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SESAME Newsletter #1 (March 2016)

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EDITORIAL: MARIANO TARANTINO (MARCH 2016) SESAME TO COLLABORATE WITH US PARTNERS ANL AND ORNL FLOW PULSATIONS AND VIBRATIONS SIMULATION PORTRAIT OF A YOUNG SCIENTIST: MEET FULVIO BERTOCCHI PHENIX DISSYMMETRIC TEST SIMULATION OF INTER-WRAPPER FLOW UNCERTAINTY QUANTIFICATION NEW INFRASTRUCTURES – THE KASOLA FACILITY

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CIRCE EXPERIMENTS

CIRCE (CIRColazione Eutettico) is the largest pool-type facility worldwide adopting heavy liquid metal (lead-bismuth eutectic, LBE) as working fluid. It is a unique facility, not only due to its size (a vessel of about 10 m with a diameter of 1.2 m), and its LBE inventory (up to 90 t), but most of all for its intrinsic features. Indeed, CIRCE is the only facility in which we can reproduce, at a relevant scale (up to 1 MW), the thermal-hydraulic behaviour of a heavy liquid metal cooled pool-type reactor, both under steady state and transients conditions.

Thanks to a 1 MW fuel pin bundle simulator, made up of 37 electrical pins (O.D. 8.2 mm, 1 MW/m2) placed in a grid-spaced wrapped hexagonal lattice, a main heat exchanger and a decay-heat-removal heat exchanger, CIRCE allows to reproduce properly the transition from the steady state forced circulation at full power to the natural circulation in decay-heat removal condition; it is operated in the framework of SESAME for this purpose.

The reproduction of this transition is of paramount importance to analyse the natural circulation onset, the mixing and stratification phenomena occurring in the pool, the coolability of the fuel pin bundle (the only fuel pin bundle worldwide operated in a LBE pool), and the coolability and stability of the whole primary system after an accidental scenario like the Fukushima event.

Up to now, several experiments have already been performed in this frame, and the first post-test analysis is ongoing.

One main goal of the experimental activity is the characterisation of forced and mixed convection in a Heavy Liquid Metal (HLM) because heat transfer significantly differs from the well-known one in water. This changed behaviour is mainly due to the Prandtl (Pr) number difference between the two media: liquid metals have a quite lower Pr with respect to water, 2 to 3 order of magnitude less. Most of the related experimental work available in scientific literature deals with sodium-potassium alloy or mercury as reference fluid. Therefore, specific experimental tests with Lead and LBE are mandatory in support of the LFR/ADS core thermal-hydraulics design. CIRCE with its unique capabilities fills this scientific gap.

Moreover, the experimental campaigns performed in CIRCE facility allow to characterise the phenomena of mixed convection and stratification in a liquid metal pool in a configuration relevant for safety concern.

Concerning natural circulation phenomena of interest in the nuclear field, once again most of the works available in literature consider water or sodium as working fluids. Furthermore, most of them neglect the thermal stratification that is instead considered one of the most important topics in the study of LFR/ADS reactors in order to increase the reactor safety and its structural integrity. In a typical accidental scenario assuming the total loss of the pumping system, the reactor is scrammed, and assuming the total loss of the pumping system, the coolant flow rate reduces drastically: large temperature variations take place and thermal stratification phenomena inside the pool are strengthened. A stiff vertical temperature gradient may induce significant thermal loads on the structure in addition to existing mechanical ones. Moreover, due to the instability of the stratification interface position, as seen in the sodium context, low frequency oscillations with large amplitude are generated. Since the thermal conductivity of HLM is 10-100 times higher than that of water (lead at 450 °C has a thermal conductivity of 17 W/m K) temperature fluctuations are transmitted with low attenuation to the structure, leading to thermal cycle fatigue on the surface of the structure materials. The phenomena must therefore be carefully investigated and CIRCE is properly the right experimental facility required.

Finally, matching the purpose of the SESAME project, CIRCE allows to investigate pool thermal hydraulics and provide experimental data for the validation of CFD models. Due to the integral nature of the facility, the test results are also extremely valuable for the validation of the system codes in mixed-convection conditions and to assess coupled STH/CFD methods, providing also data for code-to-experiment comparison.

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