KAJET experiments on pressure-driven melt jets and their interaction with concrete

Abstract

In case of a localized RPV failure, the melt expulsion into the reactor cavity may be as a compact jet for a short period, following by a dispersed release after gas break-through. The KAJET experiments were related to the short initial phase of a compact jet. The main objective of the experiments was to establish a compact jet under driving pressures up to 0.8 MPa (in performance tests up to 2.5 MPa) and to study its interaction with different substratum materials. The molten corium was simulated by an alumina-iron thermite melt. Using a revolving mechanism for the substratum samples, the interaction with both melt phases could be studied separately. The gas break-through was avoided by sharply reducing the driving pressure at the end of the melt ejection process. After a series of performance tests with water and thermite melt on the jet behaviour, seven interaction tests (KJ02 to KJ08) were carried out. The released total amount of melt per test was up to 160 kg. Driving pressures were varied from 0.3 to 0.8 MPa. Two different types of concrete were used as substratum materials, namely construction concrete and borosilicate glass concrete. Among others, the erosion rates in depth and the volume erosion were studied. In the frame of the KAJET programme, a data base has been generated for model development and testing. Two test series with different concrete types, different driving pressures and different melt phases are now available. In KJ02, KJ03, KJ04 and KJ08, siliceous construction concrete was used at pressures of 0.3 / 0.5 / 0.8 MPa. In KJ05, KJ06, and KJ07, borosilicate glass concrete was eroded at 0.4 / 0.5 / 0.8 MPa. The erosion rates in depth for iron are generally bigger than for oxide, and the rates for construction concrete are bigger than for borosilicate glass concrete. For construction concrete, these rates (in mm/s) are between 8.9 and 11.2 (iron) and between 4.5 and 10 (oxide). For borosilicate glass concrete, the rates are between 5.2 and 8.0 (iron) and 2.8 and 5.1 (oxide), respectively. In all cases the erosion rate increases with increasing driving pressure. However, volume erosion rates are similar for iron and oxide, or may be even larger for oxide. Theoretical analysis and interpretation has been performed in dose cooperation with Ruhr-Universitat Bochum and is reported separately. The melting of the structure material by the heat load of the impinging jet has been identified as the decisive mechanism for erosion.