Suppression pool thermal-hydraulics preliminary analysis of fukushima daiichi unit-2 using pool-3D module of Sampson code

On March 11th 2011, the Great East Japan Earthquake occurred generating the tsunami which has triggered a long term Station Black Out (SBO) resulting in severe damages to the Fukushima Daiichi NPS. Several analysis were performed using different accident codes in order to fully understand the accident phenomenology. One of the main issue regards the relatively fast pressure increase observed in the Pressure Containment Vessel (PCV) of Unit-2 and Unit-3 that cannot be reproduced by the classic lumped parameter approach. The PCV pressure is driven by the superficial temperature of the pool which, during an SBO, can differ considerably from the average pool temperature evaluated with lumped parameters codes. Therefore a new three dimensional module, called POOL3D and coupled with the lumped parameter containment module, has been introduced into SAMPSON code in order to obtain a complete temperature distribution inside the S/P. The aim of this work is to present a preliminary activity of coupling of a three dimensional code with a lumped parameter code for the temperature distribution in the pool and consequently the PCV pressure response. The new module has been tested on Fukushima Unit-2 simulating the first 70 hours after SCRAM when the PCV experienced a pressure increase. Using the 3D code the PCV pressure has been reproduced introducing a partial condensation model to evaluate the pool condensation efficiency and a superficial evaporation model. The thermal-stratification obtained results smaller, if compared with the experiments available in the literature, even if the PCV pressure measurements were captured. The partial condensation model needs experimental confirmation therefore validation against several experiments of POOL3D are planned to improve its capability.