Coolant mixing under natural circulation conditions following a hypothetical SB LOCA event
Boundary conditions for the ROCOM experiments
Fig. 1 shows the time records of the perturbation theta at the sensors in the upper and lower downcomer in the experiment without density differences. The figure depicts the unwrapped view of the time dependent theta distribution in the middle of the downcomer. Red arrows indicate the positions of the loops with the slugs, black arrows the two others. First of all, the time records of the perturbation at the upper sensor demonstrates, that both slugs enter the vessel at the same time. The transportation time is about 45 s and the deviation is less than 1 s, what underlines the quality of the pump control system. Further, it is clearly to be seen, that the perturbation is restricted to a sector of the vessel. The sector corresponds nearly to the fraction of the flow rate of the two loops with the slugs. A certain amount of mixing takes place with the coolant from the two other loops on the way from the upper to the lower sensor, as can be concluded from the visualization at the lower downcomer sensor. The maximum values measured at both sensors are 90.4 % and 86.1 %, respectively. That means, the perturbation is only slightly attenuated on the way through the downcomer. The shape of the perturbation in the downcomer is created by the typical velocity distribution in the downcomer, as it was found in velocity measurements. There is a minimum of the velocity directly below the four inlet nozzles, while in the middle between two inlet nozzles a maximum of the velocity is observed.
With no density difference, the weakly borated coolant almost perpendicularly flows down in the downcomer and a maximum of 64 % of the initial perturbation is detected in the core entry section below the loops where the slugs were formed (Fig. 2, the arrows indicate the positions of the loops with the slugs).
Now, the experiment was repeated with identical boundary conditions, only the density of the slugs was reduced by 2 %. This density difference impedes the down flow of the de-borated condensate. It stratifies on top of the heavier coolant and flows around the core barrel in the upper downcomer. The transfer to the core is delayed in this way. At the opposite side of the downcomer, that coolant with lower density is entrained by the heavier water (see Fig. 3). Consequently, the maximum values of theta reached at the sensor in the lower downcomer and at the core inlet (Fig. 2) are lower in comparison to the experiment without density difference. For the investigated density difference of 2 %, a value of 31 % only of the initial under-boration was measured at the core entrance. In addition to that, the position of the maximum mixing scalar theta is shifted towards the core entrance cross section part opposite to the two loops that provided the slugs.
The variation of the density difference significantly changes the mixing behavior.
Fig. 2: Distribution of the perturbation in the core inlet plane at the time of maximum (maximum: bold number in %, stepwidth: difference between two isolines)
Fig. 1: Time record of the perturbation at the sensors in the downcomer (unwrapped view) in the experiment without density differences
Fig. 3: Time record of the perturbation at the sensors in the downcomer (unwrapped view) in the experiment with a density difference of 2%
H.-M. Prasser, T. Höhne, S. Kliem, U. Rohde, F.-P. Weiss
Untersuchung der Kühlmittelvermischung an der Versuchsanlage ROCOM - Experimente und Modellierung
Annual Meeting on Nuclear Technology '03, Proc. Topical Session: "Experimental and theoretical investigations on boron dilution transients in PWRs" pp. 57-83, INFORUM GmbH Berlin, (2003)
S. Kliem, T. Höhne, H.-M. Prasser, U. Rohde, F.-P. Weiss
Experimental investigation of coolant mixing in the RPV of a PWR during natural circulation conditions
Proc. 12th Int. Conference on Nuclear Engineering ICONE-12 (CD-ROM), paper 49424 (2004)
T. Höhne, S. Kliem
Coolant mixing studies of natural circulation flows at the ROCOM test facility using ANSYS CFX
Proceedings of the OECD/NEA & IAEA Workshop: "Benchmarking of CFD Codes for Application to Nuclear Reactor Safety", CD-ROM paper A5-23 (2006)