Hydrothermal alteration of a simulated nuclear waste glass: effects of a thermal gradient and of a chemical barrier
Poinssot, Christophe; Goffé, Bruno; Magonthier, Marie-Claude; Toulhoat, Pierre
European Journal of Mineralogy Volume 8 Number 3 (1996), p. 533 - 548
published: Jun 17, 1996
manuscript accepted: Feb 6, 1996
manuscript received: Mar 16, 1995
ArtNo. ESP147050803007, Price: 29.00 €
Abstract The disposal of nuclear waste will create a thermal gradient in the near-field environment, the magnitude of which depends on the cooling delay before disposal, and on the geological environment. For anticipated conditions, such a disposal is expected to have a gradient of 100 to 200°C for 100 to 1000 years during the thermal period of disposal (P.A.G.I.S., 1988). We studied the influence of this thermal gradient on the alteration processes in terms of mass transfer and alteration phases. A simulated French nuclear waste glass (containing 8 cations) was enclosed in a gold tube filled with deionized water and put in a cold-seal vessel submitted to thermal contrasts of 320-280°C and 250-220°C between 95 to 145 days at 130 bars. The new solid phases and their spatial distribution along the tube were studied by SEM observations coupled to EDS analyses, and X-ray microdiffraction. A mineralogical zonation implying mass transfer through the tube was systematically observed whatever the experimental temperature. It allowed us to demonstrate the mass transport of elements either towards the hot or the cold point of the system; indeed, zirconium and calcium moved toward the hot extremity whereas elements of the transition series and aluminium preferentially migrated to the cold one. Moreover, the absolute temperature did not affect the trend of this mass transport and only influenced the nature of the secondary phases: the lowest temperatures favoured amorphous products and phyllosilicates vs. well-crystallized phases. In a similar experiment, the influence of a chemical barrier made of kaolinite was studied. The presence of kaolinite successfully prevented the migration of heavy elements and calcium from the glass, by inducing crystallization of oxides trapping zirconium and calcium, and by favouring precipitation of hydrated amorphous products sealing the pores of the clay. These experiments clearly demonstrate that a thermal gradient might have important consequences for the performance of the near-field of a long-term nuclear waste disposal facility. Thermally induced mass transport might prevent the release of the radioelements from the very-near field environment and thus be involved in the confinement properties of the disposal system. Lastly, the presence of an aluminium-rich environment increases the efficiency of the retention by favouring the crystallization of oxides that trap the heaviest elements.