The issues on re-operation of the first three Chornobyl NPP Units and carrying out of the required for it activities were among the most important within a plan for accident consequence elimination. They were settled simultaneously with the activities on preserving the fourth Power Unit.

After the accident the first two Power Units remained in normal operable state and were shutdown on April 27, 1986, at 1 a.m. 13 min. and 2 a.m. 13 min. respectively. The third Power Unit, technically connected with the fourth one, was shutdown on April 26, 1986, at 3 a.m. Normal cooling of the shutdown Power Units was performed.

By Order No.244 of May 02, 1986, ChNPP Power Units 1, 2, 3 were transferred into the operation mode of temporary preservation with maintaining graphite temperature at a level of 100°С. A shift in a number of 45 people works by 12-hours schedule.

After the cooling the reactors of three Power Units were transferred into the deep subcritical state by inserting all CPS rods into a reactor core and loading 20 additional absorbers into the reactors of the 1st and 2nd Power Units, and 200 absorber rods into the central FA tube for the reactor of the 3rd Power Unit. Neutron flux was controlled by operating equipment. To remove afterheat all fuel channels and primary circuit remained filled with water. The afterheat was removed in the mode of natural circulation. Water temperature in the reactor core was maintained at a level of 20-80°С, graphite temperature - 30-90°С.

The additional thermocouples, installed into the central openings of fuel assemblies at left and right reactor sides, were used to measure water temperature. Reactor and primary circuit temperature regime was provided by activating blowdown and cooldown system. A graphite stack was periodically blowdown by nitrogen or dry air with humidity of no more than 0.5g/m3. A CPS circuit was dehydrated after reactors’ complete cooling.

Recovery work was started with decontamination of the main and auxiliary buildings and structures of Power Units and equipment and personnel workplaces, located in them, as well as adjacent territories.

The most contaminated were specific horizontal sections of surfaces of a turbine hall [up to 106 β-part./(cm2 min)], as it was contaminated through the destroyed roof of the 4th Power Unit. As of May 20, 1986, dose rate of γ-radiation within the contaminated premises of the first and second Power Units was 10-100 mR/h, the turbine hall – 20-600 mR/h. Decontamination was performed using special solutions, contents of which were selected taking into consideration washed surface material, type and contamination. Jet and steam ejection methods, options of dry decontamination using polymeric coating were used. Some premises and equipment were decontaminated manually by wiping with rags wetted with decontamination solutions. The decontamination efficiency was controlled by direct measurement of γ-radiation dose rate and smear technique. As a result of the decontamination contamination of premises’ and equipment surfaces was mainly decreased up to regulation requirements.

At first two Power Units the decontamination activities were completed at the beginning of 1986 third quarter.

The decontamination activities at the third Power Unit resulted in further improvement of radiation situation at the acting Power Units. As a result of performing a part of the planned activities by the end of July 1987 the dose rate at 3rd Power Unit’s turbine hall was drastically reduced and amounted to 7-50 mR/h.

Following the completion of Shelter Object construction and a set of works on decontamination of Plant territory the radiation situation at the 1st and 2nd Power Units was finally stabilized and was brought to the fixed standards. All the required auxiliary equipment of the 1st and 2nd Power Units was maintained in ready mode. Ventilation systems were in open state before decontamination of air ducts and ventilation equipment, and installation of additional facility for purification of supply air. Fire protection system was maintained in automatic ready mode. Health physics monitoring system of the1st and 2nd Power Units was put into operation in full. In-house electric circuits provided normal power supply with readiness to take load of other stand by mechanisms. Systems of the turbine hall were maintained in preserved state. Conditions of Power Units’ reactor installations and equipment were monitored by the ChNPP operating personnel. Maintenance of Power Units’ systems and equipment in serviceable state provided the possibility in future to perform recovery work at the Power Units and commission them in a relatively short time.

On June 15, 1986, the shop commissions with involvement of experts, design and engineering organizations were created by Order No.360 in accordance with “Directive work execution schedule on ACE”, approved by Yu. Maslyukov, a Deputy Chairman of a Council of Ministers of the USSR, in order to prepare a conclusion on a possibility of further Chornobyl NPP Unit No.3 operation.   

Technical and organizational measures on improving safety of operation of acting NPPs with RBMK reactors were developed based on the analysis of reasons of occurrence and development of the accident at ChNPP 4th Power Unit.

First of all at acting NPPs were developed and implemented those measures that were targeted at:
    Reducing positive steam coefficient of reactivity;
    Implementing programs for calculation of operating reactivity margin with digital display of its current value at operator control panel;
    Preventing the possibility of deactivation of emergency protection systems when a reactor is at power operation;
    Excluding operation modes leading to reduction of temperature margin up to a coolant boiling at reactor inlet.

Reduction of steam coefficient of reactivity was carried out in two stages. At the first its value was reduced to +1β through increasing the operating reactivity margin to 43-48 manual control CPS rods.

The second stage of reducing the steam coefficient of reactivity was connected with a complete replacement of reactor fuel for the fuel with enrichment of 2.4% 235U.

Firstly CRS actuation mechanisms were replaced with a new structural system excluding columns of water at the bottom part of CPS channels by means of increasing a length of rod’s absorbing part.

Time for a complete insertion of rods into a reactor core was reduced from 18 sec to 12 sec by upgrading servodrives. As a result of performing the first two measures the emergency protection speed efficiency at first seconds of rod movement was improved by several times. The third stage of improving the emergency protection efficiency was development and introduction of high-speed emergency protection (HSEP) at all acting reactors.

