KAJET experiments on pressurized melt jets with view to their interaction with substratum materials

In the sequence of a core melt accident in a light water reactor it is assumed that, after melt-through of the reactor pressure vessel, the molten cerium is released as a pressurized melt jet which causes erosion in the substratum material of the reactor pit. Melt jet behaviour under driving pressures of up to 2 MPa and corresponding erosion rates in various substratum materials are being investigated at Forschungszentrum Karlsruhe in so-called KAJET experiments using thermite melts (either iron or alumina) as simulant materials. Masses of up to 150 kilogrammes at temperatures of about 2100 degreesC can be provided for ejection. A special feature of the tests is to prevent melt dispersal due to gas break-through by sharply reducing the driving pressure before the end of melt release. Performance tests carried out with both water and smaller melt masses (10 - 40 kg) prior to the erosion tests aimed at finding the material for and appropriate geometry of a nozzle to provide a compact melt jet. Zirconia proved to be a good material for the nozzle. Good results have been obtained with a nozzle having a length to diameter ratio of two and a contraction angle of 90 degrees. This geometry is in good a,agreement with results from vessel-wall melt-through experiments at other laboratories. The first erosion test, conducted with a melt mass of 40 kg (iron and oxide in equal shares) ejected under a pressure of 0.3 MPa onto concrete plates, gives information about erosion rates concerning the target as well as the nozzle. The erosion was larger in case of iron.