Determination of radionuclide composition of the Russian NPPs atmospheric releases and dose assessment to population.

Original data on radionuclide activities in air emissions of Russian NPPs are presented based on direct on-site measurements in vent stacks of NPPs during an extensive survey in 2017-2018. For the first time, the detailed inventories of radionuclides in the air releases were directly measured for all types of nuclear reactors, employed in the Russian NPPs. Based on the results of measurements, annual doses for local populations around the Russian NPPs were estimated. The technogenic radionuclides, detected in the air emissions, were ranked according to their contribution in the annual dose. 14C is the major contributor for the annual dose from the atmospheric releases of LWGR reactors - almost 98% for Bilibino NPP's EGP-6 reactor, and up to 86% for RBMK-1000 reactors (Smolensk NPP). For PWR reactors (VVER) contribution to the annual dose from atmospheric releases is formed mostly by tritium, 14C and noble gases. Estimated annual doses for local populations in the vicinity of the Russian NPPs do not exceed 10-5 Sv/year. Atmospheric releases of 60Co, 134Cs, 137Cs and other aerosols, involved in the routine monitoring programs of Russian NPPs, do not contribute significantly to the annual dose. Based on these results, 14C and tritium will be included in the monitoring programs of atmospheric release control at all Russian NPPs; maximal permissible levels of annual releases for 14C and tritium will be established since 2019.

Anomalously high unattached fraction of 220Rn decay products in the atmosphere of monazite storage facility.

High levels of the unattached 212Pb fraction of the 220Rn decay products in the atmosphere of the monazite storage facility were found: f1 = 0.3-0.5 at the aerosol concentration of 20 000 cm-3 and 0.1-0.16 at aerosol concentration ∼140 000 cm-3. It is shown that the sampling of aerosols in an atmosphere with a high concentration of thoron result in the local change of the equilibrium between the attached and unattached fractions near the sampling point. As a result, the measured value of the unattached fraction of thoron decay products may differ by several orders of magnitude from the mean value of unattached fraction the in the indoor atmosphere. A local increase of the unattached fraction 212Pb can take place in the worker's breathing zone in the atmosphere with a high concentration of thoron and it should be taken into account in the assessment of the inhalation intake of the thoron decay products for workers.

Comparison of radioactive aerosol size distributions (Activity, number, mass, and surface area).

Information regarding the number, mass, and specific surface area distributions of aerosols is extremely important for assessments of exposure to radiation due to the intake of radioactive aerosols by inhalation. In this study, these distributions were compared with the activity size distributions for radon decay products. The experiments were conducted in a radon chamber under controlled conditions for the aerosol concentrations and with size distributions in the range from 5 nm to 10 µm. The activity size distributions of the aerosols were measured by using a diffusion battery and two types of cascade impactors. The aerosol concentrations during the experiments were in the range from 2 × 103 to 2.5 × 105 cm-3. The measurements obtained with a diffusion battery demonstrated that at the low aerosol concentration, up to 80% of activity of the radon decay products was connected with aerosol particles with an activity median thermodynamic diameter (AMTD) ~1 nm (unattached fraction). After injecting the aerosols into the experimental chamber, the activity of the unattached fraction decreased to practically zero but we observed the presence of the activity of aerosols with AMTD values ~10-20 nm. After injecting the aerosols, the distribution of the aerosol specific surface area exhibited a bimodal distribution, where the median diameter in the accumulation mode was ~120 nm and the geometric standard deviation (GSD) was 1.3. The median diameter in the coarse mode changed with time due to the deposition of aerosols on the walls of the chamber, where it decreased from 5 µm (GSD = 1.6) at 15 min after injection to 2 µm (GSD = 2.5) at 120 min after injection. The distribution of the aerosol activity measured by the cascade impactor was virtually independent of the time from the moment when the aerosols were injected. The parameters used to assess the activity distribution comprised an activity median aerodynamic diameter (AMAD) ~500-650 nm and GSD = 1.5. Comparisons of the activity distributions with the distributions for the number, mass, and specific surface area showed that the closest correspondence was with the average distribution over the specific surface, whereas there were no exact correspondences with the other distributions.

