Thermo-fluid dynamic analysis of HLM pool. Circe experiments

This work reports a summary of the experimental and numerical activities performed to investigate thermal hydraulic phenomena occurring in response to hypothetical accidental scenarios of Protected Loss of Heat Sink (PLOHS), and Protected Loss of Flow (PLOFA) and the heat transfer in the lead–bismuth (LBE) cooled fuel pin bundle simulator (FPS) of the CIRCE facility. A series of four experiments (PLOHS-PLOFA) were carried out simulating the total loss of the secondary circuit and the coolant pump trip with the subsequent simulation of the reactor scram (reduction of the electric power supplied to the fuel pin simulator) and activation of DHR system to remove the decay heat power (∼5% of the nominal value). Tests differ from each other by the applied boundary conditions such as the electrical power supplied to the Fuel Pin Simulator, the duration of the test, the power removed by the HX etc., while test #4 also differs for the forced circulation maintained after the simulation of the accidental transient. In parallel experimental test were carried out to investigate the heat transfer in a wrapped, grid spaced FPS. The FPS is composed of 37 electrical pins placed on a hexagonal lattice with a pitch to diameter ratio of 1.8. The bundle is hosted in the CIRCE large pool facility and represents the hot source of the Integral Circulation Experiment (ICE) test section. Several thermocouples (0.5 mm O.D.) are installed in the FPS to measure both clad and subchannel LBE temperatures at different ranks and two different sections. Experimental data are used to evaluate the Nu number in the Pe range 500–3000 and obtained data are compared with Ushakov and Mikityuk correlations having a validity range containing the experimental p/d ratio and for the selected Pe range. Experimental data relating to the obtained Nu number and the thermal hydraulic phenomena such as the modification of the thermal stratification in “pool type” configuration, coolant mass flow rate adaptation in the test section occurring during the simulation of the designed accidental conditions and the capability to cool the FPS under natural circulation conditions are described in this paper. Finally, the experimental database was used for validation of the geometrical and numerical model of the FPS adopted for 3D CFD calculations. In the performed simulations a sensitivity analysis was carried out for the turbulent Prandtl number in the range between 1 and 3 and a comparison between the first order turbulence model k-ω SST and the higher order Reynolds Stress Model was accomplished.