Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

3 Publications

Coolant mixing in a PWR - deboration transients, steam line breaks and emergency core cooling injection - experiments and analyses

Prasser, H.-M.; Grunwald, G.; Höhne, T.; Kliem, S.; Rohde, U.; Weiss, F.-P.


The response of the reactor core to a reactivity perturbation caused by a decrease of the boron concentration or a decrease of temperature in one loop of the primary circuit strongly depends on the degree of mixing with the undisturbed loops. In case of an emergency core cooling (ECC) injection, the flow field can be additionally dominated by gravity effects, because the density difference between ECC water and coolant inventory is significant. The streak formation may cause thermal loads on the reactor pressure vessel wall (PTS).

In order to study coolant mixing inside the reactor pressure vessel in the flow path beginning from the inlet nozzles and ending at the core entrance the test facility ROCOM (Rossendorf Coolant Mixing Model) was built and operated during the last three years. The facility represents a German KONVOI type PWR (1300 MWel) in a scale of 1:5. ROCOM is equipped with four fully functioning loops. The circulation pumps are driven by motors with computer controlled frequency transformers. In this way, a wide variety of flow rate regimes, such as four-loop operation, operation with pumps off, simulated natural circulation modes and flow rate ramps can be realized.
In case of the experiments on ECC injection, the higher density of the injected water was simulated by adding sugar (glucose).
The evolution of the spatial distribution of the deborated coolant respectively the ECC water was measured by tracering with sodium chloride solution. Conductivity distributions were measured by electrode mesh sensors, one close to the reactor inlet nozzle (16x16 measuring points), two sensors in the upper and lower part of the downcomer (2D grids of 4 radial and 64 angular measuring positions) and one sensor at the core entrance (inlets of each of 193 fuel elements). The maximum measuring frequency is 200 Hz. The results allow the visualizations of the measured distributions at the sensor positions. In case of running main circulation pumps, the coolant from the affected loop arrives in a sector at the azimuthal position of the corresponding inlet nozzle. But on contrary to the extreme assumption of a sharp sector, the slopes of the sector are smooth and the maximum disturbance is about 70-80 % of the disturbance at the reactor inlet. Pump start-up scenarios as well as asymmetric natural circulation in one of the four loops are connected with a different picture. In this case, the water with a decreased boron concentration arrives at the core entrance at the side opposite to the azimuthal position of the inlet nozzle, in which the flow starts.
The injection of ECC water with a higher density is accompanied with the appearance of a streak of cold water streaming downwards in the downcomer. Experiments at ROCOM were carried out to identify the transition from momentum driven flow in the downcomer to a density driven regime (critical Froude number). The relative density difference was varied between 0 and 10 % and the loop flow rate between 0 and 15 % of the nominal. In case of momentum controlled flow (high Froude numbers) the ECC water reaches the core inlet first on the side opposite to the azimuthal position of the affected loop. If the density difference is large (low Froude numbers) the ECC water falls down almost in a straight line and reaches the core inlet at the azimuth of the inlet nozzle.
Parallel to the experimental work, CFD calculations were performed. Good results were achieved with the code CFX-4.2. Nozzle region, downcomer and lower plenum were discretized in a mesh of about 400 000 nodes. The calculations well reflect both shape and amplitude of the distributions at the core entrance, if a k-eps turbulence model is used. The code validation benefits from the detailed information delivered by the mesh sensors. Furthermore, a simplified mathematical model was developed to generalize experimental results using some linear properties of the transport equation for the temperature respectively the bo...

Keywords: pressurised water reactors; coolant mixing; deboration transients; main steam line breaks; emergency core cooling; experiments; computational fluid dynamics; pre-stressed thermal shock

  • Lecture (Conference)
    International Congress on Advanced Nuclear Power Plants (ICAPP), June 9-13, 2002 - Hollywood Florida, USA, Proc. CD-ROM, paper #1214.
  • Nuclear Technology 143 (2003) 37-56
  • Contribution to proceedings
    International Congress on Advanced Nuclear Power Plants (ICAPP), June 9-13, 2002 - Hollywood Florida, USA, Proc. CD-ROM, paper #1214.