Abstract
In accidents when the water level of the reactor core descends below the top of the active fuel, the cooling limit height is a key factor in determining the accident mitigation procedure and the boiling two-phase flow in a fuel-rod bundle exhibits multi-dimensional and complex flow structures during such boil-off process. A rod bundle boil-off experiment was conducted to determine the three-dimensional void-fraction distribution and axial profile of the rod-surface temperature during the boil-off process under atmospheric pressure conditions. The 5×5 rod bundle, featuring a heated length of 2 m, had an axially and radially uniform power profile, with eight pairs of sheath thermocouples embedded on the heated rod to monitor its surface temperature distribution. The void-fraction distribution was acquired with five pairs of SubChannel Void Sensor (SCVS) as time-series data. The experimental results showed the relationship between an effective cooling level and boiling two-phase flow dynamics in the rod bundle.
