Instruments and Systems: Monitoring, Control, and Diagnostics Annotation << Back Permanent in-pile reactivity controlling medium content monitoring within the VVER type reactor coolant in the course of nuclear fuel reloading S.V. VOLKOV The article discloses the physical essence of the method to permanently in-pile monitoring the current and the critical reactivity controlling medium (i.e. dissolved neutron absorber) within the coolant, filling reactor vessel volume and experimental results obtained in the course of VVER reactors nuclear fuel reloads. Keywords: VVER type reactors, nuclear fuel reloading, reactivity controlling medium, critical concentration, external and internal neutron detection , blocks. Pp. 55-58.