English - Chernobyl Nuclear Power Plant: Shelter object
Realization of this task was complicated by extremely high gamma - radiation fields, absence of reliable information on a level of destruction and on Unit 4 constructions condition, absence of similar accidents liquidation experience in global practice.
The task for the works performance connected to ChNPP Unit 4 conservation, was established by the Decision of SU CP Central Committee and USSR Council of Ministers dated June, 5, 1986 ¹ 663-194. Functions of the General contractor on this activity implementation were entrusted to the USSR Machine-Building Ministry. The object has received the name “Shelter of ChNPP Unit 4”. All-Union Research and Design institute of power technologies (VNIPIET) has been appointed the General Designer of works. Sketch design developed by VNIPIET experts under professor I.Kurnosov leadership has been taken as a basis for future "sarcophagus". The design was elaborated and changed during construction. Kurchatov Nuclear Energy Institute was the scientific manager of works.
Designing of Shelter started on May, 20, 1986. Construction continued 206 days and nights from June till November, 1986. About 90 thousand builders participated in it. During 1986 - 1987 about 200 thousand persons worked in Chernobyl area.
Process of Shelter construction can be subdivided on three stages.
The first stage since May, 20 till July, 15, 1986 was characterized by high levels of exposure rate and uncertainty of radiation fields structure. The main works, carried out during this period, consisted in manufacturing of the Shelter object constructions and elements, protecting workplaces against main sources of radiation, in these sources removal or decontamination. In such way the space for the major civil works deployment have been prepared. Concrete mixing plant, transport communications have been constructed, the railway junction has been enlarged, the social infrastructure has been developed, large-scale constructions have been produced, etc.
During the second stage from July, 16 till September, 15, 1986 the basic scope of civil work, assured necessary strength and radiation-protective properties of the Shelter object achievement has been executed. Man-hours of builders were maximal, and relative dose rates were in order lower, than at the first stage.
The third stage, during the period from September, 16 till November, 30, 1986, was characterized by constructing-and-mounting works termination.
Works sequence during "sarcophagus" construction.
1. Clearing and concreting of a territory around Unit 4.
2. Construction of protective (pioneer) walls along perimeter.
3. Construction of separation walls between Units 3 and 4.
4. Cascade wall construction.
5. Turbine Hall covering.
6. High-rise counterforce wall mounting.
7. Erection of supports and installation of a reactor compartment covering.
8. Installation of ventilation system and monitoring and measuring communications and devices.
During "sarcophagus" construction it has been placed more than 400 thousand cubic meters of concrete and 7000 tons of metal constructions has been mounted. During Shelter construction partitions and walls, separated destroyed Unit 4 from Unit 3 have been erected. Along the destroyed Unit perimeter firstly ferro-concrete "pioneer" protective walls have been erected for safety ensuring during the subsequent civil and erection works performance. Northern cascade wall has been executed from concrete as ledges in height about 12 meters. The shuttering of ledges was made of metal panels. Each subsequent ledge was carried out as closely as possible to the destroyed Unit. Worn and damaged steal constructions and containers with highly active wastes were enclosed inside ledges. A wall closed the remained western wall from the outside with buttresses in height up to 50 meters. 27 pipes with diameter of 1200 mm were used for Central Hall covering formation. Simultaneously with Shelter construction large scope of works on territory decontamination around object has been performed. Decontamination of a territory and roof has ensured fast exposure rate (ER) reduction outside of its premises.
New constructions have been built according to construction norms and rules, but term of their use is limited by periodic survey and corrosion-resistant coating renewal impossibility.
In October, 11, 1986 the Governmental Commission accepted “Conclusion on reliability and durability of a covering constructions, and radiation safety of Chernobyl NPP Unit 4 reactor compartment”. Concerning facility life time in the document it was stated: “In connection with that the shelter of reactor compartment is constructed on the destroyed constructions and in high radiation conditions, and taking into account complexity of constructions installation and control of their position, we have no a possibility to obtain the reliable data on their bearing strength”.
In the same document it is mentioned: “Taking into account low corrosion rate in operating conditions of constructions with protective coatings their life time can be ensured: for pipes 30-40 years, for beams - 30 years”.
On November, 30, 1986 State Acceptance Committee accepted for maintenance conserved Unit 4 of Chernobyl NPP.
"Shelter" peculiarity is determined by its potential hazard, significantly higher, than permitted by existing norms and the rules for facilities, containing nuclear and radioactive materials. Safety assessment of any nuclear engineering or industry facility, as a rule, is based on practical operating experience of this and similar facilities.Shelter don’t have even close analogues, and experience of its maintenance makes only 18 years.
Potential hazard of the Shelter object for an environment and for the person is determined by following factors:
• Presence of nuclear materials in approximate amount 200 tons, there are no means of active impact on their criticality, and effective physical barriers on radio-activity distribution way to an environment are absent.
Actually it is possible to consider as established, that inside the "Shelter" object remains about 95 % of fuel which was in a reactor at the moment of accident. Its total activity makes about 18 mln curies. Irradiated fuel inside the Shelter is in the form of following Fuel Containing Materials (FCM)modifications: core fragments, fuel dust, hardened lava-like Fuel Containing Materials (LFCM), dissolved forms of uranium, plutonium, americium.
Results of FCM parameters measurements show, that the incidents connected to FCM reproducing properties change are not registered by monitoring systems. Values of gamma-radiation exposure rate in places of FCM congestion (in comparison of 1994) have the low tendency to decreasing and in average make 3300 Rontgen per hour.
Destructive processes, occurring in Fuel Containing Materials (FCM), every year increase their hazard. Lava-like FCM destructs spontaneously and radionuclides containing in them are transformed from the bound state in mobile dust particles capable to live the "Shelter" boundaries with air flows. For the date the radioactive dust amount according to conservative estimations makes about 4 tons. There is a potential risk of a radioactive dust releases in an environment during probable collapse of supporting constructions, and due to water migration.
