The biggest pumps in the world?16 September 2021

Among the largest pumps made for industry are the reactor coolant pumps used in modern PWR, pressurised water reactor-type nuclear power plants such as Sizewell B in Suffolk, and Hinkley Point C, currently being built. Each 10m-tall gargantuan weighs about 100t. Here is how it works.

Within the water-filled reactor pressure vessel of a nuclear power plant, uranium fuel pellets are allowed to fission – break down atomically – releasing lots of heat into the water. The water is kept under pressure (in this case, 160 bar abs maximum pressure) to prevent it from boiling. The job of the pump is to transfer that water to the tubesheet in a tube-and-shell heat exchanger located downstream.

Within the shell of those heat exchangers is a lower-pressure second circuit of feedwater, which, as it enters the vessel, comes in contact with the hot walls and flashes to steam. The steam goes on to drive a steam turbine attached to a generator; the water in the primary circuit, now cooled, returns to the reactor. There are four loops in a single EPR reactor such as at Hinkley Point, each with its own pump and steam generator, offering a generation capacity of 4,500MW (thermal); 1,650MW (electrical).

The pumps are essential to plant operation and safety. As Framatome says, “The reliability of the primary coolant pump determines the efficiency and regularity of operations throughout the reactor coolant system.” The pumps also provide a crucial safety function in removing heat from the core. In case of an incident such as loss of electrical power, redundant safety systems, including the use of on-site diesel generators, continue to remove core heat to prevent a meltdown.

Fabricated in northern France by Framatome at its Jeumont plant and tested nearby at Maubeuge, each one consists of 1,300 parts. The build requires 2,500 manufacturing and inspection operations; some components’ tolerances are down to microns. No wonder each takes four years to complete.

In broad outline, the pumps consist of three systems: pump, motor and seals. Working from the bottom up, the entire pump rests on three columns fixed to lugs on the casing that holds the pump impeller. The casing is the only non-replaceable part of the entire pump, as it is welded to the primary coolant loop. Water – hot, radioactive and pressurised – enters from the bottom through the suction nozzle, passes through the impeller (rotating at 1,485rpm) and exits through the diffuser and discharge nozzle to one side, at a rate of from 20-28,500m3/h.

The impeller fixes to the pump shaft by a Hirth-type tooth gearing and studs around the perimeter and in the centre.

Exclusively stainless steel is used for the parts of the pump in contact with primary coolant; the casing is an integral casting from asteno-ferritic stainless steel. Partly because of its size and its complexity, it is the only cast part in the EPR primary coolant circuit; everything else is made from forgings.

Extending upward from the impeller, around the shaft is a system of three identical, hydrodynamic seals that reduce the coolant loop pressure to near ambient (see diagram below). Each stage consists of a mechanical seal, which prevents leaks around the rotating shaft, and a so-called ‘staging coil’ that acts like a bypass, distributing pressure across the three, and which receives a flow of cooling water from another system at a rate of 600 l/h. Two leak-off lines collect that flow, plus the relatively low leakage rates from the mechanical seals, of about two l/h.

Another system to help prevent leaks from the high-pressure primary loop when the reactor is shut down, is a so-called ‘standstill’ seal, which features metal-to-metal contact for greater security. When the pump stops, it is activated by nitrogen injection.

The squirrel-cage induction motor powering the pump has a capacity from 4-8MW, and operates at 10kV. Its insulation is class F thermo-elastic epoxy, and the motor is air-cooled; fans at the end of the rotor draw in air and cool the stator end windings. The heated air is pushed through air/water heat exchangers mounted on opposite sides of the pump frame. The motor is tested to 25% overspeed. There is also a flywheel at the very top of the RCP assembly.

The motor section features a solid shaft. Total rotor inertia, of pump and motor, is 5,210kg∙m2. It turns on standard bearings: a Kingsbury-type double thrust bearing, plus pad-type radial bearings. All of these are oil-lubricated, and that oil runs through water-fed coolers. In addition, a second lubrication system creates an oil film in the thrust bearing surfaces by injecting it at high pressure before and during start-up and shut-down; it also sprays oil into the upper guide bearing.

The main radial bearing is hydrostatic. An auxiliary bearing, which is intended to be a back-up in case the main one is disabled, sits near the thermal barrier that exists to reduce the effect of the temperature of the 351°C coolant. That barrier is also water-cooled.

The RCP is designed to be able to withstand a loss of water injection either at seal number one, or loss of cooling at the thermal barrier heat exchanger, without any damage, whether operating or shut-down. In addition, it is designed to be able to handle loss of both for up to two minutes until one is restablished.

The RCP pump casing and its main components are designed for a 60-year life. Wear parts, including bearings, seals and O-rings, will need regular replacement. Pump internals and seals can be accessed from underneath the motor (so it does not have to be removed) from the spool piece. Oil injection is used to assemble the pump shaft coupling. Studs and nuts are tensioned hydraulically.

BOX: Hinkley Point C update

Unit 1 nuclear island - site of the reactors plus supporting equipment - is shown in summer (image: EDF Energy).

In spring, the second of three 47m-diameter containment rings was lifted into place. The prefabricated structure measured 17m tall and weighed 308t.

The inner containment structure of the nuclear reactor is beginning to take shape following a 722m3 concrete pour, which took 27 hours.

Another recent job was the lifting of two concrete support slabs into reactor one. They measure 24m long and weigh 500t.

Will Dalrymple

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