Nuclear Science and Engineering Research

Detailed Simulation of Transfer of Radioactivity in Land Surface Ecosystems－ Development of Nuclide Transport Model for Atmosphere-Vegetation-Soil System －T exchanges in vegetation

Turbulent diffusion
Exchange throughTranslocation
Photosynthesis to other parts
stomata
AtmosphericT in leaf
T in leaf
HTO
cellular waterorganic matter(a)T concentration in leaf
cellular water (Bq m –3)

T transport in surface
atmosphere1 x 1010

T content in leaf organic
matter (MBq kg –1)

8-8

1.00

Respiration
ExchangeCondensation
Evaporation

HTO in soil airDiffusion

Uptake by roots

HTO in soil water

T transport in soil

Diffusion Advection

Deposition of atmospheric HTO to leaf cellular water
Release of HTO fromleaf cellular water toatmosphere1 x 10 9
1 x 108Obs:Height at 0-1.0 m
C a l: Height at 0.40 m
C a l: Height at 0.60 m
C a l: Height at 0.85 m

1 x 107

C a l: Average over the height1 x 10 6
1 x 10 5(b)Assimilation of T in leaf cellular water to organic matter
Translocation of T in leaf organicmatter to other plant parts

0.75
0.50
0.25
0.00

0122436487260

Elapsed time from 0:00 on the day of exposure (h)Fig.8-21 T transports and exchanges considered in thedeveloped model
The model simulates the transport of HTO in a multilayered
atmosphere and soil, and exchanges of T in vegetation, byconsidering the water and CO2 cycles.

Fig.8-22 Reconstruction simulation of experiment in whichgrape plants were exposed to atmospheric HTO vapor(a) The model simulates well the changes in T concentrationin leaf cellular water due to deposition of atmospheric HTO
during exposure (indicated by hatch marks) and release ofHTO from the leaf cellular water after exposure. (b) The model
also successfully predicts changes in T content of leaf organic
matter that are caused by assimilation of leaf cellular water T byphotosynthesis and translocation of T in the leaf organic matter.

Tritium ( 3H, T) and radiocarbon ( 14C) are importantradionuclides in the assessment of the radiological impact
to the public of nuclear industries because of the long halflives of T (12 years) and 14C (5730 years) and the biologicalimportance of hydrogen and carbon in all life forms. T and 14C
are discharged through the operation of nuclear power plants
and fuel reprocessing plants; in the natural environment, Tand 14C exist mainly in the form of tritiated water (HTO) and14CO , respectively. Thus, the discharged T and 14C are soon2
incorporated into the water and C cycles in the surrounding
environment and are assimilated to vegetation as organicmatter. The assimilated T and 14C eventually deliver a dose to
the public through ingestion pathways; therefore, the transferof T and 14C in land surface ecosystems should be precisely
predicted to assess the dose due to nuclear facilities. However,models that dynamically consider the water and CO2 cycles
and the resultant T and 14C transfer within a land surface
ecosystem have not been developed; thus, assessments of the Tand 14C-derived dose have contained a range of uncertainties.
We developed a model that predicts the transfer of T and
14C in a land surface ecosystem in detail. The dynamics ofT (Fig.8-21) and 14C in the surface ecosystem are modeled,
and the modeled T and 14C dynamics are incorporated into aland–surface water and CO2 transport model (SOLVEG-II,developed by the Japan Atomic Energy Agency). The developedmodel successfully predicted the accumulation of T (Fig.8-22)
and 14C in leaves on plants exposed to HTO or 14CO2.
For a comprehensive understanding of the land surface
T and 14C cycle, we then performed a range of numericalexperiments using the model. The results demonstrated thatduring atmospheric T deposition to the soil, the loadingsof HTO to the leaf cellular water due to root uptake of soil
HTO significantly control the T concentration in the leaf
cellular water, and hence the T content in leaf organic matter.
Our results also quantified the increase in the 14C content ofvegetation due to below-ground 14CO2 production caused bydecomposition of 14C-containing soil organic matter originatingfrom 14C-containing plant litter. Clearly, the model can be used
to clarify the role or importance of each elemental process inthe entire land surface T and 14C cycle; this useful information
cannot be obtained from field observations or laboratory
experiments.
As a more practical application of the model, we are now
investigating the transfer of T and 14CO2 discharged at a nuclear
facility over the surrounding land surface ecosystems througha simulation coupling the developed model and an atmosphericdispersion model.

Reference
Ota, M. et al., Importance of Root HTO Uptake in Controlling Land-Surface Tritium Dynamics after An-Acute HT Deposition: A Numerical Experiment,
Journal of Environmental Radioactivity, vol.109, 2012, p.94-102.
JAEA R&D Review 201391