Development of Accident-Tolerant Control Rods & Guide Tubes for PWR Fuel Assemblies

In a loss of coolant accident, the fuel rod cladding can overheat and burst open exposing the nuclear fuel within. It can also react chemically with the residual steam present in the core to release hydrogen, forming a potentially explosive mixture that can challenge containment integrity. Accident-tolerant fuel attempts to prevent these effects by replacing the zirconium-alloy fuel rod cladding with a non-reactive ceramic material such as silicon-carbide (SiC). Currently, the fuel assembly guide tubes are also made from zirconium alloy, so changing their material to SiC will support the accident-tolerant fuel program. During a reactor accident, keeping the control rods and absorber intact helps prevent a re-criticality. This would be of particular benefit if fresh water with no soluble boron has to be introduced into the vessel to cool the core during the post-accident mitigation phase. The control rod cladding is currently stainless steel that could suffer structural failure during accident conditions. Also, the silver-indium- cadmium (Ag-In-Cd) alloy that is the typical absorber material used in PWR control rods has a low melting point. So in a loss of coolant accident during which the control rods could overheat and fail, the potential exists for the absorber to melt and drain out of the core leading to a potential re-criticality. The purpose of this Phase II project is to continue development of the manufacturing processes required for SiC control rods and guide tubes and replacement absorber materials. The results of the Phase I project will be used to refine the design for the control rods and guide tubes. The analyses performed in Phase I will be updated to include test results from Phase I and additional analyses including seismic analyses and absorber swelling will be performed. A full-scale, partial-length fuel assembly and control rod assembly will be fabricated along with the required guide structure to simulate reactor internals. These components will be used to perform SCRAM tests to characterize performance.The need for irradiation tests will be determined and flow tests with the guide tube and control rod will be completed. This work is required to allow implementation of these accident-tolerant materials in commercial nuclear reactors.