The lake Druksiai serves as a natural water reservoir and supplies the plant with cooling water. Lake Druksiai is the biggest lake in Lithuania. The catchment basin of the lake is located near the foot of the east slope of the Baltic ridge, which is bordered by the Svencionys upland from the south and by the Latgal upland from the north. Such a watershed location with predominating north and south winds influences its hydrologic regime [10].
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Fig. 2.2 Configuration of lake Druksiai, location of the Ignalina NPP and permanent testing stations (1-6) [15] |
Total area of the lake, including nine islands, is 49.32 km2. The area of the biggest island is 0.23 km2, and the areas of other islands are smaller than 0.05 km2. The surface of the islands varies from one to twelve meters above the water level. All islands, except one, are located in the south part of the lake.
The biggest depth of the lake is 33.3 m, and average is 7.6 m, while the predominate depth is 12 m. The channel of the lake was formed during the movement of the glaciers by two perpendicular runways, which were extended from the north to the south and from the west to the east. Maximum depth of the first runway was 29 m, and second one - 33.3 m [8]. The largest depths are located in the middle of the lake. The most shallow water is on the south ridge of the lake, the depth of which does not exceed 3-7 m.
The length of the lake is 14.3 km, the maximum width is 5.3 km, and the perimeter is 60.5 km. Drainage area of the lake is small, only 613 km2 [15].
Total volume of water is about 369× 106 m3. It should be noted, that all data are given at the Normal Affluent Level (NAL) of water reservoir 141.6 m for the Baltic System (BS) elevation. During the exploitation of the water reservoir by multi-year regulation of discharge, the water level of the lake can decrease to an elevation of 140.7 m, that is by 0.90 m from nominal. As a result, the surface area of the lake decreases to 42 km2, and the volume of water - to 326× 106 m3. At the present the water regime of the lake is regulated by a hydro-engineering complex of a former hydroelectric power plant [10].
The hydrographic schematic of lake Druksiai is presented in Fig. 2.2, and the bathygraphic curves of the lake in Fig. 2.3. Main data of the water-cooling reservoir of the Ignalina NPP are presented in Table 2.2.
Fig. 2.3 Bathygraphic curves of lake Druksiai [15]
a - area, b - water volume
Table 2.2 Main data of hydrologic and hydrothermic regime of water cooling reservoir of the Ignalina NPP
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Druksiai lake drainage area, km2 |
613 |
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Water area of lake at NAL, km2 |
49 |
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Multiyear flow rate of water from lake, m3/s |
3.19 |
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Multiyear discharge from lake, m3/year |
100.5× 106 |
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Multiyear quantity of atmospheric precipitation, mm/year |
638 |
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Multiyear value of evaporation from water surface, mm/year |
600 |
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Normal affluent level of lake, m |
141.6 |
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Minimum permissible lake level, m |
140.7 |
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Regulating volume of lake, m3 |
43× 106 |
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Permissible drop of lake level, m |
0.90 |
A number of lakes are present in the area of the Ignalina NPP. Their total water surface area is 48.4 km2 (without lake Druksiai). Lakes occupy 15 %, swamps - 15 %, farming land - 40 % and forests - about 30 % of the surrounding area. Net density of rivers is 0.3 km/km2.
Nearly all surface discharge (74 %) flows to the south part of lake Druksiai by way of two rivers Ricianka and Drukse, the rest of the surface discharge goes to the west ridge from the tributaries of the rivers Smalve and Gulbine. Discharge from the lake goes by way of the river Prorva through the south ridge of the water reservoir. Warm coolant water of the NPP is discharged into the same place. So, the most intensive water exchange take place in the south part of the lake.
The water regime of lake Druksiai is formed by correlation of natural and anthropogenic factors. The main natural factors are the climatic conditions of the region: precipitation onto the surface of the water reservoir and natural evaporation from lake surface and watershed. Anthropogenic factors, which have an influence on the NPP operation, are the control of discharge by the hydro-engineering complex and water circulation in the lake because of the needs for cooling of the NPP equipment.
