Comparative evaluation of coolant mixing experiments at the ROCOM and the Gidropress test facilities


Comparative evaluation of coolant mixing experiments at the ROCOM and the Gidropress test facilities

Kliem, S.; Höhne, T.; Rohde, U.; Bykov, M.; Lisenkov, E.

Coolant mixing inside the nuclear reactor is the most important inherent safety mechanism against boron dilution or overcooling transients and in the case of pressurized thermal shock scenarios. In the frame of the TACIS Project R2.02/02 coolant mixing experiments have been performed at the 1:5 scaled Gidropress mixing test facility. This experimental facility consists of a model of the reactor pressure vessel (RPV) with four circulating loops. The RPV model is made of steel and reproduces in the given scale practically all the geometrical features of the RPV of the VVER-1000 at the NPP Novovoronezh-5 which are affecting the in-vessel mixing phenomena up to the core inlet, particularly the internal components such as the barrel, the lower ellipsoidal perforated shell, the core support columns and the lower core plate. In the carried-out experimental series the mixing processes during reactor coolant pump (RCP) start-up, during natural circulation conditions with the variation of density ratio differences and during stationary flow conditions with different number of RCP in operation were investigated.
At Forschungszentrum Dresden-Rossendorf, the 1:5 scaled mixing test facility ROCOM (Rossendorf Coolant Mixing Model) has been in operation since ten years. This facility models the primary circuit of the German PWR KONVOI with all four loops. The vessel of the facility is made from Perspex. The geometrical similarity between the model and the original reactor is fully respected within the region in-between the bends in the cold legs, which are closest to the reactor inlet and the core entrance. The geometry of the inlet nozzles with their diffuser segments and the curvature radius of the inner wall at the junction with the pressure vessel were modeled in detail. Similarity is also taken into account for the core support plate with the orifices for the coolant and the perforated sieve drum (flow skirt below the core barrel) in the lower plenum. Beside others, experiments have been performed at the ROCOM test facility for the first and the third classes mentioned above for the Gidropress test facility. That allows comparing the experimental results obtained at both test facilities.
The analysis of the slug mixing experiments showed comparable flow behavior. The first part of the tracer is found on the opposite side in regard to the position of the starting-up loop. In this region, the maximum tracer is measured in both facilities. These maximum values differ by about the same value as the initial slug sizes in the experiments.
In stationary experiments at both test facilities a clear sector formation at the core inlet could be observed. Coolant from the loop with the perturbation arrives nearly unmixed in both cases at single measurement positions at the core inlet. In one of the Gidropress experiments an additional counterclockwise swirl was found which is responsible for moving the sector. In the corresponding ROCOM experiment such an additional swirl is fully absent. Contrary to that, a shift of the sector is found in a four-loop experiment at reduced flow rate.

  • Contribution to proceedings
    6th International Conference "Safety Assurance of NPP with WWER", 26.-29.05.2009, Podolsk, Russland
    Proc. of the 6th International Conference "Safety Assurance of NPP with WWER", CDROM, paper 008, Podolsk: OKB Gidropress Podolsk, 9785948830926
  • Lecture (Conference)
    6th International Conference "Safety Assurance of NPP with WWER", 26.-29.05.2009, Podolsk, Russland

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Publ.-Id: 12692