Takeshi Muranaka and Nagayoshi Shima

1
AN IMPROVED ELECTROLYSER TO ENRICH TRITIUM
CONCENTRATIONS IN ENVIRONMENTAL WATER SAMPLES
Takeshi Muranaka and Nagayoshi
Shima Hachinohe Institute of Technology 88-1,
Myo, Ohbiraki, Hachinohe-shi, Aomori, Japan,
031-8501, muranaka_at_hi-tech.ac.jp
2
We designed an electrolytic cell
essentially composed of a SPE film and a couple
of porous DSEs to reduce the electrolytic
voltage in the electrolyser. A platinum mesh was
inserted between the SPE film and the anode DSE
so that the cell can be easily dismantled
without peeling of the catalyst coating from the
porous electrode. A current collector made of
thin gold plates with numerous small holes is
also included in the design to homogenize the
flow of the current passing through the SPE film.
This also allows for the electrolytic gas to be
released smoothly. Our designed device,
when operated with an electrolytic voltage lt 4 V
in a current of 6 A, achieved a tritium
recovery factor of 0.8360.021(n4) in a volume
reduction factor of five while keeping the water
bath temperature lt 2? or lower. We
simultaneously enriched sample and standard
waters by connecting two electrolytic cells in
series. We calculated the apparatus constant from
the standard water and studied tritium
concentration in the sample water, respectively,
after enriching the sample water using a
commercially-available cell with a large
electrolytic current of 50 A until the volume in
the sample water is reduced to one fifth of the
original volume. This type of
enrichment, hereafter referred to as two-stage
electrolysis, was applied to the coastal sea
waters of the Aomori prefecture, including water
samples collected in the Rokkasho area. We found
reasonable tritium concentrations, ranging from
0.2 to 0.5 Bq/L with a measuring error (i.e. a
statistical error of 1s) lt 0.03 Bq/L.
ABSTRACT
3
?. INTRODUCTION
The successive reaction test carried out in the
nuclear fuel reprocessing plant at Rokkasho in
Aomori prefecture, Japan releases
tritium-contaminated waste water into the coastal
sea, requiring a careful survey of tritium
concentration in this region. Tritium in
environmental waters is generally found at
concentrations lt 1 Bq/L, so that electrolytic
enrichment using a solid polymer electrolytic
(SPE) film is necessary before the concentration
can be measured. In Tritium 2004, we
presented a method to enrich tritium in sample
water by connecting two electrolytic cells in
series, containing standard water with high
tritium concentration in one cell and natural
tritium level in the other cell 1. The
advantage of the method is that the SPE film can
be replaced in every run to avoid memory effect
due to contaminated tritium depositing on the SPE
film. The setback, however, is that the constant
of the apparatus varies slightly each time the
electrolytic cell is dismantled. Furthermore, the
electrolytic voltage of the cells we used is
greater than that found in commercially-available
apparatuses, which use dimensionally stable
electrode (DSE). We, thus, designed a new
electrolyser to reduce the electrolytic voltage
and avoid the instability in the apparatus
constant 2. Here we describe the designs and
characteristics of the improved cell and discuss
a method of two-stage electrolysis that
combines the subsequent use of a
commercially-available electrolytic cell and our
newly-designed cells 3.
4
?. ELECTROLYSER DESIGN
The newly-designed electrolytic cell
(Fig. 1) comprises (1) a SPE film (Nafion 117),
(2) a 4040 mm long, 2.6 mm thick, porous anode
DSE, (3) an 80 mesh/inch Pt mesh, (4) a porous
cathode DSE (dimensions identical to the anode
DSE), (5) electrolytic current collectors
consisting of 0.2 mm thick gold plates, (6) and
(7) a spacer and two supporters made of acrylic
fibers, respectively.
(7)
(2)
(3)
(1)
(4)
(6)
(7)
(5)
Fig. 1b. Cross section of the electrolyser . The
symbols (1)(7) are the same as those referred to
in Fig. 1a.
