The hurried closure of ChNPP has left behind a number of unsettled problems, including the ones related to the plant’s cooling pond with a capacity of 98 million m3 stretching over an area of 22.9 km2. During ChNPP operation, the pond performed many functions: it ensured makeup of the cooling systems, provided water for further chemical cleaning, filled circulation systems, etc. Till the fatal day in April 1986, the cooling pond served even for fish farming. However, this was in the past.
Following a stoppage of the operating ChNPP units, the cooling pond lost its technological functions. Now supporting its normal operational state is economically disadvantageous, as cumulative expenditures for its maintenance exceed 2.5 million hryvnias annually.
One would think it is very easy: the pond is artificial, so the water may be drained into the Prypiat River with prior calculation of safe annual drainage volumes, and thus the problem is solved. Very easy this task would be, if it were not for the following issue: the ChNPP cooling pond being a technological object at the same time is a lake with nature-preservation functions and requires specific precautions stipulated by presence of high concentrations of the accident-originated radionuclides in its water and bottom sediments.
For the moment, if the works to lower water level in the pond are not started, then considerable costs will be required to maintain normal operational state of enclosing dams, cleanup a part of the Pripyat River’s bed needed to ensure the lake’s inflow. These activities would cost up to 10 million hryvnias. Failure to perform the abovementioned works may result into the dams’ destruction and, consequently, flooding of a huge area contaminated with radioactive isotopes and further export of radioactive materials into the Pripyat and Dnipro Rivers.
As a result of 1986 accident, a large amount of radioactive materials (RAM) has got into the cooling pond: Sr and Cs isotopes with the levels amounting to thousands of Ci. Today, they are located in the silt sediments under the water layer that serves as a powerful barrier on their way to the atmosphere. Thus, mobility of RAM is not high. As per the assessments made in 2002, (16±3)1013 Bq of Cs137, (2,4±0,9)1013 Bq of Sr90, (5,3±1,9)1011 Bq of Pu are concentrated in the pond’s bottom sediments.
The lake’s depths, radiation contamination of its bottom and types of bottom sediments were mapped and possible options of its decommissioning (lowering of the water up to the Pripyat River level) were developed.
The most preferable is an option of the water level controlled gradual lowering in parallel with rehabilitation measures performed on as-needed basis to mitigate possible consequences caused by natural water drainage from the pond.
Possible negative consequences of the water drainage may include additional contamination of the territories adjacent to the pond in the case of wind storms and spread of dust within the dewatered sectors, as well as the problems related to formation of an excessive biomass.
Special consideration should be given to changes in the ecological situation. Particularly, there are some apprehensions that in case of uncovering the bottom soils, apart from possible over-drying and spread of contaminated dust, the excessive biomass will rot in residual water bodies, which will further result in deterioration of sanitary, epidemiological, and ecological conditions within the pond location area.
Therefore, it is assumed that a comprehensive radioecological and radiobiological monitoring should be implemented during lowering the water level. Based on the data obtained during the monitoring, the decisions to correct rates of the water level lowering shall be made. The same refers to a strategy of protective measures that may possibly be required in the process of water drainage with the objective to reduce hazardous impacts (screening of the contaminated layer with clean soil, or removal of the layer, or planting vegetation, removal of the excessive biomass).
The following should be referred to positive aspects, except for economical ones, of decommissioning the cooling pond:
The report “Ecological justification of a possibility to decommission the cooling pond and prepare source data required to perform technical and economic calculations” was considered at the meeting of the Chornobyl NPP scientific and technical council (STC), which gathered representatives of numerous scientific institutions (Ukrainian Scientific and Research Hydrometeorology Institute, Center of Monitoring Research and Natural Technologies, State Scientific and Engineering Center for Control and Emergency Response Systems, Institute of Geological Sciences under Ukrainian National Academy of Sciences, Kyiv Institute ‘Energoporject’); state control and regulatory authorities (Ukrainian Ministry of Emergencies and Protection of Population against Consequences of the Chornobyl Catastrophe, Ukrainian Ministry of Health, Ukrainian Ministry of Natural Environment Protection, State Scientific and Technical Center for Nuclear and Radiation Safety). The STC decision fixed in Protocol #2 of February 16, 2007 admitted appropriateness of feasibility study (technical and economic justification) of the cooling pond decommissioning.
Further, in order to exclude influence of the cooling pond decommissioning activities onto the works being implemented at the Chornobyl NPP and with the participation of the designer general (OJSC Kyiv Scientific-Research and Design-Engineering Institute ‘Energoproject’) and scientific advisor (institute of NPP safety problems), the decision to divide the complex of activities into the following milestones was made:
Problems and specific characteristics pertaining to the cooling pond decommissioning:
Application of regulatory radiation criteria established for contaminated areas cannot be justified with due regard to the fact that the territory that surrounds the pond is contaminated as well. Removal of the contaminated bottom sediments regarded as radwaste will require multimillion costs, moreover, such types of radwaste should be removed to high-level radwaste storage facilities, as they contain fuel-containing elements. And there are no such storage facilities.
In contrast to contaminated water bodies in Russia, the ChNPP cooling pond is located 6m higher than the Pripyat River. This fact as well as availability of a makeup pumping plant allows for a flexible strategy of lowering water level in the pond (natural controlled lowering).
Later, starting with January 2010 and through to December 2012, the activities for its immediate decommissioning shall start (‘Action Plan for the Choronobyl NPP Power Units Decommissioning and Transformation of the Shelter Object into an Ecologically Safe System for 2008-2011’)
On January 28, 2009, the President of Ukraine Victor Yushchenko signed Law of Ukraine #886-VІ ‘On National Program of the Choronobyl NPP Decommissioning and Transformation of the Shelter Object into an Ecologically Safe System’. In particular, the Program defines top-priority activities planned for 2008-2012 and implementation of activities for the cooling pond decommissioning is among them.
Compiled based on the report presented by A.S. Antropov, Project Manager of the Choronobyl NPP Cooling Pond Decommissioning, at the international workshop ‘Scientific and Practical Aspects of the Choronobyl NPP Power Units Decommissioning’, Slavutych, September 9-12, 2008.