As a result of the accident the reactor core, considerable part of the technological equipment and constructions of ChNPP’s Unit 4 have been destroyed. Safety barriers and systems protecting environment from the radionuclides, containing in irradiated nuclear fuel, have been destroyed.
Therefore an issue about long-term preservation of Unit 4 by construction of a structure which would restrict release of radioactive substances and ionizing radiation outside the destroyed power Unit was arisen directly after the accident.
«Works on disposal of ChNPP Unit 4 and constructions referred to it» had been charged to the Ministry of medium mechanical engineering of USSR by Resolution of the Central Committee of CPSU and Council of Ministers of USSR No 634–188 dated 29.05.86. The facility has received the name «Shelter of ChNPP Unit 4».
Another Resolution of the Central Committee of CPSU and Council of Ministers of USSR No 663–194 dated 05.06.86 assigned functions of the General Designer of works on disposal of ChNPP Unit 4, radioactive waste and decontamination of ChNPP industrial site equipment on VNIKIET (St.-Petersburg). Kurchatov INE performed a scientific management of works on ChNPP Unit 4 disposal.
At the picture: Destroyed ChNPP Unit 4
Peculiarities and complexity of works on the emergency power unit preservation consisted in absence of experience in national and in world practice in overcoming of such large-scale accident consequences and in absence of the special normative documents at that period of time for design decisions development.
In view of critical importance of the Shelter object it was developed at conceptual level eighteen options of the design, stipulated, in particular, formation of a hill from crushed stone and concrete, construction of a huge structure in the form of a curved arch or a spherical vault and other options. But the majority of the proposed solutions required considerable construction materials consumption, and the main thing – enormous efforts and exposure doses for the personnel and long time for their construction, that didn’t correspond to the main requirement – as soon as possible to liquidate accident consequences. Separate decisions couldn’t be realized at existed at that time level of the equipment.
Taking into account material expenses and dose loadings on the personnel and tight schedule for the construction it has been accepted the final option of a protective construction which provided the maximum use of the remained supporting constructions of Unit 4 as part of the Shelter object constructional system.
Thus, building constructions of the Shelter object is a combination of «old» constructions of the destroyed Unit 4 and «new» constructions erected after the accident.
At the picture: Constructions of the supports and loading elements of the Shelter object covering
1 – B1 Beam (B2 Beam is behind it); 2 – Pipe covering;
3 – Bottom part of the wall on axis 50, reinforced with the «corset»; 4 – exhaust shaft;
5 – «Mammoth» Beam; 6 – Western support of «Mammoth» Beam;
7 – Eastern support of «Mammoth» Beam; 8 - «Octopus» Beam
Owning to such combination it has been erected the unique structure which constructions carry out extremely important function of a physical barrier on ways of radioactive substances and ionizing radiation release into environment.
The basis of physical barriers is composed of the external protective constructions built after the accident: Cascade Wall, Buttress Walls, covering of the reactor block, deaerator stack and Turbine hall.
Relatively undamaged constructions of the Unit 4 create a basic contour supporting load bearing elements of the covering of the reactor block and deaerator stack. First of all these are Northern and Southern exhaust shafts, a monolithic wall on an axis 50 with an abutting frame. The main units of beams B1 and B2 lean on these constructions. These beams together with a pipe covering create roof of the central part of the reactor block (in particular, above the Central Hall). Beam «Mammoth» and «Octopus» are supported by the deaerator stack constructions.
At the picture: External SO protective construction
1 – covering above the Central Hall; 2 - covering above the Turbine Hall; 3 –Cascade wall;
4 – Western («Big») Buttress Wall; 5 – Southern Plates; 6 – Southern «hockey - sticks» plates;
7 – Northern «hockey - sticks» plates; 8 - Northern («small») Buttress Wall
Realization of such option of a protective encasement required to resolve two the most difficult tasks:
- Survey of a technical condition of the remained constructions of Unit 4 in extremely difficult radiation conditions and assessment of possibility of their use as constructional elements of Shelter object;
- Choice of such constructional and technological decisions which would allow to reduce as much as possible terms of Shelter object construction and to minimize radiation impact on the personnel and environment.
Besides VNIPIET, design works in various directions were carried out by a number of design institutes, in particular, LenPSK (St.-Petersburg), TSNIIPSK (Moscow), UKRNIIPSK (Kiev), DneproPSK (Dnepropetrovsk) and other institutes.
To perform building and installation works on the emergency Unit preservation and Shelter object construction in the system of the Ministry of medium mechanical engineering of USSR it has been specially created Construction Management No 605.
Supporting facilities were constructed within the shortest terms: bases of supply, base on maintenance of motor transport and construction machinery, concrete plants, points of building materials acceptance and unloading and other facilities.
Children pioneer camp, recreation facilities, schools have been adapted for accommodation of the working personnel, tent towns and promptly mounted construction were created. Canteens and sanitary-and-hygienic facility have been organized.
These facilities were located taking into account radiation condition of territories and availability of transport communications. The personnel were daily delivered by «clean» transport to change point to special transport maintaining restricted area. All motor transport left a restricted area, was subject to a radiation control and, if necessary, decontamination at specialized entry-exit points.
