Stability Analysis of Natural Circulation BWRs at Low Pressure Using the Codes FLOCAL and ATHLET

The capabilities of the in-house code FLOCAL and of the code ATHLET (developed by GRS, Germany) to reproduce instabilities of natural circulation BWRs at low pressure will be shown. FLOCAL consists of a 4-equations two-phase model. This code is the thermalhydraulic module of the three-dimensional neutronic codes DYN3D, also in-house development of FZR. The code ATHLET is a general-purpose thermalhydraulic code which has the possibility of using 4, 5 or 6-equations two-phase model. This code has been coupled with DYN3D. The results of the simulation obtained with the two codes are compared with experiments carried out with facilities built at Delft University of Technology (The Netherlands) and at Paul Scherrer Institute (PSI, Switzerland). The phenomenology of the instabilities is very well reproduced by the two codes. Good quantitative agreement is found as well.