The Ignalina NPP operation has no perceptible influence on the amount of atmospheric precipitation and on the water inflow into the lake. The NPP power has an influence on the evaporation from the water surface. Evaporation processes from the water surface of lake Druksiai is very important.
Net losses of water from the lake depend on the amount of evaporation. In conditions of limited water resources this amount can limit the power of the NPP. For this reason the natural and additional evaporation from the water surface are monitored carefully. During the years 1973-1976 a total annual evaporation from the water surface of lake Druksiai was determined to be 585 mm.
The twenty-four-hour average evaporation during all this period was 3 mm. During the hottest months-June and July-the evaporation was the largest - about 115 mm per month. In general, during the May-October period the evaporation was 535 mm [8].
The slope of subsoil waters in the lake is sufficiently small : a depth of 5 m in about 150-200 m from the bank. The flow is directed to the area of drainage of lake Druksiai. Subsoil discharge is about 3 % from all the multiyear water balance of lake Druksiai.
Ignalina NPP began operation in 1984. At the start only the first turbine of the first unit was put in operation. At the end of the same summer the second turbine was introduced, which reached full power in summer of 1985. Then the power of Ignalina NPP reached 1200 MW (e). During the years 1985-1987 Ignalina NPP operated at power of 1300-1500 MW on the whole with preventive maintenance during summer months. The first turbine of the second unit reached full power in August, 1987. During the year of 1988 the two units with a total power not exceeding 2500 MW (e) operated with interruptions.
During the operation of one unit the heat load to the lake is more than 0.06 kW/m3 (i. e. the amount of heat transmitted to the lake per month is 8.7× 1015 J), and during the operation of two units - 0.11 kW/m3.
Since the time when the first turbine started operating, the NPP coolant water began adding heat to the lake, this intensified the evaporation from the water surface. With the increase of power of the Ignalina NPP and gradually rising water temperature of the lake, there were additional losses of water by evaporation [10]. During the operation of Ignalina NPP the intensity of evaporation from the surface of lake Druksiai is sufficiently high and reaches a monthly average of 5 mm/day. This corresponds to a total evaporation of 158 mm/month during the operation of one unit, and 198 mm/month during the operation of two units.
From May to October in the 1984, the evaporation from the lake surface was 627 mm, in 1985 - 720 mm, in 1986 - 712 mm, in 1987 - 684 mm and in 1988 - 788 mm. During the May-October period, the evaporation norm is 540 mm, and during all the evaporation season (end of April-November) - 600 mm. From May to October in 1984 the evaporation norm was exceed by 16 %, in 1985 - by 33 %, in 1986 - by 32 %, in 1987 - by 27 %, and in 1988 - by 46 %, which shows the influence of the operation of the NPP [10].
The other side of the effect of Ignalina NPP to the amount of evaporation from the surface of lake Druksiai is the lengthening of the active evaporation time because of the extended period during which no ice forms on lake Druksiai. During the cold period the evaporation process persists in the zone which is adjacent to the mouth of discharge channel.
During the entire season of the year 1984 evaporation was 36× 106 m3, in 1985 - 48× 106 m3, in 1986 - 45.7× 106 m3, in 1987 - 50.8× 106 m3, and in 1988 - 52.2× 106 m3. These values exceeded the multi-year average values of evaporation (600 mm) by 14 % in the year 1984 during the operation of one turbine of Ignalina NPP with power of 750 MW, and by 72 % when the power was increased to 2500 MW [10].
Predictive calculations of the former Research and Development Institute for Energy Technology, St.Petersburg, Russia, together with LEI (at that time the Institute for Physical and Engineering Problems of Energy Research, Kaunas, Lithuania), the additional evaporation was found to be 16.2× 106 m3 or 55 % from the multi-year average value of the Ignalina NPP operation at 1500 MW (e), and 32.4× 106 m3 at 3000 MW (e). During the years of 1985-1987 the measured evaporation corresponds to computed predictions.
Employing the calculated estimates of water loss by evaporation from the lake's surface - a cooling discharge equal to about 5.5 l/s km2 is obtained. This is equivalent to a discharge of the Ignalina NPP operating at 3000 MW (e) [10].
