Development of BWR regional instability model and verification based on ringhals 1 test

The mathematical model of LAPUR, the well-known frequency domain stability code, was reviewed and modified from the viewpoint of the pure thermohydraulic and neutronic-thermohydraulic interaction mechanisms. Based 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 the 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-dimensional modal analysis schemes were studied and realized as the independent two energy-groups diffusion code, ACCORD. The power iteration scheme was applied with Wielandt method. In order to resolve tightly clustered harmonic modes, the proper initial guess was given which was derived by the horizontally two-dimensional calculations taking account of the primary important first azimuthal modal shape. The adequacy of the instability evaluation system was verified based on the Ringhals 1 test data which were supplied to participants of the OECD/NEA BWR stability benchmark project. The predominant first azimuthal oscillating power shape was evaluated based on Local Power Range Monitor (LPRM) signals. The prediction by ACCORD shows an excellent agreement both in the LPRM responses and the angle of the neutral line. The measured core-wide and regional instability decay ratios were compared with the prediction by Modified LAPUR. Both the bias and standard deviation of the decay ratios are about 0.1 for both instability modes, which is acceptable in the practical design. An extensive phenomenological study was performed regarding the regional instability predominance. The dependence of the core-wide and regional decay ratios was investigated for several power shape indices. It was indicated that the newly proposed distance-weighted 2nd order axial power momentum will be a good index.