10.2172/1062632

Abstract

This is the second in a series of reports on the response of a BWR/5 boiling water reactor to anticipated transients without reactor scram (ATWS) when operating in the expanded operating domain MELLLA+. In this report ATWS events initiated by a turbine trip are considered at two points in the fuel cycle: beginning of cycle (BOC) and peak excess reactivity (PHE, close to the middle of the cycle). The effect of the gap (between the fuel pellet and clad) conductance modeling, turbine bypass fractions, and initial core flow rates are evaluated.. Two limiting values of gas-gap conductance, the lowest value of 5,000 W/m2-K at BOC and the highest value of 161,000 W/m2-K at PHE, are compared with corresponding base cases which utilized the dynamic gas-gap model. Turbine bypass fractions analyzed are 10% and 25% at PHE and these two sensitivity cases are compared with the 100% bypass base case. The reduced core flow case at 75% of nominal core flow is analyzed at PHE and compared with the base case of 85% core flow. The simulations were carried out using the TRACE/PARCS code system and models developed for a previous study at three different times during a fuel cycle with all relevant BWR/5 systems. The modeling in the core is particularly detailed (four fuel rod types included in each fuel assembly and 382 thermal-hydraulic channels to represent all assemblies taking into account half-core symmetry) in order to capture the complex neutronic-thermal-hydraulic coupling during periods of instability. The study provides insights into reactor behavior during these events, including the impact of assumed operator actions on the oscillatory behavior due to reactor instabilities and on the eventual shutdown of the reactor. It shows the effect of gap conductance, turbine bypass fraction and initial flow rate. and #8195;