Physical and chemical factors influencing fission product retention in reactor primary systems

The transport of fission products in vapor and particulate form through a reactor coolant system under accident conditions is accompanied by deposition of the fission products onto system surfaces. Deposition occurs by a number of mechanisms with the predominant mechanisms being vapor condensation and reaction with surfaces, and particle agglomeration and sedimentation. Deposition of more than 90 percent of fission products in the forms CsI, CsOH, and Te have been predicted for some accident conditions. Several computer codes have been written to perform calculations of transport and deposition and include most physical mechanisms believed to be of importance. However, there are several issues of importance to be resolved. These include processes such as chemical reactions, decay heating of fission product deposits, resuspension of deposits, and natural circulation within the primary system. It is current thinking that decay heating and chemical reactions may be important to retention at long accident times and natural circulation may significantly affect thermal hydraulic conditions. Consideration of these issues are expected to lead to revision of the transport codes. (orig.)