Transfer and Accumulation of Tritium and Radiocarbon in Terrestrial Ecosystems
Takashi TANI, Masaru NAGAI, Masanori FUJII, Yoshiko AYABE
Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Operation of the spent nuclear fuel reprocessing plant in Rokkasho, Japan, is accompanied by thedischarge of a small amount of tritium (3H, T) and 14C mainly in the forms of HTO and 14CO2, respectively.
In terrestrial ecosystems around the reprocessing plant, both radionuclides are incorporated into organic
compounds in plants mainly due to photosynthesis, followed by their supply to soil such as via dead leaves
and roots. This raises a concern about accumulation of those radionuclides in soil, because soil organic
matter is recognized as the largest carbon pool in terrestrial ecosystems. In order to predict the accumulationof those radionuclides in terrestrial ecosystems, simulation models to describe the dynamics of 3H and 14C inorganic matter and HTO concentration in soil and plant are required. We selected a Japanese radish field, ameadow, and a forest of Japanese black pine (Pinus thunbergii), which are common around the reprocessing
plant, as our targets. To construct those models, we are measuring many parameters, which will be used in
the models, through field observations and laboratory experiments.
We established an experimental Japanese radish (Raphanus sativus) field in FY 2017 in our institutefor determining the parameters of the soil moisture model. Temporal changes of the soil water content havebeen continuously measured in several soil layers during the crop cultivation period since FY 2017. Obtained
data were used to construct soil water dynamics model mentioned below.
In a 50  50 m quadrat in a black pine forest established ~5 km east of the reprocessing plant in FY
2015, the growth rate of fine roots was measured by root-ingrowth cores installed in FY 2016. Monthly
fallout rates of above-ground litter have been continuously measured from July 2015, for obtaining input rateof dead plant matter to the forest floor. For getting parameters affecting water dynamics in the forest, soil
water content and soil properties affecting water percolation were measured.
In the experimental meadow established in FY 2015 in our institute, the growth data of timothy(Phleum pratense) germinated in autumn of 2015 have been continuously obtained in FY 2018. To clarify
the long-term retention of photosynthate translocated to perennial plant parts (i.e., stem base and root),
timothy was labelled with 13C before the first harvest in FY 2018, and the concentration of 13C in the stembase and root at the second and third harvest was determined. Since significant retention of 13C in theperennial parts in the third-harvest plant was found, we decided that the cultivation of 13C-labelled timothyshould be continued in FY 2019 to establish the long-term retention of photosynthate in the parts.
Hydrological models to predict soil water dynamics in each of our targets, the Japanese radish field,
meadow and forest, were constructed by using water content data in several soil layers. The model in themeadow was validated with an experiment spraying HDO on the ground surface. Although the estimated
HDO concentrations agreed fairly well with the measured ones, further tuning of model parameters may
improve the results.

Fig. 1

Observed and estimated concentrations of deuterium in soil water of the 1-yr meadow where
deuterium-enriched water (HDO) was sprayed to the soil surface. Replicated data are separately
presented for each soil layer.