The process instrumentation required for reactor con­trol and protection includes those devices and their interconnections which measure temperature, pressure, fluid flow and fluid level in tanks vessels or pipework.

Tappings are provide for each control and protec­tion measurement on the NSSS. One instrument tap­ping of a narrow range level measurement may be shared by a pressure measurement or by a wide range level measurement, provided that the instruments con­cerned are associated with the same electrical separa­tion group.

The pressuriser water level instrumentation measures water level using differential pressure. The pressuriser has an upper and lower tapping, the upper tapping maintains a reference leg full of water by condensation of steam at the top. The transducer is connected with one side to the lower tapping of the pressuriser and the other side to the bottom of the reference leg. ^ater level is determined by the difference in pressure (between the pressuriser). The reference leg is lagged to prevent errors due to rapid changes of temperature.

There are four pairs of tappings on the pressuriser, giving four separate channels of level measurement for the PPS and station automatic control system.

The lower tapping in each measurement of pressuriser level is also used for a pressuriser pressure input to the PPS/station automatic control system.

Each steam generator has four PPS narrow range (NR) water level measurements using the differential pressure method described above for the pressuriser, with four separate pairs of tappings, one pair per separation group.

For use by the secondary protection system (SPS) each steam generator has four separate narrow range water level measurements from four pairs of separate tappings at the same levels as those of the PPS, one per separation group. Each SPS measurement channel employs a sealed reference leg with hydraulic isola­tors between the reference leg and the steam generator. The transducer is connected with one side to the lower tapping of the steam generator and the other side to the sealed reference leg. The measurement principle is identical to that of the pressuriser but without the use of a condensation pot.

For the PPS, each steam generator has one mea­surement of wide range (WR) level. The level mea­surements are derived by differential pressure in the same way as the method used for the pressuriser. The wide range measurement uses the same reference leg as one narrow range PPS measurement on each steam generator and therefore has a common reference leg tapping to the transducer, but a separate lower tap­ping to the steam generator is provided.

There are thus a total of 17 tappings to each steam generator as follows:

4 PPS reference leg connections 4 SPS reference leg connections 4 PPS narrow range connections 4 SPS narrow range connections 1 PPS wide range connection

For the RCS pressure, two separate tappings (one for the PPS, the other for the SPS) are provided at each of four branch line locations:

Loop 1 RHR suction line Loop 2 safety injection line Loop 3 safety injection line Loop 4 RHR suction line

The SPS tapping in each case is the tapping nearer to the main reactor coolant loop pipework.

For the measurement of reactor coolant flow, each coolant loop crossover leg has four tappings on the inside and four tappings on the outside of the coolant pipework elbow-. Flow is measured by differential pressure between correspondingly offset tappings on the inside and outside of the elbow, one pair for each PPS channel.

Each reactor coolant cold leg provides PPS coolant temperature measurement by means of fast-response resistance temperature detectors (RTD) mounted in appropriate pockets in the cold leg piping, and SPS measurements by means of thermocouples. The cold leg temperature sensors are located between the RCP outlet nozzle and the primary shield wall. There are 10 penetrations on each RCS cold leg allocated as follows:

— NR temperature — RTD

— NR temperature — RTD

— WR temperature — RTD

— NR temperature — Thermocouple

— NR temperature — Thermocouple spare

— NR/WR temperature — RTD spare

Each reactor coolant hot leg has a pocket with an RTD to provide a signal to the PPS guard line of the corresponding coolant loop. An installed spare hot leg RTD is provided in each loop.