Overview on coolant mixing experiments at the ROCOM test facility

In this flow regime, a constant mass flow rate over time is present at the inlet nozzles of all or a part of the loops while the quality of the coolant (temperature or boron concentration) changes in one of the running loops. Such scenarios are possible during forced convection (working main coolant pumps) as well as natural circulation in the primary circuit. A typical scenario for a temperature perturbation is a leak in one of the main steam lines, which causes an overcooling of the coolant in one loop. This overcooled water can reach the reactor before the main coolant pumps are switched off.

Einführung zu ROCOM-Experimenten - stationäres Strömungsregime

The inadvertent start of the main coolant pump in a loop, where coolant with reduced boron concentration is present, is the main topic for the investigations of flow scenarios with changing coolant flow rates in one ore more loops of the reactor. Coolant with decreased boron concentration can build-up, for instance by a malfunction of the make-up system at switched-off reactor or during a steam generator tube rupture with flow of deborated water from the secondary to the primary circuit.

Einführung zu ROCOM-Experimenten - Start der ersten Pumpe

Density differences between injected coolant and the primary loop inventory can play an important role during loss-of-coolant accidents, as the injection of the relatively cold emergency core cooling water can induce buoyancy-driven stratification. This stratification can cause high temperature gradients and increased thermal stresses in the wall of the reactor pressure vessel. Moreover, in case of inadvertent injection of emergency core cooling water with low boron concentration, a boron dilution transient could be initiated.

Einführung von Experimenten - Dichteeinfluss

During normal reactor operation, differences exist in the temperature rise of the coolant in the single fuel elements due to differences in the energy production in these fuel elements. These temperature differences are not fully vanishing during the transport of the coolant through the upper plenum. An experimental series was carried out at the test facility ROCOM for the determination of the share and the distribution of the coolant from the single fuel elements in the cross section of the different outlet nozzles of the reactor pressure vessel.

Einführung zu ROCOM-Experimenten - Oberes Plenum

It is known that under-borated coolant can accumulate in the loops and that it can be transported towards the reactor core during a loss of coolant accident. Therefore, the mixing of weakly borated water inside the reactor pressure vessel was investigated in experiments at the ROCOM test facility. Special attention was given to the mixing in the downcomer, which was observed with a measuring grid of 64 azimuthal and 32 vertical positions. The boundary conditions for these experiments were derived from thermal hydraulic experiments at the PKL test facility operated by AREVA in Erlangen.

Einführung zu ROCOM-Experimenten - Naturumlauf