3. DESCRIPTION
The fifth International Reactor Safety Research Project conducted at Marviken, Sweden, comprised a series of Aerosol Transport Tests (ATT) that were performed between 1983 and 1985. A total of five tests were performed.
The ATT project studied on large-scale the effects of low probability severe accidents in water-cooled reactors leading to overheated core materials. Such postulated accidents were represented by the vaporization of fission product simulants with or without reactor core simulants in the presence of steam, nitrogen, argon and, hydrogen. Measurements were made of the thermalhydraulic conditions present in the system, and extensive sampling was undertaken to provide data on aerosol mass concentration and particle size distribution, to identify gaseous species and to obtain a mass balance for the system.
The project has produced a large-scale data base for development and validation of analytical models used to predict radionuclide transport for a wide range of postulated accident sequences. It has further provided quantitative information on aerosol transport and deposition in a simulated water-cooled reactor primary system.
During degraded core accidents, a portion of the fuel and structural material may vaporize and mix in a gaseous form with the coolant atmosphere. Natural circulation from the hotter regions will carry the vaporized material to nearby cooler regions where aerosol particles will be formed by condensation. The potential escape of these aerosols from the reactor system constitutes the major risk from a degraded core accident. However, a significant fraction of the material may be trapped within the reactor system itself, depending on the degradation sequence. Processes such as thermophoresis, diffusiophoresis, gravitational settling and impaction may result in the aerosol particles depositing on structural surfaces. In many key sequences, radioactive aerosols pass through liquid pools which are excellent natural filters. Since small-scale tests distort important effects, such as the ratio of surface area to volume, the data from large experiments are needed to develop and improve calculational methods.
The primary purpose of the Marviken Aerosol Transport Tests (ATT) project was to create a data base on the behaviour of vapours and aerosols produced from overheated core materials in large-scale facilities representing typical water reactor primary systems and pressure vessels for risk-dominant scenarios. These data are being used by sponsoring organizations to verify theoretical methods for the prediction of the behaviour of postulated core-melt accidents for a wide range of geometries and fluid conditions. A secondary objective was to provide a large-scale demonstration of the behaviour of aerosols in primary systems. Tests 1, 2a, 2b and 7 studied the transport of fission products which might be released during a fuel damage process. High density aerosols originating from structural material were not included in these tests. A non-radioactive "mixture of caesium hydroxide (CsOH), caesium iodide (Csl) and tellurium (Te) was vaporized with plasma-arc heaters and formed a so-called "fissium" aerosol in a large-scale primary circuit. The first three fissium tests used only the portion of the facility downstream of the reactor vessel and were comparative tests for studying the effects of different temperature ranges, superheated steam, condensing steam and water. Sequences with simultaneous fuel damage and structural aerosol release were studied in Test 4, which simulated the general geometry in a PWR reactor vessel. The so-called "corium" aerosol was produced by vaporizing silver (Ag) and manganese (Mn). Fissium was also generated in this test to study the interactions between the various species and their joint transport.
The organizations represented were from Canada, Germany, Finland, France, Italy, Japan, the Netherlands, Sweden, United Kingdom and the United States.
The official documentation of the project is given in the reports and appendices, the titles of which are listed on the back cover.
