Westinghouse Realistic BWR Control Rod Drop Accidents Methodology Using POLCA-T Code

The paper focuses on the activities pursued by Westinghouse in the development and licensing of POLCA-T code Control Rod Drop Accident (CRDA) Methodology. The following aspects of CRDA Methodology are considered: • Parameter Sensitivities based on Phenomena Identification and Ranking Tables (PIRT), • Qualification data base, • Cycle specific analysis, • Limiting initial conditions for transient simulation and CRDA transient simulation, • Sensitivity studies, • Uncertainty analysis and • Estimation of the methodology conservatism. The qualification data base of the methodology consists of single parameter confirmation, separate effects and integral test. It includes the PHOENIX and POLCA7 codes qualifications; POLCA-T qualification against the NEACRP-L-335 3D LWR Core Transient benchmark; simulations of the Peach Bottom EOC 2 Turbine Trip tests and SPERT-IIIE power excursion tests. The comprehensive CRDA methodology that utilizes PHOENIX4/POLCA7/POLCA-T calculation chain foresees complete cycle-specific analysis. The methodology consists of determination of candidates of control rods (CR) that could cause a significant reactivity excursion if dropped throughout the entire fuel cycle, selection of limiting initial conditions for CRDA transient simulation and transient simulation itself. The primary parameter utilized to determine the most limiting rod drop positions and candidates is the dropped control rod incremental reactivity worth. Example of scoping calculations covering three cycle exposures beginning-of-cycle BOC, middle-of-cycle (MOC), and end-of-cycle EOC and reactor states from cold critical to the end of followed CR sequence or up to 10% of rated power are presented. The dynamic response to the dropped control rod and the subsequent consequences to the fuel are evaluated by POLCA-T code. Selection of the limiting initial conditions covers the states along the lower limit of the typical plant startup region. Considered conditions start from 1 bar pressure and 20°C inlet temperature and go up to 70 bar and 274°C. It was observed that there are initial conditions that produce an extreme peak power value. It was also realized that there are initial conditions that produce an extreme peak fuel enthalpy value. Due to the complicated feedback mechanisms those initial conditions are not necessarily the same for the extreme values of the peak power and the fuel enthalpy. The Westinghouse methodology utilizes state-of-the-art methods. Unnecessary conservatisms in the methodology have been avoided to allow the accurate prediction of margin to design bases. This is mainly achieved by using the POLCA-T code for dynamic CRDA evaluations. The code belongs to the same calculation chain that is used for core design. Thus the very same reactor, core, cycle and fuel data base is used. This allows also reducing the uncertainties of input data and parameters that determine the energy deposition in the fuel. Uncertainty treatment, very selective use of conservatisms, selection of the initial conditions for limiting case analyses, incorporation into POLCA-T code models of the licensed fuel performance code are also among the means of performing realistic CRDA transient analyses.