Advanced Material Concepts for LWR Control Rods for Improved Accident Tolerance

Abstract

In recent years efforts have been underway to improve accident tolerance of Light Water Reactor (LWR) components in loss of coolant accident (LOCA) scenarios. The primary focus thus far has been on the development of accident tolerant cladding/fuel (ATF). However, other in-core reactor components, notably the control rod assembly, also warrant accident tolerance. One option for accident tolerance is the application of a Cr coating by cold spray deposition, which has been shown to provide excellent oxidation resistance to Zr-alloy fuel cladding under temperatures and environments representing accident conditions. This study investigates cold spray Cr coatings for improving the accident tolerance of current and exploratory control rod sheath materials. The metrics studied for accident tolerance include high temperature steam oxidation resistance and the ability of the Cr coating to serve as a diffusion barrier to prevent elemental interdiffusion and potentially even the associated low temperature eutectic melting between the control rod sheath material and Zr-alloy guide tube. Preliminary evaluation of the wear resistance of the uncoated and Cr coated control rod sheath materials was performed given that the control rod system is intended to move up and down to control neutron activity. Exploratory neutron absorber materials were also investigated for accident tolerance with the focus being on interdiffusion and low temperature eutectic melting between the inner surface of the control rod sheath material and the neutron absorber. Testing and characterization were performed using high temperature steam oxidation and diffusion couple tests, pin-on-disk wear tests, nanoindentation, Vickers microhardness, scanning electron microscopy (SEM), optical microscopy (OM), x-ray diffraction (XRD), high precision weight change measurements, and optical surface profilometry. Prediction of phase evolution and elemental diffusion were conducted using Thermo-Calc software CALPHAD and DICTRA, and preliminary thermomechanical simulations were performed to evaluate the thermal stresses in the coating-substrate interface up to the operating temperature of the reactor. Three control rod sheath material substrates were investigated: 316L stainless steel (representative of the currently used control rod sheath material), and exploratory materials Mo-based

alloy TZM (Mo-0.5Ti-0.08Zr-0.02C), and Hf (99.9% pure). TZM and Hf were selected primarily on account of their outstanding high temperature strength and neutron absorption cross-section, respectively. The three materials were Cr coated via cold spray deposition using the facility at the University of Wisconsin, Madison’s Nuclear and Applied Materials Research Group (NAMRG) to yield dense, well- adhered coatings of thicknesses in excess of 60 microns and nanoindentation hardness of 5.5 GPa. High temperature steam oxidation tests conducted at 1200°C for 30 minutes showed that the Cr cold spray coating imparted notable improvements in oxidation resistance for the three substrate materials. For uncoated 316 stainless steel, the spallation of an oxide (Fe3O4/Fe2O3) layer of nearly 200 microns in thickness that formed was prevented by the Cr coating which instead formed a tenacious and protective Cr2O3 layer only about 7 microns in thickness. Weight change measurements showed that the Cr coating notably improved the already good high temperature oxidation resistance of Hf. For the TZM alloy, the volatilization of MoO3 observed for uncoated material was prevented by the Cr coating allowing for the expanded use of this alloy in high temperature oxidative environments. High temperature diffusion couple experiments were performed to determine the effects of inadvertent contact of the control rod sheath materials and the Zr-alloy guide tubes under accident scenarios, and to determine the effectiveness of the Cr cold spray coatings to serve as an effective diffusion barrier between the two materials. For the 316 stainless steel/Zircaloy-4 diffusion couple, eutectic melting initiated at temperatures as low as 930°C and complete melting was observed in a diffusion couple test conducted at 1100°C for 10 hours. Under the latter conditions, melting did not occur in the presence of a Cr coating on 316L stainless steel, and this protective effect was sustained with a coating thickness of at least 50 microns. No melting, but considerable diffusion was observed for Hf/Zircaloy-4 and TZM/Zircaloy-4 when in contact for 10 hours at temperatures ranging from 1000°C- 1200°C. The diffusion interactions reached about 460 microns for Hf into Zircaloy-4 (Zr-Hf is binary isomorphous with complete mutual solubility) and 200 microns for Mo into Zircaloy-4 at 1200°C for 10 hours. Significantly less diffusion was observed for the Cr coated versions of the two materials under

identical test conditions, with a maximum diffusion of approximately 8 microns for Cr and Zircaloy-4. Higher diffusion typically implies stronger bonding, an effect that may be deemed undesirable for components where easy relative movement is desired. Diffusion interactions between the Cr coatings and substrates themselves were minimal for Hf and TZM but were significant for 316L stainless steel, which showed complete consumption of a 50-micron Cr coating via diffusion in 10 hours at a temperature of 1200°C. These experimental studies and related observations were supported by thermodynamic and kinetic modeling using Themo-Calc software. Pin-on-disk wear testing against an inert ruby ball stylus was conducted as a preliminary evaluation of the wear resistance of the coatings compared to the uncoated substrates. For all three substrate materials, Cr cold spray coatings resulted in an improvement in wear resistance. The trends in wear resistance for the various materials surfaces investigated were consistent with the corresponding microhardness measurements. The results of wear tests, though promising, must be deemed preliminary given that in the actual control rod application, wear would occur at elevated temperatures. ABAQUS finite element analysis software was used to simulate the thermomechanical performance of the Cr coating on the control rod sheath under the operating conditions of LWR to ensure its mechanical stability. The results showed the peak stress to be below the estimated bond strength of the Cr coating. Bulk samples of three neutron absorber materials, boron carbide (B4C), hafnia (HfO2), and samaria (Sm2O3) were created from their respective powders using Field Assisted Sintering Technology (FAST) apparatus at NAMRG. XRD analysis showed that the phase purity of the powders was preserved in the compacts. Diffusion couple experiments at 1200°C for 10 hours conducted separately between 316L stainless steel/B4C and 316L stainless steel/HfO2 showed complete melting due to the interdiffusion in the former, but no melting in the latter diffusion couple indicating HfO2 to be potentially more accident tolerant than B4C.