Due to the news published in the media and wide public discussions of the potential risks of “an uncontrolled release of nuclear energy” from the fuel-containing materials located inside the Shelter after the NSC was put into the design position, we communicate the following.
Following installation of the NSC Arch into its design position over the Shelter, certain IAMS sensors were tracking increase of neutron activity in the accumulations of fuel-containing materials (FCM) located in room 305/2 in subreactor slab melting area. This situation had been predicted by the ISPNPP before the NSC Arch was slid over, and was taking place due to drying of overmoistened environment of black lava-like FCM (LFCM) layer which is located higher (at elev. +9.00 ÷ + 9.70) than potentially nuclear-hazardous FCM (NFCM). ISPNPP experts predict that the NFCM will be further constantly contributing to the neutron activity level registered by the IAMS sensors. The total neutron activity will continue to slightly rise due to decrease of water content in LFCM layer located above the NFCM, until an optimal ratio of water and fuel is achieved. Should the environment be further dried, the neutron flux density is predicted to be slightly decreased or there will be abnormally long period of constant values of neutron flux density.
For the purpose of ensuring safe state of the Shelter, the FCM accumulations inside the Shelter are being constantly monitored by technical means of the Nuclear Safety Monitoring System (NSMS) which is part of the Integrated Automated Monitoring System (IAMS) of the Shelter.
Over the whole period of monitoring since installation of the NSC into the design position, no exceeding of safe operation limits has been observed. Currently, the sensors tracking the neutron flux density show constant values in all premises with no trends to rise and the current levels do not pose threat of self-sustained chain reaction of fission. To ensure safe state of the Shelter there are safety systems in place, including systems for maintaining FCM in a subcritical state. Should the safe operation limits for neutron flux density be exceeded, neutron absorbing solution shall be sprayed to the neutron density rise area according to the Shelter operating procedure For the purpose of ongoing state assessment of the FCM contained inside the Shelter and prediction of their long-term behavior, in 2018 the ISPNPP developed «The Programme for Monitoring the Shelter Fuel-Containing Materials (Nuclear and radiation safety of the Shelter)».
The results to be obtained from the performance of works under this Programme will be used for development (if necessary) and implementation of measures to prevent adverse effects of FCM deterioration. However, it is necessary to implement FCM monitoring activities within several years once all the processes of change to temperature and humidity conditions stabilize and it is possible to predict the FCM behavior more accurately and ensure their nuclear safe state.
