Effect of Mass and Activity Heterogeneities on the Activity Measurement of Large Packages of Radioactive Waste

The activity of a radioactive waste package is usually evaluated from gamma measurements associated with transfer functions. These functions are calculated assuming that both activity and mass distributions are homogeneous. But, generally, activity and mass distributions are not homogeneous. This paper evaluates the effect of heterogeneities on the activity measurements on families of similar waste packages. An error arises, with a systematic part, leading to an overestimation or underestimation of the overall activity in a family of similar waste packages, and a stochastic part, whose mean effect on the overall activity of the family is null.

In order to evaluate the effect of heterogeneities, numerical simulation of the filling of each package has been performed. Some filling parameters are randomly varied, according to the known characteristics of the real packages, so that the mass and activity distributions are different from one package to another but are always coherent with the characteristics of the real packages.

These numerical simulations produce virtual families of packages. A way to fit and demonstrate the representativeness of the virtual family is described, so that the general results computed on this virtual family are applicable for the real family.