What are reactor coolant pumps?
Reactor coolant pumps are installed within nuclear power plants. The purpose of a reactor coolant pump (RCP) is to provide forced primary coolant flow in order to remove the heat (thermal energy) generated by the reactor core during the fission process. Even without a pump, there would be natural circulation flow through the reactor due to the variation in temperature, and consequently density, of the primary coolant. However, this flow is not sufficient to remove the heat being generated when the reactor is at power. Natural circulation flow is sufficient for heat removal when the plant is shutdown, or, when only decay heat is present.
RCPs are driven by large air-cooled electric motors designed for continuous operation. The motor is attached to a mixed-flow centrifugal impeller via a series of shafts installed in series (discussed later in this article).
Good to know - a 'reactor coolant pump' is also sometimes called a 'reactor cooling pump' although this is the incorrect definition.
Forced and Natural Circulation Example (from a watertube boiler)
Reactor coolant (primary coolant) enters the suction side of the pump from the outlet of the steam generator. The water is increased in velocity by the pump impeller. This increase in velocity is then converted to pressure in the discharge volute. A typical pressurised water reactor (PWR) power plant will operate with the primary coolant loop at approx. 155 bar (2248 psi).
After the coolant leaves the discharge side of the pump, it will enter the inlet of the reactor vessel via the 'cold leg' pipe. The primary coolant will flow through the reactor fuel assemblies to collect heat; it is then sent back to the steam generators via the 'hot leg' pipe. A typical pressurised water reactor (PWR) power plant will operate with a cold leg temperature of approx. 290°C ( 554°F) and a hot leg temperature of approx. 320°C (608°F).
Good to know - a 'cold leg' is a pipe that connects an RCP to a reactor. A 'hot leg' connects a reactor to a steam generator. A 'transfer leg', or, 'transfer pipe', connects a steam generator to an RCP. Cold and hot legs are named due to the primary coolant temperature within each leg.
PWR Power Plant Parts
Reactor Coolant Pump Design
RCP motors are of the squirrel cage type (induction motor) and usually operate at approx. 6kV or 13kV, depending upon the supply voltage. A RCP's rotational speed (rpm) is dictated by the motor's supply frequency, which is 50Hz or 60Hz depending upon geographic location. A typical RCP may rotate at approx. 1,200 - 1,500 rpm and weigh over 100 tonnes.
A typical pressurised water reactor RCP will operate at a pressure of approx. 155 bar (2248 psi) and at a temperature range of between 290°C-320°C (554°F-608°F), but these factors are ultimately dictated by the reactor coolant system.
The horsepower rating of an RCP motor will typically be from 6,000 to 10,000 bhp (4.5 MW to 7.5 MW). The large amount of power is needed to provide the necessary flow of coolant for heat removal; RCPs can achieve flow rates of 25,000 m3/hr (11,000 gallons per minute).
What are the main parts of a reactor coolant pump?
saVRee's interactive 3D model represents a mixed flow, single stage, single suction, centrifugal pump. A RCP's main parts are indicated below.
Reactor Coolant Pump Parts
Suction Nozzle
Primary coolant is drawn into the pump through this nozzle; the coolant is drawn into the pump from the transfer leg.
Discharge Nozzle
Primary coolant is discharged from the pump through this nozzle; the coolant is discharged into the cold leg.
Impeller
The impeller is a mixed-flow centrifugal impeller. The term ‘mixed-flow’ refers to the axial flow into the base of the impeller and radial flow outwards from the impeller. This type of impeller is used for medium to high flow rates, and medium to high pressures; the design is similar to that of a Francis turbine.
Shaft
The shaft transfers the rotary motion created by the electric motor to the impeller; it is held in alignment using bearings. Due to the shaft’s length, it is split into several parts, the pump half coupling, motor half coupling, and spacer coupling; these parts together are colloquially referred to as ‘the pump shaft’, although this is technically incorrect as the shaft is made-up of more than a single piece.
Guide Bearing
Radial loads exerted by the pump during operation are transferred to guide bearings. Self-aligning hydrostatic bearings are used to keep the shaft vertically aligned. Discharge pressure from the centrifugal pump impeller acts upon a bearing journal balance plate, which ensures the bearing -and shaft- alignment is correct whenever the pump is in operation.
