General gas composition The operational require­ments are summarised in Table 3.8 which indicates the concentrations of various components of the cool­ant gas.

Moisture in primary coolant In AGRs this is sig­nificant for safety since: [36]

• If not corrected, a high moisture level may lead to corrosion in the cooler part of the primary circuit.

In the case of Heysham 2, two systems are provided:

(a) Bulk moisture system

Reactor gas is bled from the bottom of the boiler annulus in quadrants A and C to two mirror dew­point instruments. These operate at a gas pressure of approximately 1.5 bar, but the pressure is not controlled. The output of these instruments is a dewpoint signal over the range — 50°C to + 40°C, and a pressure signal from an internal pressure transmitter.

Each dewpoint instrument has a dedicated mi­croprocessor. These convert the dewpoint tempera­ture signal into moisture weight parts per million concentration, using the pressure signal to give automatic compensation for the operating pres­sure. There is a 4-20 mA signal from each micro­

processor which is linear with concentration for the range 0-400 wpm.

The microprocessors also give a 4-20 mA signal which is linear with dewpoint over the range -30°c to + 40°C.

The moisture concentration and dewpoint signals are fed to the data processing system to appear on data displays (including trends with time) and alarms, the dewpoint indication being used when shut down and depressurised. Alarms are generated within the data processing system for ‘moisture level high’, ‘moisture level very high’, ‘high rate of increase of moisture level’, and ‘extra high rate of increase of moisture level’.

The moisture concentration signals are also fed via alarm amplifiers to give a grouped direct-wire facia alarm ‘moisture level very high’.

The system is not affected by oil ingress since this will plate out before reaching the instrument.

(b) Boiler leak detection system

The boiler leak detection system gives a means of identifying which quadrant has a leaking boiler in the event of an increase in moisture concentration. Reactor gas is sampled from each of eight loops which take gas from each circulator discharge back to its inlet. There are four moisture-sensitive cry­stal oscillator moisture detectors. Each detector monitors the difference in moisture concentration between two diametrically opposed points by al­ternate sampling using solenoid valves (about 30 seconds per sample):

• Detector 1 monitors gas bled from loops around circulators A1 and Cl.

• Detector 2 monitors gas bled from loops around circulators A2 and C2.

• Detector 3 monitors gas bled from loops around circulators B1 and Dl.

• Detector 4 monitors gas bled from loops around circulators B2 and D2.

Each detector is connected to its own dedicated microprocessor. While detector 1 is sampling loop Al, the datum is taken as the last reading from loop Cl, and vice versa. The output from each of the microprocessors, normally 4-20 mA for ±50 wpm, is decoded within the data processing system by means of a signal indicating which loop of the two is currently being sampled to give a non-oscillating moisture difference between the two loops.

Alarm relays raise alarms on the data processing system in the event of a difference of 50 wpm.

Oil in primary coolant This indicates whether or

not the oil leakage from the circulator motor com­partments is excessive. If this is so and the reactor is operated with a high oil concentration in the pri­mary coolant, this could lead to heavy carbonaceous deposits on the fuel pins, with a significant reduction in the heat transfer coefficient and corresponding increases in pin temperatures.

In the case of Heysham 2, the oil in the primary coolant detection system is intended to delect increases in the oil concentration in the CO; primary coolant and in this event to identify which circulator is the source. Gas is sampled from the discharge from each circulator via pipework leading out through the in­strument penetration in each quadrant to an instru­ment rack at the +3,9 m level.

There are four gas analysis systems, one per quad­rant. Each contains a photo-ionising detector cell consisting of an ultra violet lamp producing ionis­ing photons. A potential difference is applied to elec­trodes within the cell and the current carried by the ions produced by the photons is proportional to con­centration. The detector operates within an oven at 240°C and sample lines are trace heated to prevent condensation. The detector operates at a pressure of 0.8 bar.

The analysis system is differential to compensate for the effects of background organics produced from methane which is injected into the primary coolant to inhibit graphite core corrosion. The systems each contain a rotating valve and cooling coils so that the detector can give outputs for the following for comparison:

1 Circulator I gas uncooled.

2 Circulator 1 gas cooled (i. e., condensables removed).

3 Circulator 2 gas uncooied.

4 Circulator 2 gas cooled.

5 ‘Span’, i. e., reference concentration of propylene in CC>2-

6 ‘Zero’, i. e., pure CO2.

The output signal from each detector is fed to a de­dicated microprocessor. These provide 4-20 mA sig­nals for circulator 1 oil concentration (0 to 1 ppm) and circulator 2 oil concentration. The repeatability is ±0.003 ppm. The microprocessor also performs the sequence control of the rotating valve, on a settable time basis from 1 to 64 minutes per sample.

In the event of these measurements not coming up to expectations, oil ingress will be measured by hydrogen balance and discrete propene measurements at each gas circulator outlet as at existing AGRs.