До сплеску активності 137Cs в повітрі під об’єктом «Арка» 17 жовтня 2019 р.

Abstract

The construction of a New Safe Confinement (NSC) is one of the main stages in the transformation of the Chornobyl nuclear power plant 4th power unit into an ecologically safe system. On the one hand, this system is designed to ensure the safety of personnel, the population and the environment, on the other — to allow safe work with the remnants of the unit. At the same time, the project allows quite high levels of bulk activity of radioactive aerosols inside the NSC — up to 210 Bq/m3 for total β-activity. The sharp increase in the volumetric air activity, observed on October 17–19, 2019 in a number of rooms of the Shelter object and in the space under the “Arch” object in the absence of a burst of activity outside these objects, can be considered as a successful kind of spontaneous test of the NSC isolation systems. The chronology of the dynamics of volume activity according to the act of official investigation of the precedent is given. The results of studies of aerosol filters exposed in the near zone of the Chornobyl NPP before and after the burst of activity in the air under the “Arсh” object are presented. Autoradiograms of fragments of these filters demonstrate a significant (≈2 orders of magnitude) increase in the number of hot particles immediately after the fall of the concrete fragment of the floor in the room 402/3. The analysis of features of the hot particles after the collapse indicates a significant contribution to the total activity of a large number of low-activity particles (0.005÷5 Bq), and the low 137Cs/241Am ratio that indicates their fuel nature. Based on the assumption about the fuel nature of hot particles, their minimum possible aerodynamic diameters were calculated, which are in the range 2.91<d<36.7 μm, which does not exclude the presence of smaller hot particles on the filter that are not visible using this method. The contribution of particles with aerodynamic diameters less than 10 μm, determined using the impactor, did not exceed 20% of the total beta activity of the aerosol.