On July 8, 2020 a scientific and technical workshop (STW) was held in the assembly hall of the Department of Nuclear and Radiation Safety (NRS), where young scientists from the ISP NPP of the National Academy of Sciences of Ukraine presented reports on their research activities. On STW leaders and scientists of the NRS department were present. The research was carried out under the mentorship and co-authorship of leading scientists of the Institute. In the context of counteracting the spread of COVID-19 infection, all those present wore protective masks and maintained a social distance.
Presentation 1: Doroshenko A. O. “Evaluation of the effectiveness of the regulatory system for nuclear safety control of the NSC-OS complex”
According to the results of the comprehensive analysis, it has been established that the nuclear safety control system (NSCS) does not fully meet the requirements of the nuclear safety rules (NSR). According to the assessment of the adequacy of detection points of the regulatory system, it was found that there is no monitoring of clusters of nuclear hazardous fissile materials (NHFM) in the central hall and the “northern” cluster of NHFM of room 305/2. The effectiveness of monitoring the “southern” cluster of NHFM in approx. 305/2 is insufficient and does not meet the norms regulated by NSR. Also, a number of other requirements to the NSCS are not met, such as the alarm system of the spent nuclear waste repository. It is proposed to perform recomposition of measuring channels of the system, decommissioning of excess (non-informative) measuring channels, and use their equipment to organize new useful monitoring points, upgrade the software and arrange proper grounding of the system to reduce the impact of high-frequency hovering and interference.
Presentation 2: Stadnyk S. M. “Comprehensive analysis of the results of SHPN measurements in the area of localization of nuclear hazardous fissile materials”
For many years, the neutron activity of potentially nuclear-hazardous FCM clusters has been monitored inside the OS. Before the installation of the NSC-OS at various monitoring points, SSChR was determined by regular seasonal/annual trends, which were superimposed on local anomalies that accompany the processes of inflow and retention of rain, melt and technical water in the cascade of natural basins formed during the accident. After the commissioning of the NSC-OS in some channels, which are located near potentially nuclear-hazardous clusters of FCM, a significant increase in neutron flux density was registered. Changes in the rate of detectors can be caused by changes in water concentration directly to the neutron source, as metrological failures, VK SSChR NSCS easily identified by calibration and diagnostics of system nodes, and considered options to increase decrease the water concentration in the interface does not lead to significant growth of SSChR. In this case, the increase in neutron activity is caused by an increase in the number of neutrons of forced fission due to the increase in the Keff of this propagating medium (accumulation of NHFM). As a confirmation, the method of statistical processing shows the change in the ratio of variance to mean.
Presentation 3: Sushchenko K. O., “Methods for assessing the levels of subcriticality of the accumulation of FCM, located in room 305/2 of the NSC-OS complex”
For the NSC-OS complex, nuclear safety can be ensured by the availability of effective means of detecting and promptly suppressing emergency levels of subcriticality of NHFM clusters. A mathematical model for estimating the level of subcriticality of NHFM clusters was proposed. The model is based on the interpretation of the dynamics of the neutron flux density of the source, which is registered on the periphery of the clusters. The mathematical model allows to estimate the current level of subcriticality, the rate of introduction of positive reactivity in case of water loss, as well as to predict the level of subcriticality of these clusters in the observation interval up to 6 months. The use of this mathematical model in the monitoring of NFD in NSCS will ensure the early detection of dangerous levels of subcriticality to take measures to prevent a self-supporting chain reaction.