ENVIRONMENTAL PROTECTION
ESTABLISHING ROUTINE PROCEDURE FOR ENVIRONMENTAL
TRITIUM CONCENTRATION AT ICIT*
C. VARLAM, I. STEFANESCU, I. FAURESCU, I. VAGNER, D. FAURESCU, D. BOGDAN
Institute for Cryogenic and Isotope Technologies, Rm. Valcea, Romania,
E-mail: [email protected]
Received September 14, 2009
The Cryogenic Pilot is an experimental project in the national nuclear energy
research program, which has the aim of developing technologies for tritium and
deuterium separation by cryogenic distillation. The process used in this installation is
based on a combined method for Liquid-Phase Catalytic Exchange (LPCE) and cryogenic
distillation. There are two ways in which the Cryogenic Pilot can interact with the
environment: by atmospheric release and through the sewage system. In order to establish
the base level of tritium concentration in the environment around the nuclear facilities,
we investigated the sample preparation protocol for different types of samples: spinach,
spring wheat, onion, hay, grass, apple, garden lettuce, soil, milk, and meat.
For the azeotropic distillation of all types of samples two solvents, toluene and
cyclohexane, were used. All measurements for the determination of environmental tritium
concentration were carried out using Liquid Scintillation Counting (LSC), with ultralow liquid scintillation spectrometer Quantulus 1220 specially designed for environmental
samples and low radioactivity. Sample scintillation cocktail ratio was 8:12 ml and liquid
scintillation cocktail was UltimaGold LLT. The background determined for the prepared
blank samples was between 0.926 CPM and 1.002 CPM and the counting efficiency
between 25.37% and 26.10%. The counting time was 1000 minutes (50 minutes/20 cycles)
for each sample, and the minimum detectable activity according to ISO 9698 was between
8.9 TU and 9.05 TU, with a confidence factor of 3.
Key words: Tritium, liquid scintillation, azeotropic distillation, toluene, cyclohexane.
1. INTRODUCTION
Tritium, the radioactive isotope of hydrogen, behaves like stable hydrogen
and is usually found attached to molecules replacing hydrogen. Tritium is constantly
produced both by natural processes (the interaction of cosmic rays with the
atmosphere) and by human-made processes. Tritium is used in a wide variety of
consumer products such as illuminated watches, thermostat dials, and exit signs.
Both the natural and human sources contribute to a worldwide background level of
*
Paper presented at the 10th International Balkan Workshop on Applied Physics, July 6–8,
2009, Constanţa, Romania.
Rom. Journ. Phys., Vol. 56, Nos. 1–2, P. 233–239, Bucharest, 2011
234
C. Varlam et al.
2
tritium. The combined natural and anthropogenic emissions of tritium result in a
current global inventory of approximately 53×1018Bq, which is about 50 times
greater than tritium levels due to natural sources alone [1].
Tritium, even in low levels, has been linked to developmental problems.
Tritium most commonly enters the environment in gaseous form (T2) or as a
replacement for one of the hydrogen atoms in water (HTO, called tritiated water,
instead of ordinary, non-radioactive H2O). Tritiated water can replace ordinary
water in living cells (approximately 70% of the soft tissue in the human body is
water). Once in living cells, tritium can replace hydrogen in the organic molecules
in the body. Thus, despite tritium's low radiotoxicity in gaseous form and its
tendency to be released from the body rather rapidly as water its effects on health
are made more severe by its property of being chemically identical to hydrogen [2].
The Cryogenic Pilot is an experimental project in the national nuclear energy
research program, which aims at developing technologies for tritium and deuterium
separation by cryogenic distillation. The process used in this installation is based
on a combined method for liquid-phase catalytic exchange (LPCE) and cryogenic
distillation. There are two ways in which the Cryogenic Pilot can interact with the
environment: by atmospheric release and through the sewage system. The total
discharge of tritiated heavy water by sewage has a maximum activity of 5 × 1012 Bq/year,
and the tritium activity in atmospheric release of Cryogenic Pilot is around 4.5 ×
1012 Bq/year. Because the discharges by sewage don’t influence the tritium concentration found in the vegetation around Cryogenic Pilot, the only influence is the
atmospheric release.
In order to establish the base level of the tritium concentration in the
environment around the nuclear facilities we investigated the sample preparation
protocol for different types of samples: soil, hay, apple, grass, spring wheat,
spinach, garden lettuce, onion, milk and meat.
