Start-up of the first main coolant pump

This work was carried out in the frame of the project FLOMIX-R (registration number: FIKS-CT-2001-00197) funded from 01.10.2001 to 30.09.2004 by the European Union (5th Frame Work Programme).


A slug of lower borated water can be formed in the primary circuit by various mechanisms. Causes might be injection of coolant with less boron content from interfacing systems (external dilution) or separation of the borated reactor coolant into highly concentrated and diluted fractions (inherent dilution). In case of interruption of the circulation in the primary circuit, lower borated coolant can accumulate. Assuming the starting of the main coolant pump the slug of low borated water moves towards the core inlet. The mixing of these lower borated slug with water of higher boron concentration is the most mitigative mechanism against serious reactivity accidents in local boron dilution transients, and therefore, is one of the most important, nuclear safety related issues of mixing. Significant advantage in boron dilution transient analysis can be achieved, if realistic mixing data are used.

The goal of the work described in this report was the experimental investigation of the mixing of coolant with different quality on the way from the cold leg through the downcomer and lower plenum to the core inlet.


The pump in loop 1 is started, and within 14 s the nominal flow rate is reached. The deborated (tracered) coolant is driven to the reactor pressure vessel. The video shows the visualisation of the all wire-mesh sensors during the transport of the slug during the pressure vessel.

Fig. 1: Positions of the sensors in the pressure vessel

Fig. 2: Visualisation of the experiment (avi-file:1.2Mb)

In the vessel, the tracered water passes around the core barrel instead of flowing directly downstream. At the upper sensor, the tracer arrives still below the affected inlet nozzle. With growing time, the tracer spreads in the azimuthal direction. Subsequently, at the lower sensor two maxima of the tracer at azimuthal positions on the back side of the downcomer are observed. The tracer enters the plane of the sensor in the core inlet at the same azimuthal positions as it occurs in the lower part of the downcomer first.

During the transport, the slug mixes with the ambient coolant in the vessel and the perturbation reduces, as can be concluded from the decreasing maximum values reached at the different sensor positions.

Fig. 3: Time course of maximum and averaged normalized tracer concentration at each sensor in the pressure vessel (including 95.4%-confidence interval, calculated on basis of different realisations of the same experiment)

Fig. 4: Distribution of the normalized tracer concentration at the reactor inlet at the time of maximum under-boration (red arrow indicates the azimuthal position of the loop with the slug; maximum indicated by bold red number in %; stepwidth: distance between two isolines)



S. Kliem