Fig. 1a. Schematic diagram of the electrolyser
. (1) SPE film (Nafion 117), (2) Porous anode
DSE, (3) Pt mesh, (4) Porous cathode DSE, (5)
Electrolytic current collectors, (6) Spacer, (7)
Supporters
5
A couple of porous DSEs were included in the
design to reduce the electrolytic voltage. A
platinum mesh was inserted between the SPE film
and the anode DSE so that the cell can be easily
dismantled without the need for peeling off the
catalyst from the porous electrode. To
stabilize the apparatus constant, a spacer was
placed between the supporters to keep the
thickness of the electrolytic device constant. It
is indeed necessary to restrict the thickness of
the SPE film as this latter absorbs the water
from the sample and swells. A thin gold plate
pierced with numerous small, regularly
distributed holes (1mmf) as current collector
in electrodes. These two plates allowed for a
homogenous current to flow through the SPE
film and for the electrolytic gas to be released
smoothly. The four edges of the current
collector were folded in order to wrap a current
lead line made of gold and to attach the
collector to the supporters. These supporters
also had numerous small holes (1.5mmf)
distributed coaxially to the holes of the
current collectors. Our electrolytic unit was
positioned into a plastic container so that it
tilts slightly from the horizontal in order to
release the oxygen gas generated beneath the
anode. The reduction weight was measured by an
electronic balance to the nearest 0.1 g.
6
?. TWO-STAGE ELECTROLYSIS
We used a commercially-available apparatus that
can generate up to 50 A electrolytic current 4
in combination with our newly-designed devices
in a two-stage procedure (Fig. 2). At a first
step, a 1000 mL water sample is enriched to 200
mL in the large current device, A 180 mL
sub-sample of enriched water is then reduced to
60 mL in our newly-designed device. This
combined method is time-saving in the first stage
and the tritium memory effect can be avoided
using the memory-free cells in the second stage
of the procedure.
Fig. 2. Schematic diagram of the two-stage
electrolysis A) a commercially-available
apparatus is used in the first stage of the
electrolysis B) our newly-designed cells is
used in the second stage. One of the cells is
used to determine the apparatus
constant and the other is used to enrich the
tritium concentration in the water sample.
7
Two-stage electrolysis
Second stage
First stage
Large current cell
Our designed cell
T1i D1i
T1f D1f
T2f D2f
50A
6A
1000mL
200mL
180mL
60mL
A
B
Apparatus constant k1
Cell A Sample water
8
In the first stage, the tritium concentration in
the sample water T1i is calculated following
equation (1), where k1 is the constant of the
commercially-available apparatus. T and D
represent tritium and deuterium concentrations,
while the suffixes i and f refer to before and
after the enrichment, respectively 1.
T1iT1f/(D1f/D1i)k1
(1) The enriched concentration
obtained in the first stage of the procedure T1f
is calculated from equation (2) in the second
stage,
T1fT2f/(D2f/D2i)k2 (2) where k2
is the constant of the standard water cell which
is connected in series to the sample water
cell (cell B in Fig. 2). The constant k2 is
calculated as
k2ln(TS2f/TS2i)/ln(DS2f/DS2i) (3)
9
A flow chart to obtain the tritium concentration
in environmental water samples
Sampling
adding KMnO4, Na2O2 at 1 atm
Distillation
Tritium enrichment by electrolysis
Distillation
Measure tritium concentration
Measure deuterium concentration
Calculatin of the tritium concentration before
the enrichment
10
?. RESULTS AND DISCUSSION
The electrolytic voltage in our device with DSE
electrode was reduced substantially (Fig. 3)
compared to that found in conventional enriching
devices, such as the one presented in Tritium
2004 1. While the voltage approached that
of commercially-available devices, our
electrolyser presents the advantage that it
can be easily dismantled thanks to the platinum
mesh inserted between the anode DSE and the SPE
film.
Fig. 3. Electrolytic voltages during the
electrolysis. The electrolytic current is 6A
in both apparatuses. ? The commercially-available
apparatus Our newly-designed electrolyser
11
The tritium recovery factor R is defined as
follows
R(TfVf)/(TiVi) (4)
where Vi and Vf are the volumes of the water
samples before and after the electrolysis,
respectively. To verify the reliability of our
method, we compared the recovery factors obtained
using our new device, with that obtained
using the commercially-available apparatus (Table
1).
Table 1. Comparison of tritium recovery factors
obtained using (A) the commercially-available
apparatus, and (B) our newly-designed
electrolyser. VRF represents the volume reduction
factor so that, VRF3 means, for
instance, that the volume reduction factor is
three.
