Chemical processes in fission product release and transport

The central issues of radionuclide behavior in the reactor coolant system are radionuclide release from fuel, transport in the coolant system and revaporization. Modeling of the release of radionuclides from intact fuel rods has become quite detailed and recognizes the multiple mechanisms causing release. This modeling is, however, supported by a data base that does not span the entire spectrum of expected accident conditions. Few experimental studies have been conducted at elevated pressures and low gas flows characteristic of many important reactor accidents. Modeling of release from fuel in more advance stages of degradation or when hot fuel interacts with water is in a more primitive state. Radionuclide transport and deposition in the reactor coolant system is now routinely considered in accident analyses. For many accidents, significant fractions of released radionuclides are predicted to be retained in the coolant system. These predictions depend on the chemical forms of the radionuclides predicted to exist under accident conditions. Chemical forms of the retained radionuclides are now only hypothesized. The actual chemical forms assumed by radionuclides deposited in the reactor coolant system will determine whether current predictions of extensive revaporization of deposited radionuclides late in an accident are realistic