Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a method of removing
carbon-14 from particulate, ion exchange resin.
Ion-exchange resins are used in heavy water
moderated, natural uranium, nuclear reactors chiefly to
purify the heavy water coolant and moderator systems~
These resins, stored on-site, contain about 80% of the
gross ~y radioactivity produced per year in these reactors
excepting the activity within the nuclear fuel bundles.
While contaminated resins make up only 3% of the total
unprocessed reactor waste volume generated each year by
these reactors, processing of most of the other wastcs
increases this to 12%. Since these resins are contaminated
with high concentrations of the long lived isotopes Cs-137
and C-14, an efficient method for their permanent disposal
will be required. If the volume of these resins, produced
at the rate of about 130 m3 per year, cannot be reduced,
they can be expected to take up approximately 20% of the
space in a reactor waste respository.
The present invention is concerned with the
segregation of Carbon-14 which is present on the ion-
exchange resin primarily as carbonate. Some of it is
produced by neutron activation of the moderator at the
rate of 440 Ci/GW(e)-year in a 540 MW~e) heavy water
moderated, natural uranium, nuclear reactor. Oxidation
of the carbon to CO2 by oxygen radicals formed from
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radiolytie decomposition of the heavy water is the
postulated mechanism. The carbon-14 is then removed from
the moderator during passage of the moderator through a
sidestream purification system con-taining cation-anion
ion-exehange resin.
As already stated, this ion-exchange resin
eontains signifieant quantities of earbon-14 (1.7 to 7.2
TB2/m ). The long half-life of this isotope (5,730 years)
makes it diffieult to eonsider disposal of the ion-exchange
resin eontaining this isotope in a low-level repository
where the significant isotopes will be Cs-137 and Sr-90
(30-year half-life). It would be advantageous to remove
the C-14 from the resin to permit volume reduction of the
resin and to dispose of the C-14 in a repository as a
ehemieally stable solid.
Aeeording to the present invention there is
provided a method of removing earbon-14 from particulate,
ion exehange resin, eomprising contacting the particulate,
ion exchange resin with a stream of air, enriched with
earbon dioxide, while the partieulate ion exchange resin
is in eontaet with water to displaee carbon-14 from the
partieulate, ion exehange resin as gaseous carbon dioxide,
and then contacting the gaseous carbon dioxide thus
formed with at least one absorbent substance selected
from the group consisting of water soluble salts of
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calcium and barium to form as a chemically stable,
carbon-14 containing compound, a carbonate of calcium
if calcium is present in the absorbant substance, and
barium if barium is present in the absorbent substance.
In some embodiments of the present invention
the absorbent is at least one substance selected from
the group consisting of calcium hydroxide and barium
hydroxide.
In other embodiments of the present invention
the absorbent substance is an aqueous solution of sodium
hydroxide and barium chloride.
Preferably the ion exchange resin is contacted
with the air, enriched with carbon dioxide, at about
70C.
The chemically stable, carbon-14 containing
compound is preferably immobilized in cement.
In the accompanying drawings, Figure 1 is a
diagrammatic view of an apparatus used to verify the
present invention, and
Figure 2 is a graph showing the amount of C-14
released plotted against time,for scrubber solutions
produced in the apparatus shown in Figure 1 and
immobilized in various substances.
Referring now to Figure 1, there is shown a
2.5 cm. internal diameter stainless steel column 1,
having a stainless steel screen 2, has an air inlet 4
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connected to a pipe 6 and an air outlet 8 connected to
a pipe 10, an electrical heating coil 12 is disposed
around the column 1.
The pipe 6 has two branches 14 and 16. The branch
14 is for connection to an air supply (not shown~ via a
flow meter 18 and valve 20. The branch 16 is for
connection to a gaseous CO2 supply (not shown) via a flow
meter 22 and a valve 24.
The pipe 10 is connected to three off-gas
scrubbers 26 to 28 arranged in series flow. An outlet 30
from scrubber 28 is for connection to a vacuum pump (not
shown) which delivers any air therefrom to an exhaust
fume hood (not shown).
Organic mixed-bed ion-exchange resins are
commonly used in heavy water moderated, natural uranium,
nuclear reactors: Amberlite IRN-150 (trademark) is used
in the moderator system and Amberlite IRN-154 (trademark)
is used in the primary heat transport system from the
heavy water moderated, natural uranium, nuclear reactor
generating station of Pickering, Ontario, Canada. Both
of these amberlite resins are obtainable from Rohm & Haas
Co., Philadelphia, PA, USA. The major contaminants on
the resins are C-14 and Cs-137, respectively. As well as
being present in high concentrations, the Cs-137 and
especially the C-14 are, as already stated, long-lived
species and must be contained for long periods of time.
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In experiments to confirm the present invention,
Amberlite IRN-150 resin, traced with C-14 was used.
The column 1 was filled with a bed 32 of 45 g
of dewatered, C-14 traced, Amberlite IRN-150 resin in
particulate form. The resin 32 was supported on the
stainless steel screen 2. The pipe 14 was connected to
an air source (not shown)~ the pipe 16 was connected to
a gaseous CO2 source (not shown) and the outlet was
connected to a vacuum pump (not shown). The scrubbers
26 to 28 were filled with 200 mL of 2 kmol/m3 NaOH and
0.4 kmol/m BaC12.
