A Critical Flow Model for the Cathena Thermalhydraulic Code

Abstract

A two-phase flow model was developed for the calculation of critical discharge at a break. This model is compatible with the non- equilibrium, two-fluid model in CATHBNA, and is able to treat light or heavy water, with or without non-condensable gases. The model imposes a flow boundary condition at a selected location in the network to maintain the fluid mixture velocity at or below a critical velocity. The basic strategy in the model is to compare the estimated fluid velocity at a specified location in the network (a "break") with an estimate of the critical velocity for the same local conditions. Phase velocities at the break are calculated by solving the conservation equations of CATHENA as done for any network location. The critical fluid velocity is calculated by a separate two-step method. In the first step, the local thermalhydraulic conditions at the break are calculated from the upstream flow conditions using the law of isentropic expansion. In the second step, using the local conditions from the first step, the critical velocity is estimated by a nucleation delay method (which estimates the pressure undershoot at the break). Model validation was conducted against steady-state critical flow experiments: the Moby-Dick and the Super-Moby-Dick tests that use light water without non-condensables, and the Ilic tests conducted with air-water mixtures. The results of this validation work were compared with the corresponding results obtained with other two-fluid codes, and with the existing discharge model in CATBENA. It was found that this critical discharge model is in excellent agreement with the Moby-Dick and Ilic experiments, and is in good agreement with the Super-Moby-Dick experiments, except in the case where very long nozzles were used. Also, generally, the model is in better agreement with data than comparable discharge models.