Core safety measures in ESFR-SMART

A new EURATOM project, ESFR-SMART, started in September 2017. It follows an earlier project, CP-ESFR, on a commercial-size European Sodium Fast Reactor (ESFR). ESFR-SMART is devoted to ESFR safety and related studies, including new reactor core safety measures, in particular reduction of the sodium void reactivity effect. A “low void” core is expected to show a better safety behavior with respect to the CP-ESFR one. The core optimization has been performed in two steps. A preliminary configuration was established at KIT by assessing several designs, taking into account earlier studies on ESFR and ASTRID cores. Compared to the initial ESFR case, the sodium void effect is reduced by introduction of the sodium plenum above the core and reduction of the fissile core height compensated by its radial extension. In the inner core, the fissile region is shorter, while the lower fertile blanket is longer as compared to those in the outer part. Passive safety devices are foreseen to prevent core degradation. Corium transfer tubes are introduced to avoid re-criticalities after a hypothetical accident. A core reloading scheme for this configuration was developed at PSI. Then, a fine optimization of this design has been performed at EDF with an in-house multi-physics and multi-objective optimization tool called SDDS. The calculated void effect in the region including the core and plenum at the end of cycle has been reduced by 80% to a fraction of the beta-effective value while achieving a near-zero breeding gain and keeping a relatively simple axial configuration. Moreover, the selected core seems to have a satisfying behavior during both Unprotected Loss Of Service Station Power and Unprotected Control Rod Withdrawal hypothetical accidents. Further optimization studies of the control rod design and the passive safety devices will be conducted, as well as Monte-Carlo neutronics calculations. The improved safety behavior should be confirmed by further analyses foreseen in ESFR-SMART.