Monitoring fluid evolution in an Engineered Barrier System using NEO-magnets

Long-term monitoring of the evolution of the engineered barrier system (EBS) of a Geological Disposal Facility (GDF) is important for establishing the safety case for deep disposal of the UK inventory of high level radioactive waste. With a view to developing techniques for remote fluid monitoring using magnetic properties, we have examined the correlation between the corrosion properties of NEO-magnets and related changes in the magnetic properties of the alloy with fluid chemistry and crystal-chemical changes of the Na-bentonite matrix. Batch experiments comprised fragments of NEO-magnets with deionised water, saline and alkaline solution both in the presence and absence of MX-80 bentonite, and were performed in sealed vessels for durations of up to 5 months at 70°C. This study combined PXRD, thermomagnetic and hysteresis analysis to demonstrate how progressive hydrogenation of the main magnetic phase led to a maximum loss of remanence and coercitivity and increasing Curie temperature in the samples reacted with deionised water with the samples reacted in saline and alkaline solutions showing smaller changes. Semi-quantitative analysis allowed comparison of the Curie temperatures with crystal-chemical parameters. This reveals a clear positive correlation of increasing lattice parameters a and c (and cell volume) with mean hydrogens per unit formula and the Curie temperature of the product NdFeB hydrides. Precipitation of Nd and Fe hydrides/oxyhydroxides is also demonstated by the PXRD data. A crucial role is played by the transformations occurring to the smectite matrix, in particular by the cation exchange in the interlayer, which causes precipitation of highly charged K- and Ca-smectites. This study demonstrates how NEO-magnets are capable of detecting water saturation in the EBS, and that the NdFeB corrosion properties are strongly controlled by the initial fluid composition and presence / absence of the bentonite matrix.