Monitoring long-term evolution of engineered barrier systems using magnets: Magnetic response

- Nd-Fe-B magnets show time-dependent transition from ferromagnetic square-like to superparamagnetic loops upon corrosion.
- The changes in the hysteretic and thermomagnetic behaviour of Nd-Fe-B are related to the formation of hydride phases.
- SmCo magnets feature changes of the coercive field but not of the intrinsic hysteretic properties due to hydrides formation.
- AlNiCo magnets feature negligible deviations in remanence and saturation magnetization.

Remote and non-destructive monitoring of the stability and performance of Engineered Barrier Systems for Geological Disposal Facility of is gaining considerable importance in establishing the safety cases for Higher Activity Wastes disposal. This study offers an innovative use of mineral magnetism for monitoring groundwater saturation of the barrier. Four mixtures of permanent magnets (Nd-Fe-B, coated and uncoated; SmCo and AlNiCo) and bentonite were reacted for 4, 8 and 12 months with mildly-saline, high-pH leachates, representing the fluids saturating a time-evolved engineered barrier. Coupled hysteresis and thermomagnetic analyses demonstrate how Nd-Fe-B feature a time-dependent transition from square-like ferromagnetic to superparamagnetic loop via pot-bellied and wasp-waist loops, whereas SmCo and AlNiCo do not show so extensive corrosion-related variations of the intrinsic and extrinsic magnetic properties. This study allowed to identify magnetic materials suitable for shorter- (Nd-Fe-B) and longer-term (SmCo and AlNiCo) monitoring purposes.