Analysis of a steam line break accident of a generic SMART-plant with a boron-free core using the coupled code TRACE/PARCS

The Karlsruhe Institute of Technology (KIT) is engaged in safety-relevant investigations of a generic SMART-plant using available public data. Within this work, a boron-free core that fits into the reactor pressure vessel (RPV) of the SMART-plant has been developed and optimized at the beginning-of-life condition from the safety perspective that fulfils general regulatory requirements. In this paper, the analysis of the behavior of a boron-free core integrated into a generic SMART-plant under a steam line break (SLB) accident using the coupled code TRACE/PARCS is presented and discussed. The SLB-accident in a conventional PWR is an overcooling accident that could lead to re-criticality and return-to-power after the reactor SCRAM. It is characterized by an asymmetrical cooling behavior due to the break of one of the steam lines while others are intact leading to a strong radial non-symmetrical core power distortion. Hence, to capture this physical phenomenon, 3D neutronics and thermal-hydraulics codes are applied for the analysis. The performed investigation showed that the boron-free core does not experience any re-criticality and a return-to-power in comparisons with a conventional PWR due to the low severity impact of the SLB-accident. That is due to the following factors: (a) the unique helical steam generator (SG) design concept, in which there are eight SGs integrated within the RPV, and almost negligible coolant inventory of the secondary side; and (b) the flow mixing header assembly located around the core that is developed to reinforce the coolant mixing within the downcomer. The core decay heat has been proven to be removed passively after the reactor trip thanks to the establishment of natural circulation in both the primary and secondary-side, and the excellent performance of the passive residual heat removal system (PRHRS). Generally, it can be stated that the boron-free core integrated into the generic SMART-plant under an SLB-accident does not threaten the safety limits. Moreover, the SLB-accident is analyzed assuming the failure of all safety systems including the PRHRS. Such a hypothetical accident revealed that the grace time to reach core uncovery is about two hours without any human intervention and safety system actuation.