The instrumentation needed to detect an incipient accident can be divided into two classes: one for accidents that affect the whole of the core (such as reactivity changes or primary coolant pump failure), and one for accidents that affect initially only part of the core (such as a coolant flow failure in one subassembly). The latter is discussed in section 5.2.3.

It is relatively easy to detect an accident affecting the whole core by means of the instruments used to control the reactor in normal operation. The condition to be guarded against is overheating, so it is necessary to monitor the reactor power and the coolant temperature and flow-rate.

The reactor power is determined by measuring the neutron flux at convenient points. The flux-measuring instruments are normally fission chambers or boron trifluoride chambers. The main difficulty is that neutron flux has to be measured over the range from full power (maybe 3000 MW) to the shutdown level of 100 mW or less — a range of more than 1010. This cannot be done by any single instrument. If the flux is high it is possible to measure the overall ionisation current, but when it is low it is necessary to count individual pulses and determine the average count rate.

Even with these two modes of operation it may not be possible to cover the entire range with a single instrument. It is frequently necessary to have two or more sets of fission chambers in positions of different sensitivity. At low power instruments close to the core are used, whereas at high power other instruments in the neutron shield or outside the reactor vessel are brought into operation.

The high gamma flux from the radioactive primary sodium has to be allowed for. At the highest powers it may be possible to ignore it in comparison with the neutron flux because the energy of a fission event is so much greater than that of a gamma from the sodium. However, at lower powers, when the sodium activity (with a half-life of 15 hours) may correspond to earlier high-power operation, it is necessary to compensate for the ionisation caused by gammas or to discriminate against them if the pulse-counting mode is in use.

Several measuring stations round the core are necessary, partly for reasons of reliability and partly to allow for changes in the flux shape due to the movement of control rods or irregularities in the pattern of loading new fuel into the core. As burnup proceeds the sensitivity of the instruments changes as the flux at the periphery of the core and in the breeder increases relative to that at the core centre. For this reason a high power trip based on neutron flux cannot be very precise, but it is very reliable.

The signal from the neutron flux instrumentation is also used to determine the inverse period dC/Cdt, where C is the indicated flux level. The inverse period is closely (but indirectly) related to the net reactivity, and the reactor is tripped when it becomes too large. This trip system is of little importance when the reactor is operating at power because feedback keeps the net reactivity close to zero, but when the power is very low and very little heat is being generated there is no feedback. Under these conditions an inverse-period trip is a protection against accidental increases in reactivity.

Coolant temperature can be measured by thermocouples at the core outlet. The main difficulty is to ensure that a thermocouple meas­ures the mean temperature correctly, for the coolant temperature is not uniform. Coolant from the edge of a subassembly is cooler than the rest and unless the flow is mixed by some device to enhance the turbu­lence a thermocouple may be exposed to a stream of unrepresentative coolant; moreover as the coolant flow-rate changes the flow pattern at the outlet may change possibly bringing coolant from a different unrepresentative part of the subassembly to the thermocouple.

Other difficulties are caused by changes in the power generated in a subassembly by the movement of nearby control rods, and by burnup of the fuel. It is thus usually necessary to measure the temperature at a number of positions at the top of the core and use an average for the control and trip systems. The alternative of measuring the temperature farther from the core where the coolant has become more thoroughly mixed is less satisfactory because the delay allows more of the structure to experience a temperature change before corrective action is taken.

Coolant flow-rate can be monitored by flowmeters at the core outlet or by observing the rotation of the circulating pumps. The flow-rate in a pipe can be measured conveniently by an electromag­netic flowmeter, which makes use of the electromotive force induced when a conductor (the sodium) moves through a magnetic field.

It is normal to trip the reactor if the pumps stop or if the flow out of the core falls below a set value. If the reactor is to be operated efficiently at less than its full power, however, either the trip level on coolant flow has to be set low (at say 10% of the full flow-rate) or the trip level has to be altered according to the power required. This disadvantage can be avoided if the ratio of measured power to measured flow-rate is used as a trip signal.