Shelter Object 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. The Shelter has no even close analogues.
Starting from 1986 to date, not more than 60% of the Shelter (Unit “Á”) premises have been surveyed. The remaining premises are inaccessible due to high-level radiation fields, or due to permeability barriers occurred during explosion, which led to structure destruction and initiation of lava-like fuel containing materials, and during concrete pumping-in when erecting the Shelter. Thus, significant part of the facility remains unexplored area, being one of the most serious present-day risks.
Potential hazard of the Shelter Object for environment and person is determined by following factors:
• Presence of nuclear materials in approximate amount 185 tons, regarding to which there are no means for active impact on their criticality, and lack of effective physical barriers on a way of radioactivity spreading into environment.
Up-to-date, it is possible to consider that inside the Shelter about 95% of fuel remains, which was in the reactor at the moment of accident. Now, its total activity makes about 16 million curies. It is determined that irradiated nuclear fuel inside the Shelter is in the form of following modifications:
- core fragments;
- lava-like fuel containing materials (LFCM);
- fine-dispersed fuel (dust), hot fuel particles;
- secondary uranium minerals generated from FCM solutions as crystal new formations.
The results of FCM parameter measurements show that the incidents concerned with FCM reproducing property modification are not registered by monitoring systems. Values of gamma-radiation exposure rate in places of FCM accumulation (comparing with 1994) have the low tendency to decrease and make 3,300 Roentgen per hour, in average.
Destructive processes, occurring in FCM, increase their hazard every year. Lava-like FCM are destructed spontaneously, and radionuclides containing in them are transformed from the bound state into mobile dust particles capable to leave the Shelter boundaries within air flows. To date, according to conservative estimations, the radioactive dust amount makes about 1.5 tons. There is a potential risk of radioactive dust releases into environment.
The main risk of accidentally irradiated fuel (like LFCM) is its dust-generating ability being within the limits of 40 Bq/(cm2 per day) by alpha-activity, and 2,400 Bq/(cm2 per day) by beta-emitters. The dust is generated on irradiated fuel surface and taken off outside the Shelter by ventilation system.
• Water inflow into Shelter Object premises is concerned with many negative consequences capable to destabilize currently achieved level nuclear, radiation and radioecological safety of the Shelter:
• uncontrolled transfer of radioactivity;
• possible decrease of fuel containing mass sub-criticality;
• enriched uranium soluble salts wash-out and transportation from FCM with the potential accumulation in separate, uncontrolled premises;
• FCM destruction and formation of additional fuel dust amount;
• acceleration of the Shelter constructions destruction process.
Besides, the water affects normal work of diagnostic systems, interrupts research on safety improvement at the facility, and influences on electrical safety of the Shelter premises.
Water enters into chemical interaction with backfilling materials, constructive and fuel containing materials, destroys them, dissolves and transports long-lived radionuclides and fissile elements. As a result of these processes, high-level alkaline carbonate solutions are formed, so-called "Unit’s waters”. According to investigation data of 2002-2003, concentration increase in radionuclides (except for Cs137) and fissile elements in "Unit’s waters”, stipulated by destruction process of lava-like FCM with subsequent radionuclides leaching from them. These processes result in increasing radionuclides migration within the Shelter premises. The uncontrolled overflows of "Unit’s waters” into Unit 3 premises has the highest hazard. High-level "Unit’s waters” represent real ecological threat for geological environment in case of its leakage from the Shelter.
Sources of moisture intake inside the Shelter have both natural and man-caused feature:
• atmospheric precipitations penetrating inside the Object through cracks in protecting constructions within area of about 100 m2. Annual precipitations intake is estimated as 2,200 m3, approximately.
• the condensate formed during the summer period in volume up to 1,650 m3 due to temperature difference and humidity content of atmospheric air and air of premises on the bottom levels of the Unit;
• operation of regular dust-suppression system, where solution volume sprayed into space under Central Hall roof constitutes about 270 m3 per year.
There is a significant amount of the radioactive substances not isolated from the hydro-geological environment on site near the Object under the layer of ground, sand and concrete.
Long-term surveys for radionuclides content in ground waters of the Shelter local zone show lack of tendency of radionuclides spreading by underground water flows. Lower by ground water flows from the Shelter, it is not revealed neither uranium, nor plutonium though both of them are available in significant amount in "Unit’s waters”.
• Building structures of the Object don’t meet the requirements of the normative and technical documentation on safety regarding mechanical durability, structural integrity, constructive reliability and have uncertain term of operation.
As a result of accident in 1986, a part of structures of reactor unit, deaerator stack, turbine hall and others had been destroyed. In the western area, the wall has a deflection in the western direction up to 50 cm, breaks are fixed in some its places. In case of earthquake of 4.3 under Richter scale, such condition can lead to partial "sarcophagus" roof collapse. In the southern area above the level 24.3 m, deaerator stack frame supporting significant amount of fragments and constructional materials is deflected from vertical for 1.5 m, approximately. And though during sarcophagus construction this place was reinforced by metal supports, calculations show that in case of earthquake there is a risk of their collapse aside turbine hall, resulting radioactive dust release into environment through formed break. The scientists calculated that value of probability risk of such event per 1 year makes 0.24, that considerably exceeds the normative value recommended by IAEA for nuclear energy facilities.
