Analyses of Instability Events in the Peach Bottom-2 BWR Using Thermal-Hydraulic and 3D Neutron Kinetic Coupled Codes Technique

Boiling water reactor (BWR) instabilities may occur when, starting from a stable operatingcondition, changes in system parameters bring the reactor towards an unstable region. In order to designmore stable and safer core configurations, experimental and theoretical studies about BWR stability havebeen performed to characterise the phenomenon and to predict the conditions for its occurrence. In thiswork, contributions to the study of BWR instability phenomena are presented. The RELAP5/MOD3.3thermal-hydraulic (TH) system code and the PARCS-2.4 3D neutron kinetic (NK) code were coupled tosimulate BWR transients. Different algorithms were used to calculate the decay ratio (DR) and the naturalfrequency (NF) from the power oscillation predicted by the transient calculations as two typical parametersused to provide a quantitative description of instabilities. The validation of the code model set up for the PeachBottom Unit 2 BWR plant is performed against low-flow stability tests (LFSTs). The four series of LFST havebeen performed during the first quarter of 1977 at the end of cycle 2 in Pennsylvania. The tests were intendedto measure the reactor core stability margins at the limiting conditions used in design and safety analyses