Monitoring water-chemistry evolution in the bentonite buffer using magnets: Effects of corrosion on buffer stability

Abstract

Bentonite has been chosen as a buffer material by several national nuclear-waste management companies because of its swelling capacity and low water permeability, and because it retards the transport of radionuclides and corrosion products toward the geosphere. The aim of the present study was to develop a magnetic material that has the ability to detect changes in groundwater compositions in an underground nucleardeposit facility through variation in its magnetic properties with time.The present study has investigated the effect of the corrosion of NdFeB magnets on the Na-bentonite buffer under different environmental conditions. Seven different aqueous solutions were used for long-time exposure tests (708C, 5 months) with fragments of NdFeB magnets and powdered Na-bentonite. The corrosion of NdFeB alloy had no detectable effect on the mineralogy of the clay minerals, and any change on the bentonite matrix, measured through X-ray diffraction analysis, was related to the different initial chemistry of the solutions.

Magnetic Materials: Novel Monitors of Long-Term Evolution of Engineered Barrier Systems

“…The degradation of magnetic properties and signatures caused by fluid-induced corrosion of the different magnets are quantified to allow an evaluation of the sensitivity of magnetic monitoring with fluid ingress and saturation. We also summarise the effects of magnet corrosion on the bentonite matrix in order to understand if such corrosion might have a negative impact on the swelling properties of the bentonite buffer in a GDF [17].…”

“…The clay matrix evolution in the NdB experiments has been addressed in [17], which showed that the clays in close contact with intensely corroded magnets reacted in saline solutions are characterised by weak and broad powder X-ray diffraction (PXRD) peaks that most likely reflect low-temperature dissolution/reprecipitation. Such broad and weak peaks are produced by a combination of fine particle size and poor crystallinity with complex or random interstratification within the clay.…”

“…In the case of Nd-Fe-B neo magnets, comparison of the results of these experiments in the presence of bentonite with those for magnet-solution interactions alone [17] demonstrates that bentonite has a dramatic effect on magnet corrosion in the presence of saline water under static conditions (i.e., no or negligible fluid flow). In the absence of fluid flow, the presence of bentonite facilitates the development of two electric double layers with opposite polarity (one generated by the clay and the other by the corrosion on the magnet surface).…”

“…In the absence of fluid flow, the presence of bentonite facilitates the development of two electric double layers with opposite polarity (one generated by the clay and the other by the corrosion on the magnet surface). This in turn causes enhanced corrosion of the magnets compared with the corrosion of the magnets in water alone [17]. This important electrical buffering effect provided by bentonite implies that experiments on neo magnet corrosion in humid environments or in the presence of fluids alone are unlikely to be relevant to magnet corrosion in an EBS once saturation commences.…”