A new approach for nonlinear stability analysis is presented. In the framework of this approach, integrated BWR (system) codes and reduced order models (ROM's) are used as complementary tools to examine the stability characteristic of fixed points and periodic solutions of the nonlinear differential equations describing the stability behaviour of a BWR loop. Hence the methodology demonstrated in this paper is a novel one in a specific sense: We analyse the highly nonlinear processes of the BWR dynamics by application of system codes (in which numerical diffusion properties are examined in advance so far as possible) and by application of some sophisticated methods of the nonlinear dynamics (bifurcation analysis). We claim, the complementary application of independent methodologies to examine the nonlinear stability behaviour can increase the reliability of BWR stability analysis. This work is a continuation of the previous work at the Paul Scherrer Institute (PSI, Switzerland) and at the University of Illinois (USA) on this field. The current ROM was extended by adding the recirculation loop model. The necessity of consideration of the effect of subcooled boiling in an approximated manner was discussed. Furthermore, a new calculation methodology for the feedback reactivity was implemented. The modified ROM is coupled with the code BIFDD which performs semi-analytical bifurcation analysis. In addition to the ROM extensions, a new approach for calculation of the ROM input data was developed. The new approach for nonlinear BWR stability analysis is presented for NPP Leibstadt. This investigation is carried out for an operational point for which an out-of-phase power oscillation has been observed during a stability test at the beginning of cycle 7 (KKL cycle 7 record #4). The modified ROM and the new approach for the calculation of the ROM input data are qualified for BWR stability analysis in the framework of the approach (RAM-ROM methodology) demonstrated in this paper.