Investigation of coolant mixing in pressurized water reactors at the Rossendorf mixing test facility ROCOM


Investigation of coolant mixing in pressurized water reactors at the Rossendorf mixing test facility ROCOM

Grunwald, G.; Höhne, T.; Prasser, H.-M.

In Pressurised Water Reactors (PWRs) the parameters of the coolant may differ from one loop to another and change in time during transients. In case of an undesired decrease of the boron acid concentration we speak about boron dilution transients. In other cases also the coolant temperature may be affected, e.g. by overcooling of a loop if a main steam line break happens. Both kinds of transient lead to a positive reactivity insertion, which may cause a dangerous power excursion. For the analysis of the core behaviour during boron dilution transients and main steam line breaks, coupled neutron kinetic / thermal hydraulic codes are necessary. These codes must contain models of the coolant mixing on its way from the inlet nozzles to the core entrance, because the reactivity insertion strongly depends on the distribution of these parameters at the core inlet. The coolant mixing mainly happens in the three-dimensional flow field inside the complex geometry of the downcomer and lower plenum. It depends signifi-cantly on the construction of the reactor vessel. The modelling of coolant mixing was carried out by detailed computational fluid dynamics (CFD) calculations with the code CFX 4.2. For the validation of the computational results, the ROCOM test facility was built, a fluid dy-namic model of the reactor in the scale of 1:5. The reactor vessel is made of Plexiglas for flow visualisation and LDA measurements. The four loops dispose of separately controllable main coolant pumps. Plugs of coolant with low boron acid concentration are modelled by salt water injection. The use of a system of wire-mesh sensors allows to measure salt concentra-tions at over 1000 locations with a maximum time resolution of 200 Hz. Measuring locations are at the reactor inlet, at the core inlet as well as in the upper part and at the lower end of the downcomer. The experiments as well as the calculations have shown, that the mixing is in-complete. In case of stationary coolant flow in all loops a disturbance of boron concentration or temperature is transferred to a sector of the core inlet plane, which corresponds to the loca-tion of the inlet nozzle. In the steady state, the concentration respectively temperature in the centre of the sector almost reaches the amplitude of the disturbance at the inlet nozzle. A short plug of boron diluted water experiences a better mixing: due to the vortices in the downcomer different portions of the disturbance are transported with different velocities. This causes a dispersion of the front and the end of the plug (axial mixing). If the plug is short enough, the maximum concentration decreases. Further, experiments and calculations are going on to be performed for the investigation of mixing under transient conditions (start of circulation). Response functions will be calculated for the development of a fast running mixing model.

  • Lecture (Conference)
    8th International Conference on Nuclear Engineering (ICONE8), Baltimore, USA, April 2-6,2000, CD-ROM
  • Contribution to proceedings
    8th International Conference on Nuclear Engineering (ICONE8), Baltimore, USA, April 2-6,2000, CD-ROM

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Publ.-Id: 3228