2. METHOD, MATERIALS AND EQUIPMENT
There are two predominant influences on the relative distribution of HTO in
the environment [3]: the hydrogen cycling process interacting between the various
media of the release environment, and the water content in these media. The
environmental cycling of tritium follows quite closely that of natural hydrogen as it
occurs in gas, water, and organic molecules. Perhaps because of hydrogen abundance,
no sequestering processes in biota have evolved for hydrogen (or tritium, in our
case). Ingestion of homegrown fruits, vegetables, and grains from gardens in the
vicinity of Cryogenic Pilot for Tritium and Deuterium Separation from Heavy
Water must be included in the risk assessment. In order to establish the base level
of tritium in homegrown food, we collected during the April and May 2006 different
types of samples: spinach, spring wheat, onion, hay, grass, apple, garden lettuce,
soil, milk, and meat.
3
Routine procedure for environmental tritium concentration
235
In liquid scintillation counting (LSC), the preparation of biological samples
has always been treated as a particular case, because of the difficulties associated
with their preparation for counting. In these samples a major component is water,
which can be extracted and analyzed in order to determine tritium activity.
The method for establishing routine procedure for environmental tritium
concentration at ICIT uses the extraction of water by azeotropic distillation from
different types of samples. The method was applied in many laboratories for some
time, the difference consisting in the type of solvent used for azeotropic distillation
(toluene or cyclohexane). Sample preparation was made immediately after the take-off.
The sample to be analyzed is chopped, if necessary, to the consistence of a
paste and is weighted directly in the distillation flask (aprox. 100 g). The solvent is
added over the sample in the flask and then the installation is assembled, according
to Fig. 1.
Fig. 1. Azeotropic distillation installation with water bath (1), distillation flask (2),
condenser (3) and a separating funnel (4).
The ratio sample (g)/solvent (ml) was 1/5 (soil, hay) and 1/2.5 (apple, grass
and milk) using cyclohexane as well as 1/3 for all types of samples using toluene.
The temperature of the bath is set to 90ºC and the rotation speed is set to
40 rot./min. After the distillation has begun, a volume of 30 ml of water is collected
in the separating funnel. This first fraction obtained is collected, then continuing
distillation, a second fraction of 30 ml is collected and separated from the funnel
and finally a third fraction of minimum 10 ml is separated. These three fractions of
extracted water were measured using liquid scintillation method to determin tritium
activity concentration.
As tritium is a soft beta emitter (5.68 keV mean energy), liquid scintillation is
the most appropriate technique for its measurement. In this paper, low-background
liquid scintillation spectrometer Quantulus 1220 (Wallac) was used to determine
tritium in different samples obtained following azeotropic distillation. The analytical
method used to determine tritium in the water extracted from the samples was,
236
C. Varlam et al.
4
briefly, the following: 8 ml of distillate was mixed with 12 ml of scintillation
cocktail UltimaGold LLT in polyethylene vials; three background samples were
simultaneously prepared. Tritiated water with certified values of 2.51 × 106 dpm/g,
was used as internal benchmark for each type of measured sample. Samples,
backgrounds, and tritium benchmarks were stored in the system for at least one day
so that chemiluminescence, which interferes with tritium measurement, has been
sufficiently decreased. Batches of samples, backgrounds and tritium benchmarks
were counted using Quantulus 1220 during 1000min/samples, 50 min/cycle.
The background determined for tritium free water samples prepared was
between 0.926 CPM and 1.002 CPM and counting efficiency, using internal standard
method [4, 5], between 25.37% and 26.10% for a maximum figure of merit.
4. RESULTS AND DISCUSSION
From solvent properties can be observed that azeotropic mixture watertoluene contains 13.5% water (wt.) whereas azeotropic mixture water-cyclohexane
contains 8.4% water (wt.) [6]. Therefore distillation process using cyclohexane was
carried out over a longer period of time (10–12 hours) compared to the same
process using toluene (5–6 hours). After having distillated using toluene the resulting
water is opaque, because it is mixed with a small quantity of toluene. This opaque
water needs some time to be separated from toluene. From the distillation process
using cyclohexane the resulting water is clear, without any traces of cyclohexane.
