RECASS source term estimation sub-system and its application for reconstruction of the source rate of the Chernobyl accident

The work discusses the algorithm which is used in the RECASS for reconstruction of the dynamics of a multi-isotope source from outdoor measurements. The algorithm is based on applying projective geometry to minimise the objective. The contamination surface and/or space is divided into several domains of different area/volume and equal contamination. Squares of deviations of calculated and modelled contamination in these domains are minimised. The algorithm allows the use of some linear restrictions (non-negativity of source rate, assumption of dynamics of the source, known relative concentrations of isotopes, direct measurements in the source, etc). The algorithm also calculates the covariance matrix of the source rate. The way in which the software was applied to calculate the dynamics of the release of 137Cs, 131I, 140Ba, 95Zr, 100Ru, 144Ce for the early period of the Chernobyl accident, based on measurements of depositions, is discussed. (Author)