The intermediate heat exchangers, in which heat is transferred from primary to secondary sodium coolant, are normally of a shell-and-tube design. Differential expansion of tubes and shell can be accommod­ated by means of expansion bellows or by bends in the tubes. To eliminate any possibility of radioactive primary coolant leaking into the secondary circuit the pressure of the secondary coolant in the heat exchanger has to be greater than that of the primary. The tubes are usually arranged in an annular bundle, with the secondary sodium flowing down through a central duct and then upwards through the tubes.

Because the coolants transfer heat so readily it is possible to keep the temperature difference between primary and secondary coolant small and yet keep the intermediate heat exchangers reasonably com­pact. For example with tubes 20 mm in diameter and coolant velocities of about 5 m s-1 a Nusselt number of about 10 on both shell and tube sides is possible (see section 3.2.4), giving surface heat transfer coeffi­cients of about 3 x 104 Wm-2 K-1. If the wall thickness is 1 mm this gives an overall heat transfer coefficient U of about 104 Wm-2 K-1. The heat transfer rate Q, heat transfer area A, and the logarithmic mean temperature difference ATm are related by

Q = UAATm, (4.1)

so that if Q = 3.6 GW and ATm = 30 K an area of 12000 m2 is needed for heat transfer. If the tubes are 8 m long and 20 mm in diameter some 24000 of them are needed, which could for example be arranged in six separate units each containing about 4000 tubes. The diameter of each of these tube bundles, allowing for the central secondary sodium inlet duct, would be about 2 m. The design of intermediate heat exchangers is discussed by Tang, Coffield, and Markley (1978), p. 319.

In a pool reactor the primary coolant is driven through the interme­diate heat exchangers by the pressure difference due to the difference in levels between the hot coolant within the inner vessel, and the cold coolant outside it.

In the event of an accident it might become impossible to reject heat from the secondary coolant or the steam plant, and an alternative means of removing the heat due to decay of fission products in the fuel would be needed (see section 5.2.4). For this reason an auxiliary secondary coolant system is provided. In a pool reactor this may take the form of separate auxiliary heat exchangers in the vessel, in which heat can be transferred to an emergency or “decay heat removal” cooling system. In a loop reactor there may be a separate auxiliary cooling loop in the primary circuit, or arrangements for emergency cooling of the secondary circuits.