Adjustment mode and water pump selection of main water supply system of pressurized water reactor nuclear power plant

Abstract: By analyzing the thermal parameter control requirements of the secondary loop of a pressurized water reactor nuclear power plant and the operating characteristics of electric fixed-speed feedwater pumps, electric speed-controlled feedwater pumps and steam-driven speed-controlled feedwater pumps, from the perspective of economical construction, maintenance and operation, This paper discusses the main water supply system adjustment method and feed water pump selection scheme of a pressurized water reactor nuclear power plant, and puts forward suggestions on the priority of the main feed water pump configuration scheme for a pressurized water reactor nuclear power plant.
Keywords: pressurized water reactor, nuclear power plant, feed water pump, configuration plan
There are 6 nuclear power plants in operation or under construction in mainland my country, with 11 units and a total installed capacity of 8,700 MW. In the five pressurized water reactor nuclear power plants, the main feed water pump types and configurations are different. Daya Bay and Ling’ao nuclear power plants use two 50% (referring to rated water supply flow, the same below) steam-driven pumps and one 50% backup electric speed-controlled pump. Qinshan Phase I uses three 50% electric fixed-speed pumps, and Qinshan Phase II uses There are three 50% electric speed-regulating pumps and Tianwan Nuclear Power Station uses five 25% electric fixed-speed pumps. The selection and configuration methods of main feed water pumps in foreign pressurized water reactor nuclear power plants are also different. Each country has its own style and habits, but the trend is increasingly leaning towards electric feed water pumps. Although main feed water pump systems of different types and configurations can meet the safety and functional requirements of nuclear power plants, as the most important auxiliary system of the conventional island of a nuclear power plant, the economics of investment, operation and maintenance are very different. While China’s nuclear power development is facing unprecedented opportunities (the installed capacity will reach 36 to 40 GW in 2020), it is also facing the need to realize national requirements of independent design, independent manufacturing, independent construction, and independent operation, as well as reducing project costs and on-grid electricity prices, Participating in market competition is a huge challenge. Therefore, analyzing the operating characteristics of different types of feed water pumps according to the operation requirements of the main water supply system of a pressurized water reactor nuclear power plant, and determining the best type and configuration scheme of the main feed water pumps of a pressurized water reactor nuclear power plant, is important for realizing the independent design of my country’s pressurized water reactor nuclear power plants and reducing the cost of nuclear power plants. very necessary.
1. Main water supply system adjustment method
The main function of the main water supply system of a pressurized water reactor nuclear power plant is to deliver feed water with qualified temperature, pressure and water quality to the steam generator, and use the water supply system adjustment function to maintain the water level of the steam generator within a given range. It is to ensure the safe operation of the nuclear island. An important thermal system for the quality of soda and soda. Like conventional power plants, there are two types of main water supply regulation systems for pressurized water reactor nuclear power plants, namely, fixed-speed feed water pump water regulation systems and variable-speed feed water pump water regulation systems. %) adopts the principle of three-impulse adjustment control. At low loads, due to low steam parameters and small load changes, the false water level phenomenon in the steam generator is not too serious, the requirements for maintaining a given water level are not too high, and the steam flow and feed water flow are small and difficult to accurately measure, so the low The water supply under load adopts single impulse bypass regulation control method.
1.1. Fixed speed water supply pump water supply adjustment system
The fixed-speed water supply pump water supply regulation system is the simplest water supply regulation control method. The essence is that the characteristic curve of the water supply pump remains unchanged, and the flow resistance loss of the water supply pipeline is changed by adjusting the opening of the water supply valve, that is, changing the characteristic curve of the water supply pipeline changes the working point of the water supply pump. As shown in Figure 1, when the feed water regulating valve is closed, the pipeline resistance characteristic curve changes from R1 to R2, and the feed water pump operating point changes from 1 to 2, so that the feed water flow rate decreases from Q1 to Q2, thereby adjusting the feed water flow rate and steam generation. The purpose of the instrument water level.

Figure 1 Characteristic curve of fixed speed feed water pump
1.2. Variable speed water supply pump water supply adjustment system
The variable speed feed water pump water supply regulating system changes the feed water pump characteristic curve by changing the feed water pump speed when the resistance characteristic curve of the water supply pipeline is given (or basically constant), so as to achieve the purpose of adjusting the feed water flow and controlling the water level of the steam generator. As shown in Figure 2, the feed water pump speed adjustment mechanism adjusts the feed water pump group speed from n1 to n2, and the feed water pump characteristic curve will change from Q–H1 to Q–H2. Under the given water supply pipeline resistance characteristic curve (here constant value),

