In both magnox reactors and AGRs there are sources of water that include release of water already absorbed within the reactor graphite core and, in the case of the AGR, the radioiytic breakdown of methane. The radiolytic/thermal breakdown of oil entering the sys­tem and boiler leaks that allow water into the coolant circuit are causes of an increase in moisture level that are indicative of plant faults.

Gas driers are installed and blowdown is used to control moisture concentrations below that which might cause corrosion of reactor internals, or affect the performance of the failed fuel detection system discussed in Section 1.7 of this chapter.

Measurement of the bulk moisture level is neces­sary to check on the performance of the moisture control systems and to provide data used in calcula­tions of moderator integrity. Typical concentrations are quoted in Table 2.12, i. e., around 10 vpm for magnox reactors. Measurements are required within

±5% accuracy or ±2 vpm, whichever is the greater, and response time is a secondary consideration.

A number of different methods of water in CO2 measurement are available and have been used. No single method is universally applicable and develop­ment and testing continues. The principal online meth­ods are infra-red gas analysers, electrolytic, impedance, piezoelectric and dew-point hygrometers:

Infra-red gas analysers The magnox reactors were initially equipped with these devices, which depend on the principle that water vapour absorbs quite strongly in the infra-red region of the electromagnetic spec­trum near wavelengths of 1.87 and 2.7 дт, and use is made of this for the quantitative measurements of moisture in gases. Conventional infra-red absorption measuring techniques apply, the extinction being pro­portional to the partial pressure of moisture within the sample cell. Cross-sensitivity effects can usually be overcome using interference filters.

Infra-red hygrometers require calibration, but once calibrated are capable of achieving relative accuracies in the order of ±2 vpm in the range of 0-100 vpm. Moisture concentrations up to 4000 vpm can be mea­sured, whilst below 100 vpm sensitivity can be im­proved by operating the cell at pressure.

Principal disadvantages of this type of hygrometer are its relative complexity and need for high sample flow rates (several litres/minute) to achieve a reason­able response time. This is because of the long path — length cells required for moisture measurement.

Coulometric (electrolytic) The moisture in sample gas flowing through an electrolytic cell is absorbed on a film of partially-hydrated phosphoric anhydride in contact with platinum electrodes. A DC voltage greater than the decomposition potential of water is applied to the electrodes and the absorbed water is quantitatively electrolysed. The cell current produced on electrolysis is directly proportional to the rate of water absorption (Faraday’s law), and for a given flow rate the concentration of moisture in the gas may be derived on an absolute basis.

Coulometric analysers are continuous and, for most general purposes, may be regarded as providing an absolute measure of moisture concentration provided certain precautions are taken, particularly the careful control of the sample flow rate. With such precau­tions, commercially available instrumentation can be capable of accuracies better than ±5% reading or ±2 vpm, whichever is the greater.

When used for the measurement of very low mois­ture levels, for example, in high temperature helium — cooled reactors, the current is switched off for a defined period to allow moisture time to collect, thus making the device more sensitive.

Coulometric analysers have found extensive use in the CEGB, particularly for reactor coolant gas mo­nitoring. Main disadvantages, however, include the following:

• They are limited in use to moisture concentrations below 3000 vpm.

• Cells have a finite life, dependent upon operating conditions, before regeneration becomes necessary. Ceil failure is not always immediately apparent unless regular checks on performance are carried out.

• At high hydrogen (and oxygen) concentrations, platinum electrode cells suffer from recombination effects. The use of gold or rhodium electrodes has proved to be only of limited success in overcoming this.

Impedance type In this device the AC impedance of the aluminium-oxide insultant between two elec­trodes is measured, this impedance being a function of water in CO2 concentration. Limitations of this system include poor time response at low levels of water, temperature-dependent drift and memory ef­fects, but a new silicon-oxide type is claimed to have a much superior performance. These types are not absolute and require calibration.

Piezolectric hygrometers These devices rely upon the change in weight and consequent change in resonant frequency of a crystal coated with a hygroscopic film. These changes are detected on alternate exposure to a dry gas and the sample gas, and the changes in resonant frequency are processed to provide a direct readout of moisture concentration. The device is not absolute, prior calibration is necessary and periodic single point checks are required during service.

The principal advantages of this type of hygrometer are its wide range of operation and fast speed of response; in practice, the latter is limited by the flow switching cycle, which is usually 30 s. This type has potential as a differential analyser suitable for boiler leak detection or location. Early indications are that differentials of better than 5 vpm against a back­ground of 300 vpm may be determinable. The per­formance improves still further at lower background moisture concentration.

Mirror type dew-point hygrometers The dew or frost point of a gas may be determined directly by ob­serving the temperature at which formation of a dew occurs on a mirror surface cooled by a semiconduc­tor device. The dew is detected optically and control is effected by an automatic instrumentation system. High accuracy is attainable with commercially avail­able equipment, and this has led to its adoption as transfer or intermediate standards because the mea­surement is absolute.

Ab0Ve -40°C, accuracies attainable can be better than 0.2°С, but at lower dew points errors increase largely as a result of the difficulty of achieving ade­quate" dew formation.

The main disadvantages with respect to this type of h>urometer are its relatively high cost and the fact, tia[ h is vulnerable to interference from condensables other than moisture in the gas, for example, relatively minor concentrations of oil in the gas being sampled can give rise to problems.