A position detector is supplied for each control rod cluster drive rod. Each rod position detector consists of a tube, with 42 detector coil assemblies mounted on the tube and spaced out along its length at 95 mm intervals. The tube fits over the rod travel housing. Power at 12 V AC from a constant voltage source is applied to the coils. The electromagnetic flux generated by the coils penetrates the thickness of the drive rod travel housing. Since the rod within the housing is ferromagnetic, the magnetic flux rises steeply as the rod moves through a coil. The concentration of flux results in an increased impedance to the AC current flow in the coil. To prevent total loss of position in­formation due to a single coil failure, the outputs of every other coil on the tube are connected as coil set X, while the outputs of the remaining coils are connected as coil set Y.

The detector electronics samples the voltage at the junctions of coils in each set. When the rod is within either both or neither of the coils in the pair, the voltage sampled is ‘high’.

When the rod is within only one of the paired coils the sampled voltage is Mow’. A differential amplifier compares with voltage samples for each coil pair with a fixed reference, and provides an output which can be digitally encoded to represent each rod position.

11.1.4 Thermal power measurements

The N-16 power monitoring instrumentation provides an input into the station automatic control system and the primary protection system (PPS) for calculation of the thermal power.

this instrument monitors the thermal power ot the reactor by detecting the level of N-16 present in the coolant system. N-16 is an isotope of nitrogen, venerated by neutron activation of oxygen contained in the water. The level of N-16 present in the primary

oiant is directly proportional to the fission rate (and hence power) in the core. Decay of the N-16 isotope produces high energy gamma rays which penetrate the wall of the high pressure piping. Therefore the fsl-16 concentration in the primary coolant can be monitored by measuring the characteristic gamma ra­diation outside the primary coolant piping.

N-16 gamma radiation is monitored by two ion chambers located on the hot leg piping of each cool­ant loop. The ion chambers are located as close as physically possible to the reactor vessel but outside the biological shield, and shielded from reactor neutrons.

The two N-16 detector signals are processed and corrected for N-16 decay to give an average signal. The transit time around the loop (about 11 seconds) and the N-16 half-life result in a saturation signal, giving a computed signal which varies directly with reactor power.

The Troid signal is used to compensate the N-16 signal for fluid density variation and the final signal of thermal power is computed within the PPS, for use (together with system pressure) in the departure from nucleate boiling (DNB) and linear power density (kW/m) calculators of the PPS,