Designing a Giant Stress Impedance (GSI) Strain Sensor for Monitoring Intermediate Level Nuclear Waste (ILW) Packages

In this thesis the practicality and viability of a giant stress impedance (GSI) sensor was studied on three amorphous magnetic ribbons. The GMI effect between the three amorphous magnetic ribbons was investigated, initially, to understand the influence of the GMI behaviour between materials of varying magnetic properties, especially the different chemical structure and, their respective, magnetostriction coefficients (a variable that describes a magnetic material's magnetoelastic properties) (λS); Co66Si15B14Fe4Ni1 (λS = < 1x10^-6), Fe81Si13.5B13C2 (λS = 30x10^-6) and Ni40Fe40Si+B19Mo1-2 (λS = 8x10^-6). Initial characterisation of the GMI effect was difficult due to the dimensions of the samples being larger compared to previous studies investigating the GMI effect of their studied samples. It used a trial-and-error approach to improve the characterisation technique to the point it could repeatably measure a consistent GMI response of the samples. The characterisation technique for measuring the GSI effect followed a similar procedure but with little time remaining it was incomplete to achieve the desired reliability. The influence of the geometry, λS and fabrication process of the samples on their GMI behaviour was explored. It was observed that the Co-rich sample had a higher GMI response compared to Fe- and Ni-rich ribbon samples. This was related to the difference in domain structures where a negative (near zero) λS domain structure promotes transverse permeability (µT), thus having a higher GMI response. A critical aspect ratio (l/w = 20) was observed for all three samples where at the critical aspect ratio all samples exhibited their highest GMI response. In addition, it was observed the GMI response of the three samples would be impeded by the presence of permanent damages (such as plastic deformation) caused by the fabrication process. The varying GMI behaviour between the ribbon samples was discussed using the competing effects between the shape anisotropy and demagnetisation factors, influencing the ribbon sample’s transverse permeability (µT). The suitability of using the GSI effect to detect the expansion of intermediate-level nuclear waste (ILW) packages was investigated by applying stress/strain on the sensing material directly. The influence of the magnetostriction coefficients (λS) to the GSI effect of the three samples displayed similar responses to their GMI behaviours; where the Co-rich ribbon sample exhibited the highest magnitude in GSI ratio compared to the Fe- and Ni-rich ribbon samples. This implies the lower the magnetoelastic effects the higher GSI response. Although, the data suggests a more complicated interaction between the transverse permeability (µT) to the shape and stress anisotropies (magnetoelastic effects). The GSI performance between all three samples was explored at stresses/ strains up to 400 MPa/ 10x10^-3 at frequencies between 0.1 – 10 MHz. Finally, the demonstration of the feasibility of the selected material (Co-rich) as a strain sensor on monitoring globally expanding ILW nuclear waste packages was investigated. Simulating the strains that were comparable to a globally expanding ILW waste package (referenced from Sellafield Ltd) the strain sensor observed a clear noticeable trend when undergoing strain at 0.4 Ω decrease at 0.25% strain. This demonstrated a proof-of-concept of using a GSI strain sensor to monitor the expansion of a nuclear waste package using the change in the stress impedance of the sensor – where high and low impedance values signify the early and late stages of the waste package expansion. This is under the assumption the sensor will be used to monitor the waste package within an approximate time period of a decade. The experimental results and the existing literature on using the GSI effect for strain sensing applications suggest the technology is applicable for structural health monitoring for detecting very small changes of strain that are not (typically) noticeable by the naked eye. This is possible from their high sensitivity to detecting minor external changes in the material, which includes minor changes of strain. In addition, it is possible to adjust the strain-sensing capability of the material by either adjusting its magnetic or mechanical properties, such as heat treatments or Young’s modulus. As a result, this is considered a viable solution for the current application of monitoring the expansion of intermediate-level nuclear waste (ILW) packages since it has been reported by the staff at Sellafield, the expansion becomes noticeable after decades of observation [1].