Αβεβαιότητα στην εκτίμηση της παραμένουσας ραδιενέργειας ¹³⁷Cs στο έδαφος μετά από απόθεση λόγω πυρηνικού ατυχήματος

The evaluation of the remaining isotopes concentration levels in soil, both on the surface and also in depth, after a Chernobyl like nuclear accident is important for the monitoring of possible effects to the ecosystem because of the accident. Within the presented research, soil samples have been collected in 2007 at 14 different locations of the Hellenic terrain, both from the surface and also from depths up to 26cm; sampling locations were selected from the measurements pool where NTUA Nuclear Energy Laboratory had identified high levels of ¹³⁷Cs deposition during and after the year 1986. In one specific location of high deposition samples were collected following a 60m by 60m Cartesian grid with a 20m nodes distance between nodes. Single surface and in depth samples were collected from the rest 13 locations. The uncertainty components were investigated as these evolve during sample processing and isotope concentration measurements and were found mainly due to the geostatistical variation, the sampling process itself, the variation in the effective density of soil by depth, the preparation of samples for measurement and the measurement itself. A cause-effect assessment process was used to explain these components, in order to identify the most important ones and to properly apply the law of error propagation as described in ISO GUM. Moreover, besides the inventorying of the in depth migration of 137 Cs twenty years after the deposition using the collected data and appropriate fittings, the whole process demonstrated that the dominating component of the combined uncertainty is the one due to the horizontal geostatistical variation of the deposited isotopes concentration. The sampling grid seems to provide a gross evaluation of this component, mainly by the use of the semivariogram technique. Denser, optimized grid could return more accurate values for this component but with a multiplied laboratory cost, both in terms of human and material resources. Using the hereby collected data and in the case of a single depth soil sampling, this uncertainty component has been evaluated to about 23% of the measurement result for the total remaining ¹³⁷Cs deposition and to about 38% of the calculated value for ¹³⁷Cs effective penetration depth.

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