Investigation of flashing-induced instabilities at the CIRCUS test facility using the code ATHLET

The CIRCUS test facility (Circulation during startup) has been built to study the start-up phase of a natural-circulation BWR. During the start-up so called flashing-induced instabilities can arise. These instabilities are induced by flashing, due to steam production in the long adiabatic riser section, which is placed above the core to enhance the flow rate. Flashing occurring in the riser causes an unbalance between driving force and pressure losses in the natural-circulation loop, giving rise to flow oscillations.

Within the European-Union 5th Framework Programme, a project, NACUSP, has been started in December 2000, having as one of its main aims the understanding of the physics of the phenomena involved during the start-up phase of natural-circulation-cooled BWRs, providing a large experimental database and validating state-of-the-art thermalhydraulic codes in the low-pressure, low-power operational region of these reactors.

One part of this project deals with the modeling of selected CIRCUS tests using the thermo-hydraulic code ATHLET. This paper gives an overview about experimental results and the physics of the instabilities. ATHLET is used to investigate the dynamic behavior of the CIRCUS test facility and the results of the calculations are compared with the experimental data.