Passive residual heat removal system (PRHRS) for the secondary loop is one of the important features for Chinese advance pressurized water reactor (CAPWR). To prove the safety characteristics of CAPWR, serials of experiments have been done on special designed PRHRS test facility in the former stage. The test facility was built up following the scaling laws to preserve the similarity to CAPWR. A total of more than 300 tests have been performed on the test facility, including 90% steady state cases and 10% transient cases. A semiempirical model was generated for passive heat removal functions based on the experimental results of steady state cases. The dynamic capability characteristics and reliability of passive safety system for CAPWR were evidently proved by transient cases. A new simulation code, MISAP2.0, has been developed and calibrated by experimental results. It will be applied in future design evaluation and optimization works.
The passive safety feature is one important essential requirement for both integral type reactors and large scale multiloop type reactors. The steam generators (SGs) have been considered as passive safety cooling devices to provide primary loop decay heat removal capacity. There is no additional power supply needed for SGs cooling features. It is specially fit for nonloss of coolant accident applications. The steam released from SGs will be condensed in an accessory heat exchanger, which is located in a tank and submerged in low temperature coolant. To enhance the passive safety function and provide a redundant heat removal path, the secondary side passive cooling system will be necessary for advanced reactors.
In order to improve the security and reliability of reactor, some evolutionary and innovative reactors, such as AP600/1000, WWER640/407, APR+, SBWR, and SMART, employ passive residual heat removal systems [
The APR+ is a Generation III+ nuclear power plant being developed in Korea. The passive auxiliary feedwater system (SG secondary side passive residual heat removal system) is one of the advanced safety features being adopted in the APR+. It cools down the secondary side of the steam generator and eventually removes the decay heat from the reactor core by adopting a natural circulation mechanism, that is, condensing steam in the nearly horizontal passive condensation heat exchanger tubes submerged inside the passive condensation cooling tank [
The SG secondary side passive residual heat removal system (PRHRS), which removes the residual heat when station blackout occurs, is considered in Chinese advanced PWR. The passive residual heat removal system eliminates a great deal of water in the condensation tank, which is cooled by air natural circulation in the chimney. Only a small quantity of water in the emergency feedwater tank is needed for absorption of residual heat at the initial period of accident.
In order to assess the ability of PRHRS, experimental research had been done at advanced PWR PRHRS test facility in Nuclear Power Institute of China; in the meantime, the computer code MISAP2.0 has been developed and verified.
The test facility has been built according to two-phase natural circulation scaling methodology laws. The geometrical similarity, friction number, density ratio, Froude number, phase change number, and drift-flux number are the important similarity groups. And it is composed of steam-water circulation loop, emergency feedwater loop, air natural circulation loop, and safe release system. The main components include steam generator, air cooler, emergency feedwater tank (EFWT), and chimney. It has a geometrical scaling ratio of 1/390 in volume, and the total height from the top of the height-adjustable chimney to the bottom of SG is about 23 m. The maximal pressure of facility is 8.6 MPa, and the core decay heat is simulated by electric heaters with a capacity of 400 kW (DC). Figure
Schematic diagram of PRHRS test facility.
The work principle of test facility is as follows. When station blackout or other accidents occur, the isolation valves located at the outlet pipe of emergency tank are opened by a low-low water signal for the SG, so that the emergency water tank provides water to the secondary side of SG driven by gravity and maintains the water level. The water in the SG absorbs the residual heat when the water evaporates. The steam rises and passes through the air cooler where the steam is condensed into water; simultaneously, the heat is transferred into the air through the air natural circulation composed of chimney and atmosphere. Then, the condensed water returns to the SG loop driven by gravity; thereby, a continuous natural circulation flow is established.
A total of 280 sets of experimental data at steady state have been obtained and the main influence factors on heat removal capability were identified. The main operation parameters are the height of chimney, the hydraulic resistance, and the initial pressure.
More than 30 transient tests had been performed while the residual heat drops from 8% full power to 2% full power (Figure
Power comparison between test and calculation.
The natural circulation is influenced by many parameters, such as the height of chimney, the hydraulic resistance, and the initial pressure. Several tests are implemented to investigate the effect of these parameters on the passive residual heat removal system. In addition a semiempirical model was generated for predicting the natural circulation behavior of Chinese advance pressurized water reactor.
