Abstract
Nuclear reactor inspections occasionally identify degraded materials in irradiated reactor components. Although mechanical repair options are possible, these repair solutions may be cost prohibitive or impractical to implement due to access restraints and/or the severity of the degradation. Welding repair of reactor components may input excessive heat into these irradiated materials resulting in diffusion of trace amounts of helium within the grain boundaries of the weld heat-affected zone (HAZ). Intergranular HAZ cracking can then result from the combination of this helium diffusion and high localized tensile stresses generated during weld cooling. It is therefore critical to characterize these zones and understand limitations for welding highly irradiated components to prevent helium-induced cracking. To accomplish this, typical reactor structural materials including Types 304L and 316L stainless steels and nickel-based Alloy 600/182 materials irradiated within the High Flux Isotope Reactor facility at Oak Ridge National Laboratory were used in this study for welding and evaluation. A phased array ultrasonic inspection system has been developed to characterize cracking in the weld samples. It provides remote controlled scanning and minimizes handling the samples, minimizing operator dose. The samples are inspected from the side opposite of the welds. The material and weld grain noise were evaluated at 10 MHz and found to be conducive to detecting cracking in the material and welds. Inspection of the samples comprises a 10 MHz phased array probe sweeping a focused longitudinal wave from -60° to 60° while the probe is raster scanned over the sample in small increments. The collected data is analyzed using UltraVision 3. Several of the irradiated samples were inspected prior to welding. Some of the samples had what appear to be small lamination defects in them. One irradiated welded sample has been tested to date with no cracking detected, which has been confirmed by destructive examination.