On the State of the Shelter Object Nuclear Safety
Regarding the appearance in mass media and widely discussed by the society information on the Shelter object state and localized in it fuel containing materials (FCM) as well as discussion on related to it risks for an environment, the Institute for Safety Problems of Nuclear Power Plants (ISP NPP) of the NAS of Ukraine informs that:
After the New Safe Confinement (NSC) “Arch” installation into design position during over four years increase of neutron flux density was observed on the periphery of the FCM accumulation, which was localized in one of the subpile room of the Shelter object (pr. 305/2). However, to date this increase doesn’t exceed specified safety measures which are controlled by means of the nuclear safety control system (NSCS IACS).
During the last decade scientists of the ISP NPP gained knowledge and developed behavior models of the main accumulations of the nuclear hazardous fissile materials (NHFM). According to the proposed and developed by the ISP NPP specialists scientific hypothesis on the state of the FCM accumulations, in 2015 the neutron flux density increase was forecasted as a consequence of excluding the possibility of precipitation to get inside the Shelter object after the NSC “Arch” was installed into the design position. That is, current experimental data validated scientific hypothesis, developed by the ISP NPP specialists before the NSC construction.
Figure – Sampling of the water inside the premises of the Shelter object
Scientific hypothesis provides that in the subpile room 305/2 there are two overwet FCM accumulations with increased concentration of fissile materials. On the basis of analysis results of experimental calculations, the most potentially hazardous of them is “southern” accumulation that was an epicenter of the FCM formation and spread. FCM accumulation in this premise according to our data may have two-layered pore structure: top layer is black lava-like FCM, and the bottom layer is potentially critical composition with high concentration of NHFM.
Figure – Sampling of polymer coating inside the premises of the Shelter object
Before the NSC installation into the design position, FCM were fully wet and dynamics of the neutron flux density was within seasonal trends due to the regular precipitation and condensate input, i.e. there were no potentially hazardous changes of the subcriticality of these NHFM. After the NSC was installed into the design position as a result of excluding of precipitation input inside the Shelter object, evaporation and water loss started by the pore structure of lava-like FCM. This led to the registered increase of the neutron flux density.
Figure – Dynamics of the neutron flux density – (a) – near nuclear hazardous accumulation inside the pr. 305/2 of the Shelter object; (b) – in the remote periphery
On the basis of forecasts, it is expected that further increase of the neutron flux density will be observed that will be defined by the water loss in the layer of overwet environment of lava like FCM. That is why ISP NPP scientists focus their attention on the necessity of in-depth study of the FCM and factors, which have impact on the state of the Shelter object nuclear safety and therefore developed the Program of the Shelter object FCM monitoring.
The scientific hypothesis on the existence in the Shelter object of FCM accumulations with high concentration of uranium and forecasts were consistently published in domestic and foreign science publications (for example “The Shelter object in Conditions of New Safe Confinement” V. O. Krasnov, A. V. Nosovskyi, S. A. Paskevych, V. M. Rudko; edited by A. V. Nosovskyi. – Chornobyl: the ISP NPP of the NAS of Ukraine, 2021. – 344 p.) and reported at the scientific conferences. In 2021 these materials were presented in the report at the international conference INUDECO, where risks of the Shelter object nuclear safety were discussed.
Institute for Safety Problems of NPP of the NAS of Ukraine is the only scientific institution, which has been studying the state of the localized inside the Shelter object NHFM from the first years after the Chornobyl NPP accident as well as forecasting the changes of their state in time. These researches are extremely important because received data provide a basis for substantiations of radiation and nuclear safety of the Shelter object. The ISP NPP as an organization-scientific supervisor, constantly collaborates with Chornobyl NPP on issues of the Shelter object nuclear safety improvement and gives corresponding scientific recommendations.
Realizing the complexity of the problem, which stand before the Chornobyl NPP during the Shelter object transformation into an ecologically safe system, special Program of the Shelter object FCM monitoring was developed by the Institute in 2016. However today SSE ChNPP is not provided with necessary resources for the program implementation.
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