Advanced Studies and Statistical Treatment for Sodium-Cooled Fast Reactor Pin Failures During Unprotected Transient Overpower Accident

Abstract

Usually, simulation tools are validated on experimental data considering a Best Estimate simulation case and there is no quantification of this validation, which remains based on a rough expert judgment. This paper presents an advanced validation treatment of the simulation tool OCARINa, devoted to Unprotected Transient OverPower (UTOP) accidents, on two CABRI tests, considering this time, a Best Estimate Plus Uncertainties (BEPU) approach. The output results of interest are both scalar physical data such as the time and location of the pin failure and associated molten mass and vector data such as temperature axial distribution or temperature evolution versus time. This approach is a first step in quantifying the degree of agreement between the calculation results and the experimental results. It is of great interest for the VV&UQ (Verification, Validation and Uncertainty Quantification) approach, which leads to the qualification of scientific calculation tools.Within the framework of the Generation IV SFR R&D project in which the CEA is involved, OCARINa is a physical tool, relevant for performing pre-conceptual design studies, devoted to simulation of UTOP accidents on heterogeneous cores. Such accidents could not be simulated with mechanistic calculation tools such as SAS4A or SIMMER with their current capabilities; the thermomechanical models are not finalized in SIMMER tool and the SAS4A tool is only validated for homogeneous cores. The final objective aims at deriving the variability of the main results of interest to quantify the safety margins.The final use of the OCARINa tool being to perform sensitivity studies on the various possible sodium fast nuclear pre-conceptual core designs, the validation of this tool is first discussed at the pin scale (where separate effect test measurements are available) based on statistical treatment. This enables to determine the lacks and uncertainties of this tool. The modeling is then extended from local pin behavior to global core behavior adding a point kinetic neutronic model. Final simulations of UTOP accidents caused by a uniform space reactivity ramp on an SFR (Sodium-cooled Fast Reactor) core are realized taking into account the specificities of the pins of the various assemblies. The orders of magnitude of mechanical energy released are derived.