VERIFICATION OF THE 2-PHASE STRATIFIED-FLOW MODEL IN ATHLET BY SEPARATE EFFECT TESTS

The ATHLET code which is being developed by the Gesellschaft fur Anlag en- und Reaktorsicherheit (GRS) mbH is intended to cover, by means of a single code, the entire spectrum of loss-of-coolant and transient ac cidents in pressurized and boiling water reactors. The actual ATHLET v ersion Mod 1.1-Cycle A has a five-equation two-phase model based on th e conservation laws for liquid mass, liquid energy, vapor energy and o verall momentum. The relative velocity between liquid and vapor is det ermined by a full-range drift-flux model for two-phase flow in horizon tal and vertical pipes. The verification of this drift-flux model is c arried out by both large-scale experiments and single-effect tests. Th e single-effect test ECTHOR investigates stratified flow during the cl earance of a water-filled loop seal by a forced air flow through the l oop. ECTHOR is a French test for the consideration of two-phase flow r egimes in pipes for the development of the CATHARE code. The experimen ts are dedicated to investigating typical two-phase flows during small break loss of coolant accidents (LOCA) in pressurized water reactors (PWR). As a measure, the remaining water level in the loop is determin ed as a function of the air flow rate. For the verification, a compari son between ATHLET and CATHARE computations, on the one hand, and expe riments on the other hand is carried out. The results compare very wel l to each other. Test runs on different numerical grids show convergen ce to an asymptotic limit with increasing grid refinement.