In operation, a stream of air enriched with
C2 was drawn by vacuum along the pipe 6 and through the
bed 32 of C-14 traced, Amberlite IRN-150 designated 32
with the bed 32 heated to 25C or 75C by the heater 12.
The following Table 1 gives the results of
different experiments.
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TABLE 1
THE EFFECT OF CARBON DIOXIDE TO DISPLACE
C-14 FROM ION-EXCI~NGE RESIN
Experiment Time of C2 in Velocity Temperature C-14 Removal
Number Exposure Contacting of Gas of Resin Red Efficiency
to Gas Gas Stream Stream
h mL/s C %
1 2 O 2.5 25 0.05
2 2 10 2.5 25 21.8
3 4 10 2.5 25 25.8
4 2 50 2.5 25 26.5
2 100 2.5 25 11.0
6 2 10 12,5 25 54.4
7 2 10 25.0 25 7.8
8 2 O 2.5 70 0.06
9 2 10 2.5 70 101.2
l 10 ~.~ 70 62.6
It will be seen from the results in Table 1
that about 22% of the C-14 initially present on the resin was
liberated by passing air, with 10 vol % CO2, over the resin
at 2.5 mL/s for 2 hours at 25 C. The release of C-14 was
not affected by increasing the time of exposure of the
resin to the air stream, or the volume fraction of CO2 in
the air stream. However, at increased temperatures, release
of C-14 was enhanced such that at about 70C, 100% of the
C-14 was liberated in 2 hours. It was found that if resins
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are dried by passing air over the resins for 2 hours
at about 70C, no C-14 is released when CO2 is contacted
with the resins, suggesting that the presence of H2O is
important in the mechanism for release of C-14. The
mechanism for release of C-14 can probably be explained
on the basis of the hydration, exchange and dehydration
reactions described in "Advanced Inorganic Chemistry",
3rd Edition, F.A. Cotton and G. Wilkinson, Interscience
Publishers, Toronto, 1972, p. 297, and given below.
C2 + H20 _ H2C03 (1)
(RzNR3)2 CO3 + H2C3 ~ ~ (RZNR3)2co3+H2 C3 (2)
H2 CO3 ~ ~ 14co2 + H2O (3)
The simple step of con-tacting C-14 contaminated
resins with air and CO2, preferably at about 70C, accord-
ing to the present invention, can be used to recover C-14
from the resins before the resins are incinerated or
vitrified. Ash from incinerated resin may be immobilized
in cement, polyester, bitumen, glass and glass-ceramic
compositions.
The C-14-contaminated scrubber solutions must
be immobilized and disposed of. Barium carbonate waste
slurries result if C-14-contaminated NaOH scrubbing
solutions are treated with soluble salts of barium or if
Ba(OH)2 solid sorbents are used to trap C-14. To evaluate
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possible materials for disposal of these scrubber solutions,
slurries of 50 wt% barium carbonate traced with 6 GBq/L of
C-14 were immobilized in cement, polyester and bitumen.
The cement, polyester and bitumen products were leached to
measure C-14 releases. Details of the products and leaching
experiment are given in the following Table 2 and in
Figure 2.
TABLE 2
C-14 TRACED BARIUM CARBONATE SLURRY
IMMOBILIZATION MATRIX .
Speclmen
Description Portland l'olyester Resin Oxidized
Type III Ashland WEP-661-P Bitumen, Type
Cement SP-170
.
Matrix Material, wt% 60.0 50.0 60.0
Dry Amberlite IRN-150,
wt~ 10.0 12 5 40.0
Water Content, wt% 30.0 37.5 <0.5
Sample Weight, g 81.6 43.4 51.2
Sample Volume, mL 41.4 36.6 37.7
Surface Area Exposed
to Leachant, cm2 67.4 61.6 62 3
Initial Cs-14 Activity,
MBq, in Specimen 96 9 64.0 233.0
Volume of Leachant, mL 250 260 260
l Ci = 37 GBq.
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In Figure 2 leaching time (LT) in days is
plotted against cumulative fraction leached (CFL) in centi-
meters, from immobilized barium carbonate slurry, and
~ designates a 12 weight % slurry immobilized in poly~ster,
q designates a 10 weight % slurry immobilized in bitumen,
and
o designates a 10 weight % slurry immobilized in cement.
As illustrated in Figure ~ releases of C-14,
10 cm in 120 days, were lowest for the cement products.
Releases from the bitumen and polyester products were 2
and 20 times higher at 120 days after leaching started.
Since Ba(OH)2 in C-14 off-gas scrubbers will
not be completely converted to BaCO3, the effects of
incorporating Ba(OH)2 in cement were examined. A slurry,
50 wt~ water using a solid phase of 30 wt% BaCO3 and
70 wt% Ba(OH)2 traced with 6 GBq/L of C-14, was immobilized
in cement and leached to measure C-14 releases. After 30
days of leachings, the C-14 releases were the same as those
illustrated in Fig. 2 for the BaCO3 products.
Unsuccessful attempts were made to incorporate
BaCO3 into boro-silicate glass. Even at low loadings of
BaCO3, 5 wt~, the BaCO3 formed a separate phase on the
top of the glass.
