P. Dessagne

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.

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.

Neutron inelastic cross section measurements for natTi

A neutron inelastic scattering experiment was performed at the GELINA (Geel Electron LINear Accelerator) neutron source of the European Commission Joint Research Centre Geel (EC-JRC Geel) with the aim of determining the reaction cross sections for the stable isotopes of natural titanium. A 235 U fission chamber was used to monitor neutrons with energies up to 20 MeV. The GAINS (Gamma Array for Inelastic Neutron Scattering) spectrometer was employed to detect the γ rays resulting from the decay of the excited nuclei. We determined the γ-ray production cross sections of the first transitions in the 46,48,49,50 Ti isotopes. The experimental values were compared with previous reported results and also with theoretical calculations performed with the TALYS 1.8 code using the default input parameters. Uncertainties of around 5% were obtained for the strongest observed transitions.

GRAPhEME: A setup to measure (n, xnγ) reaction cross sections

Most of nuclear reactor developments are using evaluated data base for numerical simulations. However, the considered databases present still large uncertainties and disagreements. To improve their level of precision, new measurements are needed, in particular for (n, xnγ) reactions, which are of great importance as they modify the neutron spectrum, the neutron population, and produce radioactive species. In 2003, the IPHC group started an experimental program to measure (n, xnγ) reaction cross sections using prompt gamma spectroscopy and neutron energy determination by time of flight. Measurements of (n, xnγ) cross section have been performed for 235,238U, 232Th, nat,182,183,184,186W, natZr. The experimental setup, consisting of HPGe detectors and a fission chamber, is installed at the neutron beam at GELINA (Institute for Reference Materials and Measurements (IRMM), Geel, Belgium). It has recently been upgraded with the addition of a highly segmented 36 pixels planar HPGe detector. The setup is equipped with a high rate digital acquisition system. The analysis of the segmented detector data requires a specific procedure to account for cross signals between pixels. An overall attention is paid to the precision of the measurement. The setup characteristic and the analysis procedure will be presented along with the acquisition and analysis challenges. Examples of results and their impact on models will be discussed. Finally, the perspectives on complimentary experiments will be presented.