Development and Characterization of SiC-Clad Control Rods & SiC Guide Tubes for Accident-Tolerant

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 project is to develop the manufacturing processes required for SiC control rods and guide tubes. Several different replacement absorber materials will also be fabricated and evaluated. 3D CAD models of one or more control rod assembly and guide tube assembly concepts will be created. The concepts will be evaluated considering loads and functionality to determine the best design. Existing processes for fabrication of SiC fuel rod cladding will be modified as needed to fabricate control rods, guide tubes and the associated end plugs or connection parts. The process for three different hafnium compound absorber candidates (HfO2, HfB2 and HfC) will also be developed. Samples of the guide tube, control rod and the absorbers will then be characterized and tested. Based on the Phase I results, the processes will be refined and additional analyses and tests will be performed that include full-scale SCRAM tests and flow tests and irradiation tests.