Results of the latest large-scale realistic experiments investigating the post-LOCA behavior of mineral wool debris in PWRs (fragmentation, transport, deposition on sump strainers, slip through strainers, pressure losses)

During loss-of-coolant accidents in power plants fragments of the material which is used to insulate the ruptured pipe and is located in the jet impingement area of the blow down flow are released into the reactor containment. Depending on the position of the break and the direction of the jet, the insulation on nearby pipes and components may also be affected. The amount of insulant released depends on the type of material and on the break size and location. Moreover, the transportation of fragmented insulant, particularly in the sump region, its sedimentation or deposition on the sump strainers and slip through the sump strainers all depend on a range of technical plant features which can result in considerable differences between the various PWR types. This dependence on plant-specific boundary conditions, together with important correlations, for example, has been described in 'Knowledge Base for Emergency Core Cooling System Recirculation Reliability', prepared by US NRC for OECD. Analysis of the main findings has shown that integrated experimental studies should simulate specific plant boundary conditions as realistically as possible. The decision was therefore taken to conduct large-scale integrated experiments whose boundary conditions reflect the actual situation in Siemens PWRs in order to validate and expand on findings obtained from earlier, small-scale experiments. The main focus was as follows (see Figure 1): 1) Use of representative insulant material. 2) Definition of boundary conditions in the test facility representative for Siemens PWRs. 3) Determination of insulant particle size distribution in fragmentation experiments under realistic conditions similar to those in plant operation. 4) Determination of sedimentation behaviour in the containment sump, deposition processes on the sump screens and resulting pressure losses across the sump screens. 5) Determination of amount of insulant entrained into the reactor core, investigation of deposition behaviour in the reactor core and resulting increase in pressure losses. In 2001, preliminary experiments to ascertain representative insulant as well as three series of main experiments were performed under contract to VGB Power Tech Service GmbH as part of the 'Assured Sump Suction Function' research and development project. The main test series comprised fragmentation experiments, transport experiments and fuel assembly experiments. In 2003, supplementary tests were performed on a small-scaled test rig to investigate separate effects like the influence of changing the mesh size on head loss and slip for various kinds of mineral wool. (authors)