Experimental Study And Phenomenological Modeling Of CHF At The Outer Surface Of RPV Lower Head

In-vessel retention(IVR) of molten core debris via water cooling at the external surface of the reactor vessel is an important severe accident management feature of advanced passive plants. During postulated severe accidents, the water from in-containment refueling water storage tank(IRWST) will submerge the reactor vessel, and remove decay heat by natural circulation to prevent vessel failure. For this concept, it is important to keep heat load on the vessel wall surface lower than critical heat flux at any position of the lower head. The mechanism of boiling crisis in this case is not sufficient understood, and there is no reliable theoretical model proposed nowadays for predicting critical heat flux. Therefore, the main approach to obtain critical heat flux at the lower head is experimental measurement, at least for engineering applications.In this dissertation, experimental investigation on critical heat flux at downward facing heating surface is conducted and visual observation of flow boiling at different orientation condition is obtained. In the experiment, a copper block is used to simulate the pressure vessel lower head, and uniform heat flux is realized through the electrically heated rods embedded into the copper block. Boiling crisis is determined by a rapid temperature rise at the bottom of the copper block. Experimental results show that critical heat flux increases with the inclination angle of heating surface. Visual observation reveals that at small inclination angle bubbles tend to assemble close the heating surface while at large inclination angle bubbles are easier to detach the wall and enter the mainflow area. At boiling crisis, the bubbly layer thickness is greater at large inclination angle than that at small one. The bubble rise velocity is higher at larger inclination angle.Based on the experimental observation and the existing bubble crowding model in the literature, a theoretical CHF model is developed suitable for the outer surface of the reactor pressure vessel lower head. In the new model, the effect of orientation on bubble velocity and bubbly layer thickness is taken into consideration. A new method is introduced to calculate the steam quality of both layers. Comparison with the present experimental data, the new model shows satisfying prediction accuracy. The max deviation between the model prediction and experimental data is less than 10%. Parametric analysis of the new model shows that an increased reactor pressure vessel diameter will lead to a decrease in critical heat flux at the lower head outer surface when the structure of the external flow channel keeps unchanged. At the same time, heat load on the vessel wall increases with the power. So the thermal margin to keep the integrity of the lower head decreases, which will lead to failure of the lower head.The newly developed CHF model is then applied to large scale passive PWR. The traditional method to evaluate feasibility of IVR is based on the steady heat flux distribution in late stage of severe accident. But in the early stage, the transient behavior of the molten corium may impose a greater threat to the integrity of the reactor pressure vessel. This dissertation presents a transient severe accident analysis of the 1700MW-class plant and investigates the feasibility of IVR as severe accident mitigation measure. The transient procedure under severe accident conditions is simulated by the code MELCOR, to provide heat flux distribution on the lower head at different time. For the selected accident scenario, MELCOR simulation shows that at 24000 s when the steady molten pool is formed, the heat load is lower than the critical heat flux on the lower head everywhere. But at 20000 s, before the steady molten pool formed, situation is worse. At 68°and 72°position, the heat flux and CHF ratio is greater than one, which means the heat load is higher than the critical heat flux, and the IVR-ERVC strategy fails. The result indicates clearly that the traditional method to evaluate IVR feasibility based on the steady molten pool is not conservative always.