Reduction of the uncertainty due to fissile clusters in radioactive waste characterization with the Differential Die-away Technique

R. Antoni (1, 2) , C. Passard (1, 2) , B. Pérot (1, 2) , F. Guillaumin (3) , C. Mazy (3) , M. Batifol (3) , G. Grassi (4)

1 LMN - Laboratoire de Mesures Nucléaires
2 CEA-DES (ex-DEN) - CEA-Direction des Energies (ex-Direction de l'Energie Nucléaire)
3 AREVA
4 Groupe AREVA

AREVA NC is preparing to process, characterize and compact old used fuel metallic waste stored at La Hague reprocessing plant in view of their future storage (“Haute Activité Oxyde” HAO project). For a large part of these historical wastes, the packaging is planned in CSD-C canisters (“Colis Standard de Déchets Compacté s”) in the ACC hulls and nozzles compaction facility (“Atelier de Compactage des Coques et embouts”).. This paper presents a new method to take into account the possible presence of fissile material clusters, which may have a significant impact in the active neutron interrogation (Differential Die-away Technique) measurement of the CSD-C canisters, in the industrial neutron measurement station “P2-2”. A matrix effect correction has already been investigated to predict the prompt fission neutron calibration coefficient (which provides the fissile mass) from an internal “drum flux monitor” signal provided during the active measurement by a boron-coated proportional counter located in the measurement cavity, and from a “drum transmission signal” recorded in passive mode by the detection blocks, in presence of an AmBe point source in the measurement cell. Up to now, the relationship between the calibration coefficient and these signals was obtained from a factorial design that did not consider the potential for occurrence of fissile material clusters. The interrogative neutron self-shielding in these clusters was treated separately and resulted in a penalty coefficient larger than 20% to prevent an underestimation of the fissile mass within the drum. In this work, we have shown that the incorporation of a new parameter in the factorial design, representing the fissile mass fraction in these clusters, provides an alternative to the penalty coefficient. This new approach finally does not degrade the uncertainty of the original prediction, which was calculated without taking into consideration the possible presence of clusters. Consequently, the accuracy of the fissile mass assessment is improved by this new method, and this last should be extended to similar DDT measurement stations of larger drums, also using an internal monitor for matrix effect correction

Mots clés: Radioactive waste encapsulation; Nuclear fuel reprocessing; Calibration; Fuel storage; Neutrons; Binary alloys; Beryllium alloys; Radioactive waste storage; Radioactivity; Active neutron interrogation; Calibration coefficients; Differential Die-away Technique; Fissile cluster; Matrix effects; MCNP; Neutron measurements; Radioactive waste characterizations; Radioactive wastes

Domaines: Physique Nucléaire Expérimentale [nucl-ex]

R. Antoni, C. Passard, B. Pérot, F. Guillaumin, C. Mazy, et al.. Reduction of the uncertainty due to fissile clusters in radioactive waste characterization with the Differential Die-away Technique. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2018, 895, pp.144-149. ⟨10.1016/j.nima.2018.03.077⟩. ⟨cea-01992367⟩