The use of non-destructive passive neutron measurement methods in dismantling and radioactive waste characterization

The cleaning up and dismantling of nuclear facilities lead to a great volume of technological radioactive wastes which need to be characterized in order to be sent to the adequate final disposal or interim storage. The control and characterization can be performed with non-destructive nuclear measurements such as gamma-ray spectrometry. Passive neutron counting is an alternative when the alpha-gamma emitters cannot be detected due to the presence of a high gamma emission resulting from fission or activation products, or when the waste matrix is too absorbing for the gamma rays of interest (too dense and/or made of high atomic number elements). It can also be a complement to gamma-ray spectrometry when two measurement results must be confronted to improve the confidence in the activity assessment. Passive neutron assays involve the detection of spontaneous fission neutrons emitted by even nuclides (²³⁸Pu, ²⁴⁰Pu, ²⁴²Pu, ²⁴²Cm, ²⁴⁴Cm...) and neutrons resulting from (α, n) reactions with light nuclides (O, F, Be...). The latter is conditioned by the presence of high α-activity radionuclides (²³⁴U, ²³⁸Pu, ²⁴⁰Pu, ²⁴¹Am...) and low-Z elements, which depends on the chemical form (metallic, oxide or fluorine) of the plutonium or uranium contaminant. This paper presents the recent application of passive neutron methods to the cleaning up of a nuclear facility located at CEA Cadarache (France), which concerns the Pu mass assessment of 2714 historic, 100 litre radioactive waste drums produced between 1980 and 1997. Another application is the dismantling and decommissioning of an uranium enrichment facility for military purposes, which involves the ²³⁵U and total uranium quantifications in about a thousand, large compressors employed in the gaseous diffusion enrichment process.