Statistical sampling applied to the radiological characterization of historical waste

Biagio Zaffora,Francesco Paolo La Torre,Gilbert Saporta,Matteo Magistris

doi:10.1051/epjn/2016031

Abstract

The evaluation of the activity of radionuclides in radioactive waste is required for its disposal in final repositories. Easy-to-measure nuclides, like γ-emitters and high-energy X-rays, can be measured via non-destructive nuclear techniques from outside a waste package. Some radionuclides are difficult-to-measure (DTM) from outside a package because they are α- or β-emitters. The present article discusses the application of linear regression, scaling factors (SF) and the so-called “mean activity method” to estimate the activity of DTM nuclides on metallic waste produced at the European Organization for Nuclear Research (CERN). Various statistical sampling techniques including simple random sampling, systematic sampling, stratified and authoritative sampling are described and applied to 2 waste populations of activated copper cables. The bootstrap is introduced as a tool to estimate average activities and standard errors in waste characterization. The analysis of the DTM Ni-63 is used as an example. Experimental and theoretical values of SFs are calculated and compared. Guidelines for sampling historical waste using probabilistic and non-probabilistic sampling are finally given.

Highlights

The evaluation of the activity of the radionuclides in radioactive waste is required for its disposal in final repositories
These methods can be separated into 2 classes: – Authoritative and stratified sampling allow us to estimate an average content of Ni-63, which is identical for each single package of the batch. – Geometric scaling factor (SF), mean SF and linear model allow us to estimate the specific activity of Ni-63 in each package, scaled by the activity of Co-60
We tested and compared different techniques to sample historical waste. These methods were used to estimate the concentration of Ni-63 in copper after studying the correlation between a key nuclide and the DTM nuclide of interest

Summary

Introduction

The evaluation of the activity of the radionuclides in radioactive waste is required for its disposal in final repositories. The characterization of radioactive waste includes establishing the list of radionuclides, together with their specific activity, inside each package. For historical waste, which is defined as waste collected before the implementation of a traceability system [1], the radiological characterization process is complex. Some of the radionuclides are easy-tomeasure (ETM) from outside the waste package by means of nuclear non-destructive assay, such as g-spectrometry. Other radionuclides, such as pure-b, a and low-energy X-rays, are difficult-to-measure (DTM) or impossible-to-measure (ITM) by non-destructive techniques. When an experimental statistical correlation can be established between an ETM and DTM radionuclides, the scaling factor (SF) method can be applied to quantify the specific activity of DTMs [2]. ETM radionuclide statistically correlated to a DTM is defined the tracer or the key nuclide (KN)

Methods

Findings

Discussion

Conclusion