The SPOT is 16 air cooled heat exchangers-condensers (four ones for each steam generator), located outside around containment. The heat exchanger is canned in the box connected with exhaust stack above to ensure the air circulation. The design basis of the SPOT is the removal of 60 MW heat at the maximum design temperature of the external air (plus 50°C) taking into account failure of one train, so the power of one heat exchanger is 5MW. The air flowrate is controlled by partial opening of the air gates to ensure that the power of the heat exchanger should not be excessive in the minimum design temperature of external air (minus 40°C). Under standby conditions the air gates are closed, but the heat exchangers are in the hot state due to air leakage through air gates and heat transfer through the building constructions. These thermal losses are estimated in the design by value less than 0,1% of reactor rated power. The levers with load open the gates under de-energization of the electromagnets keeping the air gates in the closed position, and the air natural circulation through heat exchangers begins.

To confirm the design solutions mentioned above and the system characteristics, the extensive experimental investigations have been performed in OKB Gidropress. The investigations were carried out at the external air temperature from minus 19°C to plus 30°C and the pressure in the steam-condensate circuit from 0,5 MPa to 6,4 MPa. The start-up of the system from hot standby condition is performed, the heat losses are determined at the closed air gates, the system power against external air temperature and steam pressure is determined, the heat removal control is checked by the partial opening of the air gates. The investigations have given the convincing substantiation that the SPOT design functions are performed for WWER-1000/V-392 in the real conditions.

The test rig constructed in OKB Gidropress in 1991, includes the full scale heat exchanger of the SPOT system with representation of air and steam-condensate circuits as shown in Figure

4. The heat exchanger consists of about 200 flat spirals having small slope in relation to horizontal. The tube bank about 2 m high and total surface more than 300 m has two inlet and two outlet collectors. On the sections with cross-sectional air flow, the tubes have ribs for intensification of the heat transfer to the air.

The tube bank is canned in heat-transfer box of square cross-section, which forms the heat exchanger air flow circuit. The lower and upper part of this box are connected with horizontal boxes for air inlet and outlet accordingly. The box upper outlet is connected to exhaust stack. The air gates are installed before of the heat exchanger and after one in pairs in one plane, each of these overlaps half of the cross-section of the circuit. The gate is the flat plate rotated about one’s own axis. The lever with load which is retained by the electromagnet, when the gate is closed, is attached to the axis. The horizontal inlet box, the box of the heat-transfer bank, the horizontal outlet box and exhaust stack together create the circuit for air natural circulation. In this circuit (see Figure 6) the elevations of its components (including the height from inlet box axis up to top of the exhaust stack) and cross sections of all air circuit elements per one real heat exchanger are represented in the full scale. Whole air circuit, starting from the heat exchanger, has been covered by thermal insulation and sheathed by aluminium sheets to create adequate conditions concerning to the heat losses of the real SPOT system.

During the tests, the parameters necessary both for the confirmation of the SPOT design operation and for the development and validation of calculation models for the whole system and its separate elements have been measured. The set of the measured parameters includes:

• flow rate, pressure and temperature of the superheated steam,

• feedwater flow rate,

• flow rate and temperature of the condensate,

• flow rate, pressure and temperature of the saturated steam supplied to the heat exchanger,

• pressure and temperature of the atmospheric air,

• pressure difference along the heat exchanger in the steam-condensate circuit,

• air temperature in the air circuit elements,

• temperature of the heat-transfer tube surface,

• temperature of the thermal insulation.

These measurements have been shown in the control room of the test rig and the registration of the parameters have been realized by the data acquisition system IMPACT 3590 with subsequent treatment by a personal computer.

The investigations to define the heat losses in hot standby condition with the closed air gates have been performed from 07.04.92 to 16.02.93 (totally 30 experiments). In experiments, the steam pressure before the heat exchanger accounted from 0,6 MPa to 6,39 MPa, the steam temperature before the heat exchanger accounted from 159°C-280°C and external air temperature accounted from minus 8°C to plus 18°C. It was found that the direct recalculation of the test results to define the system heat losses results in a larger losses than it is anticipated by the SPOT design (less than 0,1% of the reactor rated power). It was also determined that temperature of the external surface of the heat exchanger thermal insulation exceeds significantly the design value 45°C. The reasons of these differences for the test rig have been found and the recommendations have been developed for the design modification of the SPOT system thermal insulation for WWER-1000/V-392.

