Out-of-core instrumentation

Reactor operation and safety require nuclear power to be controlled by measuring the radiation emitted by the core. In all cases, power is characterized by its overall value and its period over the entire operating dynamic of the reactor. In the case of large reactors, it is also necessary to monitor its distribution throughout the core volume.

The neutron level varies considerably between shutdown and power operation. When the reactor is at power, the neutron level is 5 × 10 ¹⁰ times the level corresponding to shutdown. This highly dynamic situation calls for the use of several detectors to monitor the power level. To illustrate, if we take the centimeter as the unit corresponding to the neutron level at shutdown, then the nominal power level corresponds to 5 × 10 ¹⁰ cm or 500,000 km, roughly the distance from the earth to the moon.

If, during operation, limits are reached on power, evolution speed or even core distribution, the reactor must be shut down quickly by dropping the control absorbers.

Depending on the size of the reactor and the choices made by the reactor designer, nuclear parameters are monitored via two distinct instrumentation systems:

• the out-of-core instrumentation system, which covers the entire dynamic, and relies on detectors placed outside the core, or more precisely the reactor vessel for power reactors;

• the core instrumentation system, which can be either fixed and permanent, or mobile and periodic.

This article presents out-of-core instrumentation systems based on neutron measurements, which help control the power of nuclear reactors. It complements the articles on the electronics associated with radiation detectors and on detectors.

The focus is on achievements in France, but other projects are also presented, such as instrumentation for VVER reactors.