The CALIST facility is developed to characterise the drops sizes and velocities from one or more spray nozzles, initially those of pressurised water nuclear reactors (PWR), as well as to study gaseous entrainment by sprays.
Figure 1. Panoramic view of the CALIST facility © IRSN
Study of drop interaction
This experimental facility was built in 2010 in the framework of a PhD thesis (Arnaud Foissac) to study the interactions between sprays obtained from nozzles like those used in nuclear reactors in case of severe accident.
Figure 2. Operating layour of the CALIST facility © IRSN
During this type of accident, between 200 and 500 spray nozzles located at the top of the reactor containment vessel are activated in order to reduce the pressure and gas temperature in the containment vessel and to capture fission products. They may also mix/dilute the potentially explosive hydrogen that may be present.
Figure 3. Production of the hollow cone spray by a PWR nozzle © IRSN
The droplet characteristics, in particular their size, can affect the wash-out, cooling and mixing efficiency. But the droplet sizes change in the containment vessel. This variation is due to both thermo-hydraulic conditions that lead to evaporation of the drop and/or condensation of water vapour on the drop and to interactions between drops (collision of drops, which can lead to their fragmentation in the event of high-speed collisions or to gravitational coalescence during their long vertical fall).
Figure 4. Study of collision outcome of drops: different regimes are observed according to the sizes and velocities of drops and impact conditions (Weber number and impact parameter) © IRSN (PhD thesis of Christophe Rabe, 2009)
Evaporation/condensation phenomena have been studied for over 15 years, using in particular the TOSQAN facility and the former CARAIDAS facility (PhD theses of Pascal Lemaitre and William Plumecocq). Studies of interactions between drops are more recent and are performed on the COLLGATE (Figure 4, collision of two drops) and CALIST (Figure 5, crossed sprays) facilities (theses of Christophe Rabe and Arnaud Foissac).
Figure 5. Crossed sprays on the CALIST facility © IRSN
Study of gas entrainment by a spray
The other effect of the nuclear spray systems studied on the CALIST facility is the gaseous entrainment by sprays. This phenomenon was studied in the TOSQAN containment vessel with nozzles of small size, and the CALIST facility allows the gaseous entrainment to be considered for nozzles of “actual” size (PWR nozzles; a spray in a so-called “hollow cone” shape can be seen in Figure 3).
Figure 6. High-speed camera image of the spray, showing the drop atomisation zone © IRSN
Modelling the mixing by such a spray is difficult, especially on the scales considered; part of the exchanges occur in the atomisation zone of the liquid jet, and the density of drops in the spray formation area is high (see in Figure 6 a photo of the spray atomisation zone). This leads to strong retroactive coupling between the drops and the gas in which turbulence also plays a role. Tests in CALIST are currently undergoing benchmarking in the frame of the SARNET-2 network of excellence on severe accidents (see an example of CFD calculations in CALIST, Figure 7).
Figure 7. 3D view of the drops trajectories of a spray from a PWR nozzle in the CALIST facility
The CALIST facility includes:
- an optical bench including a Phase Doppler Anemometer that can be moved along two axes over several metres and allows the sizes and velocities of drops to be measured (3 components for the velocity);
- a structure for the vertical motion of the spray nozzles by up to 3 metres height;
- a hydraulic system supplying water to 1 to 4 PWR spray nozzles (1 to 4 kg/s);
- a hydraulic system supplying atomizers generating 4 mist sprays of fine droplets;
- a water storage tank.
Other nozzles can be studied on this facility, such as sprinkler-type nozzles used to protect facilities against fire (PRISME-2 European project).