Tritium Metabolism in the Human Body
Tsuyoshi MASUDA, Kensaku MATSUSHITA, Yasuhiro TAKO, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
In the radiation safety assessment for nuclear facilities including the first commercial spent nuclear fuelreprocessing plant in Rokkasho, Japan, the internal radiation dose of the public due to tritium has been estimatedusing the dose conversion factors based on the ICRP metabolic model of hydrogen in the human body. The ICRP
metabolic model is very simple and is comprised of a free water tritium (FWT) compartment and an organicallybound tritium (OBT) compartment having biological half-lives of 10 and 40 d, respectively.
Although the biological half-life of tritium water (HTO) in the human body was examined in several casessuch as accidental intakes or experimental administrations, actual data on the metabolism of OBT are quite limited.
The objective of this research program is to establish experimentally the metabolism of tritium including OBT in
the human body for more realistic dose estimation. In the experiment, the stable isotope of hydrogen, deuterium(D), was used as a substitute for tritium.
In FY 2012, volunteers were administered with 4 g of D-labeled palmitic acid (1 g each in each of four dailylunches consumed); then their urine and breath samples were collected up to 16 weeks after the first administration,
and analyzed with a mass spectrometer to get the isotopic ratios of D/H. All procedures of the experiment wereapproved by the IES Review Board for Human Subject Experiments, and written informed consents were obtained
from all volunteers. The D isotopic ratios in the urine and breath samples increased after each administration, andthen decreased. The urine data obtained were used to construct a compartment model of metabolism of D
ingested as palmitic acid. The model had a compartment of free water D (FWD) and two compartments oforganically bound D (OBD1 and OBD2), and those compartments were connected to each other in series. Thetransfer rate constants between those compartments and the excreting rate constant from the FWD compartment
were determined by a least square fitting method using measured data. The excreting rate constant of D from theFWD compartment was 7.5×10-2 d-1 which was comparable with that of the ICRP metabolic model, 7.1×10-2
d-1. Transfer from the OBD1 to the OBD2 compartment was rapid with a rate constant of 24 d-1, while thedegradation rate constant from the OBD1 to the FWD compartment was 1.8 d-1. The OBD2 compartment had a longmean residence time of >100 d. Retention of D in each compartment calculated by the model showed that most of
the administered D remained in the OBD2 compartment at the end of the experimental period while the Dconcentrations that remained in the FWD and OBD1 compartments were 2 and 4 orders of magnitude smaller thanthat in the OBD2 compartment.

FWD
MFWD(t)k2MFWD(t)I(t)

Table 1 Transfer rate constant of D in model

k5MFWD(t)

（d ）

k1

1.8

k2

2.6 x 10-5

k3

2.4 x 101k4

7.6 x 10-3

k5

7.5 x 10-2

k1MOBD1(t)k3MOBD1(t)OBD2
MOBD2(t)OBD1
MOBD1(t)-1Transfer rate

k4MOBD2(t)Fig. 1 Three-compartment model of metabolism of D ingested as palmitic acid
FWD, OBD1, OBD2: compartment names, MFWD(t), MOBD1(t), MOBD2(t):BConcentration of D ( μmol D mol H-1 )AConcentration of D ( μmol D mol H-1 )D amounts in each compartment (g). k1 to k5: transfer rates.

D

Amount of D ( g )CTime after administration initiated (d)Time after the first administration (d)Amount of D ( g )Time after administration initiated (d)Time after administration initiated (d)Fig. 2 Estimation of retained D in each compartment of the model
A, B: D concentration in water in urine after D-labeled palmitic acid administration. Solid line: estimated values bythe model. C, D: Retained D amount MFWD(t), MOBD1(t), MOBD2(t) (g) in FWD, OBD1, and OBD2 respectively. Green
Green line: MFWD(t), blue line: MOBD1(t), red line: MOBD2(t), purple line: total D retained. A, C: until 4 days afteradministration initiated.

B, D: until 112 days after administration initiated.