Aqueous alteration in CR chondrites: Meteorite parent body processes as analogue for long-term corrosion processes relevant for nuclear waste disposal
Résumé
Aqueous alteration of carbonaceous chondrites is one of the fundamental processes on accreting planetesimals that changes pristine materials from the formation of the Solar System. The study of mineralogical, petrological and chemical changes resulting from this alteration provides insight into the physical and chemical setting of forming planetesimals. CR chondrites provide samples for all stages of aqueous alteration, from type 3 to 1 (entirely hydrated), and are thus suited to study the alteration of pristine materials in a coherent sequence.
Vitrification is a common way to store and stabilize fission products and minor actinides resulting from the reprocessing of nuclear spent fuel in a nuclear boro-silica glass in steel containers. The waste material has to be stored safely for a period of at least 105–106 years in a clay-rich geological repository. Laboratory experiments being too short to follow the long-term evolution of these materials, we analyzed the mineralogical, petrological and chemical changes in a series of CR chondrites (Renazzo CR2, Al Rais CR2, and GRO 95577 CR1) to serve as analogues. Rims of secondary materials around metal grains in contact to the fine-grained matrix serve as analogue to the interface between steel containment and the surrounding clay-rich geological layer, while chondrule glassy mesostasis is used as a proxy of the nuclear glass.
With increasing degree of aqueous alteration in the sequence, Renazzo → Al Rais → GRO 95577, the size of the rims increase. Fe-rich alteration rims are ∼10 μm in thickness around metal grains in the fine-grained matrix in Renazzo. In Al Rais, multi-layered structures of interchanging Fe, S and P/Ca-rich layers appear, with a thickness of up to ∼30 μm. In the highly altered GRO 95577, extensive inner and external rims of secondary phases reach up to ∼200 μm into the surrounding matrix. In chondrules, metal in contact with the altered mesostasis shows similar trends, but with thinner alteration rims.
Altered mesostasis is similar to the surrounding fine-grained matrix, consisting of serpentine and smectite. However, the chemistry has not completely equilibrated, as indicated by e.g. a remaining high Al contents (>2.2 wt.%). Fine-grained matrix and altered mesostasis show a trend towards decreasing Fe with progressing alteration. Matrix is also the main source of S in the alteration system.
Most of the dissolved species stays close to the source. The main source for Fe, Ni, P and also Ti, Cr and Mn is the altered metal. These findings confirm the model for long-term corrosion of nuclear disposal sites: alteration rims of iron oxides form around the steel containments, whereas dissolved species (Fe, Al, and Si) diffuse into the surroundings.