For the calculation of moisture content from the analyzed samples, two
methods were used: azeotropic distillation (with toluene and cyclohexane) and
drying to the moisture-testing oven (105–110ºC). For drying in the oven, samples
were also chopped to the consistence of a paste and an amount of aprox. 100 g was
used for humidity determination. The obtained data are reviewed in Table 1.
Humidity of the samples was determined using the equation:
% Humidity =
mwater
msample
⋅ 100
(1)
where mwater – amount of water obtained from the drying process computed as the
difference between the initial amount of sample and the amount of sample after
drying, g; and msample – initial amount of sample, g.
It can be observed that both methods for humidity determination are viable
and can be applied in establishing the routine procedure.
For the three collected water fractions (f1, f2 and f3) the value of pH and
conductivity were measured at a given temperature (Table 2). For these measurements
a multiparameter pH/ conductivity WTW 340i apparatus was used.
5
Routine procedure for environmental tritium concentration
237
Table 1
Distillation efficiency and humidity of the analyzed samples
Sample type
Soil
Hay
Apple
Grass
Spring wheat
Spinach
Garden lettuce
Onion
Milk
Meat
Drying
14.0
35.2
88.8
86.1
77.1
85.6
87.9
90.7
90.6
65.1
Humidity, [%] wt.
Toluene
Cyclohexane
14.0
13.3
34.9
30.1
87.2
84.0
86.0
82.0
76.5
75.9
85.0
84.1
86.1
85.8
90.0
88.9
90.2
86.0
65.0
60.8
Distillation efficiency, [%]
Toluene
Cyclohexane
100
95.0
99.1
85.4
98.1
94.5
99.8
95.2
99.2
98.4
99.2
98.2
97.9
97.6
99.2
98.0
99.5
94.9
99.8
93.3
Table 2
Characteristics of the fractions collected from the studied samples
Sample Type
Soil
Hay
Apple
Grass
Spring wheat
Spinach
Garden lettuce
Onion
Milk
Meat
pH
Solvent
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
f1
5.25
5.20
7.45
7.02
4.91
4.89
7.43
7.40
6.70
6.68
6.48
6.40
6.42
6.50
8.58
8.60
4.58
4.49
4.56
4.59
f2
6.77
6.91
7.90
7.47
5.89
5.92
7.98
7.87
6.82
6.93
6.85
6.91
6.86
6.92
8.05
8.00
5.98
6.02
5.88
5.91
f3
6.01
5.98
6.98
6.90
3.98
3.80
8.02
8.00
4.47
4.80
4.40
4.78
4.46
4.81
8.91
8.95
3.83
3.99
5.90
5.91
Conductivity
[µS/cm]
f1
f2
f3
16
11
12
17
10
12
19
15
51
21
14
48
70
17
34
60
15
31
105
43 125
198
40 111
19
15
51
21
13
60
22
11
12
20
9
11
21
10
12
22
9
12
83
41 149
79
38 165
120
43 210
116
38 198
40
21
98
39
18
84
Temperature
[ºC]
f1
f2
f3
23.1 23.1 23.5
24.0 24.2 24.0
23.7 23.5 23.7
23.5 23.8 23.9
24.1 24.1 24.4
23.8 23.9 24.1
23.3 23.3 23.5
24.1 23.9 24.3
23.4 23.0 24.0
23.4 23.8 24.1
23.8 23.7 24.0
23.9 23.9 24.0
23.8 23.9 24.2
23.8 23.8 24.1
22.8 22.9 23.2
23.0 23.5 23.5
24.2 24.1 24.2
24.3 24.1 23.9
24.0 24.3 24.3
24.0 23.9 24.4
The first and the last distilled fraction of extracted water present lower values
for pH and higher values for conductivity than the middle fraction that has pH
around 7 and lower conductivity.
The tritium specific activity obtained for the three distilled fractions of different
types of samples is reviewed in Table 3. Tritium concentrations measured in extracted
238
C. Varlam et al.
6
water were between 27.0 ± 2.4 TU and 40.8 ± 2.5 TU for the first distillation fraction.
The second distillation fraction had values between 12.9 ± 2.2 TU and 22.9 ± 2.3 TU.
The last distillation fraction recorded values between 26.1 ± 2.3TU and 40.1 ±
2.5 TU. Tritium concentration in water extracted regardless of the type of sample
had the following means: 34.9 TU for the first distillation fraction, 18.4 TU for the
middle distilled fraction and 32.4 TU for the last distilled fraction. Knowing that in
the vicinity of our institute there is no source of tritium, we assume that in the
measured samples there can be tritium only from precipitation.
