Coolant Mixing Studies of Natural Circulation Flows at the Rocom Test Facility Using Ansys Cfx

Partial depletion of the primary circuit during a postulated small break loss of coolant accident can lead to the interruption of one-phase flow natural circulation. In this case, the decay heat is removed from the core in the reflux-condenser mode. In case of the partial non-availability of the safety injection system, weakly borated condensate can accumulate in particular in the pump loop seal of those loops, which do not receive this safety injection. After refilling of the primary circuit, natural circulation re-establishes and the lower-borated slugs are shifted towards the reactor pressure vessel (RPV). Mixing in the downcomer and the lower plenum is an important phenomenon mitigating the reactivity insertion into the core in this postulated scenario. Therefore, mixing of the lower-borated slugs with the ambient coolant in the RPV was investigated at the four loop 1:5 scaled ROCOM mixing test facility. In these experiments, the length of the flow ramp and the initial density difference between the slugs and the ambient coolant was varied. From the test matrix an experiment with 2% density difference between the de-borated slugs and the ambient coolant was used to validate the CFD software ANSYS CFX. To model the effects of turbulence on the mean flow a Reynolds stress turbulence model was employed and a hybrid mesh consisting of 3.6 million nodes and 6.4 million elements was used. The experiment and CFD calculation show a stratification in the downcomer. The less dense slugs flow around the core barrel at the top of the downcomer. At the opposite side the lower borated coolant is entrained by the colder safety injection water and transported to the core. The validation proves that ANSYS CFX is able to simulate appropriately the flow field and mixing effects of coolant with different densities. Introduction A small break loss of coolant accident (SBLOCA) can lead to the interruption of one-phase flow natural circulation in the loops of a pressurized water reactor (PWR). This could happen when the high pressure safety injection (HPSI) should be partly unavailable. In such cases, a part of the decay heat is removed from the core in the reflux-condenser mode. This leads to the production and accumulation of low borated coolant in the primary circuit. The primary system will be filled up again and one-phase natural circulation re-establishes. The one phase natural circulation forwards the underborated condensate towards the core. By theoretical analyses using the thermal-hydraulic system code ATHLET, the Gesellschaft für Reaktorsicherheit (GRS) revealed a special scenario which could possibly occur after a SBLOCA