Impact of Heterogeneity on Gamma-Spectrometry Data at Nondestructive Assay of Radioactive Waste

The uncertainty of the direct gamma-spectrometry method at a nondestructive assay of radioactive wastes is considered by taking into account the heterogeneity (over the container’s volume) of the activity and of the gamma-attenuation factor distributions. Since it is impossible to obtain in a real assay the estimations of the uncertainty in individual cases, the “maximal uncertainty” approach to characterize the method is introduced in this study. It is shown that the relative maximal uncertainty of the assay is comparatively small in a rather wide range of container’s volume values (at least for bulk volumes up to $5–8{\text{m}}^3$ this uncertainty does not exceed 200–400%). That uncertainty, owing to the heterogeneity of activity, gives a main contribution in the total value. Nonuniformity of the attenuation factor (i.e., waste matrix’s density and/or effective atomic number heterogeneity) gives a comparatively less contribution to the total uncertainty, the average value of measured gamma flux turns out to be positively shifted relating to the flux value for the homogeneous waste matrix, and leads to overestimating of the activity value at the assay of heterogeneous wastes.