ULTRAFINE AEROSOL INFLUENCE ON THE SAMPLING BY CASCADE IMPACTOR.

Cascade impactors based on inertial deposition of aerosols are widely used to determine the size distribution of radioactive aerosols. However, there are situations where radioactive aerosols are represented by particles with a diameter of 1-5 nm. In this case, ultrafine aerosols can be deposited on impactor cascades by diffusion mechanism. The influence of ultrafine aerosols (1-5 nm) on the response of three different types of cascade impactors was studied. It was shown that the diffusion deposition of ultrafine aerosols can distort the response of the cascade impactor. The influence of diffusion deposition of ultrafine aerosols can be considerably removed by the use of mesh screens or diffusion battery installed before cascade impactor during the aerosol sampling.

A primary standard source of radon-222 based on the HPGe detector.

The present paper describes the prototype of a calibration standard system for radon concentrations to be used in establishing the traceability of radon concentration measurements in dwellings. Radon gas was generated with a radium-226 solid source in a certified volume as a closed system. The activity of the radon that was released in the closed system was determined from the difference between the absolute activity of the standard radium solid source and the residual radon decay products (214Bi or 214Pb). A high-purity germanium (HPGe) detector, which was calibrated using gamma reference standard sources, was used to measure the activity of a radium solid source and radon decay products (214Bi or 214Pb). The emanation factor of the 226Ra source was controlled online with the HPGe detector. Radon activity was achieved at ~1500±45Bq from the radium source at 3.95±0.2kBq under equilibrium conditions. After this activity, the radon gas was transferred into the closed system producing radon activity concentrations of 31.1±0.3kBq/m3. Systematic errors were found of less than 4% with a random error around 0.5%. The random error is generally associated with the estimation of the count rate of the measured radon progenies (214Po and 214Po for alpha measurements or 214Pb and 214Bi for gamma measurements), but systematic errors are associated with the errors introduced by the instrumentation and measurement technique. The system that was developed has a high degree of accuracy and can be recommended as a national or regional prototype standard of radon activity concentration to calibrate different working radon measurement devices.

Prototype of a primary calibration system for measurement of radon activity concentration.

To calibrate measurement devices for monitoring the activity concentration of (222)Rn in air, a prototype of a calibration facility is tested using a solid (226)Ra source and a high-purity germanium (HPGe) detector. An emanation box was mounted on the detector for online gamma measurements. Inside this box, a 32.8 kBq ±3% (226)Ra standard source was placed. An AlphaGUARD control radon monitor was connected to the emanation box with a pumping air system in an open flow mode as a reference monitor. The emanation coefficient of the source was controlled online by comparing the gamma activity of (214)Bi (Eγ=609.3 keV), progeny of (226)Ra, to that of the calibration source. A standard (137)Cs source, installed within the emanation box, was used as a reference for gamma spectroscopy using the HPGe detector, with a total systematic error of 4% and a random error less than 2%. The ratio between gamma measurements and AlphaGUARD was 0.94±0.4; which is within the 9% uncertainties of AlphaGUARD calibration.

Calibration system for radon EEC measurements.

The measurement of radon equivalent equilibrium concentration (EECRn) is very simple and quick technique for the estimation of radon progeny level in dwellings or working places. The most typical methods of EECRn measurements are alpha radiometry or alpha spectrometry. In such technique, the influence of alpha particle absorption in filters and filter effectiveness should be taken into account. In the authors' work, it is demonstrated that more precise and less complicated calibration of EECRn-measuring equipment can be conducted by the use of the gamma spectrometer as a reference measuring device. It was demonstrated that for this calibration technique systematic error does not exceed 3 %. The random error of (214)Bi activity measurements is in the range 3-6 %. In general, both these errors can be decreased. The measurements of EECRn by gamma spectrometry and improved alpha radiometry are in good agreement, but the systematic shift between average values can be observed.