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Chornobyl : | Operating History | Scope of DOE Activities | Accomplishments | Initiatives |

Chornobyl Operating History

All of Chornobyl's four Units are shut down. Unit 4 was destroyed by the 1986 accident, and Unit 2 has not operated since a fire occurred in its turbine building in October 1991. Unit 1 was shut down in 1996 and Unit 3 was shut down December 15, 2000.

Also on this page:
- Plant Incidents
- Plant Operations
- The Chornobyl Accident
- Inspections
- Condition of the Unit 4 Sarcophagus

Plant Incidents

In January 1993, two small fires occurred at the plant, one in a building housing auxiliary electrical equipment, and the other in a ventilation center in Unit 4's sarcophagus. Both fires were classified as Level 0 on the seven-level International Nuclear Event Scale.

In April 1994, two incidents occurred on successive days. One involved a drop in cooling system water levels after a short circuit in a cable as workers were reconnecting Unit 3 to the grid following planned maintenance; it was classified as Level 1 on the International Nuclear Event Scale. The other incident involved the failure of a controlling arm while nuclear fuel was being moved in Unit 1; it was classified as Level 0.

In October 1994, a through-wall crack in the upper part of a fuel channel tract in Unit 3 was classified as Level 1 on the International Nuclear Event Scale.

In November 1995, radioactive contamination of the floor of the Unit 1 central operational room was found to exceed acceptable levels. Several other problems were identified within the restricted access zone, as well, where floors were contaminated. The source was found to be process medium contaminated during fuel assembly reloading procedures using a refueling machine. The room was decontaminated and the levels reduced to regulatory compliance.
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Plant Operations

In 1993, 150 of the plant's highly skilled employees--many of them Russian--left. The loss continued in 1994, with 13 supervisory staff departing in the first three months of the year, including the plant's director. According to Chornobyl's new director, the plant had adequate personnel to fully staff the six shifts needed to operate the two units.

In February 1994, Russia halted the delivery of nuclear fuel to Ukraine because that country had not yet signed the Nuclear Nonproliferation Treaty. At the time of the Russian move, Chornobyl had only about one to two weeks of fuel remaining. The Russians resumed deliveries to Chornobyl in late February 1994.

In early April 1994, the Ukrainian government approved the restart of Unit 2. A few weeks later, plant management officially applied to the Ukrainian State Committee for Nuclear and Radiation Safety (GANU) to restart Unit 2. In June 1994, GANU adopted a policy on restart that would require plant management to present an annual report of actions planned to increase plant safety. GANU further said it would make a decision on restart only after the unit had been upgraded to meet current safety standards and a technical safety report had been submitted as a basis for licensing. But in December 1994, GANU was abolished, and its functions were assumed by the newly created Ministry for Environmental Protection and Safety of Nuclear Power Utilization.

According to a July 1994 Ukrainian news agency report, repair work and preventive maintenance were not being carried out at the Chornobyl plant because of a shortage of funds. The same news agency reported that the plant director suspended the plant's chief engineer in September 1994 because of a week- long delay in repairs to Unit 3.
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The Chornobyl Accident

The 1986 accident at Chornobyl Unit 4 resulted from a combination of design and technical deficiencies and operator error. However, In January 1993, the IAEA issued a revised analysis of the Chornobyl accident, attributing the root cause to the reactor's design and not to operator error. The IAEA's 1986 analysis had cited the operators' actions as the principal collapse of the accident.

In response to the accident, the Soviet government initiated a major backfitting program to upgrade existing RBMK nuclear units, increasing control rod scram speed from 24 seconds to 10-12 seconds, improving core physics and increasing the uranium fuel enrichment from 2 percent to 2.4 percent.

Radioactive wastes created as a result of the Chornobyl accident have been buried in temporary storage facilities, which are noncompliant with current requirements. Registered temporary Chornobyl radwaste sites have a total activity of about 300,000 curies.
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The most recent inspections by teams from the International Atomic Energy Agency (IAEA), in 1994, found serious safety deficiencies in Units 1 and 2 (now closed). The problems existed in four areas of reactor operation: design, inspection, fire protection, and radiological protection.

    In design, the team found the following deficiencies:
  • poor separation between control and protection systems
  • main steam lines located directly above the control rooms
  • no emergency control rooms in either unit
  • no filtered ventilation in control rooms
  • emergency core cooling systems that cannot cope with breaks in pipes greater than 300 mm in diameter and lack of dedicated emergency core cooling system pumps
  • pressure relief capability from the reactor cavity limited to breaks in no more than four pressure tubes (out of a possible 1660)
  • no check valves in the group distribution headers to protect against a break in a header or in the pressure header of the main circulation pumps
  • lack of an accident localization system, so radioactive steam is released directly to the atmosphere in case of overpressure in the reactor building
  • leak rates from hermetic compartments as high as 40% per hour at 40% overpressure
  • lack of redundancy and separation in parts of the service water system, making the entire system sensitive to common mode failures.
Serious deficiencies in fire protection at Unit 1 included plastic floor coverings in the turbine and reactor buildings that can generate toxic smoke or fumes and a lack of systematic analyses to determine needed fire protection and mitigation measures.

Radiation protection was found to be deficient in the training, safety culture, calibration of instruments, individual dosimetry and exposure control, adequacy of procedures, and contamination control.

The team was also concerned about the plant's ability to obtain modern equipment and spare parts and about the condition of the sarcophagus around Unit 4.

The team was so concerned about the condition of Units 1, 2, and 4 that it called a meeting of international experts and Ukrainian representatives to review the plant's safety situation.

A second IAEA team focused on the plant's performance in preventing incidents. It reviewed 243 events reported between January 1989 and December 1993: 110 were considered to be of current concern, with 12 events classified as Level 1 on the International Nuclear Safety Event Scale, 2 as Level 2, and 96 as Level 0. The team was particularly concerned with fuel- handling operations. Among the recommendations to prevent future incidents were the following:

  • Improve the maintenance and testing of fast-acting emergency core cooling system gate valves.
  • Improve cooperation and teamwork between operations and maintenance personnel.
  • Ensure that all new or revised criteria for testing equipment be included during the revision of testing procedures.
  • Schedule corrective actions as soon as possible after detection of latent weaknesses.
  • Give further training in root cause analysis and operational feedback to plant staff.
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Condition of the Unit 4 Sarcophagus

The concern of the IAEA inspectors was confirmed by a 1995 report of the Ukraine Ministry of Environmental Safety (Status Report: Nuclear Radiation Safety in Ukraine, 1994). The report cautions that "our knowledge of the location of the placement of fuel-bearing masses within the sarcophagus and their nuclear-physics parameters is not adequate to prevent a spontaneous chain reaction. The existing sarcophagus is not sealed, and the reliability of the surviving structural works at the destroyed unit, including those used to support the sar cophagus, cannot be reliably determined." The lack of information and operations control systems is being addressed first, so that more accurate assessments of the danger posed by the sarcophagus can be made.

Source: Source Book: Soviet-Designed Nuclear Power Plants in Russia, Ukrane, Lithuania, the Czech Republic, the Slovak Republic, Hungary, and Bulgaria, 4th edition. Nuclear Energy Institute. 1996. (online)

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Chornobyl : | Operating History | Scope of DOE Activities | Accomplishments | Initiatives |


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