Abstract
A usual way of radioactive waste drums characterization combines gamma spectrometry measurements with passive neutron measurements. The CEA facility for waste drum characterization hereby described has been operated for more than 30 years. In this framework, a large variety of waste drums has been characterized in terms of spectra, densities, materials and radioactivity levels. As the facility was first dedicated to measure Intermediate-Level Long-lived Waste, the neutron background was not significant compared to expected neutron emitters from waste packages. These last years, Dismantling and Decommissioning operations have been advanced in this CEA site to the point where they are now associated with mostly Very Low Level Waste production. Therefore, neutron background is becoming significant. Using the large variety of past characterized drums brings the opportunity to study this background. The present study has been led over a sample of almost 1500 drums over a wide waste density range. These drums have been selected over the last 20 years by taking into account only one criterion: without any expected neutron emitters from the waste itself. This work first presents the technical settings of the measurement facility before describing the raw data of the measurements. Next, a statistical study over raw data enables to better acknowledge the neutron spallation background behavior in terms of time, density and materials. Ensues a way of using this new knowledge in order to improve how to take into account neutron spallation background in passive neutron measurements of packages of low actinides activities and high densities.
Highlights
The combination of gamma spectrometry with passive neutron measurement has so far been a usual way to characterize radioactive waste [1]
The first device enables to quantify radioactivity in a waste package spectrum through the identification of its gamma emitters, whereas the latter allows the measurement of nuclear material mass through the neutron count rate associated with spontaneous fission of fissile emitters (e.g. 240Pu, the usual neutron tracer for plutonium assay) and to confirm the measurement data obtained by gamma spectrometry
Some difficulties are encountered when alpha radionuclides activities are so low that they cannot be measured in reasonable measurement time durations in gamma spectrometry
Summary
The combination of gamma spectrometry with passive neutron measurement has so far been a usual way to characterize radioactive waste [1]. The first device enables to quantify radioactivity in a waste package spectrum through the identification of its gamma emitters, whereas the latter allows the measurement of nuclear material mass through the neutron count rate associated with spontaneous fission of fissile emitters (e.g. 240Pu, the usual neutron tracer for plutonium assay) and to confirm the measurement data obtained by gamma spectrometry. Expressed as the equivalent mass of 240Pu, the nuclear material detection limit is constrained by the cosmicray background of spallation neutron events taking place in the vicinity and body of the assay chamber, as well as in the waste itself This background cannot be removed by a “simple” coincidence counting. D&D operations is coming in this CEA site, the same electronics have been laid out in order to keep the same raw data format and shall remain so until the end of operations
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