To ensure efficiency and safety of works on preservation of the emergency Unit prior to the major constructional and installation works it has been performed the complex of measures for decontamination of surrounding territory. Fragments of the reactor core (fragments of fuel assemblies, graphite and constructional materials of the reactor), fire fighting vehicles and other equipment, elements of the destroyed constructions and the technological equipment, the top contaminated layer of earth have been removed. These works were carried out using the special engineer vehicles created on the basis of tanks and equipped with protective shielding, clamshell, technical television and devices for detection of local ionizing radiation sources and the bulldozers equipped with protective shielding. After completion of works on removal of a radioactive waste the territory around the emergency power unit was covered with a concrete layer in the thickness up to 0.5 m.
The construction along the perimeter of the emergency power unit so-called pioneer walls which with shielding function was other essential factor of radiation conditions improvement. Besides, the space behind pioneer walls was used for disposal of a radioactive waste, collected from surrounding territory.
Works on decontamination of surrounding territory and creation of protective pioneer walls allowed to start performance of the main complex of constructional and erection works for the Shelter object construction.
Construction of the Shelter object in extremely difficult radiation conditions required to develop and introduce such organizational and technological decisions which as maximum as possible would provide radiation protection of the personnel.
The main actions for radiation protection of the personnel consisted in realization of radiation survey of working areas, use of various types of shielding and application of remote controlled technologies for performance of works in the most hazardous radiation conditions.
The technology of installation with use of the large-sized constructions, which were assembled in «clean» zone was extremely effective and permitted remote installation. Constructions were designed with connection units for which it wasn’t need to perform the operations connected with presence of people directly in an installation zone.
To manage installation process the central operative station was established. The information from the television cameras mounted directly at levers of cranes and special at towers installed in places with the maximum overlook come to this station.
Special technologies of timbering installation and concreting with remote use of pumps for concrete mix submission have been also introduced.
To ensure radiation protection of the personnel the complex of organizational, radiological and hygienic and technical measures was carried out, in particular:
- permanent monitoring of radiation conditions at ChNPP area and at adjacent territories;
- the organization of a sanitary access control;
- personnel provision with necessary individual protection equipment (working clothes, respirators and another);
- individual radiation control;
- shielding of a vehicle cabs and devices;
- dust suppression in works performance zones and at surrounding area;
- decontamination of vehicles and mechanisms;
- arrangements for nutrition in «clean» zone.
Construction and erection works were carried out with use of unique for that time vehicles
and mechanisms, in particular: caterpillar cranes «Demag» with weight-lifting capacity on the main lever up to 650 tons and on auxiliary lever – 112 tons with out-in 78 m; automobile cranes «Liebherr«, pumps for concrete mix submission of firms «Schwing», «Putzmester», «Worthington» and other cars and the mechanisms which have been equipped additionally with remote control and protection frames.
The general view of the Shelter object construction is provided at Fig. 1.5 (look the coloured inset).
In process of Shelter object construction it has been laid down about 345 thousand m3 of concrete mix and mounted 7 thousand tons of metal constructions.
Beside the construction and erection works the considerable scope of works has been executed for creation of necessary systems for Shelter object safe operation (ventilation, power supply, firefighting system, monitoring system and others).
Designing and construction of the Shelter object have been carried out for record short term – only for half-year. The Certificate of the State Commission about Shelter object acceptance for maintenance has been signed on November 30th, 1986.
Construction of the Shelter object became the essential result of urgent measures implementation for minimization of the consequences of beyond designed accident at ChNPP Unit 4.
At the same time the Shelter object isn’t the facility created according to designing, construction, commissioning and operation rules and norms referring not only to nuclear installations or Radioactive Waste Management facilities, but also to usual industrial facilities. Its building constructions don’t meet the safety requirements specified in normative and technical documents in terms of structural integrity and reliability and have uncertain term of operation.
Following basic demerits are characteristic to Shelter object building constructions:
- supporting constructions of support contour (remained constructions of Unit 4) and connection joints are significantly damaged, overloaded by the weight of the dropped constructions and the equipment, and by materials used during the accident liquidation. The bared armature of reinforced concrete constructions and metal constructions are exposed to corrosion;
- reliability and durability of supporting constructions of a support contour can’t be credibly identified in the absence of access to many elements and units and hazardous radiation conditions doesn’t allowing to perform their detailed survey;
- the structures constructed after the accident are separated – not connected among themselves, freely lean on the supporting constructions without physical connection and are kept in designed position (welding or bolted connections of supporting parts of constructions are absent);
- difficult access to metal constructions elements and units for periodic survey and corrosion-resistant coating refreshment.
These lacks lead to the Shelter object safety level decreasing in time. Process of building constructions degradation continues. There is a high probability of constructions collapse, which can lead to considerable radioactive contamination of surrounding environment, and personnel and population exposure.
All that demands permanent supervision for the status of Shelter object constructions, important for safety, and intrusion in case of threat occurrence of their condition dangerous deviation from stable. Therefore surveys of building constructions condition and implementation of urgent measures for their strengthening have been started immediately after completion of Shelter object construction.