• Energy Research

A comprehensive approach for modeling the pulse height spectra of gamma-ray detectors from passing radioactive cloud in a case of accident at NPP has been developed. It involves modeling the transport of radionuclides in the atmosphere using Lagrangian stochastic model, WRF meteorological processor with an ARW core and GFS data to obtain spatial distribution of radionuclides in the air at a given moment of time. Applying representation of the cloud as superposition of elementary sources of gamma radiation the pulse height spectra are calculated based on data on flux density from point isotropic sources and detector response function. The proposed approach allows us to obtain time-dependent spectra for any complex radionuclide composition of the release. The results of modeling the pulse height spectra of the scintillator detector NaI(Tl) Ø63x63 mm for a hypothetical severe accident at a NPP are presented.

The article represents the approaches introduced in the RELTRAN code developed under the FTP NRS-2 to modeling the radiation consequences for the population caused by possible releases of radioactive substances into the atmosphere during the planned work on the decommissioning of nuclear facilities. One of the tasks of the RELTRAN code is to perform calculations of the consequences of radioactive releases for the population for the tasks of justifying radiation safety. Several modeling options are considered: calculations based on average concentrations over several years, calculations based on average concentrations over several years, considering the schedule of the decommissioning process, case-study modeling approach based on historical meteorological series analysis and case-study modeling approach based on historical meteorological series analysis, considering the schedule of the decommissioning process. It was obtained that, depending on the duration of the planned process and depending on the season, the results differ significantly, for medium-term work, their seasonality becomes an important factor, at the same time, work with conditionally constant emissions lasting more than 6 months has a negligible uncertainty associated with time start of work.

The recovery of non-uniform surface contamination density from measurements of instrumental spectra performed by the airborne survey method is examined. An algorithm based on Tikhonov’s method of regularization is proposed for solving the inverse problem. The results of testing the chosen algorithm on model problems of recovering the 137Cs contamination of a soil surface in the form of spots with different activity and size are presented. The influence of the regularization parameter on the recovery error is analyzed, and the optimal values of the regularization parameter are obtained.

The results of modeling of the spreading of radionuclides as a result of the reactivity accident in Bukhta Chazhma are presented. The computational results are compared with the measured 60Co fallout density and the γ-ray dose rate along the axis of the track. The PROLOG model, developed at the Institute of Problems in the Safe Development of Nuclear Energy (IBRAE) and taking account of the rise height of the radioactive cloud, dispersity, and local orographic features, was used.

The PROLOG software is used to reconstruct the radioactive track formed during the 1957 accident at the Industrial Association Mayak. Using the PROLOG model of a fictitious source and varying the disperse composition of the emissions, satisfactory agreement was obtained between the measured and computed values along the axis of the track and the geometric characteristics of the fallout regions on the surface. However, the discrepancy between the measured and computed dose rates at the populated points closest to the accident site reaches a factor of 1000. A hypothesis is advanced concerning the reason for the discrepancy between the results.

The radioactive track formed on March 15, 2011 during the accident at the Fukushima-1 nuclear power plant (Japan) is reconstructed using the PROLOG–SOKRAT/V3 software system. The emission of radionuclides into the atmosphere and the radiation conditions are estimated taking account of the complex relief of the location. Comparing the computed activity, radionuclide composition and fallout density showed satisfactory agreement with actual measurements.