Table 3
Tritium specific activity in water extracted from different types of samples
Sample Type
Soil
Hay
Apple
Grass
Spring wheat
Spinach
Garden lettuce
Onion
Milk
Meat
Solvent
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Toluene
Cyclohexane
Tritium specific activity [TU]
First distilled
Second distilled
Last distilled
fraction f1 (approx.
fraction f2
fraction f3
30 ml)
(approx. 30 ml)
28.9 ± 2.4
19.8 ± 2.3
27.2 ± 2.4
31.1 ± 2.4
20.1 ± 2.3
30.0 ± 2.4
32.8 ± 2.4
22.7 ± 2.3
32.2 ± 2.4
31.6 ± 2.4
21.9 ± 2.3
30.9 ± 2.4
31.8 ± 2.4
20.7 ± 2.3
28.7 ± 2.4
30.1 ± 2.4
19.6 ± 2.3
29.6 ± 2.4
27.2 ± 2.4
16.4 ± 2.2
26.3 ± 2.3
27.0 ± 2.4
17.1 ± 2.2
26.1 ± 2.3
38.4 ± 2.5
15.4 ± 2.2
35.3 ± 2.5
36.3 ± 2.5
16.7 ± 2.2
40.1 ± 2.5
29.2 ± 2.4
12.9 ± 2.2
25.7 ± 2.3
31.0 ± 2.4
14.7 ± 2.2
26.9 ± 2.3
34.7 ± 2.4
18.4 ± 2.2
30.8 ± 2.4
38.9 ± 2.5
16.5 ± 2.2
37.5 ± 2.4
40.8 ± 2.5
14.1 ± 2.2
29.2 ± 2.4
37.7 ± 2.5
17.1 ± 2.2
28.8 ± 2.3
32.1 ± 2.4
20.8 ± 2.3
30.9 ± 2.4
29.8 ± 2.4
19.4 ± 2.3
28.7 ± 2.4
32.0 ± 2.4
21.3 ± 2.3
31.3 ± 2.4
32.8 ± 2.4
22.9 ± 2.3
32.7 ± 2.4
Tritium concentration in precipitation for Europe [7] or for our Institute [8]
ranges between 4.2 TU in the winter months and 18.9 TU in late spring and early
summer. Comparing the published data for tritium concentration in precipitation
and our established means, we conclude that tritium specific activity for the middle
distilled fraction is representative. Because the presence in the first and the last
distilled fraction of some volatile organic compounds or other organic compound
streamed with water-solvent azeotrope vapour, demonstrated in the fraction’s low
pH and high conductivity value, there is a possibility for the chemiluminiscence
phenomena to appear and to lead to inaccurate tritium specific activity determination in
these fractions.
7
Routine procedure for environmental tritium concentration
239
5. CONCLUSIONS
All experiments have been carried out in order to establish a routine
procedure for the determination of tritium concentration in environmental samples
at ICIT. Distillation efficiency obtained for toluene ranged between 98.2 and
100%, and for cyclohexane, the second solvent used, between 85.5% and 98.4%.
Considering that toluene is a very good “extractant” for water from the given
samples in a short time, as well as the fact that toluene is more toxic compared to
cyclohexane, the use of toluene for samples containing more than 40–50% (wt.)
water (short time sample treatment) and the use of cyclohexane for samples
containing less than 40% (wt.) water is proposed. Measured tritium activities from
different types of samples are around environmental values proving that the results
are not influenced by the used solvent. The presence in the first and the last
distilled fraction of some volatile organic compounds or other organic compound
streamed with water-solvent azeotrope vapour, demonstrated by the fraction’s low
pH and high conductivity value, can lead to chemiluminiscence phenomena and to
inaccurate tritium specific activity determination.
The measured tritium specific activity ranged between 27.0 ± 2.4 TU and
40.8 ± 2.5 TU in the case of the first distilled fraction, between 12.9 ± 2.2 TU and
22.9 ± 2.3 TU in the case of the second distilled fraction and between 26.1 ± 2.3TU
and 40.1 ± 2.5 TU in the case of the last distilled fraction. Comparing the results
and also, the probable inaccurate determination of tritium in the first and last
distilled fraction, the second distilled fraction is considered representative for
tritium determination and it is emphasized that the first and last distilled fraction
must be discarded.
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