Principal Research Results

Restraint of Radinuclides Migration by Cementitious Materials
containing Fly AshBackgroundIn the concept of sub-surface repository system for the low-level radioactive waste disposal in Japan, cementitious materials
are considered for use as an engineered barrier to restrain the migration of radionuclides (Fig. 1). Especially, organic carbon-14 andiodine-129 have potentially large contributions to the dose evaluation, but they are not sorbed so strongly on cementitious materials.
Low-heat Portland cement containing 30 wt% fly ash (FAC) is a candidate cement material for the construction of sub-surface repository because of its high density structure. Therefore, the diffusion of radionuclides in FAC materials is a very important parameter forthe performance assessment when considering the release of those radionuclides from wastes and its migration in a cementitious
repository environment.

Objectives
The purpose of this study is to estimate effective diffusion coefficients for organic carbon and iodine by diffusion experiment, and clarify the barrier effects of FAC materials.

Principal Results
1. Change of diffusion rates in FAC
The diffusion of acetate, which is one of typical chemical form of organic carbon, and iodide in FAC hardened cement paste
was examined by the through-diffusion experiment (Fig. 2). As a comparison, ordinary Portland cement (OPC) was also used for thediffusion experiments. The water-to-cement weight ratio was 0.35. For OPC hardened cement paste, diffusion rates of trace ions wereconstant (Fig. 3(a)). On the other hand, diffusion rates for FAC gradually became smaller (Fig. 3(b)). This is probably due to theaddition of pozzolanic materials such as fly ash, and it is possible that the structure of FAC gradually becomes more complicated dueto the pozzolanic reaction.
2. Effective diffusion coefficients
Effective diffusion coefficients, De, of trace ions for FAC were estimated to be 10-13 m2 s-1 order at the beginning of diffusion experiments. Then, the diffusion of trace ions slowed down over the experimental period of 4-18 months, and De were estimatedto be 10-14 m2 s-1 order. On the other hand, the De for OPC were estimated to be 10-12 m2 s-1 order.
These suggest that FAC can have further effective barrier performance for the diffusion of trace ions as time passes.

Future Developments
As future works, it is necessary to clarify the mechanism of the restraint of radinuclides migration and consider the relation between
the long-term pozzolanic reaction and the diffusion behavior of trace ions in FAC materials.
Main Researcher: Taiji Chida, Ph. D.,
Senior Research Scientist, Radiation Safety Research Center, Nuclear Technology Research Laboratory
Reference
T. Chida and D. Sugiyama, 2008, “Diffusion of Organic Carbon and Iodine in Low-heat Portland Cement containing Fly Ash”,
CRIEPI Report L07018 (in Japanese)865. Nuclear

Rubber stopper

Concrete pit

Backfill

Sample holder

Acrylic cell

Diffusion

Concretestructure

TracerAcetate orIodide
0.1 mol dm-3

High dense
concrete layer
Bentonite
Cement filler

Cement-equilibrated
water
200 cm3

200 cm3

Radioactivewastes

RockO-ring

Fig.1 Concept of sub-surface disposal.

Hardened cement paste sample

Fig.2 Experimental apparatus for thethrough-diffusion experiments.

200(b)FAC35

OPC3510150

Iodide concentration / ppm

Iodide concentration / ppm(a)

The diffusion rate
was constant.
10010-12 m2 s-1 order

50FAC35010-14 m2 s-1 order
The diffusion of trace ion
became slowly.

510-13 m2 s-1 order

The structure of FAC became
more complicate.
00500

1000

1500

2000

0Time / hour

1000

2000

Time / hourFig.3 The diffusion of iodide through the hardened cement pastes.

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