Figure 2 Variable speed feed water pump characteristic curve
The working point transitions from 1 to 2, which reduces the feed water flow from Q1 to Q2, thereby realizing the adjustment of the feed water flow and the water level of the steam generator.
There are two main types of prime movers for variable speed feed water pumps: small steam turbines and electric motors.
The steam-driven feedwater pump is driven by a small steam turbine. The small steam turbine receives the flow or speed demand signal of the water supply regulation system, and adjusts the speed of the feedwater pump through the small steam turbine inlet steam flow adjustment mechanism. Under normal operating conditions, the incoming steam of the small steam turbine comes from the extraction steam of the main steam turbine, and under low load conditions comes from the fresh steam of the reactor. When the load of the main turbine increases, the extraction pressure of the small steam turbine also increases accordingly. When the valve opening changes little, the output of the steam-driven pump will automatically increase. Therefore, the steam-driven feed water pump has a certain self-balancing ability in the feed water flow control.
The electric speed-regulated feed water pump adjusts the speed of the feed water pump through a hydraulic coupling installed between the motor and the feed water pump. The hydraulic coupler receives the flow or speed demand signal of the water supply regulating system, and adjusts the feed water pump speed by changing the oil filling amount of the hydraulic coupler through the scoop tube control mechanism. Since the hydraulic coupling can only decelerate, a gear increaser is needed before the coupling to increase the speed to the upper limit in advance.
Since the 1980s, advanced industrial countries, especially the United States, have gradually used variable frequency speed-adjustable motors for variable-speed water supply pumps in large power stations (feed water pump power ranges from 4 to 20 MW), but our country is still in its infancy in this regard.
1.3. Pressure water reactor nuclear power plant steam generator pressure control requirements
The heat transfer temperature difference of the heating surface of conventional power station boilers is as high as hundreds of degrees. The boiler design is not subject to too strict volume restrictions, and there are no special requirements for nuclear safety. The thermal design has a large degree of freedom, so it can be maintained at a constant pressure (the main steam pressure is basically inconvenient to maintain) The operation mode (Fig. 3) design can also be designed according to the sliding pressure (main steam pressure increases with the increase of load) operation mode (Fig. 4).

Figure 3 Conventional boiler constant pressure operation curve

Figure 4 Conventional boiler sliding pressure operating curve
This is not the case in a pressurized water reactor nuclear power plant. The temperature of the pressurized water in the primary circuit (can be regarded as the furnace temperature of a conventional boiler) is low, the steam in the secondary circuit is always in a non-superheated state, the heat transfer temperature difference between the primary and secondary circuits is small, and the logarithmic average temperature difference is about It is only over 50℃ and is arranged in a cement containment with limited space. To meet the strict requirements of nuclear safety, thermal design freedom is very limited. In the thermal shutdown state, the reactor output power is close to zero, there is basically no temperature difference between the primary and secondary circuits of the reactor, and there is no temperature difference between the reactor core inlet and outlet, that is, the average temperature of the primary circuit and the steam temperature of the secondary circuit are basically the same. When the load of the secondary circuit increases, in order to ensure normal heat conduction on the heat exchange surface of the steam generator, the average temperature of the primary circuit must increase or the steam pressure of the secondary circuit must decrease. If the steam pressure (temperature) of the secondary circuit remains unchanged (line B-E in Figure 5), the average temperature of the primary circuit must remain large enough to ensure an appropriate logarithmic average temperature difference between the primary and secondary circuits. At this time, the outlet pressure of the feed water pump is as shown in the figure Lines A-F of 5. ​​

Figure 5 Relationship between steam generator outlet steam pressure and load
However, an excessive increase in the average temperature of the primary circuit will bring a series of problems to the safety design of the entire nuclear island. First of all, after the average temperature of the primary circuit increases, the volume of the voltage regulator must be increased. The increase in the volume of the voltage regulator will directly lead to an increase in the water content of the primary circuit, that is, an increase in the total latent heat of vaporization of the coolant in the next circuit due to a large breach in the primary circuit. In order to ensure a high degree of safety of a pressurized water reactor nuclear power plant in the event of a large primary circuit breach accident, the containment volume must be increased accordingly, which is technically and economically unfeasible.
In order to avoid the above problems, the average temperature change range of the primary circuit is currently limited in mature pressurized water reactor designs (the average temperature change range of the M310 reactor is not greater than 20°C). In this way, under the premise of ensuring nuclear safety requirements, in order to maintain normal heat power transfer between the primary and secondary circuits, the steam pressure or temperature of the secondary circuit must decrease as the load increases as required (line B-C or line B-G-C in Figure 5). At this time, the outlet pressure of the feed water pump remains basically constant (line A-D in Figure 5). 2. Economic analysis of various water supply pump configuration methods
2.1. Electric speed-controlled water supply pump and pneumatic speed-controlled water supply pump
When the capacities are the same, the main differences between electric speed-regulated feed water pumps and pneumatic speed-regulated feed water pumps are different forms of prime movers, different energy input methods, and different speed regulation methods. Assuming that both feed water pumps can meet the water supply regulation requirements in terms of speed regulation range, the economic difference between the two speed regulation methods of feed water pumps is mainly reflected in investment, operation and maintenance.
2.1.1. Operational economic analysis
​ It is assumed that the starting point of energy transmission is the extraction point of the main turbine that supplies steam to the small turbine (see Figure 6). Then the energy transmission and conversion links of the steam-driven speed-regulated feedwater pump include the extraction pipe from the steam extraction point to the steam inlet of the small steam turbine, the heat-mechanical conversion process of the small steam turbine flow part and the pre-pump deceleration mechanism. The energy transmission and conversion links of the electric speed-regulated feedwater pump mainly include the heat-mechanical conversion process of the flow part after the steam extraction point of the main engine, the electromechanical conversion process of the generator, the factory transformer, the switch cable, the electromechanical conversion process of the motor, and the hydraulic coupling. converter and its speed-increasing mechanism or motor frequency conversion system. Since the losses of small steam turbine steam supply pipelines and switch cables are relatively small, they are not included in the analysis and comparison factors here.
When the total power N of the turbine-generator unit is the same and the use of a steam-driven speed-controlled feed water pump is compared with the use of an electric speed-controlled feed water pump, the unit power increase ΔNn is a positive value, indicating that the use of a steam-driven speed-controlled feed water pump is economical in operation. , on the contrary, it shows that the use of electric speed-regulated feed water pump is economical in operation.