Since the Rasmussen study in 1975, various potential containment failure modes giving rise to radioactive releases have been examined. Source terms have been identified at various stages according to the delay for containment failure and the potential for delayed release through some pathway with some possibility of retention, see for example WASH 1400 (1975).

In this case, the source terms have been classified into releases associated with:

— an early containment failure with a pathway for direct release;

— a delayed containment failure (24 h) with a pathway for direct release;

— a delayed release through a pathway including some radionuclide retention.

Experimental studies are being undertaken within the EC 5th Framework programme to quantify fission product and core materials released from molten corium during the late phase of a severe accident. This would be at a time when the integrity of the containment vessel might be threatened. This work has been carried out within the PHEBUS programme. A schematic of the facility is shown in Figure 15.2.

In addition to promoting understanding, another important objective of the PHEBUS programme is to provide well-instrumented data for the validation of integral severe accident computer codes. The main processes that effect the degradation of fuel

and control rods, the release of fission products and aerosols, their transport in the primary circuit and the source term to the containment, are all included within the scope of the experiments.

Generally a good understanding has been gained of the releases of the more volatile fission products from intact fuel. However, the database for the release of less volatile fission products, core materials, aerosols, etc. from a degraded core is much less complete. This is particularly true for releases from a molten core. Previous experiments (Benson et al., 1999) have partially improved the database for the behaviour of metallic and ceramic pools. Additional data are required on the effects of fission product release in sparging and on the formation of crusts.

A current EC experimental programme (Bechta et al., 2001) is underway to examine such behaviour of fission product release from metallic and oxidic melts. The experiments will provide more understanding of species chemistry during the late phase of an accident. There are also tests aimed at examining the long-term behaviour of previously liquefied melt with an overlying water pool.

Metallic melt experiments are providing a better understanding of the important mechanisms affecting fission product and core materials releases up to 2000°C. They cover

the effects of temperature, oxygen potential, sparging, slug formation, two phase pools and composition of melt.

Oxidic melt experiments are in progress, concerned with studying the volatilisation of uranium oxides, fission products and boron oxide from melts of different compositions of UO2/ZrO2/SiO2/FeOx. These experiments are for both air and inert atmospheres, and with different temperatures of corium.

The main chemistry interests of the work concern tellurium, ruthenium, barium and strontium and the influence of steam on the volatility of the refractory fission products and actinides. The experiments focus on the generation of these elements at both high temperature, 1000°C and low temperature 25 °C.

Immersed core experiments are being carried out to determine the leaching and suspension rates of solid melts immersed in water under prototypical accident conditions. These use prototypical materials composed of UO2 and ZrO2 and other oxides.