Short description
(German)
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AEROSOL TRAPPING IN STEAM GENERATOR (ARTIST)
N INVESTIGATION OF AEROSOL AND IODINE BEHAVIOUR IN THE SECONDARY SIDE OF A STEAM GENERATOR
BR>Steam generator tube rupture (SGTR) incidents, such as those, which occurred in various operating pressurized, water reactors in the past, are serious operational concerns and remain among the most risk-dominant events. Although considerable efforts have been spent to understand tube degradation processes, develop improved modes of operation, and take preventative and corrective measures, SGTR incidents cannot be completely ruled out.
BR>Under certain conditions, high releases of radionuclides to the environment are possible during design basis accidents (DBA) and severe accidents. The severe accident codes' models for aerosol retention in the secondary side of a steam generator (SG) have not been assessed against any experimental data, which means that the uncertainties in the source term following an unisolated SGTR concurrent with a severe accident are not currently quantified.
BR>The accident management (AM) procedures aim at avoiding or minimizing the release of fission products from the SG. The enhanced retention of activity within the SG defines the effectiveness of the accident management actions for the specific hardware characteristics and accident conditions of concern. A sound database on aerosol retention due to natural processes in the SG is not available, nor is an assessment of the effect of management actions on these processes. Hence, the effectiveness of the AM in SGTR events is not presently known.
o help reduce uncertainties relating to SGTR issues, an experimental project, ARTIST (AeRosol Trapping In a Steam generaTor), has been initiated at the Paul Scherrer Institut to address aerosol and droplet retention in the various parts of the SG. The test section is comprised of a scaled-down tube bundle, a full-size separator and a full-size dryer unit.
BR>The project will study phenomena at the separate effect and integral levels and address AM issues in seven distinct phases: Aerosol retention in 1) the broken tube under dry secondary side conditions, 2) the near field close to break under dry conditions, 3) the bundle far-field under dry conditions, 4) the separator and dryer under dry conditions, 5) the bundle section under wet conditions, 6) droplet retention in the separator and dryer sections and 7) the overall SG (integral tests).
BR>SHORT INTRODUCTION TO THE ARTIST CONSORTIUM PROJECT
BR>Since 1998, contacts have been established with more than a dozen organizations in order to build a consortium to perform SGTR-related tests in the ARTIST facility. After review of the available data and models, it was decided that several open issues warranted further investigation in the framework of the ARTIST project. A preliminary ARTIST test matrix was proposed to potential partners in June 2000. Based on the suggestions and comments received, the test matrix has been modified and the latest version fixed at the beginning of 2001.
BR>1. Phase I: Aerosol retention in SG tubes under dry conditions. In this phase, in-tube aerosol deposition/resuspension will be studied under high flow conditions. Tube length, bend curvature, and aerosol type, size and concentration will be varied.
BR>2. Phase II: Aerosol retention in the break vicinity under dry conditions. In this phase, aerosol deposition/resuspension at very high velocities will be addressed. The break gas flow rate and break type (fish-month, double-guillotine, vertical) will be varied.
BR>3. Phase III: Aerosol retention in the bundle far from the break, under dry conditions. In this phase, aerosol deposition due to thermophoresis and impaction is studied at velocities, which are relatively small because the flow has evened out across the secondary side flow area. The gas flow rate and the gas-structures temperature differential will be varied.
BR>4. Phase IV: Aerosol retention in the separator and dryer under dry conditions. This phase studies aerosol impaction and interception due to complex 3D flows in the upper components of the SG. The gas flow rate and the gas-structures temperature differential will be varied.
BR>5. Phase V: Aerosol retention in the bundle section under flooded pool conditions. This phase investigates condensation-induced aerosol scrubbing by the SG water pool as well as inertial impaction upon the structures. The break flow rate, pool submergence, carrier gas steam content and pool subcooling will be varied.
BR>6. Phase VI: Droplet retention in separator and dryer sections under dry conditions. This phase deals with Design Basis Accident (DBA)-type phenomena, i.e. the potential for "primary bypass", whereby a break at the top of the tube bundle sprays fine primary liquid droplets that might find their way to the environment through, for example, a stuck-open safety valve. Air-liquid nozzles that create droplets with typical diameters have already been tested. In this phase, carrier gas flow rates and droplet sizes will be varied.
BR>7. Phase VII: Integral tests. The seventh set of experiments is in integral nature and is focused on aerosol retention in the whole model steam generator. The conditions of the tests will be determined based on insight gained from the results of the previous phases.
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