Aging Effect on Speciation and Soil-to-Plant Transfer of Cs and I in Soil
Akira TAKEDA, Hirofumi TSUKADA, Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Aging effect of radionuclides in soil after deposition is an important factor for their speciation andsoil-to-plant transfer. Laboratory experiments for the aging effect of Cs and I using stable nuclides werecarried out for periods of 7 - 20 months and 0 - 7 months, respectively. The stable Cs or I were added to asoil sample, which was collected from a grassland area in Rokkasho where the first commercial spent
nuclear fuel reprocessing plant in Japan is located. Iodine was added to the soil as I- or IO3- to examine theeffect of chemical form on the behavior in soil. The soil sample was put into cultivation pots, and thesoil-filled pots were stored in an artificial climate chamber. Orchardgrass or red clover plants were cultivatedin the pots for 28 d several times during the experiments, and the soil-to-plant transfer factors of Cs and I
were determined. Extractability of Cs and I in the soil sample was occasionally determined by pure water or
1 M ammonium acetate solution for Cs, and only pure water for I. Decreasing patterns of their
extractabilities were compared with the transfer factors.
We have already reported that the soil-to-plant transfer factor and the extraction yield of Cs in the soil
decreased with time up to 7 months after its addition. Their further decreases were not observed from 7 to 20
months after the addition. Both the soil-to-plant transfer factor and the extraction yield of I decreased withtime during 7 months after its addition as I- or IO3-. The transfer factors of IO3- were higher than those of Iup to 86 d after the addition, however, those at 177 d after the addition were similar to each other. Higher
extractability of IO3- than I- was also observed during a similar period after the addition. The decreasing
pattern of the extraction yield of Cs with pure water agreed with that of its transfer factor. Although theextraction yield of I with pure water showed a fairly similar pattern to that of its transfer factor, its similarity
was less than that shown by Cs.
When the soil-to-grass transfer factors of Cs and I in the present study were compared to those whichwere used for the safety review of the spent nuclear fuel reprocessing plant in Rokkasho, the latter werefound to overestimate the transfer beginning at 4 months after their addition to the soil.

1Transfer factor

Extraction yield

1

0.1

0.01

0.1

0.01

0.001
0100

200

300

400

500

600

0700

100

Extraction yield of Cs added to the soil.
○, water extraction; Δ, ammonium
acetate extraction. A standard deviation(n=3) is within the range of each symbol.

Fig. 2400500600700

Soil-to-plant transfer factor of Cs
determined by pot experiments. ◆ ,Orchardgrass;□, Red clover. Error
bars indicate a standard deviation(n=3). The arrow indicates thesoil-to-grass transfer factor of Cs
which was adopted for the safety
review of the spent nuclear fuelreprocessing plant in Rokkasho. Data
are shown as dry weight basis,

10Transfer factor

Extraction yield

1010.10.01

10.1

0.010Fig. 3300

Elapsed time (d)Elapsed time (d)Fig. 120050100

150200250050100

150200250

Elapsed time (d)Elapsed time (d)Extraction yield of I added to the soil bywater extraction. Δ, I added to soil as I-;□, I added to soil as IO3-. Error bars
indicate 1 standard deviation (n=3).

Fig. 4 Soil-to-plant transfer factor of I determined bypot experiments (Orchardgrass). Δ, I added tosoil as I-; □, I added to soil as IO3-. Error bars
indicate 1 standard deviation (n=3). The arrow
indicates the soil-to-grass transfer factor of I
which was adopted for the safety review of thespent nuclear fuel reprocessing plant. Data areshown as dry weight basis, assuming 10% watercontent in grass.