Comparison of a Hypothetical Strongly Asymmetrical Severe Accident Using the Standard and the New Nodalization Method with the Code System AC2

The module ATHLET-CD (Analysis of THermal-hydraulics of LEaks and Transients with Core Degradation), which is part of the code system AC(2) (ATHLET, ATHLET-CD, COCOSYS) is designed to simulate the reactor coolant system thermal-hydraulic response during severe accidents, including core damage progression as well as fission product and aerosol behaviour, to calculate the source term for containment analyses, and to evaluate accident management measures. The whole code system is constantly being developed by GRS. One of the main goals for the further development of ATHLET-CD was to improve the code usability for strongly asymmetrical nuclear accidents. Currently, the released version of ATHLET-CD and other similar major severe accident codes are well applicable in case of accidents with symmetrical characteristics. For core nodalization these codes divide the core into concentric rings. Within each ring it is assumed that all the fuel rods behave identically, there are no differences in the parameters in azimuthal direction. There are, however, scenarios that do not have an azimuthally symmetrical characteristic. A new nodalization scheme was developed for ATHLET-CD which allows the representation of azimuthal differences inside the core. The departure from the cylindrically symmetrical nodalization of the core required some model changes. These were successfully implemented into the developer version of ATHLET-CD. To demonstrate the effects of the new nodalization and model changes two simulations of the same hypothetical severe accident with strongly asymmetrical characteristics are going to be presented. First, using the standard, ring-like core nodalization and then using the new nodalization method.