Features of Neutron Flux Density and Gamma-Radiation Exposure Dose Rate Dynamics in ChNPP Shelter Object after the New Safe Confinement Commissioning
Authors:   Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine, Mykhailov O. V. , Saveliev M. V. , Sushchenko K. O. , Dmytryshyn V. V. 
Published: December 2023

The nuclear safety monitoring system (NSMS), which is a part of the integrated automated monitoring system (IAMS), was created within the framework of measures aimed to build the New Safe Confinement — Shelter Object complex (hereinafter NSC-SO) before the protective arch itself was slide into design position. During NSMS operation period, the attempts were repeatedly made to analyze accumulated measurement data, and a number of important conclusions were obtained regarding the peculiarities of formation of time trends in the dynamics of neutron flux density (NFD) and gamma-radiation exposure dose rate (GDR). To reliably detect any NFD and GDR trends and to predict its further development, special software was developed, deployed and tested at the Institute for Safety Problems of Nuclear Power Plants of the National Academy of Sciences of Ukraine in 2022. The purpose of this work was to summarize the data obtained by special software and to analyze the peculiarities of NFD and GDR dynamics in monitoring point around the nuclear-hazardous clusters of fuel-containing materials (NHC FCM) localized in the NSC-SO, for the period after commissioning of NSC. The research carried out allowed identifying and comparing the general trends in NFD and GDR dynamics, at the same time, for 16 measurement assemblies of NSMS (32 monitoring points located in NSC-SO rooms). It has been established that the control parameters dynamics had two main types according to their manifestation form. The first type is characterized by NFD growth on the background of gradual decrease in the GDR, for the second one — the same character of NFD and GDR changes in the form of simultaneous drop in their mean annual values. Estimated regression equations enabled obtaining quantitative characterization of identified trends regarding NFD/GDR drop or growth and confirmed previously established general trends regarding the behavior of control parameters around the NHC FCM. For majority of monitoring points, gradual GDR drop is noted, but in room 305/2 of NSC-SO, mean annual rate of GDR drop can significantly differ from decay rate of 137Cs radionuclide to the downside. The results obtained in this work allow stating with sufficient confidence that the presence of synchronous NFD growth against the background of synchronous but opposite nature of GDR drop is typical for monitoring points located near the NHC FCM. At the same time, the following phenomenon is typical for monitoring points group, where significant NFD growth is present, and this monitoring area is located in close vicinity to the NHC FCM boundaries: the greater mean annual rate of NFD growth, the slower GDR declines as compared to decay rate of 137Cs radionuclide. The proposed hypothesis is based solely on the results of statistical study of NSMS data and requires further, more in-depth study in order to establish physical nature of the discovered phenomenon.