Sodium-cooled fast reactor pin model for predicting pin failure during a power excursion

International audienceWithin the framework of the Generation IV Sodium-cooled Fast Reactor (SFR) in which the CEA (FrenchCommissariat à l’Energie Atomique et aux Energies Alternatives) is involved, the French innovative reactordesign behavior under severe accidents conditions has to be assessed. Such accidents have mainly been simulatedwith mechanistic calculation tools (such as SAS-SFR and SIMMER-III). As a complement to these codes,which provide reference accidental transient calculations, a new physico-statistical approach is being developedat CEA; its final objective being to derive the variability of the main results of interest to quantify the safetymargins. This approach requires fast-running tools to simulate extended accident sequences, by coupling modelsof the main physical phenomena with advanced statistical analysis techniques. The tool enables to perform alarge number of simulations in a reasonable computational time and to describe all the possible scenario progressionsof the hypothetical accidents. This general approach, combining mechanistic codes and evaluationtools, has already been conducted for some accidental initiator families (USAF – Unprotected SubAssembly Fault(Marie et al., 2016) and ULOF – Unprotected Loss Of Flow (Droin et al., 2017). In this context, this paperpresents a physical tool (numerical models and result’s assessment) dedicated to the simulation of the beginningof the primary phase of the Unprotected Transient OverPower accidents (i.e. before failure of sub-assemblywrapper).At the beginning of this primary phase, the fast increase of nuclear power induces a strong temperature rise inthe fuel pellets leading to strong mechanical and thermal loads on the cladding which could lead to clad failureor/and fuel meltdown. These phenomena are described and modelled analytically in single pin geometry inaccordance to the level of details required to catch all the decisive phenomena.Slow power increase transients, such as control rod withdrawal, and fast power increase transients have beeninvestigated in the past. Experimental validation on CABRI (experimental reactor dedicated to safety studies)and CESAR (Circuit d’Etude de l’ébullition du Sodium lors d’un Accident de Réactivité) experiments were carriedout focusing on the amount of molten fuel formed during the transient, on the propagation of the void front inthe channel in case of sodium boiling and on the pin failure mechanisms. Furthermore, a comparison of thephysical tool calculation results was performed against reference accident SIMMER-III calculations.The tool is demonstrated to be able to predict the radial propagation of the molten zone in the fuel pin, the pinfailure mechanism and void front propagation with a discrepancy of less than 10%. In the future, this physicaltool, associated with a point kinetic neutroni