WOSUB : a subchannel code for steady-state and transient thermal-hydraulic analysis of BWR fuel pin bundles. Volume III. Assessment and Comparison

Abstract

The WOSUB-codes are spin-offs and extensions of the MATTEO-code [1]. The series of three reports describe WOSUB-I and WOSUB-II in their respective status as of July 31, 1977. This report is the third in a series of three, the first of which [2] contains all the information about the models, solution methods and constitutive equations and the second [3] being the user's manual of the code. This report summarizes the assessment of the WOSUB- code against experiments and compares its results with the results of other subchannel codes. The following experiments are used for the purpose of the assessment of the code under steady-state conditions: 1) 9-rod GE-tests with radially uniform and non- uniform peaking factor patterns. 2) 16-rod Columbia tests with slight power tilts. 3) Planned 9-rod Swedish tests with very strong power tilts. 4) Actually performed 9-rod Swedish tests with power tilt. 5) 9-rod GE-CHF experiments. The comparison with these data shows that WOSUB is capable of predicting the lower-than-average behavior of the corner sub- channel and the higher-than-average behavior of the center subchannel for both quality and mass flux. None of the other well-known subchannel codes is indeed capable of specifically predicting the correct corner subchannel behavior. These codes seem to inherently suffer from major deficiencies associated with their incorporated mixing models. Therefore, it is con- cluded that only improved models for the description of two- phase flow phenomena are capable of handling these situations and that the vapor drift flux model together with the vapor diffusion model as incorporated into WOSUB is doing a good job. The fact that WOSUB does not perfectly match the experimental results over the whole spectrum of experimental evidence can be attributed to the vapor diffusion model which was originally fitted to air-water test results in a geometry consisting of two subchannels only. Obviously, this geometry leads to over- emphasizing the importance of the vapor diffusion as compared to what actually happens in a multi-rod geometry. WOSUB gives the user the option of calculating the critical power as a function of the boiling length - a concept which is especially useful to easily account for axially nonuniform power profiles and which closely resembles the procedure now used by GE. Furthermore, the code determines four heat transfer coefficients around the circumference of the fuel pin, thus giving the user the possibility of selecting the minimal one for the purpose of hot spot calculations. Overall, the assessment and comparison presented in this volume show that the WOSUB-code has to be considered a valuable tool for BWR bundle and PWR test bundle analysis with a potential for further improvements. The commonly used concept of power-to-flow ratio fails to explain most of the test data used for comparison in this report. The WOSUB-code is still in the stage of evolutionary development. In this context, the results presented in this present report have to be considered preliminary. They reflect the development as of July 1977.