Regional instability evaluation of Ringhals unit 1 based on extended frequency domain model

The frequency domain model has been extended for the regional instability e valuation while retaining its practicality and improving the reliability of major influential numerical models. The unified friction and local pressur e loss model of the original LAPUR was modified considering the different d ynamic characteristic of two pressure loss mechanisms. The detailed ex-core recirculation loop model was implemented and the neutron point kinetics mo del was also modified to reflect the inter-mode void reactivity interaction . The neutron flux modal analysis code, ACCORD-N, was developed based on th e nonlinear iterative nodal method. Efficient schemes were proposed to give the higher mode initial flux guess. The modified code system was verified based on the Ringhals unit I stability test data. Extensive studies were pe rformed to identify influential factors in the regional instability. A depe ndence of the decay ratio was investigated with regard to the sub-criticali ty of the first azimuthal mode, Nyquist plots and several power shape indic es. It seemed reasonable to conclude that the regional instability was stro ngly influenced by the thermal hydraulic mechanism. Including the simulatio n results of other reactors, the distance weighted axial power momentum, na med the AS-value, gave a good account of both core-wide and regional instab ility modes. (C) 2000 Elsevier Science S.A. All rights reserved.