Water inflow in Shelter object premises is connected to many negative consequences, capable to destabilize the level of achieved Shelter object nuclear, radiation and radioecological safety:
· uncontrollable radio-activity transfer;
· possible Fuel Containing Masses sub-criticality decreasing;
· enriched uranium soluble salts scavenging and transportation from FCM with possible their accumulation in separate, uncontrollable, premises;
· FCM destruction and additional fuel dust amount formation;
· acceleration of Shelter object constructions destruction process.
Besides water affects normal work of diagnostic systems, impedes researches on object’s safety increasing, influences an Shelter premises electrical safety.
Water enters in chemical interaction with filling materials, constructive and Fuel Containing Materials, destroys them, dissolves and transports long-lived radionuclides and fissionable elements. As a result of these processes highly active alkaline carbonate solutions are formed, so-called "block" waters. According to researches data in 2002-2003 increase in radionuclides concentration (except Cs137) and fissionable elements in the "block" waters, caused by lava like FCM destruction process with the subsequent radionuclides leaching from them is marked. These processes result in radionuclides migration increase on SO premises. The uncontrollable "block" waters overflows to Unit 3 premises represent highest hazard. Highly active "block" waters represent real ecological threat for the geological environment in cases of their leakage from Shelter object.
Sources of a moisture penetration inside the object have as natural, and man-caused character:
· the atmospheric precipitation penetrating inside the object through cracks in protecting constructions with the area about 100 m2. Annual precipitations penetration is estimated approximately in 2200 m3.
· the condensate formed during the summer period in volume up to 1650 m3 due to a temperature difference and atmospheric air humidity content and air of premises on the bottom levels of the Unit;
· work of regular dust suppression system at which the solutions volume sprayed in space under the central hall roof makes about 270 m3 per year.
At the site near the object under a ground, sand and concrete layer there is a significant amount of the radioactive substances not isolated from the hydro-geological environment.
Long-term radionuclides content observation in subsoil waters of the Shelter object’s local zone shows absence of the tendency to radionuclides distribution by underground waters flows. Lower on subsoil waters flows from Shelter object it is not revealed neither uranium, nor plutonium though both of them are available in significant amounts in "block" waters.
· Object’s constructions don’t meet the requirements of the normative and technical documentation on Safety if the field of mechanical durability, structural integrity, constructive reliability and have undetermined operational term.
As a result of accident in 1986 the part of constructions of reactor block, deaerator stack, turbine hall and others has been destroyed. In the western zone the wall has a deflection in the western direction up to 50 cm, breaks are fixed in some its places. Such condition in case of earthquake 4,3 Richter number can lead to partial "sarcophagus" roof collapse. In a southern zone above the level 24,3 m a deaerator stack frame keeping significant amount of fragments and constructional materials is deflected from a vertical approximately on 1,5 m. And though during sarcophagus construction this place was reinforced by metal support, calculations show, that in case of earthquake there is a risk of their collapse aside turbine hall, that will entail a radioactive dust release in an environment through the formed break. Scientists have calculated, that such event probability risk per 1 year makes 0,24, that considerably exceeds the normative value recommended by IAEA for Nuclear Engineering facilities.
Due to high humidity level in the "Shelter" premises ferro-concrete constructions are water saturated that results in their degradation. Metal constructions are significantly damaged as a result of corrosion processes. Deformation process proceeds. The analysis of a constructions status testifies to the critical zones presence, sensible even at insignificant earthquakes, tornado and other natural phenomena, probable in ChNPP site location conditions.
· At the moment it is impossible to ensure completely monitoring of a facility condition, including - nuclear and radioactive materials conditions because of the limited access in some premises which is complicated by high radiation levels. As a result of insufficient investigation of object it was not carried out reliable quantitative estimations of different types of danger.
It is impossible to consider as sufficient the scope of monitoring nuclear and radiation parameters, carrying out by existing monitoring systems. In structure of the " Shelter" monitoring systems there is no regular monitoring system of construction there are no fire safety systems and inside "Shelter" there are near 2000 tons of flammable materials - graphite, cables, elastron, wooden constructions. Deterioration of monitoring systems of Fuel Containing Materials condition has came to a critical level. For example, "Finish" system is in operation since 1988 on which repair – restoration works were carried out many times.
· Radioactive aerosols of Shelter object
Now the Shelter object does not impact appreciably on the general radioactive contamination of the surrounding air environment, which basically is determined by superficial contamination of adjacent territory as a result of accident in 1986.
Actually the radioactive formations arisen at the moment of accident and existing inside the construction as a dust, and formed during physical and chemical FCM destruction are taken out from Shelter object.
Carrying out of radioactive aerosols occurs by two main ways – "nonorganized" and "organized".
"Nonorganized" release of aerosols occurs through construction cracks, which total area makes now about 100 m2. "Organized" release is carried out by Shelter object natural ventilation system through ventilation shaft of ChNPP Unit 3. The maximal estimation of the total release from Shelter last years makes 1,1 % from an allowable limit.
With the purpose of radioactive aerosols releases reduction in surrounding environment the stationary dust suppression system for various chemical compounds coating on a "ruin" surface was commissioned at the object from the end of 1989, that has allowed to reduce approximately in 10 times "nonorganized" activity release from the Shelter object. The system has been upgraded in 2003 that has allowed to increase the area coating by dust suppressive compositions.
Concentration of radioactive aerosols in a surface atmosphere at Shelter object site is insignificant. However, investigations have shown radioactive aerosols concentration in a surface atmosphere, significant activity increasing is possible at excavations.
19 сентября 2008