Interactions between concrete and claystone in unsaturated conditions during ventilation phase in deep

geological ILLW repositoryP. Thouvenot(1), O. Bildstein(1), I. Pointeau(1), B. Cochepin(3), I. Munier(3), X. Bourbon(3) and E. Treille(3)

(1) CEA, DEN, DTN/SMTM/LMTE, 13108 Saint-Paul-lez-Durance – France

(2) ANDRA - 92298 Châtenay-Malabry Cedex - France

Context : behavior of materials in ILLW cells during ventilation phase

Concepts of deep geological repository of intermediate level long-lived radioactive waste (ILLW ) largely involve concrete materials. During the operating period (up to 100 years), part of the concrete components will be subject to ventilation with air drawn from the surface, in order to maintain operating safety and contribute to the evacuation of residual heat from waste.The interactions between concrete and claystone during this period lead to (1) the hydrolysis, sulfatic and carbonated attacks of the concrete components, processes which trigger a progressive lowering of pH inside the cement paste and (2) to the alteration of the claystone submitted to the migration of a high pH plume especially in the excavation damaged zone (EDZ). These processes may occur in unsaturated conditions involving intricated couplings between transport of water and CO2 in the gas and liquid phases, capillary processes during the desaturation of the concrete and clay stone, and chemical reactions between minerals and aqueous species.

Conclusions

Carbonation in satured and unsatured conditionsModel description

Modelling results show that the water saturation reaches a value close to complete saturation (98%) in the EDZ within the first year and then progressively drops to 35 % in a time span of 100 years. The system remains unsaturated from the shaft (concrete) to the EDZ and is therefore characterised by a high mobility of CO2 through the gas phase.

pH increases in the claystone close to the interface with concrete by one pH unit during the same period. In the claystone, precipitation of calcite and dolomite, sepiolite, hydrotacite and zeolites (K-philipsite, laumontite) is predicted at the expense of siderite and celestite , while sulphate migrates into the concrete to form ettringite. The alteration front is limited to few centimeters over this period of 100 years.

Simulations of carbonation processes are performed with Toughreact v1.2 – 1D radial geo-metry – 100 years of ventilation – a variety of secondary phases in order to evaluate the alteration extension of both media.

During drying of the concrete, atmospheric carbonation in unsaturated conditions involves intricate couplings between capillary flow, transport of both vapour and liquid water as well as aqueous and gaseous CO2. Chemical reactions lead, in the same time, to the alteration of the cement hydrates (reactions with dissolved CO2).

Primary mineral are destabilized and secondary minerals are formed according to kinetics laws.

2 different approaches are compared for the complete multiphase reactive model : an unsaturated case and a saturated case

Waste disposal package

Disposal cell

Protection airlock

(dual gate system) Access drift

Note that the transport properties depend on the liquid saturation :

D = D0 a+1 Sl b (Millington Quirk )

Profile of primary minerals in the micro-fractured and fractured EDZ at initial time

Complete dissolution at the concrete/argilites interface of illite, microcline, dolomite, pyrite, kaolinite, siderite, and celestitePrecipitation of calcite and magnetite

Precipitation of secondary minerals : hydrotalcite, sepiolite, straetlingite CSH 1.2, dawsonite, phillipsite, and laumontite. After 50 years precipitation of secondary ettringite

Processes are exarcerbed in satured compared to undersaturated conditions (as shown here for laumontite precipitation

(Andra, 2005)

Profile of primary and secondary minerals in the micro-fractured and fractured EDZ at 50 years of simulation

Claystone primary minerals Volume fraction

Quartz alpha 0,185Calcite 0,263Dolomite 0,0116Pyrite 0,0067Célestite 0,0006Sidérite Fe0.85-Ca0.15 0,0059Montmorillonite-Na 0,021Montmorillonite-Ca 0,041Illite-Mg 0,221Kaolinite 0,0086Microcline 0,0556  Total = 0,82Porosity 0,18

Concrete primary minerals Volume fraction

Portlandite 0,050

CSH 1.6 0,120

Ettringite 0,031

Hydrotalcite 0,003

C3FH6 0,018

Monocarboaluminate 0,020

Calcite 0,627