In a nuclear reactor, one of the accident management measures that take place during a loss-of-coolant accident is an emergency water injection into the primary circuit to ensure the cooling of the reactor core. Under such two-phase flow conditions, saturated steam and water inside the reactor pressure vessel (RPV) are mixed with the low water temperature of the emergency core cooling (ECC) system. Thermal stratification phenomena occur in several parts of the RPV. This fact raises a question about computer codes' capability of modelling adequately the thermal mixing phenomena. In order to help answering the previous question, detailed experimental investigations have been carried out within the OECD/NEA PKL-2 and PKL-3 projects. For this purpose separate effect tests related to the single-phase thermal mixing phenomena were addressed in the ROCOM facility experimental campaign. One of the goals was to assess the mixing phenomenon in the reactor pressure vessel.

In the current study only a code-to-code comparison is shown. The boundary conditions from ROCOM experimental test T2.3 are used to compare, on one hand, thermal-hydraulic (TH) system codes with 3-D capabilities (TRACE, CATHARE and ATHLET) and, on the other hand, the CFD code STAR-CCM+, in simulating the mixing phenomenon in the RPV. The main challenge in simulating the thermal mixing phenomenon with the best-estimate codes is a correct modelling of 3-D effects that occur under natural circulation flow regimes. In particular, unlike the CFD, TH system codes do not model turbulent mixing. In fact, one of the driving forces for the coolant mixing is related to numerical diffusion. To address the adequacy/accuracy of different numerical schemes in predicting single-phase thermal mixing phenomena, a comparative study where quantities of interest like the mixing scalar (MS), as well as the axial velocities, in the downcomer (DC) and core inlet (CI) plenum are presented. The results emphasize the capabilities and the differences between the considered computational codes as well as their suitability for such purposes.