[en] After the accident at the Fukushima Daichi nuclear power plant, the stability of nuclear power plants against external influences were tested all around the world with stress tests, and it was declared that a high level of operational safety had been reached, in accordance with “international standards.” However, given the experience of two accidents - the Armenian power plant in 1982 and Chernobyl in 1986 (in the aftermath of which the author took part) - we shouldn’t declare such things with confidence because there remain hidden, internal factors that are outside the field of view that don’t lend themselves to being known before a specific situation, and because some documents such as “Probability Safety Analysis” or “Safety Analysis Report” haven’t been taken into account. Many of these hidden factors could be discovered safely if performed at the modular/block level with In-core Monitoring Systems (IcMS), with early forecasting and detection of anomalies in the technical process of the reactor station. For this, it’s necessary to raise its status and quality; above all, it’s in formativeness, the reliability of metrological characteristics, and the stability of its mathematical software. In this sense, as was shown by an analysis, the majority of stationary In-core Monitoring Systems are based on neutron (for example, rhodium Rh-103) detectors that don’t meet today’s requirements for any of these indicators. The root cause of this failure is the need to account for burnout processes, neutron flux depression, and the change in the spectrum of neutrons. Accounting for these requires regular recalibration of integral balance reactor indicators and the existence in stationary In-core Monitoring Systems of interprocess communications with cumbersome computational operations. All of which can result in the failure of a stationary system to provide the information needed to avoid a crisis. In order to achieve the above-mentioned level of control (in-core monitoring), a new concept was introduced by us, and in 1985 an IcMS system project was conducted on the second block of the Armenian nuclear power plant, the physics of which were based on monitoring the gamma-field of the core, and on calorimetric gamma-detectors developed as primary incore sensors. The theoretical and experimental results of this project were presented in a report, including: — The concept and principles for building the system; — The construction of gamma-detectors and measurement channels (probes); — A description of physical processes and control algorithms; — The results of metrological, operational and resource/endurance tests; — The results of system control algorithms and programs in general.