5.0
BIOLOGICAL HALF-LIFE OF TRITIUM
Tritium rises radiological safety concern for internal exposure due to inhalation, ingestion or
skin absorption. The human body may be exposed by elemental tritium or ingestion of
tritiated water or vapor in the form of HTO. Moreover tritium is one of most useful
radionuclides in medical, fusion reactor, neutron generator, electronic applications, defect
dynamics, particle sensor, smoke detector, cold electron sources, radio-luminescent devices
and research. The committed internal dose to a human body depends upon the effective
half-life of nuclide. In this chapter, biological half-life of tritium for some subjects of a PHWR
type nuclear power plant has been discussed. The nuclear power plant is situated near the
coastal region of Karnataka, India.
5.1
SAMPLE ANALYSIS
Tritium in urine sample is estimated using Liquid Scintillation Analyser (LSA), Perkin Elmer,
Tri-Carb-2900 TR. Daily instrument performance assessment (instrument background,
counting efficiency, sensitivity and reproducibility with standards 3H, 14C and background) is
ensured before the sample analysis. The scintillation solution used for analysis comprises of
binary dioxane and naphthalene, 2, 5 diphenyloxazole as primary solute. Ratio of sample to
scintillation cocktail 1:5 is chosen with LSA counting efficiency of order 30%. The chemical
quenching and color quenching in the samples are taken into account during the tritium
activity estimation. The biological half-life, BHL of tritium for each subject has been
calculated using the eq (4.3). The estimation of biological half-life of tritium has been
reported by Singh et al.[154].
5.2
ANALYSIS OF DATA
Tritium uptake data of the subjects are analysed for consecutive three years (2007-2009) for
estimation of BHL of tritium. The BHL of tritium for the subjects is found to be log-normal
distribution as shown in the figure 5.1.1.The distribution of BHL has been plotted using
software @ Risk. The graph plotted for distribution of the BHL shows that BHL varies from 1
to 16d and two subjects show 25d and 35d. Also thee subjects are found to be having BHL
up to 121d. These three subjects show the presence of organically bound tritium (OBT). The
OBT is found to be when tritium bounds to O, N, P or S atoms and is incorporated into
carbohydrates, lipids, proteins, nucleic and acids in a carbon-hydrogen bond. Low BHL up to
16 d is possibly due to free water tritium (FWT) which attains equilibrium in body fluid
instantaneously and completely in the soft tissues. The BHL of OBT is longer than FWT
because it is difficult to remove tritium from C-H bonds.
42
5
Nothing in life
is to be feared.
It is only to be
understood.
-Madam Curie
1.96
0.12
16.38
Frequency
0.1
0.08
0.06
0.04
0.02
40
35
30
25
20
15
10
5
0
0
days
Figure 5.1.1: Log-normal distribution of biological half-life of tritium
5.3
RESULTS AND DISCUSSION
Effective half-life, teff of tritium has also been estimated for those subjects and is found to be
average 8.19d as given in Table 5.1.1. Average values of teff of tritium by different
investigators are also given in the Table 5.1.1. From the Table 5.1.1, it is to be noted that the
estimated teff in the present work is comparable with the different literature whereas slightly
higher than Indian reference (5 d) reported by Dang et al.[24] and lower than ICRP reference
man (10 d)[23] .
References
Singh et al.[154]
Butler and Leory[155]
Osborne[156]
Fallot et al.[157]
Foy and Schnieden[158]
Pinson and
Langham[159]
[160]
Rudran
[161]
Trivedi et al.
[162]
Wylie et al.
[163]
Balnov et al.
[164]
Richmond et al.
[165]
Lambert et al.
Moghissi et al.[166]
* Present study
Subject
202
310
30
20
10
Avg. teff (d)
8.19
9.5
10.5
8.5
7.5
Remark
9
11.3
---------
8
8
7
5
5
1
1
6.0
8.4
8.5
12.0
9.5
9.1
9.0
Occupational workers
Acute tritiated water Intake
--------Exchange kinetics of HTO
Mammal’s study
Accidental tritium intake
---------
*
PHWR workers
-------Tritiated water vapor in man
-------Virtual tritium space in man
Table 5.1.1: Comparison of effective half-life of tritium by different investigators
The estimated teff of tritium for the subjects are given in the Table 5.1.2. It is found to
be that the teff are roughly independent on the ages of the subjects.
43
Age (y)
20-24
25-29
30-34
35-39
40-44
45-55
No. of cases
45
99
34
10
9
5
Tritium (µCi/l)
3-57
3-62
3-55
4-16
3-54
5-14
teff (d) ±σ
7.638±3.390
7.098±2.144
6.982±5.151
6.824±3.097
7.200±4.818
7.120±3.429
Table 5.1.2: Statistics of age, tritium uptake and effective half-life of tritium
BHL of tritium for the same subjects has also analysed for different seasons (summer,
rainy and winter) and given in the Table 5.1.3 along with temperature and relative humidity
(RH). The seasonal variation of BHL is to be noted and found that during summer season
the BHL reduces by half as compared with the winter season. The BHL of tritium for single
component exponential disintegration has been reported 10 d[167,168]. The mean BHL of
tritium in adult male students under tropical conditions has been reported 7.5 d[158] which is
comparable with the present work.
Month
Apr- May
Jun-Oct
Nov-Mar
Season
Summer
Rainy
Winter
Temp (°C)
19-40
18-38
13-39
RH
70-73
84-88
62-70
BHL(d) ±σ
3.09±1.48
5.73±0.76
6.87±0.58
Table 5.1.3: Seasonal variation of biological half-life of tritium
From the present investigation, it is to be observed that the effective half-life of tritium
for the coastal region is in the range of the world average. The seasonal variation of the
biological half-life of tritium is observed and the value reduces by half in the summer as
compared with the winter. The distributed of biological half-life of tritium is log-normal.
44