Atalante research facility implementation of a rule of fractions for the management of reflecting materials in mass-limited units

International audienceATALANTE,located in Marcoule,is one of the main Nuclear Facilities of the French CEA.In terms of criticality risk prevention, the facility is divided into work units mostly managed through a mass control mode.The limit authorized is 350 g of fissile materialwith 239Pu-H2O as reference fissile medium. This mass limit was determined considering a reflectionby 20 cm of water.Under these conditions, some neutronic reflectors, which are more efficient than watercan be authorized only in limited quantities. In ATALANTEthe reflecting materials identified as requiring a specific management are lead, uranium (235U/Utotal = 1%), graphite, heavy water and beryllium.Initiallya maximum permissible mass was determined for each of these materials taken separately. However, this method requires that when different reflectors are present simultaneouslythe sum of the masses of all these reflectors must be less than the limit specified for the most penalizing of them.This rule has proved to impose too many re-strictions on the operator.A new rule has therefore been implemented:the rule of fractions.To demonstrate that this rule is acceptable, criticality calculations have been performed. The geometric configurations studied are a sphere of 350 g of 239Pu moderated by water and re-flected by various masses of 2 to 6 reflectors (successions of concentric shells) followed by 20 cm of water.It has been concluded that compliance with the new rule makes it possible to ensurecriticality safety in the case of the simultaneous presence of different reflectors

First Industrial Tests of a Matrix Monitor Correction for the Differential Die-away Technique of Historic Waste Drums

International audienceAbstract The fissile mass in radioactive waste drums filled with compacted metallic residues (spent fuel hulls and nozzles) produced at AREVA NC La Hague reprocessing plant is measured by neutron interrogation with the Differential Die-away measurement Technique (DDT). In the next years, old hulls and nozzles mixed with Ion-Exchange Resins will be measured. The ion-exchange resins increase neutron moderation in the matrix, compared to the waste measured in the current process. In this context, the Nuclear Measurement Laboratory (LMN) of CEA Cadarache has studied a matrix effect correction method, based on a drum monitor, namely a $^3$He proportional counter located inside the measurement cavity. After feasibility studies performed with LMN's PROMETHEE 6 laboratory measurement cell and with MCNPX simulations, this paper presents first experimental tests performed on the industrial ACC (hulls and nozzles compaction facility) measurement system. A calculation vs. experiment benchmark has been achieved by performing dedicated calibration measurements with a representative drum and $^{235}$U samples. The comparison between calculation and experiment shows a satisfactory agreement for the drum monitor. The final objective of this work is to confirm the reliability of the modeling approach and the industrial feasibility of the method, which will be implemented on the industrial station for the measurement of historical wastes

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

International audienceAREVA 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 correctio

Status of the nuclear measurement stations for theprocess control of spent fuel reprocessing at AREVA NC/La Hague

International audienceNuclear measurements are used at AREVA NC/La Hague for the monitoring of spent fuel reprocessing. The process control is based on gamma-ray spectrometry, passive neutron counting and active neutron interrogation, and gamma transmission measurements. The main objectives are criticality-safety, online process monitoring, and the determination of the residual fissile mass and activities in the metallic waste remaining after fuel shearing and dissolution (empty hulls, grids, end pieces), which are put in radioactive waste drums before compaction in stainless steel containers. The whole monitoring system is composed of eight measurement stations which will be described in this paper. The main measurement stations n°1, 3 and 7 are needed for criticality control. Before fuel element shearing for dissolution, station n°1 allows determining the burn-up of the irradiated fuel by gamma-ray spectrometry with HP Ge (high purity germanium) detectors. The burn-up is correlated to the $^{137}$Cs and $^{134}$Cs gamma emission rates. The fuel maximal mass which can be loaded in one bucket of the dissolver is estimated from the lowest burn-up fraction of the fuel element. Station n°3 is dedicated to the control of the correct fuel dissolution, which is performed with a $^{137}$Cs gamma ray measurement with a HP Ge detector. Station n°7 allows estimating the residual fissile mass in the drums filled with the metallic residues, especially the hulls, from passive neutron counting (spontaneous fission and alpha-n reactions) and active interrogation (fission prompt neutrons induced by a pulsed neutron generator) with proportional $^3$He detectors. So far, large campaigns of reprocessing of the UOX fuels with a burn-up rate up to 60 GWd/t have been performed at AREVA/La Hague. This paper presents a brief overview of the current status of the nuclear measurement station