MEASUREMENT AND CALCULATION OF CONTROL ROD WORTHS IN MASURCA J.L. Kloosterman, Y. Rugama and M. Szieberth 1 Delft University of Technology, Interfaculty Reactor Institute, Mekelweg 15, NL-2629 JB Delft, Netherlands J.L.Kloosterman@iri.tudelft.nl C. Destouches (on behalf of the MASURCA team) CEA/DEN/CAD, F-13108 Saint Paul-Lez-Durance, France ABSTRACT In this paper, an alternative derivation of the inverse point-kinetics equation is given based on the concept of micro-kinetics, which describes the fission rate in terms of prompt fission chains instead of single fission events. The concept is applicable to fast reactors with a short generation time and has been applied to measurements in the MASURCA reactor in Cadarache. In this reactor, two series of rod drop experiments have been conducted. Big discrepancies exist between the reactivity worth measured with different detectors, and spatial correction factors are needed to correct the measured count rates for the flux depression induced by the rods. These factors were calculated with the two-dimensional diffusion code FX2 applying an XY model of the reactor. The corrected control rod worth measured with monitor B differs significantly from the values measured with the other three monitors, and has been discarded from the analysis. The measured reactivity worth equals 11.7±0.2$ for rod BC1 and 13.5±0.4$ for rod BC2. The reactivity worth of BC1 calculated with MCNP is significantly lower (10.8±0.2$), while the calculated value for BC2 agrees quite well within the margins of the experimental error (13.2±0.2$). Surprisingly, there is quite a large difference for the reactivity worth of BC2 calculated with data from JEF-2.2 and ENDF/B-VI (13.6$ and 12.9$, respectively). This needs further research. Taking into account the approximate solution method and geometrical models, the results from the XY diffusion calculations are in reasonable agreement with the Monte Carlo results (11.1$ and 14.3$ for rods BC1 and BC2, respectively). 1. INTRODUCTION The MUSE project aims at a better understanding of the physics phenomena of sub critical reactors, with the ultimate aim to develop a reliable reactivity monitoring system for accelerator- driven systems. To this purpose the GENEPI deuterium accelerator has been placed adjacent to the MASURCA reactor in Cadarache, and a target (either deuterium or tritium) has been placed at the center of the reactor core. Many experiments are being conducted at this system, until now without the external neutron source in operation. In this paper we describe some control rod drop experiments performed at MASURCA, and analyzed with inverse point-kinetics. Some interesting spatial effects were observed that obviously couldn't be explained by point kinetics and that needs a more sophisticated analysis. In section two of this paper, an alternative derivation of the inverse point-kinetics equations is given 1 On leave from Budapest University of Technology and Economics, Institute of Nuclear Techniques, H-1111 Muegyetem rkp 9, Budapest