By the decision of Government Commission, at this phase of accident management, started the works on availability of possible core melt at the bottom of protective plate of reactor and also for the creation of supplementary cooling horizon (a special heat-exchanger) under the foundation of reactor plate for the guaranteed excepting of radioactive materials and melted fuel hit into the soil and ground water. The construction of plate started on June 3 and completed on June 28. But the process of accident development didn’t come to the possible melt of foundation plate and this special heat-exchanger wasn’t operated.
For more detailed assessment of reactor state and assuring access the passageways to the destroyed Unit were cleaned; the fragments of fuel assemblies, graphite columns and elements of structures thrown out by explosion were cleaned. Simultaneously the construction of shielding protection wall between Unit 3 and Unit 4 started.
When the danger of subsequent development of accident processes in the destroyed reactor disappeared, the efforts of Government Commission were directed for the organization of accident recovery and decontamination work, waterproof and antifiltration measures and also for the insulation and entombment of reactor installation together with destroyed structures of ChNPP buildings and facilities.
On the proposition of Government Commission two regulations of Central Committee of CPSU and Council of Ministry of USSR were adopted at the end of May 1986. By these regulations the implementation of measures on decontamination of industrial site, buildings and facilities of Chornobyl NPP and also on recommencement of Units 1 and 2 operation was allowed, including:
- decontamination of industrial site, town of Pripyat’ and roads;
- construction of stations for decontamination and special processing (SDSP);
- construction of interim storages for low-level waste;
- construction of new construction base;
- construction of residential village;
- construction of water-supply pipeline and gas pipeline to the village;
- construction of base for workers provision;
- construction of waterside drainage along Pripyat’ river;
- construction of vertical borehole drainage along cooling pond;
- construction of facilities for purification drainage and waste water;
- preservation of the main building;
- preservation of debris;
- closure of reactor compartment of Unit 4;
- start-up of two concrete plants;
- concreting of the territory and sites;
- concreting of cascade walls;
- assembling and concreting of protective wall on axis 51;
- completion of roof;
- completion of ventilation system and cleaning of internal gas volume;
- concreting of protective walls;
- closure of turbine hall;
- construction of partition wall between Units 3 and 4;
- separation of stage 1 buildings from stage 2 buildings;
- Units preparation for winter 1986-1987;
- Units 1 and 2 preparation for operation.
The accident had a large-scale radioactive contamination of territory, buildings, facilities, roads not only near the plant, but far from it. The radioactive contamination had very serious danger for population and also for the people who took part in the elimination of accident consequences; negatively influenced on the ecological state of territories. In order to avoid the overirradiation and radioactive substances transfer beyond 30-km zone, beginning for the first days after the accident, the decontamination works at ChNPP and adjacent territory were organized, namely:
- decontamination of territory, buildings, facilities of NPP, settlements, transport and other technique;
- collection, removal, maximal sterilization and radioactive waste disposal;
- sanitization of people, decontamination of cloths and means for individual protection, decontamination of roads and dust suppression.
The planned decontamination of territory, buildings and facilities of NPP and Pripyat’ town started on May 6 1986 by military units of chemical forces, civil defence, sappers and also Ministry of Energy of USSR. The top-priority task was to clean the territory near the destroyed Unit from high-level parts of core, thrown from reactor, their collection, transportation and disposal. The works on cleaning the territory were carried out twenty-four hours a day by shifts, the total number of people was about six thousand and about two thousand of them before September 1986 were replaced because they had maximum permissible dose of exposure (25R). And already on May 10 1986 due to the removal of most active fragments, the level of exposure dose rate of gamma radiation was decreased in 10-20 times and to prepare more front.
The upper layer of soil on the industrial site and industrial zone was thrown up in 1986-1987 during carrying out decontamination works. At the same time with the purpose of lowering of radioactive contamination level the collection of nuclear fuel fragments, soil removal, crushed stone backfilling and its concreting was carried out. The crushed stone backfilling and concreting were carried out at all north part of the territory adjoining to the building of Units 3 and 4 and also along western and southern part of turbine hall. The thickness of covering was 0.5m and in some parts it was 8m. The territory adjoining to Unit 4 was filled up with crushed stone, sand, dry concrete mixture and also the bulky shuttering panels were installed. Up to June 15 1986 at the main facilities of ChNPP the values of exposure dose rates were decreased to 10R/h that allowed provide the supplementary front and to work at Units 1 and 2. On date August 10 1986 it was decontaminated 862000m² of internal premises of the main building of NPP, more than 500000m² of other buildings at the industrial site were cleaned, it was removed 25000m³ of soil, by the railway flagstones the territory of 187000m² was covered.