Sealing Assembly
The sealing assembly of the pump is usually the area of the pump requiring the most maintenance interventions because it forms a dynamic pressure boundary. It is essential that a reliable seal between the hydraulic side of the pump and the main drive shaft is maintained. If the seal integrity is compromised, a leakage of primary coolant will occur.
Shaft sealing is accomplished in the upper part of the pump housing using a removable seal cartridge which contains three mechanical seals installed in series. Mechanical seal springs press the titanium carbide and stationary graphite faces together and ensure the faces remain aligned. The torturous flow path from the lower seal to the upper seal ensures little leakage through the shaft sealing assembly. To ensure parts of the mechanical seal do not fail due to high temperatures, seal cooling is provided. Underneath the sealing assembly, an auxiliary impeller is keyed to the main impeller. Seal water flow through the seal cooler is provided by the auxiliary impeller.
Good to know - shaft seals have several different designs and the one described above is only one such type.
Diffuser
After liquid is discharged from the impeller, it flows through a diffuser. The shape of the diffuser causes the liquid’s velocity to decrease and its pressure to increase (Bernoulli’s principle); a volute casing performs the same function.
Electric Motor
An electric motor is used to rotate the impeller. RCP motors are alternating current squirrel cage induction type motors with a typical operating voltage of between 6kV to 13.8 kV (design dependent).
Space Coupling
The space coupling is installed between the pump half coupling and the motor half coupling; it can be removed to give personnel easier access to the shaft seal and thermal barrier.
Component Cooling Water (CCW)
The component cooling water (CCW) circuit is fed to an integral heat exchanger and the pump thermal barrier. CCW removes heat from the pump internals to prevent overheating. Both the integral heat exchanger (cooled primary loop water / CCW), and the CCW cooler (CCW / external cooling water circuit), use shell and tube heat exchangers.
Metallic O-Rings
O-rings and mechanical seals are installed at various places within the pump to seal/close unintended flow paths. Two O-rings are installed between the pump casing and casing cover. If the inner O-ring fails, hot cooling water will flow via a drain path to a containment sump. Temperature sensors installed in the drain path alert personnel of any coolant leakage flow.
Flywheel
A flywheel is a heavy, disc shaped, piece of metal, that smooths vibration. It does this by storing rotational energy and then using this energy to resist changes to the machine's rotational speed. The amount of energy stored in the flywheel is the square root of its rotational speed. A RCP flywheel also ensures that the pump continues to rotate even if the reactor is tripped, thus providing adequate reactor cooling even when no electrical power is available.
External Heat Exchanger
An external shell and tube heat exchanger transfers heat from the component cooling water (CCW) circuit to a heat sink e.g. cooling tower.
Motor Half Coupling
The bottom of the motor half coupling connects to the space coupling. Rotational motion from the motor half coupling is transferred to the space coupling.
Pump Half Coupling
The top of the pump half coupling connects to the spacer coupling. Rotational motion from the spacer coupling is transferred to the pump half coupling.
Upper Radial Bearing and Thrust Bearing
The upper radial bearing and thrust bearing are located near the top of the electric motor. Radial bearings handle radial loads (loads acting perpendicular to the pump shaft), whilst thrust bearings handle axial loads (loads acting parallel to the pump shaft).
Air Cooler
Air is cooled in an air cooler prior to being delivered to the space between the pump cover and motor housing; cooling air removes waste heat.
Air Duct
Cooling air is transferred via air ducts.
Lower Radial Bearing
The lower radial bearing caters for loads perpendicular to the motor shaft.
Auxiliary Impeller
The auxiliary impeller serves two main purposes: It provides pressure to the self-aligning journal bearing and provides a small amount of flow to the pump seals; flow to the pump seals is drawn through the thermal barrier.
Standpipe
To provide a means to determine seal leakage, a seal collection system is utilised. Leakage past the third mechanical seal is collected in a standpipe that surrounds the pump shaft. Leakage into the standpipe is constantly monitored.
Restriction Bushing
The restriction bushing provides a restriction between the fluid being pumped by the impeller, and the mechanical seal area, so that the flow is reduced or controlled.
Additional Resources
https://www.nuclear-power.net/reactor-coolant-pump
https://www.ksb.com/centrifugal-pump-lexicon/reactor-pump/191846