Summary and recommendations

Substantial progress has been made in understanding and predicting the behaviour of iodine in the containment building of a reactor following an accident, such as a loss-of-coolant accident. Collaboration between scientists and engineers in designing and analyzing integral and fundamental tests is necessary for further progress in this field. In spite of recent progress, fission product chemistry is still one of the main contributors to source term uncertainties; further research is needed in this field; in particular in the areas of organic compound interactions and surface effects. Dialogue between safety analysts and chemists is essential to identify the importance to risk of chemical phenomena and to prioritize safety issues involving fission product chemistry. Much of the homogeneous solution chemistry is understood; particularly important exceptions are rates and mechanisms for iodine hydrolysis, hydrogen peroxide reactions and the role of organics. Interactions between iodine vapour and bulk material aerosols can occur and have significant effects on iodine compound and transport. Iodine volatility within containment increases with decreasing pH, but, in the presence of radiation, it is not sensitive to the initial aqueous iodine species. There is further need for the development/validation of existing models and codes against realistic integral tests. However separate effects experiments must also be performed to both guide and help rationalize the data for the large integral tests. A variety of surface interactions involving iodine can and probably will affect its transport and volatility in accident situations. Dealing with surface reactions was identified as one of the most significant uncertainties in models and codes. Suspended CsI aerosols can be oxidized to iodine during hydrogen burns under dry condition. However, modest steam concentration (10% ) suppresses CsI oxidation to I2. Additional research on this issue was recommended. Intervention remains an important issue. It has been shown that high pH is favourable, but the minimum pH required to suppress volatility is still uncertain. An evaluated kinetic database is being issued and should be maintained. Some efforts should be devoted in the future to assess the temperature dependence of the reactions included in this database. Also, a sensitivity analysis should be conducted to reduce the number of chemical reactions to the most important ones. Studies should be broadened to examine fission products other than iodine that are important to risk such as cesium and tellurium. It is recommended that the next Workshop, if held, should also focus on these fission products. The need for and the scope of another Workshop should be reviewed in two years time