MELCOR, Review and Ongoing Development of the MELCOR Computer Code for the Analysis of Severe Accidents in Light Water Reactors: the MELCOR computer programme is used to simulate severe accidents in light water reactors. It maps accidents from the introductory event until the release of radioactive substances into the surrounding area, and is continuously adapted in line with the latest findings from accident research.

One aspect regarded as particularly significant for the description of severe accidents is oxidation of the fuel cladding tubes in the event of fuel rod dryout in the reactor vessel or the fuel storage pool. This phenomenon can result in accelerated core degradation and the increased release of fission products. Typically, oxidation takes place very quickly at the outset under oxygen-rich conditions, changing to a linear progression after several minutes as the protective oxide layer develops. In case of abrupt changes in temperature, however, parts of the thicker oxide layer can break off, causing the oxidation reaction to accelerate again at these points. On the basis of experiments at the Karlsruhe Research Centre, a more accurate model for this physical process was developed at the PSI. The model is now implemented in MELCOR and is being tested to determine its compatibility with the standard MELCOR model.

Effect of temperature ramp rate on oxidation in steam – breakway occurs on E110 at 5 and 10 K/min only.