Analysis of Cliff Effects and Thermal Hydraulic Instabilities in the PLANDTL-2 Sodium Experiment Transient Tests
Résumé
Separate Effect Tests (SET) and Integral Effect Tests (IET) are commonly used in support of reactor designs, both to analyze physical phenomena and to validate simulation codes. In the framework of the Franco-Japanese collaboration on Research and Development for Sodium Fast Reactors (SFR) thermal hydraulics, transient tests were performed in the IET named PLANDTL-2 test facility in Japan. This IET's instrumented test section is composed of an electrically heated core and a hot pool with a Dipped Heat Exchanger (DHX). The Intermediate Heat Exchanger (IHX) and the Electro-Magnetic Pump (EMP) are located in a deported primary loop. Studied transients consist in transition from forced to natural convection, in the hot pool and in the primary circuit, under various decay heat removal operations using the DHX. It was observed that in the long term, a cliff effect occurs, meaning that the apparent steady natural convection is perturbed if a threshold is reached. Instabilities and flowrate oscillations from positive to negative values in the primary loop are observed after a period of smooth natural circulation. The unstable behavior results from the competition between IHX and DHX cooling, the latter leading to an increase in thermal stratification in the hot pool. This paper aims to analyze this phenomenon, bring a comprehensive criterion for the onset of unstable behaviors and give some general guidelines to avoid such effects for accidental transient management.
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