1. MVE – “Rate Drop” experiment: progress report on an in situ glass dissolution test at the Andra’s URL.
P-03-08
Yannick Linard1, Christelle Martin2, Michel L. Schlegel3, Patrick Jollivet4
1 - Andra, DRD, CMHM, Bure, France / 2 - Andra, DRD, Chatenay-Malabry, France
3 - CEA, DEN/DPC/SEARS/Laboratoire d’Ingénierie des Surfaces et Lasers, Gif-sur-Yvette , France
4 - CEA, DEN/DTCD/SECM/Laboratoire d'étude du Comportement à Long Terme, F Bagnols-sur-Cèze , France
Introduction: as part of the evaluation of the safety of the geologic disposal of high-level vitrified radioactive waste, an integrated in situ experiment, named MVE-Rate Drop, is being performed in the Andra’s underground research laboratory (Bure, France). In this in situ experiment, the long-term performance of SON68 glass – inactive surrogate of the French R7T7 HLW glass – is studied in realistic disposal conditions.
The experimental setup is made of an interval of a vertical descending borehole filled by a mixture of glass powder and iron filings. The test interval is then closed, saturated with a synthetic water simulating the porewater chemistry of the surrounding Callovo-Oxfordian argillaceous rock (COx) and the pressure raised to 40 bars. The transport of dissolved elements is mainly diffusive. In contact of the same water, four cells fixed at the extremity of rods are filled with the same glass/iron mixture to allow us to sample at four times the solid phases. Around the vessel, a heater system maintains a temperature of 50° C. A closed water circulation is maintained at flow rate equal to 5 mL/h between the interval and a module located in the drift. This module is made of a circulation pump, a flowmeter, several sampling cells used to get samples of the leachate and monitor its chemical evolution, and different sensors to measure inline pH, Eh and electrical conductivity.
Progress report: the MVE-Rate drop test has been installed and the test interval has been saturated with water on July 2, Thus, the test provides a 5-year survey that can be exploited to understand the kinetics of in situ SON68 glass alteration. The first sampling rod was extracted on June 18, 2012, i.e. 713 days after the water saturation. The second one was extracted on December 3, 2014 , i.e days after the water saturation, and the other rods are still in place in the borehole.
Water chemistry
Initial decrease in sulphate concentration reflect a bacterial activity (which has been confirmed by microbiological analysis)
Initial increase in chloride content is due to the impact of drilling operations (including the drying of the borehole wall)
pH values are mainly neutral
(6.8 – 7.5) along five years at 50°C.
After 120 days, the rate of boron release slows down, reflecting a decrease of the alteration rate. Initial rate (r0) is estimated at g.m-2.d-1, close to most r0 values measured in pure water in conventional laboratory and slightly inferior to the r0 measured in synthetic COx porewater at 50°C (about g.m-2.d-1). In longer term, the rate is about g.m-2.d-1, similar in magnitude to the residual rate observed for classical experiments in pure water with the same S/V (about 400 cm-1) at 50 ° C. This is surprising because the dissolved concentrations of Si do not indicate that a steady state is reached.
Tests carried out by conventional laboratory with COx porewater indicate larger differences in the kinetics with pure water tests. Interpretation of the results of conventional laboratories concerning the comparison of SON68 glass dissolution in pure water and in COx porewater highlights the role of Mg: a strong anti-correlation was found between the amount of altered glass in COx porewater and Mg concentrations in the leaching solution. The decrease in the concentration of Mg in the leachate is explained by the precipitation of Mg-silicates at the surface of glass altered layers. In our test, this anti-correlation is also present without result in high kinetic difference with pure water tests.
The evolution of the B concentration, present only at trace levels in the rock and its porewater, reflects the progress of the dissolution reaction of the glass. However, measured B concentration has to be corrected for losses due to diffusive transport in the geological formation (estimated from tracers diffusion).
Solid phase characterization
performed ​​on the glass-iron sample altered during 713 days in the first extracted rod
The alteration products of glass are made of a gel layer, a newly formed layer, and loose precipitates associated with the corrosion products of iron. The gel incorporates Fe at various amounts. The newly formed layer contains crystalline phases and precipitates of many different chemical compositions: rare earth compounds associated with Ca and Si (apatite phase) or borates, presence of ZnS and FeS that could possibly be related to bacterial activity. There is a wide variation in Si / Fe and Al / Fe ratios in this layer.
Schematic representation of corrosion products. The glass grains and their alteration layers have been omitted for simplicity.
Corrosion products are made of siderite and chukanovite, hydrated Fe-silicate phases, and locally some Fe sulfide (mackinawite). The Fe-silicate had a highly disordered structure, probably with silicate groups forming embryonic sheets acting as precursors of larger phyllosilicate particles.
The characterizations provide information on the composition and morphology of the alteration layer and corrosion products. The gel thickness suggests (assuming a constant glass dissolution rate during the 713 days of leaching) a mean alteration rate of g.m-2.d-1, different from that calculated from NL(B). A significant retention of B in the newly formed layer could, at least partially, explain this difference.
The scarcity of corrosion products in the middle of the sample probably reflects the occurrence of gradients
in Fe and/or ligands (carbonate, silicate) concentration. Fe was however present in the alteration
layers of glass everywhere in the sample, suggesting that local corrosion was significant,
but that dissolved oxidized Fe was scavenged by these layers. No passive layer
in contact with metal Fe was observed, hence any layer, if present, would
be no thicker than a few nm, meaning that they would be
extremely prone to dissolution.
D.PO.ASCM