A mathematical dynamic modeling and thermal hydraulic analysis of boiling water reactors using moving boundaries.

Abstract

A new development and practical application of a mathematical dynamic modeling for simulating normal and accidental transient analysis for the boiling water reactor system is presented in this dissertation. The mathematical dynamic modeling represents a new technology based on a moving boundary concept. The mathematical model developed for fluid flows is based on a set of the four equation mixture model, one-dimensional, single channel with a drift flux model in the two-phase flow regime. The four conservation equations used in the mathematical model formulation include the vapor phase mass equation, the liquid phase mass equation, the mixture energy equation, and the one-dimensional mixture momentum equation for the boiling channel. The formulation of the core thermal-hydraulic model utilizes a transient moving boundary technique which tracks the movements of the phase change and boiling transition boundaries. Such a moving boundary model has been developed to allow a smooth representation of the boiling boundary movement based on empirical heat transfer correlations and the local thermal-hydraulic conditions of the coolant flow along fuel pin channels. The mathematical models have been implemented to accommodate three-dimensional reactor kinetics, with detailed thermal conduction in fuel elements. Also, an accurate minimum departure from nucleate boiling ratio (MDNBR) boundary is predicted during transients. Several test calculations were performed to assess the accuracy and applicability of the moving boundary model. Comparison between the calculated results and the experimental data are favorable. Overall system studies show that some thermal margin is gained using the transient MDNBR approach vs the traditional quasi-static methodology. The model predicts accurate void fraction profiles for kinetic feedback and boiling stability analysis for the BWR. The moving boundary formulation and improved numerical solution scheme are an efficient and suitable tool which can be useful for realistic simulation of degraded nuclear power plant transients.