Monte Carlo MSM Correction Factors for Control Rod Worth Estimates in Subcritical and Near-Critical Fast Neutron Reactors

The VENUS-F reactor core is very modular and its reactivity can range from deep subcritical to critical by varying the number of fuel assemblies in the core. It is also equipped with six safety rods, one absorbent rod for rod drop experiments and two boron carbide control rods which allow for a finer tuning of the reactivity. Ten fission chambers, spread throughout the reactor reflector and the core, allow recording count rates during steady-state and time-dependent measurements. In order to test on-line subcriticality monitoring techniques, the reactivity of all the VENUS-F configurations used must be known beforehand to serve as benchmark values. Thus the reactivity worth of the control rods must be known as accurately as possible so that the reactivity of every new reactor configuration created by moving the control rods can be estimated correctly. Although reactor asymptotic period measurement is a usual technique to determine the reactivity worth of control rods, it is limited to a very small reactivity range (from  -0.3 $ to +0.3 $) and consequently does not always allow measuring the total reactivity. Furthermore it is obviously inapplicable to control rod worth measurement in deep subcritical reactors. This is the reason why the Modified Source Multiplication Method (MSM) was used as an alternative method for estimating the reactivity worth of the VENUS-F control rods. The unknown reactivity is determined by comparing detector count rates driven by an external neutron source in the configuration of interest with those obtained in another subcritical configuration whose reactivity is known (reference configuration). However, to account for the flux shape differences between the two reactor configurations, some position-dependent correction factors (the so-called MSM factors) must be calculated using a neutron transport code. In this paper, we will present calculations of MSM factors performed with the Monte Carlo neutron transport code MCNP for estimating the VENUS-F control rod worth when the reactor is subcritical and near critical. (Dis-) advantages of using a stochastic code instead of a deterministic one will be discussed. General trends in the MSM factor behavior depending on neutron source and detector locations, as well as on the subcriticality level will be outlined. Finally the calculated MSM correction factors will be applied to detector count rates measured for various control rod heights, with the GENEPI-3C source in subcritical configuration and with an Am-Be source in near-critical configuration. Consistency between reactivity values given by all detectors will be discussed