The European project FLOMIX-R: Description of the slug mixing and buoyancy related experiments at the different test facilities(Final report on WP 2)

The goal of the work described in this report was the experimental investigation of the mixing of coolant with different quality (temperature, boron concentration) in nuclear reactors on the way from the cold leg through the downcomer and lower plenum to the core inlet in a systematic way. The obtained data were used for the clarification of the mixing mechanisms and form a data basis for the validation of computational fluid dynamics (CFD) codes.
For these purposes, experiments on slug mixing have been performed at two test facilities, modelling different reactor types in scale 1:5, the Rossendorf and Vattenfall test facilities. The corresponding accident scenario is the start-up of first main coolant pump (MCP) after formation of a slug of lower borated water during the reflux-condenser mode phase of a small break loss of coolant accident (LOCA). The matrices for the experiments were elaborated on the basis of the key phenomena, being responsible for the coolant mixing during pump start-up. Slug mixing tests have also been performed at the VVER-1000 facility of EDO Gidropress to meet the specifics of this reactor type.
The mixing of slugs of water of different quality is also very important for pre-stressed thermal shock (PTS) situations. In emergency core cooling (ECC) situations after a LOCA, cold ECC water is injected into the hot water in the cold leg and downcomer. Due to the large temperature differences, thermal shocks are induced at the reactor pressure vessel (RPV) wall. Temperature distributions near the wall and temperature gradients in time are important to be known for the assessment of thermal stresses.
One of the important phenomena in connection with PTS is thermal stratification, a flow condition with a vertical temperature profile in a horizontal pipe. Due to the fluctuating character of the flow, this may cause thermal fatigue in the pipe. Besides of thermal fatigue, a single thermal shock can also be relevant for structural integrity, if it is large enough, especially in the case, that the brittle fracture temperature of the RPV material is reduced due to radiation embrittlement. Therefore, additional to the investigations of slug mixing during re-start of coolant circulation, the mixing of slugs or streams of water with higher density with the ambient fluid in the RPV was investigated. The aim of these investigations was to study the process of turbulent mixing under the influence of buoyancy forces caused by the temperature differences. Heat transfer to the wall and thermal conductivity in the wall material have not been considered.
Experiments on density driven mixing were carried out at the Rossendorf and the Fortum PTS facilities.