Assessment of a BWR analytical reduced order model against the system code RAMONA: applicability and limitations

A novel analytical, reduced order model (ROM) has been developed recently to simulate the different instabilities encountered in heated channels and Boiling Water Reactors (BWRs), viz. density wave oscillations (DWOs), as well as in-phase and out-of-phase oscillations in the reactor core. The complete model comprises three main parts: spatial lambda-mode neutron kinetics with the fundamental and first azimuthal modes, fuel heat conduction dynamics, and core thermal-hydraulics based on a drift flux model representation of the two-phase flow. The principal aim of the present paper is to make a comparison between results of the developed reduced order model and those obtained using the system code RAMONA for a specific operational point of the Swiss nuclear power plant (NPP) Leibstadt. This is done in order to assess the ROM's applicability and limitations in a quantitative manner. Results show that, although the ROM could adequately predict certain characteristics, it is not able to correctly predict some others because of the highly simplified reactor core geometry, the uncertainties in evaluating the design and operating parameters, and the limitations of some model assumptions employed, e.g. negligence of subcooled boiling. The main conclusion to be drawn in this context is that, although reduced order models do indeed allow an in-depth understanding of the complex processes determining BWR stability, they need to be considered as valuable complementary tools to complex system codes, and not as alternatives. (authors)