TRACG analyses of flashing instability during start-up

The Simplified Boiling Water with natural circulation is being designed by GE. The proposed design includes a long ({approx}8m) chimney region on top of the core region. The special interest in this case is the possibility of a thermal-hydraulic instability which may occur during reactor start-up with natural circulation conditions. Detailed data for natural circulation flow corresponding to the start-up period was obtained at the CRIEPI experimental facility. Thermal-hydraulic instability similar to geysering was observed. Based on the experimental data it is concluded that the observed instability has a substantial nonlinear behavior. A computer simulation of the CRIEPI experimental facility has been performed using the TRACG code. The TRACG model and the results of the computer simulation are discussed in the paper. A power-temperature stability map for system pressure 0.2 MPa is presented and good agreement between CRIEPI experimental data and TRACG results is shown. It is found that within the analyzed instability region there is no vapor generation due to boiling in the heated channels and vapor is generated in the chimney region due to flashing. The mechanism of the geysering-type thermal-hydraulic instability observed at CRIEPI test facility is controlled by two major phenomena: enthalpy propagation in the channel and chimney regions and flashing in the chimney region. (author)