VEERA FACILITY

The VEERA facility was designed for the investigation of the behavior of an aqueous boric acid solution during PWR LOCAs. The reference reactor was the modified Soviet VVER-440 design of the Loviisa plant. The main design principle was the accurate simulation of the rod bundle geometry ' and the geometry of the structures above the rod bundle. The exact simulation of these structures was necessary in order to be able to simulate accurately the mixing and concentration processes of an aqueous boric acid solution and to study the effect of boric acid crystallization on the flow behavior of the coolant in the core.

The reactor vessel is simulated by a stainless steel U-tube structure consisting of a downcomer, lower plenum, core, core outlet throttle and upper plenum. The primary loops with horizontal steam generators are not simulated in the VEERA facility.

The exhaust steam line consists of a moisture separator, condenser and condensate tank. Two large tanks are connected to the downcomer to feed the aqueous boric acid solution. Simulation of the main circulation pumps, pressurizer, auxiliary systems and secondary side has not been considered necessary in the intended experiments.

All elevations in the reactor vessel simulator below the hot leg connection are scaled 1:1. The scale of the volumes and flow areas is 1:349 referring to the number of fuel rod simulators in the facility and the number of fuel rods in the reference reactor.

The test section includes one full-scale copy of a VVER-440 reactor rod bundle. It consists of 126 full-length electrically heated rod simulators and an unheated cold center rod, which replaces the support tube of the actual fuel bundle. The bundle is enclosed in a thermally insulated hexagonal shroud. The inside distance of the opposite walls of the hexagonal shroud is 139 mm and the wall thickness 4 mm. Viewing windows on the side walls of the channel box can be used for visual observation of boric acid crystallization and possible blockage formation. They can also be used for taking photographs of phenomena or for taping with a video cassette recorder camera. The area of the viewing windows is quite small compared to the size of the shroud itself. Small windows were used in order to avoid extra heat losses and cold surfaces, which could disturb the phenomena and processes under investigation.

The heating coils in the rods are inside stainless steel cladding in a magnesium oxide insulation. The heated length, the outer diameter and the lattice pitch of the fuel rod simulators, as well as the number and construction of the rod bundle spacers, are the same as in the reference reactor. In order to simulate better the actual power profile of the reference reactor a nine-step chopped cosine axial power distribution was adopted.

The structures near the core top, i.e. the core outlet nozzles, are accurately simulated. The flow area of the perforated plate inside the core outlet nozzle corresponds to that of the upper tie plate of the reference reactor. This was done because the aim of the first experiments was to study the crystallization process, which always happened at the top of the core in the earlier REWET-II experiments. The structures below the core are not simulated exactly due to the electric connections to the rod simulators.

The moisture separator consists of 10 lamellae. The condenser is made of two pipes, one inside the other. Cold water flows in the shell side and hot water in the tube side. Condensed steam is collected into the condensate tank. By measuring the boric acid concentration in this condensate tank it is possible to calculate the boric acid distribution coefficient, i.e. the ratio of concentrations in steam and water.

Sheathed electrical heaters with a power range from 200 to 2500 W have been attached, by means of metal bands, to the outer surface of the downcomer, lower plenum and boric acid tank walls to heat up the structures and so reach suitable initial conditions and avoid unwanted condensation. The total power of the surface heaters is 8833 W. The whole facility housing, except the viewing windows, is thermally insulated with 100 mm thick mineral wool.

The main measurements in the experiments are coolant and cladding temperatures, system pressure, pressure differences, boric acid concentrations and heating power. Thermocouples are used for temperature measurements. Most of them are in the rod bundle at different radial and axial locations. Thermocouples measuring coolant and cladding temperatures are of the NiCr-Ni type with a 0.5 mm diameter. Wall temperatures are usually measured with 1.0 mm or 1.5 mm diameter thermocouples. System pressure is measured with a sensor in the upper plenum. Pressure taps and transducers are used for pressure difference measurements. Boric acid concentrations are measured by taking samples of the coolant at different locations of the facility and by titrating the samples manually or automatically with an apparatus designed for the purpose.

Modified VEERA facility

After the first test series the VEERA facility was slightly modified in 1988 in order to simulate better the mixing processes of aqueous boric acid solutions between the core section and the lower plenum. In the design of the modified version of the facility, much attention was paid to the exact geometric similarity of the core lower structures between the simulator and the reference reactor, a feature prevented earlier by the electric connections to the rods. This was realized by turning the rod bundle upside down and by adding the core inlet nozzles. The upper plenum was equipped with a flow orifice to simulate pressure losses in the structures above the core. As part of the modification process the rod bundle of the original VEERA facility was broken into parts. The fuel rod simulators were reconstructed and the thermocouples were either spot welded or tied onto the rod cladding. The number of thermocouples measuring cladding temperatures was increased. The locations of some thermocouples outside the core region were also changed. Because the bundle was now upside down the electric connections to the rods had to be designed differently. Especially the measurement locations of the boric acid concentrations in the lower plenum and core region had to be reconsidered.

VEERA facility in reflood experiments

The VEERA facility was modified for the second time in 1993 in order to use it for reflood studies. Owing to the full size rod bundle the facility gives a good opportunity to study different phenomena occurring during the reflood phase of a LOCA. Areas of interest include top to bottom reflooding (falling films), the effect of spacer grids (early rewetting) and different modes ofECC injection. The viewing windows on the side walls of the channel box can be used for observing the quench front progression and the behaviour of thermocouples attached on the outer surface of the rods.

For the reflood experiments the rod bundle was turned to its original position and the structures above the core (=core outlet throttle) were added back to the facility. Four different ECC injection ports were built. These are located in the upper plenum, lower plenum, top of the downcomer and bottom of the downcomer. A bypass possibility exists for adjusting the ECC flow rate in advance, before the test. Cooling water can be preheated up to 100 °C in the ECC tank. A pump is used for injecting water into the facility. In order to avoid pressure build up in high heat flux tests, where steam formation is strong, the rather small diameter pipe from the condenser to the condensate tank was replaced with a larger diameter pipe leading from the condenser straight to the laboratory roof. Sheathed electrical heaters were added to the outer surfaces of the hexagonal shroud and upper plenum for preliminary heating of the core region. The total power of the heaters was increased from the previous 8833 W to 14633 W.

Few additions and changes to the instrumentation were made. The vertical sectional view of the VEERA test bundle gives the axial positions of the eleven grid spacers as well as the measurement levels of the rod surface temperatures. Coolant temperatures in the test loop are monitored at 17 locations: one measurement in the downcomer, three in the lower plenum, elevenin the core and two in the upper plenum. Wall temperature measurements were added onto the channel box and upper plenum walls. ECC flow rate is measured with a magnetic flow meter. There is also a pressure transducer and a thermocouple in the ECC line. Four pressure difference measurements were added to the core region by using the points of boric acid concentration measurements as connections for pressure taps. One pressure difference measurement was added both to the lower plenum and downcomer. The pressure difference measurements are used for level indication and (local) pressure loss evaluation. The total number of temperature measurements and pressure and differential pressure transducers is 55 and 8 respectively.

The last experiments with the facility have been carried out at 1999 related to severe accidents. The facility is still available for experiments. Until now, 68 experiments have been carried out.