Release of Fission and Activation Products During Light Water Reactor Core Meltdown

The most relevant open questions combined with activity release during hypothetical core meltdown accidents refer to the chemical behavior of the highly reactive elements iodine, cesium, and tellurium, to the release characteristics of the medium-volatile fission and activation products, to the properties of the resulting aerosol particles, and to various phenomena during steam explosion and melt/concrete interaction. To answer some of these questions, experiments are conducted at the melting facility SASCHA in which a representative core material mixture (corium) is induction-heated to temperatures of 3000 K. The released material is analyzed by use of gamma-ray spectrometry and electron scanning microscopy. Some results of the first series of experiments in air are given below:

1. The fractional release of cesium is <1% at 1500 K and 26.2% at 2400 K when a heatup rate of 110 K/min is applied.
2. The release of selenium (and probably also of tellurium) is strongly reduced by a chemical reaction with the Zircaloy cladding material.
3. The deposition characteristics of the released products show that the relative amount of low volatile species (U,Zr) is much higher in the vicinity of the hot crucible system than on the more distant glass fiber filters.
4. The size distribution of the aerosol particles is trimodal, with maxima at diameters of 0.17, 0.30, and 0.73 µm.