Main coolant pump, also called reactor pump and forced circulation pump. In nuclear power stations, main coolant pumps are vertically arranged, single-stage centrifugal pumps whose job it is to circulate the coolant (H₂O, D₂O, sodium, helium etc.) needed for conveying the heat of nuclear disintegration. Main coolant pumps vary in design according to the type of reactor (nuclear reactor). In the case of pressurized water reactors – with pressures up to around 160 bar and a temperature of 350 °C – the main coolant pumps are equipped with conventional electric motors; between 2 and 4 such pumps are used, depending on the facility's output. The pumps feature pressure-resistant pot-type or spherical housings. Shafts are sealed either with a combination of hydrostatic split-ring seals and downstream hydrodynamic mechanical seals, or with several successive hydrodynamic mechanical seals. In either case, each hydrodynamic mechanical seal is designed for the full pressure of the primary system and can withstand this pressure reliably for several hours in the event of the upstream seal failing. A special buffer water auxiliary system cools and lubricates the mechanical seals. Antimony- or resin-impregnated carbon against tungsten carbide has proven to be a successful face material combination in many years of service. The choice of face materials for new main coolant pumps and their mechanical seal arrangements is decided by the results of tests carried out on both the newest and proven materials. In the case of boiling water reactors – with pressures up to around 75 bar and a temperature of 300 °C – it is possible for two different concepts of main coolant pump to find application. For reactor pressure vessels with built-in jet pumps, the main coolant pumps welded into the outer pipelines are designed as driving water pumps. The latter convey only that part of the coolant stream that drives the jet pumps circulating the rest of the coolant in the pressure vessel. In the second concept, several (between 8 and 10) main coolant pumps are inserted in the reactor pressure vessel as immersion pumps where they perform their circulation function directly. Immersion pumps are fitted with submersible motor drive as well as with conventional electric motor and shaft seals. This sealing arrangement consists mainly of two hydrodynamic mechanical seals (pressure reduction in 2 stages, with each seal designed for the full operating pressure) and a downstream safety seal, which in the event of damage first ensures that the pump shuts down properly and then takes over the sealing function with the pump at standstill. Nowadays the most used face materials are antimony-impregnated carbon against reaction-sintered silicon carbide. In the case of fast breeder reactors (fast breeders), provision is made between the actual pump chamber and the first of two mechanical seals for a large space filled with protective gas on account of the risk of explosion (should any sodium come into contact with water).

Given the low system pressure of 12 bar maximum (approximately 550 °C), there is no need for pressure reduction over several stages. With heavy water reactors it is recommended to design the safety seal downstream from the single-acting main mechanical seal as a double-acting mechanical seal in order to stop toxic tritium entering the reactor chamber. As such, the mechanical seal does not seal off sodium but protective gas (inert gases).

Seal arrangement for a pressurized water reaction with 2-stage pressure reduction and positive-response, open safety seal

Seal combination for stationary circulating pumps in a boiling water reactor with 3-stage pressure reduction. At standtstill, the seal on the atmosphere side takes over the sealing function.