Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Probabilistic evaluation of the energetics upper bound during the transition phase of an unprotected loss of flow accident for a sodium cooled fast reactor by using a Phenomenological Relationship Diagram Fabrizio Gabrielli a, ⁎ , Werner Maschek a , Rui Li b , Claudia Matzerath Boccaccini a , Michael Flad a , Simone Gianfelici a , Barbara Vezzoni c , Andrei Rineiski a a Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany b Faculty of Applied Natural Sciences and Industrial Engineering, Deggendorf Institute of Technology, Dieter-Görlitz-Platz 1, D-94469 Deggendorf, Germany 1 c Framatome, 1, Place Jean Millier, 92084 Paris, France 1 ARTICLE INFO Keywords: Risk analyses Nuclear reactor safety Severe accidents Sodium-cooled fast reactors Probabilistic safety assessment Phenomenological Relationship Diagram ABSTRACT One of the main research goals of the GEN-IV systems is enhancing their safety compared with the former Sodium-Cooled Fast Reactor (SFR) designs. A key issue is the capability of accidents prevention as well as of demonstrating that their consequences do not violate the safety criteria. In order to fulfill such requirements, risk analyses of severe core disruptive accidents are performed. Since the beginning of the SFR development, Hypothetical Core Disruptive Accidents (HCDAs) have played an outstanding role. Numerous safety analyses have been performed for developing and licensing past SFR designs and nowadays a large database of results is available. In particular, a large amount of results of the mechanistic SIMMER-II and SIMMER-III/IV analyses for various core designs and different power classes is available at the Karlsruhe Institute of Technology (KIT). The current paper describes the probabilistic approach based on the Phenomenological Relationship Diagram (PRD), which is used to evaluate the Probability Distribution Function (PDF) of the thermal energy release during the transition phase of an unprotected loss of flow accident scenario for a SFR. The technique allows taking into account the mechanistic nature of the accident scenario. In fact, the available results of the mechanistic analyses of HCDAs in SFRs are used to assess the PDFs of the dominant phenomena affecting the thermal energy release, which are propagated in the PRD by employing a Monte Carlo method. 1. Introduction The analyses of Hypothetical Core Disruptive Accidents (HCDAs) for Sodium-cooled Fast Reactors (SFRs) play an outstanding role in the safety assessment since the beginning the SFR development in the ‘50 s (Bethe and Tait, 1956). Nowadays, the Integrated Safety Assessment Methodology (ISAM) is employed for the Generation IV nuclear systems to enhance the SFR safety compared with the former concepts (GIF IV, 2002, 2014). The final goal aims at achieving a robust architecture to prevent severe accident conditions and to demonstrate that their con- sequences do not violate safety criteria. The defense-in-depth-concept is the key to guarantee the robustness in the safety assessment of the system (Fiorini, 2009). Within this framework, the probabilistic eva- luation of the most severe consequences of a given accidental scenario plays a fundamental role. The upper bound energetics during the Transition Phase (TP) of an Unprotected Loss of Flow (ULOF) accident is one of the key parameters of the safety analyses of HCDAs, the ULOF transient being usually considered as the key Beyond Design Basis Accident (BDBA) initiator (Bohl, 1979; Maschek and Asprey, 1983; Kondo et al., 1985; Theofanous and Bell, 1985). As described e.g. in (Maschek, 1982), the TP is characterized by a progressive core disruption where local multi- phase fuel/steel pools grow radially after hexcan destruction. The analyses performed in the past for oxide cores of different power classes (Bohl, 1979; Maschek and Asprey, 1983; Kondo et al., 1985; Theofanous and Bell, 1985) did show that, in such conditions, fuel re- compaction phenomena, due to coherent material motion (sloshing) in the core might occur. Since the fuel in SFRs cores is not arranged in its neutronically most reactive configuration, recriticality events may re- sult due to the motion of the molten fuel in core regions characterized https://doi.org/10.1016/j.nucengdes.2018.11.004 Received 30 August 2018; Received in revised form 29 October 2018; Accepted 2 November 2018 ⁎ Corresponding author. E-mail address: fabrizio.gabrielli@kit.edu (F. Gabrielli). 1 Current address. Nuclear Engineering and Design 341 (2019) 146–154 0029-5493/ © 2018 Elsevier B.V. All rights reserved. T