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Abstract: The MEGAPIE project, aiming at the construction and operation of a megawatt liquid lead-bismuth spallation target, constitutes the first step in demonstrating the feasibility of liquid heavy metal target technologies as spallation neutron sources. In particular, MEGAPIE is meant to assess the coupling of a high power proton beam with a window-concept heavy liquid metal target. The experiment has been set at the Paul Scherrer Institute (PSI) in Switzerland and, after a 4-month long irradiation, has provided unique data for a better understanding of the behavior of such a target under realistic irradiation conditions. A complex neutron detector has been developed to provide an on-line measurement of the neutron fluency inside the target and close to the proton beam. The detector is based on micrometric fission chambers and activation foils. These two complementary detection techniques have provided a characterization of the neutron flux inside the target for different positions along its axis. Measurements and simulation results presented in this paper aim to provide important recommendations for future accelerator driven systems (ADS) and neutron source developments. [Copyright &y& Elsevier]

Highlights • The "current-to flux" method for monitoring ADS subcriticality was successfully tested experimentally. • Spatial effects affecting the reactivity estimation can be corrected with Monte Carlo simulations. • The robustness and precision of the correction technique is demonstrated. Abstract In this article, we evaluate the quality and robustness of a method envisaged for the on-line monitoring of the subcriticality of an ADS, called the "current-to flux" (CTF) method. For this evaluation, we performed a dedicated experiment at the GUINEVERE facility. It is hosted at the SCK-CEN and consists of the subcritical VENUS-F reactor coupled to a continuous external neutron source provided by the GENEPI-3C accelerator. During this experiment, the reactor control rods were moved in various patterns, and the subsequent dynamical evolutions of the reactor reactivity were monitored using nine fission chambers (FCs). The space-energy effects that bias the reactivity values are corrected using a procedure based on simulations computed with the Monte Carlo neutron transport code MCNP. We investigate the precision of this correction procedure by comparison with a reactivity value extracted with the beam interruption technique and we demonstrate its insensitivity to the simplifications made on the VENUS-F reactor modeling and to a simulation key parameter such as the boron carbide density filling the control rods. [ABSTRACT FROM AUTHOR]

The GUINEVERE project was launched in 2006 in order to study the feasibility of transmutation in Accelerator Driven subcritical Systems (ADS). This facility, hosted at the Belgian Nuclear Research Center SCK·CEN, couples the fast subcritical lead reactor VENUS-F with an external neutron source provided by the GENEPI-3C accelerator. In order to test on-line subcriticality monitoring techniques, an independent measurement of the reactivity of VENUS-F was achieved using the Modified Source Multiplication Method (MSM). The unknown reactivity of the subcritical configuration of interest was determined by comparing detector count rates driven by an external neutron source in this configuration with those obtained in a slightly subcritical configuration whose reactivity was determined by rod drop experiments. To account for spatial effects, MSM correction factors were calculated for all the detectors using the Monte Carlo neutron transport code MCNP. The corrected reactivity values of all the detectors were found to be consistent. This led to a final estimate of −5.28 ± 0.13 $ for the reactivity of VENUS-F. The MSM factors calculated with MCNP were found to be insensitive to the assumptions made for modeling the reactor and the detectors. Thus the MSM method appears to be a robust technique for measuring large subcriticality values with good accuracy. [ABSTRACT FROM AUTHOR]