Design-oriented tool for Unprotected Loss Of Flow simulations in a Sodium Fast Reactor: Validation and application to stability analyses

Abstract

Within the framework of the Generation IV Sodium-cooled Fast Reactor (SFR) R&D program of CEA (French Commissariat à l’Energie Atomique et aux Energies Alternatives), the reactor behavior in case of severe accidents is assessed through experiments and simulations. Such accidents are usually simulated with mechanistic calculation tools (such as SAS-SFR and SIMMER-III). As a complement to these codes, which give reference accidental transient results, a new Best Estimated Plus Uncertainty approach has been developed by CEA; its final objective being to derive the variability of the main outputs of interest for the safety. This approach involves a fast-running description of extended accident sequences coupling physical models for the main phenomena with advanced statistical analysis techniques. It enables to perform a large number of simulations in a reasonable computational time and to describe all the possible bifurcations of the accident transient. In this context, this paper presents a physical tool (models and results assessment) dedicated to the primary phase of an Unprotected Loss Of Flow accident (ULOF) in a SFR, i.e. before the first hexagonal can failure. This paper focuses on the modeling of neutronic phenomena and of the pin degradation and on their validations, the two-phase sodium thermal–hydraulic behavior having largely been assessed in a previous paper. The physical tool called MACARENa is described before presenting the comparison of its results with experimental tests results for the pin degradation behavior modelling and with mechanistic SIMMER-III code results for the global core behavior evolution during an ULOF transient. This tool is demonstrated to be capable of reproducing the mass-flow rate, reactivity, power evolution, and global pin degradation behaviour during an ULOF with a low discrepancy regarding the same transient simulated with SIMMER-III. In particular, observed discrepancies concerning mass flow rates and power evolutions are under 3% before boiling occurs and, later during the transient, the power excursion starts only 2s earlier in the MACARENa simulation than in the SIMMER-III one. The fast-running tool has then been used for safety-informed design and flow stability analyses of fast reactor systems, more specifically dealing with the ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) core concept. It allowed to emphasize the main dominant phenomena and the significant trends for safety assessment. This work has pointed out the strong impact of the modelling of reactivity feedback due to thermal expansion on the resulting core degradation during an ULOF. It has also highlighted the main scenario parameters and models influencing this transient evolution.