To define the dependence of the thermal power removed by the heat exchanger against the external air temperature and steam pressure, the tests have been performed under stationary conditions in the range of external air temperature from -19°C to +30°C and steam pressure at the heat exchanger inlet from 0,54 MPa to 6,39 MPa. During the period from 27.03.92 to 15.02.93, thirty experiments have been performed under positive external air temperature and ten experiments under negative one; the air gates were open partially in eleven experiments to evaluate possibility of the SPOT power control. It was determined that the heat exchanger removes from 5 MW to 7.4 MW at the pressure 6,3 MPa and the external air temperature from plus 30°C to minus 19°C. It was confirmed that the heat exchanger design geometrical characteristics ensure necessary power of the heat exchanger at the maximum design temperature 50°C and fully open air gates. For evaluation of the removed power control by means of partial closing of the lower air gates, two series of the experiments have been performed at the external air temperature plus 18-19°C and minus 11-16°C and steam pressure 6,3 MPa. The gates have been deviated on 10, 20 and 30 degrees in relation to horizontal. It was determined that the design configuration of the air gates ensures the power control of the heat exchanger under turn angles 0-40 degrees, after that the regulating capacity is lost. For example, the heat exchanger power at the turn angle of the gates 10 degrees is approximately 2,5 times less than under fully open gates, and at the angle 30 degrees the power is only approximately 15% less than the power at fully open gates.

To investigate the influence of the non-condesable gas on the heat exchanger operation, two experiments have been performed with supply of the different nitrogen quantity to the steam generator model. The gas quantity in these experiments was approximately 5,5 and 1,3 times more than quantity which corresponds to the real conditions of the reactor plant operation. A number of effects related to the nitrogen supply have been noted. In particular, the condensate temperature is reduced concerning saturation temperature due to decreasing of the partial steam pressure. The nitrogen is distributed on the collectors unequally, according to their location concerning steam pipeline. Discharge of a part of nitrogen with condensate flow was observed. The experiments have been performed with putting the heat exchanger into operation at the fresh steam supply and at the different quantity of nitrogen injected. It was determined that in all cases the heat exchanger power is stabilized in 40-50 seconds, and the value of the stationary power does not depend on quantity of the nitrogen injected and is only determined by the external air temperature. The tests performed have shown that the nitrogen is discharged gradually with the condensate. So, at the injection of the nitrogen quantity, which exceeds possible injection in real conditions about 5,5 times, the nitrogen has practically discharged completely from the heat exchanger in 10 minutes after the air gates opening.

The investigation of the stability of the SPOT heat exchangers parallel operation for the different loops has been performed on TDU-1 facility in one of the institutes of the Science Academy of Belorussia. The experiments have been performed on the two-loop three-circuit test rig by 1 MW power. The steam generator and the SPOT air heat exchanger-condenser were modelled in each loop. It has allowed to model operation of two train of the SPOT in parallel at the different loading of the heat exchangers. The experiments have been performed at the air temperature from plus 5°C to plus 31 °C and at the steam pressure from 0,6 MPa to 5,4 MPa. The experiments have demonstrated steady operation of the heat exchangers, the variations of the condensate and heat transfer tubes temperatures were not noted. In the same institute the experiments have been performed on the SPOT-2 facility, which models the circulation circuit WWER-1000 and the SPOT circuit in scale 1:5500 in relation to the power and ensures the hydraulic similarity and real difference of the equipment elevations. These experiments have confirmed the possibility of the long-time passive heat removal in case of the main circulation pipeline rupture and station blackout.

The containment model have been developed and constructed in scale 1:80 for the experimental investigation of the possible influence of the wind on the SPOT effectiveness. The investigations have been performed for the wind speed from 0 to 90 m/s (from the calm atmosphere to hurricane) at the wind direction from 0 to 360 degrees in relation to the reactor building axes. These experiments have shown the absence of the circulation reversal in the exhaust air stacks and have confirmed the design solution correctness, at which the SPOT trains are connected by the common inlet collector and the common outlet collector with the deflectors.

5. CONCLUSION

Broad objectives for advanced nuclear power plants have been documented [7] by the International Atomic Energy Agency. With regard to the safety enhancements, this document states that the plant design should seek to take the maximum, feasible advantage of inherent safety features, and efforts should be made to utilize passive safety systems to the extent that they can be shown as reliable and cost effective as active systems for the same function. Following these recommendations, a reasonable balance of active and passive systems based on the weighing of their advantages and disadvantages with regard to the designated functions, overall plant safety, and construction and operation costs has been found in WWER-1000/V-392 and WWER-640/V-407 designs.

A number of the relatively innovative passive safety means are used in new Russian plant designs with V-392 and V-407 reactors to fulfil the fundamental safety functions, such as reactivity control, fuel cooling and radioactivity confinement. Their implementation allowed to significantly increase the power plant safety in terms of the expected severe core damage and excessive radioactivity release frequencies. For example, the estimated core melt frequency of WWER-1000/V-392 is three orders of magnitude less than for the operating power units with WWER-1000/V-320 reactor.

As the sufficient operational experience for some passive systems and components is absent, the extensive experimental investigation and tests have been carried out or planned to prove the functioning of these systems under plant conditions. In particular, the experiments are already performed for residual heat removal system (SPOT), quick boron supply system, the system to keep the rarefied atmosphere in the containment wall’s gap, emergency core cooling system, and some others.

The experimental investigations and tests performed have confirmed the design functioning of the passive safety means proposed and allowed to optimize their general configuration and initiating signals. These investigations have also created the necessary experimental data base for the modeling of the passive safety means by the system thermohydraulic codes. Further investigations are being planned for additional verification of the passive safety systems and for the optimization of their design.