A long-term stay of the first two Power Units at shutdown stage, effects of radiation exposure and decontamination substances required thorough inspection and diagnostic test of all main and auxiliary equipment and automatic systems, repair activities and a set of pre-commissioning work. Scope of work on preparation and commissioning of the Power Units was specified by a Program of Comprehensive Testing and Commissioning of Chornobyl NPP Units 1 and 2, and satisfied the requirements made for a newly commissioned Power Units. According to this program during the pre-starting period the unit test of Power Unit systems was performed. It included performance check of reinforcement, control instrumentation, symbolic circuits, protection, block systems, alarm system and automatic load transfer, system activation, bringing of work environment quality to operational standards, acting test of systems and mechanisms on emergency protection signals. Under the results of the repair and recovery work and unit test an Act of system readiness for a Power Unit starting and General Act of readiness of equipment, systems, technical documentation and Chornobyl NPP personnel for the Power Unit starting were drawn up.

The specific tasks of the first two Power Units commissioning consisted in checking the efficiency of the performed measures on safety improvement of RBMK NPPs.
To settle these tasks the Power Units were commissioned in three stages:
1st Stage – formation of reactor core and first criticality;
2nd Stage – uptake of Power Unit capacity at a rate of 700 MW along with comprehensive operational testing of Unit’s equipment and systems;
3rd Stage – uptake of specified rated power.

The objective of the safety improvement measures was first of all the reduction of steam coefficient of reactivity and enhancement of emergency protection operation speed. To achieve these objectives and considering reactor’s characteristics prior to the first criticality 50% of rods were inserted into the reactor core at 1.4 m level according to a position indicator, a number of shortened absorbers was increased to 32 and their movement was restricted by the interval from 1.2 m to 3.5 m according to the position indicator.   

During the first criticality process the additional absorbers were installed instead of some FAs. The operating reactivity margin during reactor operation was set equal to 43-48 rods. The formed initial fueling of 1st Power Unit reactor included 1648 FAs (124 out of them were fresh), 30 additional absorbers, 14 fuel channels with water and one blanked off cell. The initial fueling of 2nd Power Unit reactor included 1610 FAs (313 out of them were fresh), 81 additional absorbers, 2 fuel channels with water. A number of additional absorbers in the 2nd Unit reactor was increased for purposes of further reduction of the steam coefficient of reactivity.

Order No.449 “On additional measures on ensuring emergency activities and in pursuance of Order No.253 of the USSR Ministry of Energy dated 05.05.1986” was issued on July 22, 1986.

This document specified organizational measures on safety assurance:
1.    ChNPP Power Units should work only in base-load operation.
2.    Refueling and gaining reactor power rate should be performed only in the presence of Plant Chief Engineer (PChE) and Deputy Chief Engineer for Operation.
3.    To forbid any research and experimental work at the acting Power Units.
4.    Programs of non-typical tests of the main equipment, installations and Power Units as a whole should be considered and approved at the ChNPP Technical Council chaired by PChE. To forbid strongly departures from programs.

Following the completion of reactors’ first criticality the Power Units systems and equipment were prepared and put into operation in accordance with process regulations; started the activities on equipment settings and tests, check of process parameters, uptake of 700 MW power and regular running of turbogenerators ТГ-1(3), ТГ-2(4) to 500 MW power.

In the process of power ascension and comprehensive tests at a rate of 700 MW the considerable shortages in operation of the main and auxiliary equipment at the first two Power Units were not detected. Quality of primary circuit and CPS coolant loop water and feed water was complied with the standards. Power Unit controllers maintained the process parameters set by the regulations. Reactor core’s characteristics remained satisfactory.

Power density distribution in operating channels was measured with a sufficient reliability. It was easily maintained within standard deviations. Radiochemical composition of primary circuit coolant met the regulation requirements. Radiation situation within premises enabled to commission the first two Power Units. The data, received during the comprehensive Power Units testing at a rate of 700 MW, enabled to make a conclusion about the possibility to increase the power to a design rated level of 1000 MW and perform the 3rd Stage of Power Units commissioning. The power was increased gradually by 10% N along with the comprehensive equipment testing at each grade.

At the 3rd Power Unit, in contrast to the 1st and 2nd Power Units, a great scope of recovery work was performed. The ChNPP personnel cut off cable paths, water and gas communication lines that connected the 3rd and 4th Power Units. Some ASRC premises were concreted.

A protection and separation wall within Block V was concreted and clad in lead above a level + 12.5 to reduce the radiation background. Additional protection walls were erected at other Block V sections. The equipment, pipelines, ventilation ducts and cable paths within the premises of 3rd Power Unit with high levels of γ-background (especially adjacent to the separation wall of Block V) were dismantled to reduce the radiation background.

October 1, 1986 – post-accident commissioning of 1st Power Unit.

November 5, 1986 - post-accident commissioning of 2nd Power Unit.

December 4, 1987 - post-accident commissioning of 3rd Power Unit.

Considering the presence of a zone with radioactive contamination around the Plant, it was important to create personnel living arrangements. The solution of these issues was found in Chornobyl NPP operation under a rotation system.

From July 1, 1986, the ChNPP personnel were transferred to a work under a shift system by Order of the USSR Ministry of Energy No.394.

The shift system was about the following:
- During shift (working) days operating and operational personnel were living at a shift settlement located beyond the boundaries of 30-km zone;
- Operational personnel spent free days in cities of Kiev and Chernihiv where they were provided with the necessary conditions.

Duration of a working day for operating personnel was accepted equal to 12 hours (from 8.00 to 20.00 and from 20.00 to 8.00), for operational personnel and all plant staff (as for all staff working within the 30-km zone) – 10 hours (from 9.00 to 19.00).

Duration of a shift for operating personnel was accepted equal to 5 days with 7 free days. For all other staff the duration both of a shift and rest was 13 days.

The work rotation system under the radiation conditions within 30-km zone proved its value and enabled to create conditions both for personnel work and rest.