The equation of momentum for passive residual heat removal loop is
Equation of momentum for air loop is
The air cooler heat transfer equation is
In steam side of the cooler,
In air side of the cooler,
Coupling with (
The effect of the height of chimney in the passive residual heat removal system has been identified at the initial height of chimney of 9.7 m and 27.1 m. In this group of sensitive studies, the hydraulic resistance hardly keeps the same value under different condition, which is maintained at less than 10%. And the temperature of air at the inlet of chimney and pressure are unchanged values with only the height being allowed to change to find the effect.
Table
Effect of the height of chimney.
Number | Height of chimney (m) | Test | Calculation | ||||
---|---|---|---|---|---|---|---|
Natural circulation |
Temp. of condensate (°C) | Power (kW) | Natural circulation flow rate (kg/h) | Temp. of condensate (°C) | Power (kW) | ||
1 | 9.7 | 204.4 | 280.2 | 86.7 | 210.3 | 277.09 | 90.8 |
2 | 14.1 | 226.4 | 279.6 | 96.5 | 236.7 | 278.66 | 101.7 |
3 | 14.8 | 230.9 | 280.9 | 97.2 | 235.6 | 273.87 | 102.8 |
4 | 21.1 | 238.5 | 263.1 | 107.4 | 249.5 | 256.16 | 115.1 |
5 | 27.1 | 249.0 | 243.7 | 119.1 | 259.3 | 237.96 | 125.8 |
The height between air cooler and SG is 14.5 m, the hydraulic resistance is 105, the water in SG is 5.1 m, the pressure is 6.4 MPa, and the temperature at the inlet is 20°C.
The effects of the hydraulic resistance (frictional resistance and local resistance) in the passive residual heat removal system are performed in this study (Table
Effect of the hydraulic resistance.
Number | Hydraulic resistance (m) | Test | Calculation | ||
---|---|---|---|---|---|
Natural circulation flow rate (kg/h) | Power (kW) | Natural circulation flow rate (kg/h) | Power (kW) | ||
1 | 86 | 215.6 | 89.4 | 227 | 92.7 |
2 | 122 | 211.0 | 88.5 | 221 | 91.9 |
3 | 153 | 210.1 | 88.2 | 220 | 92.7 |
4 | 157 | 209.4 | 87.9 | 219 | 92.5 |
5 | 171 | 206.7 | 87.6 | 217 | 91.7 |
The height between air cooler and SG is 14.5 m, the height of chimney is 10.5, the water in SG is 5.1 m, the pressure is 6.4 MPa, and the temperature at the inlet is 20°C.
The initial pressure in the passive residual heat removal system changes from 3.53 MPa to 7.30 MPa, and other parameters are fixed except for the hydraulic resistance with a 10 percent deviation. As the pressure ascends, the increase of power and natural circulation flow rate is 22.3% and 25%, respectively (Table
Effect of the initial pressure.
Number | Pressure (MPa) | Test | Calculation | ||
---|---|---|---|---|---|
Natural circulation flow rate (kg/h) | Power (kW) | Natural circulation flow rate (kg/h) | Power (kW) | ||
1 | 3.53 | 151.0 | 76.1 | 153.2 | 74.7 |
2 | 4.51 | 160.2 | 80.8 | 161.6 | 80.2 |
3 | 6.82 | 170.3 | 88.0 | 163.0 | 86.2 |
4 | 7.80 | 188.3 | 93.6 | 176.7 | 89.5 |
The height between air cooler and SG is 14.5 m, the height of chimney is 12.8, the water in SG is 5.1 m, the hydraulic resistance is 105, and the temperature at the inlet is 33°C.
When the station blackout accident happens in the Chinese advance pressurized water reactor, it adopts the passive residual heat removal system to mitigate consequences of accidents. The passive residual heat removal system startup modes include warm and cold patterns when the emergency feedwater is on or off, respectively. If there is a tiny flow rate to keep a little natural circulation in both steam-water circulation loop and air natural circulation loop before the startup of transient test, this type of startup mode is defined as warm startup. On the other hand, if the initial condition is cold, or there is no flow in both steam-water circulation loop and air natural circulation loop, this type of startup mode is defined as cold startup.
