​The TOSQAN experimental facility was commissioned in the early 2000s to study the main phenomena governing hydrogen distribution in the containment of a reactor (condensation on the wall, exchanges of mass and heat caused by the sump or the containment spray system) and the associated formation of flammable mixtures. For the last ten years, the facility’s scope of application has been extended to other areas of nuclear safety such as studying the risk of aerosol dispersion during corium removal from the Fukushima reactors and loss of vacuum accidents through water or air ingress into the vacuum vessel of the future ITER fusion facility. The experimental data acquired in TOSQAN contribute to the validation of computer codes used in nuclear safety.

​TOSQAN consists of a closed cylindrical vessel (volume 7 m3, height 4 m, internal diameter 1.5 m) in which controlled injections of steam and various non-radioactive and non-explosive gases and aerosols are performed to simulate the thermo-hydraulic conditions of a core melt accident affecting a nuclear reactor (maximum design basis pressure and temperature: 6bar –160°C).

​Schematic diagram of TOSQAN facility vessel

 Modifications made for the study on dust resuspension in the ITER fusion installation

​TOSQAN’s instrumentation is designed for the characterization of multiphase flows under hostile experimental conditions. The facility has a high level of instrumentation in terms of both density and diversity, as a result of specific developments with partner universities (CNRS CORIA). These means of measurement, mostly based on innovative optical methods, can locally and non-intrusively characterize multiphase flows consisting of gaseous mixtures (air, water vapor and helium used to simulate the presence of hydrogen), water droplets and aerosols simulating a release of fission products.

Optical diagnostics, a specialism of TOSQAN

For measuring droplet velocity:

For measuring droplet size:

out-of-focus imaging (interferometrics laser imaging for droplet sizing (ILIDS)), which is used to measure diameter by analyzing the interference fringes created by the interaction of a laser and a droplet. See video below.
 

For measuring droplet temperature:

rainbow refractometry
 

For measuring concentrations of gases:

Spontaneous Raman Scattering (SRS) spectrometry, which uses the inelastic interaction between an incident photon from the laser and a polyatomic molecule.
 

Other measurements performed

Gases: concentration, temperature (K), velocity, turbulence, pressure

Droplets: diameter, velocity, temperature

Aerosols: diameter, concentration

​The research programs carried out in TOSQAN were initially part of research being conducted at IRSN on severe accidents likely to affect nuclear reactors. TOSQAN was designed in particular for studies of hydrogen distribution: the violent hydrogen combustion generated by reactor core degradation can damage the reactor containment or cause the loss of its safety systems. Today, the facility’s scope of application has been expanded to areas involving the characterization and analysis of similar physical mechanisms.

Current programs

ITER: dust dispersion in the vacuum vessel of a fusion reactor

As part of the ITER project for fusion research, dust dispersion in the tokamak’s vacuum vessel is being studied. TOSQAN can simulate certain accident scenarios (loss of vacuum due to air or water ingress) that could occur in the ITER Tokamak, to acquire experimental data that will be used to validate computer codes.

Videos below:

​Fukushima decommissioning: characterization of aerosols from the laser cutting of corium

IRSN is involved in a project being run by the ONET group in collaboration with the CEA, to assess the feasibility of laser cutting to recover the core of the damaged reactors at the Fukushima plant, as part of the decommissioning process. IRSN is participating in this project by studying the problems related to the dispersion of the aerosols generated by cutting. By identifying the aerosol source term suspended during laser cutting and by proposing and assessing the effectiveness of strategies to contain the aerosols, IRSN is helping to evaluate the radiological risk to humans and the environment.

​Study of the spraydown by precipitations of aerosols emitted into the environment in accident situations

This program, which is still running, used the TOSQAN facility between 2009 and 2012 to determine the phenomenological laws describing the deposition of aerosols by different types of rainfall, by means of highly controlled and heavily instrumented analytical experiments. The TOSQAN facility can simulate certain conditions representative of atmospheric deposition: millimeter-scale droplets, terminal fall velocity, rainfall intensity. A thesis directed by the Physical Meteorology Laboratory (LAMP) in Clermont Ferrand was carried out as part of this program (see the thesis of A. Quérel).

The ​flagship research behind TOSQAN: the study of hydrogen risks in the event of a severe accident at a nuclear reactor 

The mechanical effects resulting from combustion of the hydrogen emitted during reactor core degradation can damage the reactor containment or lead to the loss of its safety systems. The TOSQAN program, followed by the ERCOSAM program (FP7) (both run between 2000 and 2010), studied the main phenomena influencing hydrogen distribution in the containment of a reactor, which can lead to the formation of flammable mixtures. In particular, the TOSQAN spray program studied evaporation/condensation of the water droplets generated by the nozzles and the trapping of aerosols in water droplet sprays representative of the situation in a reactor. The experimental data acquired over a period of approximately 10 years, in more than 300 analytical experiments, helped to validate the different physical models embedded in the nuclear safety computer codes used or developed at IRSN (P²REMICS, Astec), and thus to verify the ability of the computer codes to simulate hydrogen distribution.

ALIDS, development of airborne optical instrumentation

The ALIDS project, coordinated by IRSN in the context of the FP7 EUFAR network, concerned the development of airborne optical instrumentation installed in aircraft to measure the drop size of water droplets in clouds. The ALIDS probe is based on the out-of-focus imaging optical diagnosis technique used in the TOSQAN facility to characterize the size of spray droplets.