DEVELOPMENT OF BWR REGIONAL INSTABILITY MODEL AND VERIFICATION BASED ON RINGHALS-1 TEST

The mathematical model of LAPUR, the well-known frequency domain stabi lity code, was reviewed and modified from the viewpoint of the pure th ermohydraulic and neutronic-thermohydraulic interaction mechanisms. Ba sed on the ex-core loop test data, the original combined friction and local pressure loss model was modified taking account of the different dynamic behavior of these two pressure loss mechanisms. That is, the perturbation terms of the two-phase friction multiplier and mass flux correction factor were adjusted semi-empirically. With reference to th e neutronic-thermohydraulic interaction, the new regional instability model was introduced considering not only the sub-criticality but also the shape of the higher neutron flux mode. The efficient three-dimens ional modal analysis schemes were studied and realized as the independ ent two energy-groups diffusion code, ACCORD. The power iteration sche me was applied with Wielandt method. In order to resolve tightly clust ered harmonic modes, the proper initial guess was given which was deri ved by the horizontally two-dimensional calculations taking account of the primary important first azimuthal modal shape. The adequacy of th e instability evaluation system was verified based on the Ringhals 1 t est data which were supplied to participants of the OECD/NEA BWR stabi lity benchmark project. The predominant first azimuthal oscillating po wer shape was evaluated based on Local Power Range Monitor (LPRM) sign als. The prediction by ACCORD shows an excellent agreement both in the LPRM responses and the angle of the neutral line. The measured core-w ide and regional instability decay ratios were compared with the predi ction by Modified LAPUR. Both the bias and standard deviation of the d ecay ratios are about 0.1 for both instability modes, which is accepta ble in the practical design. An extensive phenomenological study was p erformed regarding the regional instability predominance. The dependen ce of the core-wide and regional decay ratios was investigated for sev eral power shape indices: It was indicated that the newly proposed dis tance-weighted 2nd order axial power momentum will be a good index. (C ) 1997 Elsevier Science Ltd.