ELSY Neutronic Analysis by deterministic and Monte Carlo methods: an innovative concept for the control rod systems

This paper deals with the neutronic design of ELSY (the European Lead-cooled SYstem), a 600 MWe Fast Reactor developed within the 6th EURATOM Framework Programme. ELSY aims at being an “adiabatic” system (as far as possible) in order to fulfill both the requirements of sustainability and proliferation resistance. It represents the European solution for the Lead Fast Reactor (LFR), one of the six candidate typologies proposed by the Generation-IV International Forum (GIF). The analysis of the ELSY reference configuration, with typical pure MOX loading, is here presented. An introductory investigation of the adiabatic and, possibly, the burner options viability is also achieved by providing a rough estimate of the Minor Actinides (MAs) equilibrium concentrations and time constants. One of the main challenge-points in the design of the core, made up of wrapper-less square Fuel Assemblies (FAs) according to the common scheme of PWRs, is the small delta-T between the coolant average outlet temperature (480 °C) and the allowable cladding one (550 °C): it requires a rather flat radial power distribution, obtained by segmenting the core in three zones with different enrichments. Three different control sets have been introduced in order to achieve the required reliability for reactor shutdown and safety systems: eight traditional concept Control Rod (CR) assemblies together with two independent systems of sparse control “Finger Absorber” Rods (FARs), small B4C rods that can be inserted, in principle, in the center of each FA. One of the two finger absorber systems includes a subset of rods devoted to the regulation of the criticality swing during the cycle: their number can be limited indeed since the small reactivity swing (some hundreds pcm) due to the about unitary breeding ratio. Such an innovative solution can also be positioned in order to maintain an optimal power flattening during the fuel cycle. To verify the feasibility of this solution, a very detailed neutronic analysis, adopting both deterministic and stochastic approaches, has been carried out. It becomes crucial indeed to estimate accurately the self-shielding phenomenon of the innovative FARs in order to achieve the aimed performances (a reactivity worth of about 3000 pcm for scram).