How REP works. Reminder
Internal reactor instrumentation

The measurement of a neutron flux, i.e. of uncharged particles, always involves a neutron/matter interaction, which in turn creates particles capable of producing a measurable electric current. Each time a neutron is absorbed, the atoms of this sensitive material are transformed and no longer exist as such.

The material chosen to undergo this interaction will be consumed and disappear at a rate that depends on the intensity of the neutron flux and the probability of occurrence of the reaction (effective absorption cross-section). The higher this probability, the higher the current supplied, but, on the other hand, the faster the sensitive material will disappear, ultimately necessitating rapid replacement of the sensor....