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International audience; In the framework of the GUINEVERE (EUROTRANS-IP FP6) and FREYA (FP7) projects, a two-level procedure is developed to measurethe time variations of an ADS reactivity ρ(t). First, the time variations of the reactor power and neutron source intensity are monitored online,allowing to extract the fast fluctuations of ρ(t) around a reference value. Then, absolute measurements of ρ are periodically performed toreadjust the relative measurements. These calibration measurements require the analysis of the decay of the neutron population triggeredby programmed beam interruptions. In this paper, we describe an innovative method developed to measure the prompt multiplicationfactor, kp, of a subcritical reactor (and therefore its absolute ρ). Unlike conventional methods used for reactivity measurements, ourapproach only relies on the prompt decay of the neutron population, therefore enabling the use of shorter beam interruptions (< 50 μs) thanusual (~ 1 ms). After a review of the method’s mechanics, we apply it to measurements performed at the GUINEVERE facility, with theVENUS-F reactor, a zero power lead subcritical fast reactor. During these measurements, the neutron source was operated in pulsemode. Our results, promising, are compared with reference reactivity values provided by a MSM analysis.

International audience; In order to incinerate minor actinides and thus to reduce the issues linked to nuclear waste management, Accelerator-Driven Systems(ADS) are being under study. An ADS consists in the coupling of a particle accelerator with a sub-critical fast reactor. The on-line reactivitymonitoring is a serious issue regarding safety, therefore several methods to estimate the reactivity of such sub-critical systems have tobe investigated. Here, we present one method based on the study of the neutron population evolution during beam interruption experimentscarried out in the framework of the FREYA FP7 program [1,2] at the GUINEVERE facility, which couples the fast lead sub-critical reactorVENUS-F with the deuteron accelerator GENEPI-3C at SCK-CEN in Mol, Belgium. After describing the facility, the analysis based on pointkinetics theory and preliminary results of the reactivity measurements will be presented. Then, spatial effects that are not taken into accountby point kinetics theory will be highlighted using MCNP simulations, and correction factors to raw results will be calculated. In the end, finalresults will be compared to reference reactivity values obtained with the Modified Source Multiplication (MSM) method.

International audience; The GUINEVERE facility hosted at the SCK·CEN site in Mol (Belgium) couples the fast subcriticallead reactor VENUS-F with an external neutron source provided by interactions of deuteronsdelivered by the GENEPI-3C accelerator and a tritiated target located at the reactor core center.Part of the experimental program is devoted to the study of subcriticality monitoring techniques.It includes the analysis of Pulsed Neutron Source experiments using the so-called Area Methodtechnique which is based on Reactor Point Kinetics. Experimental results show that the extractedreactivity depends on the detector position inside the reactor. A time-dependent multigroup diffusionapproach using a simplified description of the VENUS-F reactor is developed to computecorrection factors which should allow one to overcome the shortcomings of the Area Method technique.Preliminary results are very promising since the application of the correction factors leadsto an overall reduction in the spread of the area ratio reactivity measurements.KEYWORDS: ADS, area method, multigroup diffusion equation, modal expansion.

International audience; With Accelerator-Driven Systems (ADS), the incineration of large quantities of minor actinides is envisaged by coupling a particleaccelerator to a subcritical core. Despite the subcritical operation of the core, a monitoring tool is indispensable in an ADS for both safetyand operational purposes. Several experimental techniques are under investigation today within the European FP7 project FREYA (FastReactor Experiments for hYbrid Applications) at the VENUS facility at SCK-CEN, where the lead fast zero-power VENUS-F core is coupledto the GENEPI-3C deuteron accelerator. The proposed monitoring concept combines an initial reactivity determination, e.g. by the pulsedneutron source technique, a long-term relative reactivity monitor, by the current-to-flux (CTF) technique, and an interim cross-checking,by the source jerk technique. In this paper the CTF technique is presented as a candidate for continuous reactivity monitoring. Staticmodal analysis is applied to evaluate the point kinetics relationship between accelerator source neutrons and the neutron flux. As a firsttest case, the zero-power VENUS-F SC1 core is considered. Spatial correction on the CTF monitor is evaluated by measurement results forthe control rods worth at different detector positions. Good agreement between the modal and experimental spatial correction factor (SCF) isfound with 9 modes. Limited SCF are found in the VENUS-F reflector, far away from the perturbation. Modal analysis is envisaged for themonitoring of burn-up in demonstrator ADS like MYRRHA.