Predictions of one-group interfacial area transport in TRACE

In current nuclear reactor system analysis codes utilizing the two-fluid model, flow regime dependent correlations are used to specify the interfacial area concentration (a _i). This approach does not capture the continuous evolution of the interfacial structures, and thus, it can pose issues near the transition boundaries. Consequently, a pilot version of the system analysis code TRACE is being developed that employs the interfacial area transport equation (IATE). In this approach, dynamic estimation of a _i is provided through mechanistic models for bubble coalescence and breakup. The implementation of the adiabatic, one-group IATE into TRACE is assessed against experimental data from 50 air-water, two-phase flow conditions in pipes ranging in inner diameter from 2.54 to 20.32 cm for both vertical co-current upward and downward flows. Predictions of pressure, void fraction, bubble velocity, and a _i data are made. TRACE employing the conventional flow regime-based approach is found to underestimate a _i and can only predict linear trends since the calculation is governed by the pressure. Furthermore, trends opposite to that of the data are predicted for some conditions. In contrast, TRACE with the one-group IATE demonstrates a significant improvement in predicting the experimental data with an average disagreement of ± 13%. Additionally, TRACE with the one-group IATE is capable of predicting nonlinear axial development of a, by accounting for various bubble interaction mechanisms, such as coalescence and disintegration.