Principal Research Results

Development of Evaluation Method for Chemical Alteration ofCementitious Materials in a TRU Waste Disposal Facility−A Coupling Transport and Chemical Equilibrium Calculation Code toPredict the Alteration Behaviour of the Fractured Cementitious Materials−
BackgroundResearch and development on specifications of disposal concepts and a reliability of safety assessments of TRU waste
disposal are addressed by the Federation of Electric Power Companies and Japan Nuclear Cycle Development Institute, as given in
their responsibility for management of TRU waste. Cementitious material is a potential waste packaging, backfilling and constructing
material for the disposal of TRU waste (Fig.1) and is expected to provide both chemical and physical containment. For the long term
performance assessment of the containment properties of cementitious materials, it is necessary to develop a series of predictive calculation models of the long-term evolution of the cementitious repository system. In past performance assessment studies in Japan, theevolution of cementitious materials was discussed only using the homogeneous porosity transport model, however, an evaluation
method to discuss the alteration of cementitious materials in a fractured repository should be also developed from a realistic
viewpoint.

Objectives
To develop a coupling transport and chemical equilibrium calculation code to predict the chemical alteration behaviour ofthe fractured cement material in the TRU waste repository environment.

Principal Results
1. Development of calculation code to predict the alteration behaviour of fractured cementitious
materials
Coupled Chemical equilibria-mass Transport code for Fractured-media (CCT-F), was developed to predict the alteration
behaviour of fractured cementitious materials in a groundwater flow condition. In CCT-F, the thermodynamic incongruent dissolution/precipitation model of calcium-silicate hydrate (C-S-H) developed by us is coupled with a quasi- two-dimensional transportmodel in which the transport in the cracks is described by only advection and dispersion and transport within the cement solid matrix
is constrained to be diffusive and perpendicular to the direction of the crack (Fig.2).
2. A flow-through leaching experiment using an artificial fractured cement hydrate and verification ofthe CCT-F code
A flow-through leaching experiment using an artificial fractured ordinary Portland cement (OPC) column sample anddistilled water was carried out and the composition of the discharged solution and the distribution of components in the solid matrix
were analysed. It showed that calcium leached from the surface due to dissolution of Ca(OH)2 and C-S-H and the Ca/Si ratio of thesolid decreased in the surface region (Fig.2). As the amount of discharged water increased, calcium concentration decreased and silica
concentration increased with time due to the incongruent dissolution of C-S-H, with decreasing Ca/Si ratio (Fig.3). The alteration
behaviour of the artificial fractured OPC was reasonably well predicted with the proposed model, therefore, we can conclude thatCCT-F code is applicable to the discussion on possible alteration of cementitious materials along a crack.

Future Developments
Sophisticated model and code considering a network of plural fractures will be developed based on the above model, andwill be applied to predict the chemical alteration behaviour of the fractured cementitious facility in the TRU waste repository. Long
term performance of the containment properties of the facility will be also evaluated to take radionuclide retardation into account.
Main Researchers:
Daisuke Sugiyama and Tomonari Fujita, Research Scientists, Radiation Safety Sector, Nuclear Technology Research Laboratory
References:
D. Sugiyama, et al., “Chemical Alteration of Cement Materials in a Radioactive Waste Repository Environment (1), -Thermodynamic
Modelling for the Dissolution and Precipitation of Calcium Silicate Hydrates-”, CRIEPI Report T01007, Nov. 2001 (in Japanese)
D. Sugiyama, et al., “Chemical Alteration of Cracked Cementitious Materials (1) -Flow-through leaching experiment using a simple
artificial cracked cement column sample and development of reactive transport calculation code”, CRIEPI Report L05002, Sept. 2005
(in Japanese)865. Nuclear - Nuclear fuel cycle

Backfill

Backfill

Bentonite
BufferConcentration / mol dm-3

Host Rock10101010101010

Waste Package

ConcreteStructure

Fig.1 Concept of the TRU waste
disposal facility-1

OPC hydrate (w/c=0.35)
Flow rate: 2.5 cm3 hr -1-2-3-4-5

Experiment Modelling
CaCaSiSi-6-70500 1000 1500 2000 2500 3000 3500
Time / hr

Fig.3 Measured and predicted aqueous compositionsof discharged water
Measured Ca and Si concentrations in the flow-through leachingexperiment using an artificial fractured OPC hydrate werereasonably well predicted by CCT-F.

Waste

Fig.2 Schematic representation of transport model in fractured cement materials and experimental
design of flow-through leaching experiments with calcium concentration profile at crosssection of leached OPC sample
A prototype coupling transport and chemical equilibrium calculation code was developed by describing the transport in fractured
cement solid as a quasi- two-dimensional transport in which the transport in the cracks is described by only advection anddispersion and the transport within the cement solid matrix is constrained to be diffusive and perpendicular to the direction of thecrack.
In the flow-through experiment, leaching of calcium from the surface and the distribution of Ca/Si ratio in the solid matrix wasobserved.

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