Facility

The ThAI facility is a unique technical scale experimental facility for research in the area of nuclear reactor containment safety. The acronym ThAI stands for Thermal Hydraulics, Aerosols and Iodine. The facility enables to simulate various thermal-hydraulic scenarios ranging from turbulent free convection to stagnant stratified containment atmospheres. It is equipped with innovative measuring, sampling and data acquisition tools including a radiological control area to utilise radiotracer I-123 in the experiments. It allows to investigate safety relevant phe-nomena and component behavior under thermal hydraulics typical for severe accidents, including hydrogen phenomena, e.g. combustion and/or recombiner effects. ThAI is also equipped for aerosol investigation.
Program

Safety assessment and accident management in nuclear power plants necessitate investigating complex phenomena and processes with sufficient accuracy.

In support of such activities, the experimental ThAI program, sponsored by the Federal German Ministry of Economy and Labor, aims at answering open questions on fission product behavior in reactor containment.
ThAI provides experimental data bases for lumped parameter and CFD (Computatio-nal Fluid Dynamics) computer simulation programs in the reactor safety area. Coupling of thermal hydraulics, fission product transport behavior and fission product chemistry is another main focus, especially in the case of iodine with its high radiological importance.

ThAI is particulary aimed at the validation of the containment code COCOSYS (Containment Code System) presently under development at GRS. COCOSYS is mainly based on mechanistic models for the comprehensive simulation of all relevant phenomena, processes and plant states during severe accidents in the containment of light water reactors, also covering design basis accidents. In future, dedicated applications of CFD codes like STAR-CD or CFX will become more frequent in reactor safety analysis. ThAI is prepared to perform experiments for the validation of detailed models, e.g. for atmospheric stratification, boundary layers or jet dispersion.
Test Vessel

Fig. 1 shows a sectional view through the ThAI facility including auxiliary rooms. The 60 m3 test vessel is made of 22 mm stainless steel, its height being 9.2 m including sump, its diameter 3.2 m. It can be operated up to 180 °C and 14 bar. The inner multicompartment structure can be seen from fig. 2. The vessel is thermally isolated. The cylindrical part of the vessel is equipped with three independent heating/cooling jackets over the height for controlled heating or cooling of the walls by means of an organic liquid. A large top flange and two man holes provide access (see fig. 3). Measuring flanges on five levels at five circumferential positions allow the installation of in-situ optical and conventional instrumentation.
Fig. 1: Sectional view through the ThAI facility
Instrumentation

Considerable efforts were made to monitor local and large scale flows. Instrumentation includes Particle Image Velocimetry; a 2-D Laser-Doppler Anemometer, equipped to stepwise measure radial profiles of the vertical and circumferential velocity compo-nent; an innovative Micro Radio Acoustic Sounding System which provides instant height profiles of the vertical velocity component; and hotwire gas velocimetry for experiments with forced convection flow.

Furthermore, innovative dew point based relative humidity sensors are applied, specially designed to measure under near-to-saturation conditions at elevated temperatures.
Fig. 2: Interior of the ThAI test vessel

Radioactive labelled iodine is injected in a gaseous form directly into the vessel atmosphere. Various sampling devices and sampling lines are operated to measure local iodine concentrations in the vessel atmosphere, in liquid films at vertical walls and pools. Iodine deposition on surfaces is monitored online by a collimated NaI scintillation detector, controlled remotely. Gas samples are taken in-situ by small gas scrubbers installed inside the vessel. Sampling lines are connected to filter stacks outside the vessel devoted to discriminate Iodine species (Maypack). Chemical analyses are performed on site and in the Radiochemical Laboratory of Framatome ANP.

For aerosol experiments, ThAI is equipped with an aerosol generator and aerosol measuring equipment like Condensation Nuclei Counter, Electrostatic Classifier, Low Pressure Impactors, Automated Filter Stations and Aerodynamic Particle Sizer.
Fig. 3: Top view into the open test vessel
Performed Experiments

The 1998-2002 program (ThAI Phase I) in-vestigated large-scale transport and distribution phenomena.

Major emphasis was laid on:
 Running Test Progam

The ongoing 2002 – 2007 program (ThAI Phase II) concentrates on:
  • interfacial mass transfer of volatile mole- cular iodine between pools and atmosphere
  • multi-compartment transport of iodine
  • iodine and ozone
  • wet resuspension from boiling pools
  • aerosols and iodine behaviour during hy- drogen combustion
  • influence of hydrogen recombiners upon thermal hydraulics and on depletion of aerosols and iodine.
  • atmosphere stratification and convection
  • mass transfer of iodine at the gas/liquid and gas/structure interfaces
Experience

Until now, 15 thermal hydraulic tests and 14 iodine tests have been performed in the ThAI facility.

Members of the ThAI team are internationally acknowledged experts, having more than two decades of experience with large scale containment experiments like the HDR (Heiß-Dampf-Reaktor) and the Battelle Model Containment, relevant to hydrogen and aerosol behavior, further lab- and largescale experience in iodine experiments and chemistry, as well as in code development, application and assessment. They participate continuously in numerous international projects and working groups in the field of severe accident such as recent OECD International Standard Problems No. 41, 46, and 47, iodine status report writing group, numerous projects of the EU Framework Programmes on Nuclear Safety up to the current SARNET and in PHEBUS-FP working groups.

The ThAI program is performed by Becker Technologies GmbH, Eschborn, in close cooperation with Framatome ANP GmbH, Erlangen, and the technical departments of Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, Cologne.
Contact