2. Neutron level measurement

The simplest way to quickly measure reactor power without being subjected to the constraints of the core is to measure the intensity of neutrons leaving the core. The parameter used is the neutron fluence rate or neutron flux. Neutron detectors are placed outside the core and even outside the reactor vessel.

The only detectors that can be used in such conditions are those based on gas ionization phenomena: proportional counters and ionization chambers. These detectors are sensitive to neutrons and gamma rays, and the radiation leaving the core is a combination of neutrons and gamma rays, not all of which are representative of the reactor's instantaneous power.

To clarify this point, we recall that the radiation leaving the core includes neutrons and gamma rays, which can be classified into five categories:

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