Scope

Specific types of reactor, e. g., magnox and AGR, and specific examples of the two types are provided with a wide variety of C and l systems to suit their par­ticular needs.

Magnox Reactors

Flux scanning Equipment, described in Chapter 2, is provided to enable the axial neutron flux distri­bution to be determined at a number of points across the reactor core. The frequency of flux scans is stated in the Station Operating Instructions and is typically six months to two years. For example, at Hinkley Point A power station with magnox reactors, flux scanning is normally carried out once per year follow­ing the return to power of each of the two reactors from its biennial overhaul: this is the only time when it is possible to cover the complete operational range of control and rod positions.

Delayed neutron detection The status of delayed neu­tron monitoring equipment on CEGB magnox stations is:

(a) Installed, active, and normally capable of tripping the reactors at:

Berkeley Bradwell Dungeness A Hinkley Point A Sizewell A

Due to operational problems and electrical inter­ference effects the equipment may be vetoed under conditions which may include refuelling, electric welding in certain areas, local lightning storms and equipment faults.

(b) Installed but vetoed, hence never capable of trip­ping reactors at Trawsfynydd.

(c) Not installed in concrete PV reactors, i. e., Oldbury and Wylfa.

(d) Equipment installed as (a) and (b) above is vir­tually identical in all cases.

AGRs

Liner leakage and penetration sampling Liner leakage detection and penetration sampling equipment is pro­vided to confirm the absence of significant leakage of primary coolant.

In the case of Heysham 2, the liner leakage holes are normally capped at their upper end and open to the safety shutdown room at their lower end. When required the portable liner leakage sampling trolley is connected to each hole in turn. The upper end of a hole will be uncapped manually and a pump will draw air down the hole and through the analyser. The measured CO; concentration on a 0 to lOO^o scale may be multiplied by the measured flow — to yield the leakage rate.

The trolley is connected via flexible couplings and discharges to the H and V extraction ducts.

The sampling holes can be fitted with additional pipework to provide suitable disposal of leaking gas if a significant leak were to be detected. The trolley is equipped with flow and inlet pressure indication. The portable system is designated Safety Class 3 and QA level 4.

The CO2 in air monitoring system allows detec­tion of leakage which may occur prior to more severe failure of the monitored parts which are pressurised with CO;, thus giving the operators a chance to reduce the economic and radiological consequences. Alarms are raised in the CCR only, unless a sample also serves a personnel protection role.

The installed CO;-in-air monitoring 12-point ana­lyser samples the safety shutdown room H and V extraction and would alarm a major leakage.

Seismic instrumentation Instruments are provided to fulfil the following functions:

• To raise an alarm in the CCR and E1C to warn the operator that a seismic event in excess of the operator shutdown earthquake (OSE) {0.05 g) has occurred.

• To allow the operating staff to assess the situation following a seismic event.

• To provide information for any subsequent analysis of a seismic event.

This section does not apply to the seismic switches used to inhibit secondary shutdown action which are part of the safety system. All seismic instrumentation discussed here performs a monitoring function, alert­ing the operator when some investigation is required. It does not initiate automatic actions.

The seismic instrumentation has no direct safety role. Failure of the instrumentation does not affect reactor safety, generation or the ability to shut down. It does have a role in assessing the integrity of plant following an earthquake and is thus assigned Safety Class 4 and QA level 4.

The equipment is designated Seismic Category A. This is both to give assurance that the information on the earthquake is available and to ensure that those items located in the safety room do not collapse onto safety equipment.