Inspection of nuclear assets with limited access

Traditionally the inspection of nuclear canisters and their welds, is highly challenging, time-consuming, and expensive due to the complexities and logistics of the environment and process. This problem is further compounded when the asset lifetime is increased beyond their original design intent, and when accessibility to the asset is obstructed. One such problem being faced by Sellafield LTD in the UK is the storage of products arising from the reprocessing of spent nuclear fuel. This material is stored in a multi-package configuration with outer containment provided by grade 1.4404 stainless steel canisters with a Resistance Seam Weld (RSW) sealing the lid to the canister body. This project seeks to investigate the inspection of the welds located on these canisters at 1) the point of manufacture, 2) at periodic inspection intervals and 3) through in-situ inspection in their storage environment. When specifically considering the in-situ inspection scenario, the storage arrangement only allows for partial circumferential access of the complete RSW. This research is concerned with the development of an ultrasonic screening method of the lid and cannister RSW whilst in storage. The Semi Analytical Finite Element Method (SAFEM) has been utilised to predict the existence of a guided wave contained only in the welded region known as a Feature Guided Wave (FGW). A novel curve tracing algorithm has been developed to curve trace the dispersion relationships uncovered from the SAFEM, dramatically increasing the speed of the post processing solutions generated. A flexural FGW was found to be most strongly confined to the welded region and was similar in its dispersion relationship to that of the fundamental A0 lamb wave mode. Various transduction techniques were investigated and the SAFEM results were validated by utilising a Polytec MSA-100 3D laser Doppler vibrometer. Several pulse echo experiments were performed initially on flat mock samples and then representative curved canister samples where strong intensity reflections from various artificial defects were observed, confirming the efficacy of this method as a screening tool for RSW joints. RSW can be frequently found in automotive, aerospace and piping components implying to the wider application of the concept and methodology can be extended beyond the nuclear industry.