12
Water samples electrolyzed with a 6 A current
to volume reduction ratios of three, five and
ten in our device produced recovery factors of
0.91, 0.84, and 0.78, respectively. In
comparison, the factor of a water sample
electrolyzed with a 6 A current to a volume
reduction ratio of five in the commercially-availa
ble apparatus is 0.67. The tritium recovery
factor in our device was, therefore, greater than
that obtained using the commercially-available
device, probably as a result from the low water
temperature in our electrolytic cell.
Our results suggest that tritium recovery factor
obtained through the two-stage electrolysis
may become greater to some extent than that using
only the commercially-available apparatus 3.
13
Tritium concentrations in water samples
collected from various sites (Fig. 4) along the
Pacific coast, in Aomori prefecture, Japan,
in January and June 2006, were surveyed by the
aforementioned two-stage electrolysis method.
Water samples were distilled repeatedly until
their electric conductivity became lt10µS/cm,
following the guidelines issued by the Japan
Chemical Analysis Center5.
Fig. 4. Sampling sites. 1.Shirahama
beach in Hachinohe 2. Misawa fishing port 3.
Lake Obuchi The cross indicates the
nuclear fuel reprocessing plant at Rokkasho
14
Table 2. Tritium concentrations at some costal
sea sites, in Aomori prefecture, Japan.
The sites ?, ?, ? are the same as those
referred to in Fig. 4.
In the present study, we measured tritium
concentrations ranging from 0.2 0.5 Bq/L at the
different sampling sites, (Table 2). Since
the statistical error of 1s was lt 0.03 Bq/L, ca.
10 of the tritium oncentrations in the costal
sea water, the accuracy of data is sufficient to
investigate variations in tritium concentration
in environmental water influenced by
tritium-contaminated waste water.
15
?. CONCLUSION
We designed an electrolyser composed essentially
of a SPE film and DSE electrodes to enrich
tritium concentration in environmental waters.
We presented the electrolytic characteristics
of the newly-designed cell, as well as a
two-stage electrolytic method that combines a
commercially-available apparatus with our
newly-designed device. We showed that
1. The insertion of a platinum mesh between
the SPE film and the anode DSE allowed us to
dismantle the cell without the
deteriorating the anode DSE. In addition, the
electrolytic voltage was reduced by lt 4 V
at an electrolytic current of 6 A in the cell.
2. The presence of electrolytic current
collectors made of thin gold plates pierced with
numerous small holes allowed the current
to flow homogenously through the SPE film and the
electrolytic gases to be released
smoothly. 3. The tritium recovery factor of
the cell presented in our study was higher than
that obtained with a commercially-available
apparatus.
16
4. We also presented a two-stage electrolytic
method where two electrolyser were used
subsequently, which proved to be time-saving and
highly accurate (in terms of final volume
measurement) method to enrich tritium
concentration in environmental water. 5.
Tritium concentrations in the costal sea water of
Aomori, Japan, including Rokkasho area, were
measured using the two-stage electrolysis method.
The concentrations ranged 0.20.5 Bq/L with
an accuracy lt 0.03 Bq/L for 1s allowing us to
survey satisfactorily variations in tritium
concentrations in costal sea water influenced by
the tritium-contaminated waste water.
REFERENCES
1 T.MURANAKA, N.SHIMA, H.SATO, A study to
estimate tritium concentrations of 1 Bq/L or
lower in water samples, Fusion Science and
Technology, 48, 516(2005). 2 N.SHIMA,
T.MURANAKA, Characteristics of a Newly Designed
Electrolyser to Enrich Tritium in
Environmental water, RADIOISOTOPES, 56, 455
(2007) (in Japanese). 3 N.SHIMA, T.MURANAKA,
Two-stage electrolysis to enrich tritium in
environmental water, Proceedings of the
International Symposium on Environmental Modeling
and Radioecology, Rokkasho, Japan, 247
(2007). 4 PERMELEC ELECTRODE LTD. Documents
to show characteristics of an electrolyser,
TRIPURE, the type of XZ027 (2005) (in
Japanese). 5 Japan Chemical Analysis Center,
Instruction manual of Tritium Analysis (2002) (in
Japanese).