Elsevier

Applied Radiation and Isotopes

Volume 109, March 2016, Pages 114-117
Applied Radiation and Isotopes

Metrological tests of a 200 L calibration source for HPGE detector systems for assay of radioactive waste drums

https://doi.org/10.1016/j.apradiso.2015.11.039Get rights and content

Highlights

  • Large (200 L) volume drum source designed, produced and certified as CRM in 2007.

  • Source contains 448 identical sealed radioactive 152Eu sources (modules).

  • Two metrological inspections in 2011 and 2014.

  • No statistically significant changes of the certified characteristics over time.

  • Stable calibration source for HPGe-gamma radioactive waste assay systems.

Abstract

In this work we present test procedures, approval criteria and results from two metrological inspections of a certified large volume 152Eu source (drum about 200 L) intended for calibration of HPGe gamma assay systems used for activity measurement of radioactive waste drums. The aim of the inspections was to prove the stability of the calibration source during its working life. The large volume source was designed and produced in 2007. It consists of 448 identical sealed radioactive sources (modules) apportioned in 32 transparent plastic tubes which were placed in a wooden matrix which filled the drum. During the inspections the modules were subjected to tests for verification of their certified characteristics. The results show a perfect compliance with the NIST basic guidelines for the properties of a radioactive certified reference material (CRM) and demonstrate the stability of the large volume CRM-drum after 7 years of operation.

Introduction

Proper activity measurements of gamma-ray emitting radionuclides are needed for complete characterization of radioactive waste (ISO, 2007). The accurate activity estimation strongly depends on the efficiency calibration of the assay system. The precision of the efficiency calibration in its turn depends on the precision of the calibration method. The methods usually applied for calibration of gamma-ray assay systems are: experimental, using radioactive certified reference material (CRM) and/or computational (including Monte Carlo simulations, e.g. Sima et al. (2004)). The experimental method requires a CRM of the same form and size as the sample and, generally, with uniform activity distribution. In the case of measurements of radioactive waste where large volume drums (about 200 L) are usually used, the experimental calibration methods often use ‘quasi-CRM’ whose geometrical form and size are the same as that of the drum, but a uniform activity distribution is simulated by consecutive measurements of a point or linear source placed at several suitable positions in the drum (Bruggeman et al., 1999, Toma et al., 2007). In this case, the only certified source is the linear (or point) source and the so-called ‘calibration source-drum’ relates to the calibration method rather than to a certified reference material. In this work we comment on a real large volume calibration source-drum which is validated as a radioactive CRM.

The large volume source (drum) was designed and produced in 2007 (Mitev et al., 2010). A total of 470 modules were prepared as sealed radioactive sources from a standard solution of 152Eu with activity certified by the Laboratoire Etalons d’Activité of the Compagnie pour l’Etude et la Réalisation de Combustibles Atomiques (LEA CERCA). The modules were produced from polycarbonate material in cylindrical shape (ϕ 25×56 mm) with an activity spot (ϕ 14×2 mm) centred at the geometrical centre of the module (Mitev et al., 2010). The activity per module was 68.6 kBq at the reference date of 18.04.2007 with a 2.1% relative expanded uncertainty (k=2). The activity was estimated through the solution mass activity and the mean mass of the solution dropped in each module. The uncertainty includes the uncertainty of the certified solution (1.5%, k=2) and the uncertainty of the mass of the solution dropped in a module (1.4%, k=2). After a number of tests the modules were certified as Certified Reference Materials (CRM-modules) by the Bulgarian National Metrological Institute in 2007. Of these 470 modules, 448 were chosen to be built into the drum. Thus, the large volume source consists of 448 identical small radioactive 152Eu sources (CRM-modules) with 30.7 MBq total activity and a 2.1% relative expanded uncertainty (k=2). The modules were apportioned in 32 transparent plastic tubes which were placed in holes drilled in a homogeneous wooden matrix which filled the 200 L drum (Fig. 1). A specially designed arrangement of the CRM-modules in the drum ensures that the emission of the drum is practically the same as the emission of a drum with homogenously distributed activity as fully described in Mitev et al. (2010).

After a number of tests the drum was also certified as Certified Reference Material (CRM-drum) by the Bulgarian National Metrological Institute in 2007. Since then the CRM-drum is in operation in the Bulgarian state enterprise “Radioactive waste”. The entire construction of the CRM-drum ensures easy and stable usage over time while allowing the tubes with the radioactive modules to be easily removed from the drum and tested.

The objective of this work is to assess the long term stability of the certified characteristics of the CRM-modules and consequently of the CRM-drum. The results of the metrological tests are presented and discussed.

Section snippets

Method and material

A full description of the CRM-drum, including the characteristics of the modules, container and of the wood matrix, have been described previously (Mitev et al., 2010). The proper usage of the CRM-drum requires periodic tests of its certified characteristics. Essentially the verification of the CRM-drum requires the verification of the CRM-modules themselves. If the certified activity of the modules is demonstrated, then, by design, the verification of the drum requires only a simple check of

Results from the leak test

None of the swabs measured during the inspections in 2011 and 2014 gave a statistically significant net count rate (Fig. 3). The critical level (1.25 min−1, α=0.05) was estimated according to Currie (1968) and the estimated MDAs (a posteriori estimates) were each less than 0.5 Bq. The results show that the modules are leaktight in the sense of ISO (1992).

Results from the activity test

The activity of each of the measured modules was evaluated as a mean value A¯of the activity estimates Ai by 152Eu gamma lines at energies Ei:

A new application of the SRM modules

A total of 470 modules were prepared and certified as radioactive CRM and 448 of them were loaded in the CRM-drum. Each of the remaining modules could be used for efficiency calibration of HPGe detectors. In a recent study, we found that the geometry of the modules is very convenient for HPGe measurements of radon absorbed in small-size thin foils. The HPGe measurement of 222Rn activity absorbed in thin polymer foils is a useful technique in the studies of radon absorption in plastics (

Conclusions

This work demonstrates that the design of the large-volume CRM-drum with many easy-to-remove sealed sources (CRM-modules) makes easier the metrological tests of its certified characteristics and offers years of stable operation. The results of the two inspections show a perfect compliance with the NIST basic guidelines for the properties of a radioactive CRM: the CRM-modules are stable – the test results show no statistically significant changes of the certified characteristics; the entire

Acknowledgments

This work is supported in part by the Bulgarian National Science Fund under contract DFNI T02/13”POLYRAD”. The tests in 2011 and 2014 are performed in the framework of contracts between “Three sigma” LTD and SE “RAW”. We acknowledge the participation of Mr. L. Minev in the gross beta measurements of the swabs.

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Present address: Faculty of Physics, St. Kliment Ohridsky, University of Sofia James Bourchier Blvd., 1164, Sofia, Bulgaria.

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