Post Test Calculations of NOKO Bundle Experiments

The safety concept of the new innovative boiling water reactor
SWR1000, which is developed by Siemens AG, is aiming at the
increase of safety margins and of the grace period for operator
actions during accidents by increasing the water inventory
inside the pressure vessel, by the arranging large water pools
in- (core flooding pool) and outside of the containment (dryer-
separator storage pool), simplifying the (emergency) cooling
systems and further decreasing the failure probabilities of
safety systems. Therefore, active safety systems are substituted
by passive ones or combined with passive safety systems in cases
where a replacement is not possible. The function of passive
systems is directly based on the physical phenomena: gravity,
natural convection and/or evaporation. Their effectiveness is in
dependent of operator actions. Therefore, smaller failure rates
are assigned to passive safety systems. The probability of
severe accidents of the new SWR is reduced significantly.

For the experimental investigation of the operation mode and the
effectiveness of these passive safety systems the multipurpose
thermohydraulic test facility NOKO was constructed at the
Forschungszentrum Jülich (FZJ) within a German collaboration
sponsored by the German Federal Ministry of Education, Sciences,
Research and Technology (BMBF) and German Utilities (EVU). The
facility has a maximum power of 4 MW for steam production and a
maximum operating pressure of 10 MPa.

In the EU BWR R&D Cluster six test series with an emergency
condenser test bundle and four test series with a single tube
were performed in 1996. Within the Physics and Thermalhydraulics
Complementary Actions to the BWR Cluster (BWR/CA) the Forschungs
zentrum Rossendorf (FZR) e.V. has performed 10 post test calcula
tions and an additional blind test calculation of NOKO bundle
experiments. The tests were selected by FZJ. The results of
these calculations are presented in this paper.

Post test calculations of NOKO experiments were performed with
an improved version of ATHLET. To calculate the heat transfer
coefficients during condensation in horizontal tubes the module
KONWAR has been developed and implemented in ATHLET. KONWAR is
based on the flow regime map of Tandon and includes several semi-
empirical correlations for the determination of the heat trans
fer coefficients. The post test calculations presented here
prove qualitatively and quantitatively the expected operation
mode and the capacity of the emergency condenser. Additionally
the blind calculation proves, that the improved version of
ATHLET (ATHLET with the extension KONWAR) is able to perform
proper calculations.