Transient 3D simulation for heating and melting process of PWR core after SBO

In the heat transfer between the core and coolant in a reactor pressure vessel (RPV), a bounding structure such as a shroud has an important influence on the heating and melting process of the core. To study the 3D transient heat transfer and melting process of the Hualong 1 PWR using an IVR strategy, a numerical analysis model has been established based on the 2D transient heat conduction of each rod, which is coupled considering the residual heat of the core, the convective and radiation heat transfer inside the core, and natural convection heat transfer of the outer water chamber. A transient 3D numerical simulation code, 3DTMCOR, has been developed to analyze the melting process of an IVR reactor core with high precision. The effects of heat generation from a zirconium-water reaction, radiation heat transfer, and the adoption of an IVR strategy on the heating and melting process of the reactor core have been investigated. It was found that the first melting time for the same 100 MW PWR from the 3DTMCOR code is about 2500 s later than that of MAAP. The core temperature will first decrease with a decrease in core power within 100 s, then increase as the water level drops down with a zirconium-water reaction of 100 s to 1000 s, then decrease again with a decrease in core power from 1000 s to 4300 s, and increase a second time when water in the RPV is evaporated after 4300 s. At 4860 s, a continuous melting area first appears in the core top. Radiation heat exchange is very important for transferring heat from the fuel rods to structures such as the control rods and bounding structures, which is also the main cooling mode in the core after water is evaporated. The first melting time with the precise radiation exchange model will occur 200 s later than in the results with the empirical model. The water chamber outside the RPV does not have a remarkable effect on the heating and melting process of the core.