Bulk coolant gas temperatures are measured in each of the reactor gas circuits, using thermocouples sit­uated in the ducting reactor-side of the main isolating valves. These temperatures are used by the operator to help calculate the maximum operating tempera­tures as previously described. The temperatures are displayed on the low speed scanner and individual circuit temperatures can be monitored on recorders situated in the central control room (see Table 3.4). In addition the temperatures can be obtained from a 16-point recorder in the reactor annexe. This gives sufficient information to the operator so that loss of individual temperature measurement would not cause any problem.

In addition to channel gas outlet (CGO) tempera­tures, measurements are also made at other important points in the individual gas circuits. All circuit inlet and outlet temperatures are measured and displa>ed on a 16-point recorder on the reactor control desk; each circuit also can be individually selected onto an­other recorder (on the reactor control desk) to give a continuous reading during single circuit outages or reinstatement (see Table 3.4),

Reactor core graphite temperatures

The reactor core consists of graphite blocks with chan­nels for fuel elements and control rods. Regular mo­nitoring of graphite temperatures is important for the reasons given in Chapters! and 2. The maximum tem­perature allowed for each section of the graphite core is defined by the station’s ‘Operating Rules’ which are a requirement of the site licence. A large number of thermocouples are connected to the low speed tempera­ture scanner (LSS) and a printout of their readings can be obtained on demand. A selection of graphite temperatures are indicated on recorders in the annexe of the CCR and trends can easily be detected. As there are 243 graphite thermocouples per reactor, loss of any individual thermocouple or recorder would not cause significant problems for the operator. Loss of all these temperatures is not considered possible as this would require a complete failure of the instrument and safety supplies system which is a secure supply.