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In the framework of the GUINEVERE (EUROTRANS-IP FP6) and FREYA (FP7) projects , a two-level procedure is developed to measure the 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 to readjust the relative measurements. These calibration measurements require the analysis of the decay of the neutron population triggered by programmed beam interruptions. In this paper, we describe an innovative method developed to measure the prompt multiplication factor, k p , of a subcritical reactor (and therefore its absolute ρ). Unlike conventional methods used for reactivity measurements, our approach only relies on the prompt decay of the neutron population, therefore enabling the use of shorter beam interruptions (< 50 μs) than usual (~ 1 ms). After a review of the method's mechanics, we apply it to measurements performed at the GUINEVERE facility, with the VENUS-F reactor, a zero power lead sub-critical fast reactor. During these measurements, the neutron source was operated in pulse mode. Our results, promising, are compared with reference reactivity values provided by a MSM analysis.

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... There is no protocol on how to best perform, and what can be learned from, measurements on increasingly complex reactor systems, such as zero-power pin-type pool research reactors. Furthermore, these types of measurements can also inform protocol for future subcriticality monitoring measurements on accelerator driven reactor systems (Dulla et al., 2014;Chabod et al., 2014;Uyttenhove et al., 2014). ...

A new series of subcritical measurements has been conducted at the zero-power Walthousen Reactor Critical Facility (RCF) at Rensselaer Polytechnic Institute (RPI) using a ³He neutron multiplicity detector. The Critical and Subcritical 0-Power Experiment at Rensselaer (CaSPER) campaign establishes a protocol for advanced subcritical neutron multiplication measurements involving research reactors for validation of neutron multiplication inference techniques, Monte Carlo codes, and associated nuclear data. There has been increased attention and expanded efforts related to subcritical measurements and analyses, and this work provides yet another data set at known reactivity states that can be used in the validation of state-of-the-art Monte Carlo computer simulation tools. The diverse (mass, spatial, spectral) subcritical measurement configurations have been analyzed to produce parameters of interest such as singles rates, doubles rates, and leakage multiplication. MCNP®6.2 was used to simulate the experiment and the resulting simulated data has been compared to the measured results. Comparison of the simulated and measured observables (singles rates, doubles rates, and leakage multiplication) show good agreement. This work builds upon the previous years of collaborative subcritical experiments and outlines a protocol for future subcritical neutron multiplication inference and subcriticality monitoring measurements on pool-type reactor systems.

The neutron multiplication factor k-eff is a key quantity to characterize subcritical neutron multiplying
devices and for understanting their physical behaviour, being related to the fundamental eigenvalue of
Boltzmann transport equation. Both the maximum available power - and all quantities related to it, like, e.g.
the effectiveness in burning nuclear wastes - as well as reactor kinetics and dynamics depend on k-eff . Nevertheless, k-eff is not directly measurable and its determination results from the solution of an inverse problem: minimizing model dependence of the solution for k-eff then becomes a critical issue, relevant both for practical and theoretical reasons.

In this paper we examine an alternative method for determining the neutron multiplication factor keff in fast subcritical reactors, based on the observable obtained by calculating the ratio between the ``fast`` component of the neutron flux (in this case by fast we mean neutrons with kinetic energy above 0.5 MeV) and the whole flux at all energies. The assumption behind the present study was that the above mentioned observable would be only sensitive to keff, provided a peripheral fuel rod is considered (i.e. far from the neutron source in subcritical systems) and keff is sufficiently high. Indeed, our results show that, for keff> 0.7, the ratio calculated for LFR or GFR models turns out to show a quite similar trend well approximated by a straight line, while a different slope is observed for a sodium reactor. In our opinion, the results obtained show that such observable, after appropriate calibrations on the real systems under consideration, may be used for determining keff in all operational conditions, in particular for subcritical systems with different types of neutron source and different values of the beam current.

The GUINEVERE project is part of the EUROTRANS Integrated Project of the 6th EURATOM Framework Programme. It is mainly devoted to ADS on-line reactivity monitoring validation, sub-criticality determination and operational procedures (loading, start-up, shut-down, ...) as a follow-up of the MUSE experiments. The project consists in coupling a fast lead core, set-up in the VENUS reactor at SCK•CEN Mol (B), with a GENEPI neutron source under construction by CNRS. To accommodate the accelerator in a vertical coupling configuration, the VENUS building is being heightened. The fast core will be loaded with enriched Uranium and will be moderated and reflected with solid lead (zero power experiment). For the purpose of the experimental programme, the neutron source has to be operated not only in pulsed mode but also in continuous mode to investigate the current-to-flux reactivity indicator in representative conditions of a powerful ADS. In this latter mode it is also required to make short beam interruptions to have access to the neutron population decrease as a function of time: from this spectrum it will be possible to apply different analysis techniques such as “prompt decay” fitting techniques and “source jerk” techniques. Beam interruptions will be repeated at a programmable frequency to improve time spectra statistics. Different sub-criticality levels (keff=0.99, 0.97, 0.95, ...) will be investigated in order to obtain a full set of data points for the final overall validation of the methodology. This paper describes the status of the experimental facility assembling, and the foreseen experimental programme to be started.

MUSE collaboration, 5th EURATOM FP-Contract #FIKW-CT-2000-00063

- F Perdu

F. Perdu et al., Prog. in Nucl. Energy 42 pp 107 (2003)
MUSE collaboration, 5th EURATOM FP-Contract #FIKW-CT-2000-00063. Deliverable # 8:
Final Report (2005)

ENEA, 2011), X. Doliguez (CNRS, 2013), private communications

- M Carta

M. Carta (ENEA, 2011), X. Doliguez (CNRS, 2013), private communications