Siemens AG is developing the new innovative boiling water reactor concept SWR1000. New features are passive safety systems, such as emergency condensers, building condensers, passive pressure pulse transmitters, and gravity-driven core flood lines.

For experimental investigation of emergency condenser effectiveness, the NOKO test facility has been constructed at Forschungszentrum Jülich in cooperation with Siemens. This test facility has an operating pressure of 10 MPa and a maximum power of 4 MW for steam production. The emergency condenser bundle consists of eight tubes and is fabricated with planned geometry and material of the SWR1000. In more than 200 experiments, the emergency condenser power was determined as a function of pressure, water level, and concentration of noncondensables in the pressure vessel as well of pressure, water level, and temperature in the condenser.

Posttest calculations of NOKO experiments were performed with an improved version of ATHLET. To calculate the heat transfer coefficients during condensation in horizontal tubes, it was necessary to develop the KONWAR module and to implement it in ATHLET. KONWAR is based on the flow regime map of Tandon and includes several semiempirical correlations for the determination of the heat transfer coefficients. The comparison between calculations and experiments shows good agreement.