Some early fast reactors used electromagnetic pumps to circulate the coolant, which have the advantage that no moving part penetrates the sodium containment. The sodium is pumped either by passing an electric current through it in the presence of a transverse magnetic field (a conduction pump) or by subjecting it to a moving magnetic field (an induction pump). However it proved difficult to scale elec­tromagnetic pumps up to the size needed for large reactors and now mechanical pumps are used universally. The problem of penetrating the sodium containment can be met by means of electric motors with totally enclosed, “canned”, rotors, but the usual method is to allow the shaft to pass through the containment above the sodium level. The penetration is thus exposed to the argon cover gas containing sodium vapour but not to liquid sodium, and oil seals have been found satisfactory in most cases.

The pumps are thus driven by motors situated on the roof of the reactor vessel (in the case of the primary pumps of a pool reactor) or the pump vessel (in the cases of a loop reactor and the secondary circuits of either style), via vertical shafts with thrust bearings at the top and sodium-lubricated sleeve bearings at the bottom. The long shafts of the primary pumps of a pool reactor are usually tubular with a large diameter to avoid whirling. The variable-speed motors are fitted with auxiliary or “pony” motors that are capable of turning the pump fast enough to maintain adequate flow to keep the fuel cool when the reactor is shut down. In the case of power failure these can be energised from a standby source such as a diesel generator to guarantee emergency cooling.

The pumps have to provide a large volume rate of flow at a relat­ively low pressure rise. A typical 3600 MW(heat) reactor would have three primary pumps each delivering about 8 m3 s-1 at 500 kPa. Single­stage centrifugal impellers of conventional design are normally used. In most cases the primary pumps deliver the coolant vertically down­wards and the pump volutes are designed so that the resulting axial thrust, which can be as much as 10 tonnes, is borne by the pump casing rather than the shaft.

The main difficulties in design are to cope with sudden changes in temperature and to prevent cavitation, and in these respects loop and pool reactors pose different demands. The coolant temperature has very little effect on cavitation, because even at 600 °C the saturation pressure is only 7 kPa. The important factor is the pressure at the pump inlet (the “net positive suction head”). The pressure of the cover gas above the sodium in the reactor or pump vessels is limited to some 1-200 kPa gauge to minimise leakage of radioactive material and sodium vapour. The pump inlet pressure is this gas pressure plus the hydrostatic head of the sodium above the pump. The risk of cavitation can be minimised by increasing the depth of immersion, decreasing the rotational speed (which implies increasing the rotor diameter) or shrouding the inlet to make the flow uniform. For the primary pumps of a pool reactor all these options imply increases in the dimensions of the primary vessel.

In a loop reactor, however, the suction pressure depends on whether the pump is located before or after the heat exchanger (i. e. whether it is in the “hot leg” or the “cold leg”). If it is before the heat exchanger there is a loss of pressure due to the flow in the pipe from the reactor vessel to the pump. If it is after the heat exchanger there is an additional loss of pressure due to the flow through the heat exchanger and more pipe. It is not easy to compensate for these pressure drops by positioning the pump at a lower level to increase the hydrostatic head, and anyway doing so would require still longer pipes. Thus the conclusion is usually reached that cavitation is avoided more easily if the pump is placed in the hot leg, before the heat exchanger. The relative advantages of hot leg and cold leg pumps are discussed by Campbell (1973).

Primary or secondary pumps may be exposed to sudden temperat­ure changes in the event of emergency shutdowns (“trips”) of either the reactor or the steam plant. This is not usually a problem in the case of the primary pumps of a pool reactor because they draw from the large mass (of the order of 2000 tonnes or more) of cold coolant filling the vessel. In a loop reactor, however, the situation is quite dif­ferent. If the reactor is tripped a hot leg pump is subject to a rapid fall in temperature, and if a secondary heat exchanger is shut down because of a steam plant trip a cold leg pump is subject to a rapid rise. These temperature transients can be designed for, but they constitute a disadvantage of the loop layout.