On 19 May, nuclear and radiation safety issues of Object Shelter were discussed during an online meeting of the Parliamentary Committee on Environmental Policy and Nature Management.
Participants of the discussion, in addition to members of the Committee, included Acting Director General of SSE Chornobyl NPP Valeriy Seyda, Head of the State Agency of Ukraine for Exclusion Zone Management Serhii Kostiuk, Director of the Institute for NPPs Safety Problems Anatolii Nosovskyi, Chairman of the State Nuclear Regulatory Inspectorate of Ukraine Hryhorii Plachkov, and other experts on NPP safety.
The meeting was primarily focused on the issues regarding probability of self-sustained chain reaction in fuel-containing materials (FCM) of Object Shelter.
Valeriy Seyda in his speech noted: «The neutron flux density could rise due to the change in the composition of the environment which lies between a neutron source (nuclei of fissile elements in the FCM) and the detection area of ionizing radiation which arises from the interaction of neutrons with nuclei of the fissile material in the fission chamber.
Following sliding of the NSC Arch into the design position, penetration of rainwater to the monitored FCM accumulations has completely stopped, and water content continues to recede in the FCM structure due to drying. Since water is a moderator, its decrease in the FCM causes more neutrons to reach the detection unit».
It should also be noted that after the NSC was installed into the design position, there was slight increase in the neutron flux during the years 2016-2018 according to 4 out of 19 radiation monitoring sensors, however starting from 2019, the values have adjusted and keep being stable until this day.
To sum up his presentation, Valeriy Seyda noted that currently the situation is not threatening, but most of the processes taking place in the Shelter require constant R&D support: «Over the whole period of monitoring since installation of the NSC Arch to the design position, no exceeding of safe operation limits was observed. At the moment, 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 fission reaction».
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.
Director of the Institute for NPPs Safety Problems Anatolii Nosovskyi for his part brought into focus the fact that 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. 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. However, full implementation of the Programme requires sufficient and continued financing, but unfortunately there is lack of it.
SNRIU chairman Hryhorii Plachkov stated: «Taking into account the need for constant monitoring of the situation, the New Safe Confinement has to be taken from pilot into full-scale operation mode as soon as possible».
During discussions, members of the parliamentary committee suggested requesting for additional funding for survey activities on FCM of the Shelter and/or making relevant provisions for these activities in the National Programme for Chornobyl NPP Decommissioning and Object Shelter Transformation into an Environmentally Safe System, as well as found it necessary to speed up signing procedure for this Programme.
FYI: The issue of the probability of self-sustained chain reaction in the fuel-containing materials of the Shelter arose following publication of an article in the Science magazine based on the information published during the 6th International Conference on Nuclear Decommissioning and Environmental Recovery INUDECO-2021. The article referred in particular to the fact that after installation of the NSC Arch into its design position over the Shelter, certain IAMS sensors were tracking the 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 is 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).