High purity water is used in nuclear systems as a primary reactor coolant (pressurised water reactor), for steam raising in a secondary circuit (advanced gas cooled reactor), or fulfilling both roles (boiling water reactor). This section deals with the applications where the water passes through the core but does not con­stitute the steam raising circuit. It is specific therefore to the PWR, where the reactor coolant is high tem­perature water held in a single phase at a pressure above its saturation vapour pressure, for the purposes of neutron moderation and heat transfer.

Typical reactor coolant conditions for steady state full power operation of a 1200 MW(e) PWR are shown in Table 1.17, and the main thermodynamic and physical properties of water at 155 x 105 N/m2 (155 bar) taken from Grigull et at (1984) [25] over the tem­perature range of interest are summarised in Table 1.18. On exit from the reactor there is, in principle, an approximately 20°C margin to boiling and the system therefore is single phase, although in some de­signs it is expected that a degree of boiling will occur at the top of the fuel elements. Maintenance of sys­tem pressure and adjustment of the coolant inven­tory are achieved by the pressuriser (Section 9.2.1 of this chapter) which contains coolant under SVP con­ditions of 155 x 105 N/m2 and 344.8°C in an atmos­phere of hydrogen.

It should be noted that the 240 m3 coolant in a typical PWR undergoes a significant specific volume change over the temperature range of 25°C at atmos­pheric pressure (1 x 105 N/m2) to 30O°C at operating pressure (155 x 105 N/m2), as can be seen from the specific volume data in Table 1.18.