6.6.1 Water purity in feedwater and steam circuits

In order to control corrosion and deposition on the surfaces of boiler and turbine plant, stringent con­trol of water chemistry is essential. This control is achieved by a combination of treatment techniques and monitoring. The latter provides evidence that the control has been maintained, monitors the operation of the chemical control processors and initiates alarms if there is a marked departure from acceptable con­ditions, e. g., after a condenser leak.

The later magnox and AGR stations have once — through boilers which introduce special problems be­cause they cannot be blown down to remove the water impurities in a way similar to that used for drum type boilers.

6.6.2 Other water circuits

Other water circuits that require chemical monitoring include generator stator water cooling, and pressure vessel and reactor ancillary water systems. For the latter two, conductivity, pH and CO: in water are used for the detection and location of leaks.

Important aspects of the measurements are the sampling technique which has to provide a represen­tative sample, and the reduction of pressure and tem­perature to make it suitable for the measuring device.

6.6.3 Steel/graphite measurements

The long-term performance of the core is monitored by measurements on steel/graphite samples and is described in detail in Chapter 1.

There would be no problems with interaction be­tween the graphite core and its steel restraint and support structures if:

• The core restraint is temperature compensated to expand at the rate of graphite.

• The graphite is decoupled from the support structure (diagridj.

The degree to which this is designed into magnox reactors varies. An example of where extra tempera­ture instrumentation is necessary to prevent damage to the graphite from uneven expansion rates is Oldbury, where neither of the above is achieved. In some re­actors, where the temperature compensated core re­straint no longer acts as designed due to mild steel oxidation problems, additional displacement instru­mentation has been installed.

6.6.4 Use of computers

Modern digital computers of the type described in Volume F, Chapter 7 provide the most effective means of collecting, analysing and recording data. They are used extensively to provide the basis for the long term monitoring systems required to operate a nuclear power station.