Silver-Indium-Cadmium Control Rod Behavior in Severe Reactor Accidents

Silver-indium-cadmium (Ag-In-Cd) control rod behavior in severe reactor accidents is examined with a goal of improving the methodology used to estimate reactor accident source terms. Control rod behavior in both in-pile and out-of-pile experiments is reviewed. A mechanistic model named VAPOR is developed that calculates the downward relocation and simultaneous vaporization behavior of the Ag-In-Cd alloy expected after control rod failure in a severe reactor accident. VAPOR is used to predict the release of silver, indium, and cadmium vapors expected in the Power Burst Facility (PBF) severe fuel damage (SFD) 1-4 experiment. In addition, a sensitivity study is performed to examine the effects of system pressure and flow rate on control rod vapor release. Although cadmium is found to be the most volatile constituent of the alloy, all of the calculations predict that the rapid relocation of the alloy down to cooler portions of the core results in a limited release for all three control rod alloy vapors. Results of the control rod and aerosol behavior in PBF test SFD 1-4 are presented. VAPOR calculations are found to compare much better with the control rod material release in test SFD 1-4 than empirical models that do not consider relocation of the alloy away from the hotter portions of the core. The timing and magnitude of control rod material release and the potential for control rod aerosol/fission product interactions during the early phase of a severe accident are dependent on the system pressure. A better understanding of control rod material behavior during the later in-vessel phase of the accident is needed to define more accurately both the magnitude of the aerosol source and the initial composition of molten material exiting the vessel in the event of lower vessel head failure.