Further development and application of a method for assessing radionuclide surface activity distribution and source location based on measurements of ambient dose equivalent rate

Abstract

A method has been developed for solving the Fredholm equation in the barrier geometry for reconstructing the surface radioactive contamination density from the results of measuring the ambient dose equivalent rate (ADER). Inclusion of the barrier geometry means that the method takes into account the shielding effect of buildings and structures on the contaminated site. The method was based on the representation of the industrial site, buildings and radiation fields in the form of a raster and the use of the visibility matrix of raster cells to describe the barrier geometry. The developed method was applied to a model industrial site with a size of 200x200 conventional units for 4 types of surface activity density (SAD) distribution over the surface of the industrial site: "fragmentation", "diffuse", "uniform" and "random". The method of Lorentz curves was applied to estimate the compactness of the distributions of surface activity density and the ADER for the considered radiation sources. It was shown that the difference between the Lorentz curve for SAD and ADER means that the determination of the spatial distribution of SAD over the industrial site by solving the integral equation is essentially useful for determining the location of radiation source locations on the industrial site. The accuracy of SAD reconstruction depends on the following parameters: resolution (fragmentation) of the raster, the height of the radiation detector above the scanned surface, and the angular aperture of the radiation detector. The measurement of ADER is simpler and quicker than the direct measurement of SAD and its distribution. This represents a significant advantage if SAD distribution needs to be determined in areas with high radiation dose-rate during limited time. The developed method useful for supporting radiation monitoring and optimizing the remediation of nuclear legacies, as well as during the recovery phase after a major accident.

Further development and application of a method for assessing radionuclide surface activity distribution and source location based on measurements of ambient dose equivalent rate. Examples for Andreeva Bay, Chernobyl NPP and Istiklol

“…where matrix coefficients of the SLAE are in matrix F, the measured ADER values are in vector-column P, and unknown SAD values-in vector-column A [7].…”

“…Lorenz curves were generated as described in [7] and used to assess the compactness of the SAD distributions on the basis of ADER.…”

“…In other words, in reality, the industrial site may be contaminated with radioactive substances over the surface and in the depth of the soil, and containers with SRW or individual large-sized fragments of SRW may be located on the surface of the industrial site. However, the approximate solution of the integral equation according to equation (1) 12 from the first part of this article, which is submitted for publication and is under review by J Radiol Prot (JRP-102885.R1) [7] reduces the real picture of radioactive contamination of the industrial site to the distribution of 'effective' SAD.…”

“…, where P(x, y, L) is the DR at the height L (m) above the surface at the point of the space with coordinates (x, y) (m) on the surface from all point radiation sources at the area of radioactive contamination (units are given by column 3 in table 1 [7]); Vis(x, y, x ′ , y ′ ) is the visibility function that takes into account the transmission of ionising radiation through the walls of buildings and constructions (unitless); А(x ′ , y ′ ) is the surface activity of radiation sources localised in the neighbourhood of the point with coordinates (x ′ , y ′ ) (Bq); W is a normalising factor ( [7], table 1]); Q(x, y, x ′ , y ′ , L) is the air kerma rate at the point at L (m) height above the point on the surface with coordinates (x, y) from the isotropic point source without taking into account the scattered radiation with coordinates (x ′ , y ′ ).…”

“…This paper presents work carried out under the regulatory cooperation programme between Federal Medical Biological Agency (FMBA of Russia) and Norwegian Radiation and Nuclear Safety Authority (DSA). It follows up on earlier work on the topic of development and application of a method for assessing radionuclide surface contamination density based on measurements of ambient dose equivalent rate (ADER) [6] and the further development of the technique described in the first part of this article, which is submitted for publication and is under review by J Radiol Prot (JRP-102885.R1) [7].…”