The typical results of cold and warm startup tests with emergency feedwater are shown in Figures
The pressure of cold and warm startup mode.
The natural circulation flow rate.
The feedwater flow rate.
The pressure fluctuation when water hammer occurs.
The effect of temperature of feedwater in the passive residual heat removal system has been identified at the initial temperatures of 32°C and 52°C and the other parameters are kept at a constant value. The results are shown in Figure
Water hammer influenced by feedwater temperature.
In this group of case studies, the resistance of feedwater loop is changed by +50% of the reference case, and the other parameters are the same. The results are shown in Figure
Water hammer influenced by resistance of feedwater loop.
MISAP2.0 code is a typical code that is used to analyze steady and transient performance of the secondary passive residual heat removal system.
A basic assumption is that one-dimensional approach is used.
Equation of continuity is
Equation of momentum is
Equation of energy is
Equation of continuity is
Equation of momentum is
Equation of energy is
Consider
Momentum equation is
In view of the fact that two loops closely couple with each other due to energy and momentum interactions, the two loops are solved together. Figures
MASAP nodalization diagram of the test facility.
MASAP nodalization diagram of SG.
During the steady state calculation, several parameters, such as natural circulation flow rate, flow rate of air, temperature of condensate, and air at outlet of chimney, are calculated. And the calculations of MASIP2.0 are compared with the experimental data shown in Table
Comparison between the calculation of MISAP2.0 and experimental data.
Number | Test | Calculation of MISAP2.0 | ||||||
---|---|---|---|---|---|---|---|---|
Natural circulation flow rate (kg/h) | Temp. of condensate (°C) | Flow rate of air (kg/s) | Temp. of air at outlet (°C) | Natural circulation flow rate (kg/h) | Temp. of condensate (°C) | Flow rate of air (kg/s) | Temp. of air at outlet (°C) | |
1 | 184.0 | 218.5 | 0.581 | 184.3 | 194.6 | 218.3 | 0.600 | 184.1 |
2 | 165.8 | 194.9 | 0.582 | 185.5 | 183.4 | 216.9 | 0.595 | 177.6 |
3 | 198.8 | 255.2 | 0.579 | 181.2 | 219.9 | 249.6 | 0.609 | 202.1 |
4 | 164.6 | 229.3 | 0.515 | 193.0 | 180.6 | 246.3 | 0.550 | 188.6 |
The transient calculation results of cold startup model test are compared with experimental data in Figures
The pressure comparison between calculation and test.
The flow rate comparison between calculation and test.
The passive residual heat removal system (PRHRS) characteristics of Chinese advance pressurized water reactor (CAPWR) have been experimentally investigated. Based on the investigations, the following conclusions are drawn. A total of 280 sets of tests at steady state have been implemented to investigate the effect of some parameters on the passive residual heat removal system. The result shows that the height of chimney has great influence on natural circulation flow rate. And the effect of the initial pressure and hydraulic resistance is small or negligible. Based on the experiments, semiempirical model for analyzing passive residual heat removal system is established, and the calculation shows good agreement with experimental data. It can be applied to system arrangement design for PRHRS of Chinese advanced PWR. In case of station blackout accident, the transient characteristics of passive residual heat removal system were studied. The natural circulation and injection of feedwater are very useful for removal of decay heat. The increase of feedwater’s initial temperature and resistance of feedwater loop is useful to avoid the water hammer. A code MISAP2.0 has been developed, and the transient tests are used to verify the prediction of MISAP2.0. The calculated parameter variation trend is reasonable. However, the flow fluctuation and water hammer cannot be simulated by MISAP2.0. Now the direct contact condensation is under development for next version of MISAP.
Flow area (m2)
Friction factor
Gravity acceleration (m/s2)
Specific enthalpy (J/kg)
Loss coefficient
Emergency feedwater tank water lever (m)
Pressure (Pa)
Heat flux (W/m2)
Time (s)
Heated perimeter (m)
Wetted perimeter (m)
Mass flow rate (kg/s)
Special coordinate (m)
Void fraction
Density (kg/m3).
The authors declare that there is no conflict of interests regarding the publication of this paper.