Dr. Sören Kliem

Reactor Safety
Phone: +49 351 260 2318

The ROCOM Test Facility

The test facility ROCOM (Rossendorf Coolant Mixing Model) was erected for the investigation of coolant mixing in the reactor pressure vessel of pressurized water reactors (PWR). ROCOM is a 1:5 model of the PWR KONVOI.

The test facility was designed for the investigation of a wide spectrum of mixing scenarios.
Experiments are carried out to measure the time-dependent distribution of the transport variables coolant temperature and boron concentration inside the reactor pressure vessel. The leading input variables are the time history of the flow rates in the four loops of the primary circuit as well as the coolant temperature or boron concentration in the inlet nozzles, respectively. The differences in either boron concentration or coolant temperature are modeled by means of a salt tracer solution, which influences the electrical conductivity.

The test facility is equipped with measurement instrumentation, which allows a high resolution measurement of the transient tracer concentration in space and time.

Design Parameters

The design parameters of the test facility are presented in the following table together with the data of the original reactor (Comparison original PWR - ROCOM with water at 20°C)

value unity original ROCOM
inner diameter of the pressure vessel mm 5000 1000
height of the pressure vessel mm ~12 000 ~2400
inner diameter of the inlet nozzle mm 750 150
width of the downcomer mm 315 63
coolant flow rate per loop m3/h 23 000 350 (max.)
185 (nominal)
coolant inlet velocity m/s 14.5 5.5 (max.)
2.91 (nominal)
velocity in the downcomer m/s 5.5 2.1 (max.)
1.1 (nominal)
Reynolds-number in the inlet nozzle - 8.4 · 107 8.3 · 105 (max.)
4.4 · 105 (nominal)
re downcomer - 2.7 · 107 2.6 · 105 (max.)
1.4 · 105 (nominal)
re original/re ROCOM - 1 ~100 (max.)
~190 (nominal)
coolant travelling time original/ROCOM - 1 1 (nominal)


The reactor pressure vessel manufactured from acrylic glass (see Figure above) is the main part of the test facility. Beginning from the bends in the cold leg which are closest to the reactor inlet until the core inlet, the geometrical similarity according to the given scale is respected between model and original reactor. 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 was modeled in detail. Similarity is also taken into account for the perforated sieve drum and the core support plate with the orifices for the coolant. The entry into each fuel assembly is a orifice with a diameter of 30 mm, containing one measurement position of the integrated core inlet wire mesh sensor. Further, all inlet nozzles of the reactor pressure vessel are equipped with sensors. Two different types of sensors can be installed in the downcomer of the test facility. The first sensor typ measures the tracer concentration in one radial plane. One of this type can be installed at the inlet (just below the nozzle plane) and at the outlet of the downcomer. 64 measurement positions along the circumference are distributed in a distance of 5.625°. At each of these positions the conductivity is measured at four measuring points over the width of the downcomer (pitch: 13 mm). Recently, a new sensor was developed for an improved visualization and quantification of the mixing processes in the downcomer. It can be installed instead of the two radial sensors. This new sensor now spans a measuring grid of 64 azimuthal and 32 positions over the height of the downcomer. The signal measured at each of the 2048 positions is the conductivity that is averaged over the width of the downcomer. Sensors can be installed at further positions along the flow path (e.g. into the outlet nozzles or at the position of the emergency core cooling water injection).
The reactor core itself is represented by a hydraulic resistance of the fuel elements, only. A core basket is inserted being a hydraulic short circuit between core inlet and outlet. In the current design, the model of the pressure vessel is equipped with a plane vessel head, which can be replaced by a spherical head according to the original reactor. The upper plenum does not contain any internals.
ROCOM is equipped with four loops (see Figure below) with a speed controllable circulation pump in each of them. These circulation pumps are controlled by individual frequency transformers. That allows to realize nearly each desired combination of flow rates in the single loops. For natural circulation conditions, the corresponding pumps are operated at reduced rotation speed. The geometry of the loops can be seen in the fig. The boundary of full similarity between original reactor and test facility is located behind the first bends in the cold and in the hot legs closest to the vessel. That means, these bends are included into the similar representation of the original reactor in the given scale. The volume ratio of vessel and loop is identical between test facility and original reactor to ensure identical coolant travelling times.

schematic view of the facility


An overview about the carried out coolant mixing experiments can be found here: Einführung von Experimenten


An overview about post test calculations using ROCOM data can be found here:   Geschwindigkeitsfeld