Due to high humidity level in the "Shelter" premises, reinforced concrete structures are saturated with water resulting their gradual damage. Metal structures are significantly damaged as a result of corrosion. Deformation process is continuing. Analysis of building structure condition testifies about critical zones presence sensible even at insignificant earthquakes, tornado and other natural phenomena, potential under ChNPP site location conditions.
• At the moment, it is impossible to ensure completely monitoring for the facility condition, including conditions of nuclear and radioactive materials due to limited access in some premises complicated by high-level radiation levels. As a result of insufficient investigation of the Object, reliable quantitative estimations of different types of danger were not performed.
It is impossible to consider the scope of monitoring nuclear and radiation parameters as sufficient, performed by available monitoring systems. The structure of the "Shelter" monitoring systems has no regular monitoring system for building structures, no fire safety systems, and there are graphite, cables, plastic compounds, and wooden structures near 2,000 tons of flammable materials inside the "Shelter".
• Radioactive aerosols of the Shelter
Nowadays, the Shelter Object does not visibly impact on general radioactive contamination of ambient air, which basically is determined by surface contamination of adjacent territory as a result of accident in 1986.
To date, radioactive formations, generated during the accident and existing inside the facility in the form of dust, and formed during physical and chemical FCM destruction, are taken out from the Shelter.
Radioactive aerosols are carried over by two main ways – non-organized and organized.
Non-organized release of aerosols occurs through construction cracks, total area of which is currently about 100 m2. Organized release is carried out by the Shelter natural ventilation system through ventilation shaft of ChNPP Unit 3. The maximal estimation of the total release from the Shelter for last years constitutes 1.1% from allowable limit.
In order to reduce radioactive aerosol release into environment, the stationary dust-suppression system for coating breakdown surface with various chemical compounds was commissioned at the Object since the end of 1989, enabled to reduce non-organized release of activity from the Shelter 10 times, approximately. The system has been upgraded in 2003 enabled to increase the area of coating with dust-suppressive compositions.
Concentration of radioactive aerosols within near-surface atmosphere on the Shelter site is insignificant. However, as surveys of radioactive aerosol concentration near-surface atmosphere showed, significant activity increase is potential during excavation works.
In spite of making efforts during whole period of Shelter operation, condition of the facility safety is reducing with time:
• Gradual degradation of building structures is taking place;
• Equipment and systems, actually worked out their resource, need to be replaced or updated completely;
• Under influence of microclimatic conditions inside the Shelter, FCM proceeds to be destroyed leading to increase of radioactive dust volume;
• There is a potential for collapse of separate building structures, which could lead to significant radioactive contamination of ChNPP site and adjacent area and personnel irradiation;
• There is a potential for contamination of adjacent areas, surface and ground waters with radioactive outflows from the Shelter;
• There is a potential for origin of self-sustaining chain reaction;
• Negative impact of humidity and water on the Shelter is continuing (conditioned by penetration of atmospheric precipitations, water condensation and its movement at the Shelter).
Currently, safe operation of the Shelter is achieved by the complex of organizational and technical measures, including:
• Control of Shelter status;
• Limitation of radioactivity spreading outside the Shelter;
• Violation prevention of the Shelter safe operation limits;
• Support of systems, equipment and structures of the Shelter in good condition;
• Emergency preparedness.
The Shelter status is controlled by ChNPP specialists (visual monitoring and instrumental measurements of building structure careen), as well as involving contract organizations (geodesic monitoring of shifts and concretion of the Shelter building structures).
In order to limit radioactivity spreading outside the Shelter, ChNPP personnel perform daily decontamination and dust-suppression of premises and territory of the Shelter local zone, and work performance areas. During generating or intake of radioactive waste, its gathering and removal is constantly carried out for disposal or temporary storage.
In order to determine Shelter impact on environment with involving contract organizations, radiation monitoring is carried out, including the control of following:
• Non-organized gas-aerosol releases through Shelter leakiness;
• Amount of activity and radionuclide structure of ground waters;
• Condition of air environment within the Shelter area impact.
Violation prevention of the Shelter safe operation limits is provided by constant readiness of management systems of Shelter beyond design basis accident to introduce neutron-absorbing solutions into FCM accumulation places, supply dust-suppressing and dust-fixing solutions into under-roof space of the Shelter, localize radioactive substances and ionizing radiation sources, and control hydrogen.
Except for current operation, the most important task is organization of safe work performance by off-site organizations, which take part in implementing projects on Shelter transformation into ecologically safe system. To perform these activities at Chornobyl NPP, there is qualified personnel having long-term experience in radioactively hazardous conditions of the Shelter.