Influence of neutron moderating materials in the characterization of 200 L radioactive waste drums by neutron activation analysis

A prompt and delayed gamma neutron activation analysis facility using a 14 MeV neutron generator wasdeveloped at Forschungszentrum Jülich GmbH at a level of a prototype facility for the determination ofnonradioactive toxic and reactive elements in 200 L standard waste drums [1]. Analytical performance andsensitivity of this facility called MEDINA (Multi-Element Detection based on Instrumental Neutron Activation)were investigated for a 200 L steel drum filled with a concrete matrix [1,2] as concrete often is used asembedding material in waste conditioning. However the waste may also contain additional inhomogeneouslydistributed neutron moderating and absorbing materials and/or gamma shielding materials which hamper thereliability and accuracy of the analytical results.In this work, the influence of the presence of neutron moderating material in a concrete matrix on thedetermination of the waste matrix elemental composition is being investigated. Measurements were performedfor various axial and radial distributions of polyethylene and concrete bodies placed in 200 L steel drums. Theneutron generator was operated in a pulsed mode at a neutron emission of about 8·107 n·s-1. The neutron pulsewas set to 50 μs and the repetition period of the neutron pulses to 1 ms. The 14 MeV neutrons are moderated andreflected by the graphite walls of the irradiation chamber and thermalized by the sample as well. The gamma-rayspectra were acquired between the neutron pulses over a counting time of 940 μs after a waiting time of 10 μsafter end of the neutron pulses. The gamma-ray spectra were recorded for a total counting time of 3600 s. Thenet areas of the gamma-ray peaks for the isotopes of interest (1H, 10B, 12C, 28Si, 40Ca and 56Fe) are obtained bymanual analysis of the spectra using the Gamma-W spectroscopic software. The mean thermal neutron flux wasdetermined from the prompt gamma-rays of 56Fe induced by the activation of the steel drum [3]. The elementalcomposition of the PE/concrete matrix is calculated assuming that the drum is filled only with concrete. Thiscase would correspond to the real assay of inhomogeneous historical waste drums with no a priori knowledge ofthe content except the waste density. Discrepancies between measured and expected elemental concentration areevaluated and discussed involving MCNP5 simulations results.References[1] E. Mauerhofer, A. Havenith, The MEDINA facility for the assay of the chemotoxic inventory of radioactive waste packages, J.Radioanal. Nucl. Chem. (2014) 302:483-488[2] A. Havenith, Stoffliche Charakterisierung radioaktiver Abfallprodukte durch ein Multi-Element-Analyseverfahren basierend auf derinstrumentellen Neutronen-Aktivierungs-Analyse – MEDINA – Schriften des Forschungszentrum Jülich, Energie & Umwelt / Energy &Environment, Band/Volume 248, ISBN 978-3-95806-033-2[3] International Patent Application WO 2012/010162 A1; Australian Patent AU201128201