reactor operational reactivity margin (ORM) but this value was not readily available to the operators and they were not aware of the safety significance of ORM on void and power coefficients.
  However, regulations did forbid operating the reactor with a small margin of reactivity. Yet "post-accident studies have shown that the way in which the real role of the ORM is reflected in the Operating Procedures and design documentation for the RBMK-1000 is extremely contradictory", and furthermore, "ORM was not treated as an operational safety limit, violation of which could lead to an accident".
  Even in this revised analysis, the human factor remained identified as a major factor in causing the accident; particularly the operating crew's deviation from the test programme. "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." This included operating the reactor at a lower power level than the prescribed 700 MW before starting the electrical test. The 1986 assertions of Soviet experts notwithstanding, regulations did not prohibit operating the reactor at this low power level.
  INSAG-7 also said, "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 cu
caption: Reactor hall No. 1 of the Chernobyl Plant
exif: {}