S. Chabod

Effective delayed neutron fraction measurement in the critical VENUS-F reactor using noise techniques

This paper present the measurements of VENUS-F kinetic parameters using the Rossi-Alpha methods. The VENUS-F reactor is a zero-power reactor based in Mol, Belgium at SCK-CEN [1]: its fuel is made of metallic enriched uranium with pure lead in order to simulate the behavior of a lead fast reactor. The reactor can be operated in a sub-critical state when it is coupled with the GENEPI-3C neutron source [2]. At the beginning of 2014, a measurement campaign was performed in the critical state in order to estimate the kinetic parameters of the reactor. In this paper, two measurements are analyzed at two different powers (approximately 2W and 30W) with 7 different fission chambers (with a 235-U deposit that varies from 1g to 10mg). All the correlation functions needed for the Rossi-Alpha method have been built for each possible set of two detectors for the two power levels and values of the effective delayed neutron fraction obtained are then compared. Experimental results show the importance to operate at a very low power. The final value for the effective delayed neutron fraction is finally estimated to be (730 ±11) pcm and the prompt neutron generation time is estimated to be equal to (0.41 ± 0.04) μsec.

Experimental results from the VENUS-F critical reference state for the GUINEVERE Accelerator Driven System project

The GUINEVERE (Generation of Uninterrupted Intense NEutron pulses at the lead VEnus REactor) project was launched in 2006 within the framework of FP6 EUROTRANS in order to validate online reactivity monitoring and subcriticality level determination in accelerator driven systems (ADS). Therefore, the VENUS reactor at SCK-CEN in Mol, Belgium, was modified towards a fast core (VENUS-F) and coupled to the GENEPI-3C accelerator built by CNRS. The accelerator can operate in both continuous and pulsed mode. The VENUS-F core is loaded with enriched Uranium and reflected with solid lead. A well-chosen critical reference state is indispensable for the validation of the online subcriticality monitoring methodology. Moreover, a benchmarking tool is required for nuclear data research and code validation. In this paper, the design and the importance of the critical reference state for the GUINEVERE project are motivated. The results of the first experimental phase on the critical core are presented. The control rods worth is determined by the positive period method and the application of the Modified Source Multiplication (MSM) method allows the determination of the worth of the safety rods. The results are implemented in the VENUS-F core certificate for full exploitation of the critical core.

Measurements of the mass and isotopic yields of the 233 U(n th , f) reaction at the Lohengrin spectrometer

Over the last 10 years, a vast campaign of measurements has been initiated to improve the precision of neutron data for the involved key nuclei (232Th, 233Pa and 233U) of the innovative Th−233U cycle. This latter might indeed provide cleaner nuclear energy than the present U-Pu one. New measurements of charge and mass distributions of the fission products have been achieved at the Lohengrin spectrometer of the Institute Laue-Langevin (ILL) during fall 2010 to complete the experimental data of 233U(n, f) that exist mainly for light fission fragments. That is why we performed measurements of mass and isotopic yields with a special focus on the heavy fission fragment part. Mass yields were measured by ion counting with an ionization chamber after separation by the Lohengrin spectrometer. Isotopic yields were derived from gamma spectrometry of mass-separated beams using HPGe clover detectors. This paper will present the results of these fission yield measurements along with details on the experimental set-up and the chosen analysis method.

Monitoring fast neutron sources for accelerator driven subcritical reactor experiments

In the framework of nuclear waste management, minor actinides could be incinerated in subcritical reactors driven by an accelerator coupled to a spallation target. For safety reason, this so-called Accelerator-Driven System (ADS) requires on-line and robust core reactivity monitoring. In such a system, a simple proportional relationship exists between the reactivity and the ratio of the beam current to the thermal power of the reactor core. This relationship is planned to be exploited as a relative reactivity indicator by the measurement of both the beam current delivered by the accelerator and the core neutron flux. The GUINEVERE experiment facility, which is being built at SCK-CEN in Mol (Belgium), will be devoted to the study of such reactivity measurement techniques. This zero power experiment will consist of the coupling of a subcritical fast core to an external source of 14-MeV neutrons originating from fusion reactions between a deuteron beam and a tritium target. In this case, the target evolution over time and target inhomogeneities preclude from using the beam current for reliable reactivity monitoring and therefore the external neutron source intensity must be monitored directly. This paper presents the systems envisioned to achieve such a monitoring and the results of test experiments.