The article presents the results of the analysis of the validity of simplifications and assumptions accepted by researchers in the models of neutron-physical characteristics of fuel-containing materials (FCM) in the "Shelter" object (SO). The analysis and substantiation of the FCM nuclear safety will be one of the important issues during the SO operation for a long time, until the moment of retrieval and disposal of nuclear materials from the ChNPP Unit 4 destroyed in 1986. A significant number of nuclear materials in the SO are uncontrolled, and the geometry of their location is also little known. Therefore, the use of traditional methods of nuclear safety analysis to determine the conditions for reaching criticality in the volume of FCM can lead to significant uncertainties, depending on the assumptions and simplifications made in FCM models. In contrast to traditional nuclear facilities with design characteristics and control systems, there is no possibility to deterministically influence the FCM, and accordingly to determine their dynamic characteristics in the SO. In such conditions, it is necessary to take a responsible approach to obtain and interpret the values of FCM parameters by control systems of the SO. For example, in 1990, one of the neutron flux detectors recorded an abnormal change in the neutron pulse count rate for about 100 hours. This event is often called a “neutron incident”, and it formed the basis of some models, in which the possibility of reaching criticality in one of the FCM clusters is assumed. The article presents the results of the study on the physical possibility of the anomalous event in 1990, as well as the results of the analysis of simplifications and assumptions made in models of FCM criticality. An important result of the conducted research is the justification of the need to include in the FCM criticality model the effects of reactivity on fuel and water temperature, as well as to take into account delayed neutrons.
