Note: Descriptions are shown in the official language in which they were submitted.
CA 03088286 2020-07-13
METHOD FOR SEPARATING CESIUM AND TECHNETIUM
The present invention relates to a method for separating cesium and technetium
from mixtures
of radioactive substances and to a device for carrying out the method.
Prior art
When spent fuel elements from reactors for civil or military use are
reprocessed, solutions of
highly radioactive waste occur (High Active Waste Concentrate, HAWC; High-
Level Waste,
HLW or HLLW and HAW), these solutions containing fission products, generally
dissolved
in nitric acid solution. According to the current prior art, one way of
conditioning this waste
in a form suitable for final disposition is to integrate it safely in a glass
matrix (vitrification).
The waste, in a concentrated form, is poured into molten glass and then safely
immobilized in
the waste glass created. Stored in stainless steel canisters, in this form the
highly radioactive
waste is suitable for final disposition in deep geological rock formations.
The short-lived and gaseous fission products with half-lives of less than one
year (e.g. iodine-
131) play almost no role in the final disposition and radioactive activity,
since these nuclides
have completely decomposed or outgassed during the reprocessing and storage
period. The
very long-lived nuclides with half-lives of more than 10,000 years, such as
technetium-99
(Tc-99, 211,500 year half-life) necessitate safe long-term containment of the
glass molds in
the repository, as their radioactivity decays only slowly. However, their
contribution to the
total radioactivity of the HAWC solution and the molds is minor, since these
nuclides only
decay at low decay rates. In contrast, of crucial importance are the nuclides
cesium-137 (Cs-
137) and strontium-90 (Sr-90); with half-lives of 30 years and 28.9 years,
respectively, they
decay relatively rapidly, and therefore make a critical contribution to the
total radioactivity of
the HAWC solution and glass molds.
The nuclides of cesium (that is, e.g., Cs-137, Cs-133, Cs-134, Cs-135) also
have the property,
under certain conditions, of forming cesium pertechnetate (CsTcO4) together
with the fission
product Tc-99. This happens primarily in reprocessing plants under oxidizing
conditions in
solutions with nitric acid. However, cesium pertechnetate is considered to be
extremely
problematic with respect to final disposition due to its high water solubility
as well as its high
volatility in steam. It also has strong oxidizing properties and high thermal
volatility. It has
been observed that, for example, when the HAWC solution is metered into it,
cesium
pertechnetate undesirably volatilizes out of the glass melt during the
vitrification process and
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is deposited in cold areas of the exhaust system and thus partially clogs the
latter. Attempts
are therefore generally made to prevent the formation of cesium pertechnetate
in the
vitrification plant.
The problem of the cesium pertechnetate deposited in the vitrification plant
has so far
regularly been countered by removing, either mechanically or by dissolving
with water/acid,
the cesium pertechnetate formed during the vitrification of HAWC solutions,
e.g., at glass
melt temperatures of greater than 600 C, and deposited in the exhaust system,
and thus the
escaped cesium and technetium nuclides are returned to the vitrification
process. However,
these measures often produce unsatisfactory results and their effectiveness is
usually short-
lived.
However, re-metering these solutions or solids with an increased cesium
pertechnetate content
does not solve the problem in a sustainable manner, since increased deposits
in the exhaust
system of the vitrification plant also result due to high volatility with an
elevated added
cesium concentration. In most vitrification plants, continuous vitrification
operation therefore
leads to long-term accumulation of cesium in the exhaust system. It is assumed
that on
average approx. 15% of the cesium inventory and at least as much of the
technetium-99
inventory is not added to the glass product. In principle, this technical
problem exists with all
HAWC vitrification plants worldwide. In addition, pertechnetates (i.e.,
technetium
compounds in oxidation state VII) are also quite problematic in the finished
glass of the mold.
That is, in contrast to lower oxidation states (e.g. in oxidation state IV for
Tc02), they are
easily leached out of the solidified glass melt with water.
DE 26 09 223 describes a process for producing aqueous solutions of
radioactive
pertechnetate. Technetium is separated from molybdenum-99 via a column,
however, no
mention is made of cesium.
WO 97/37995 describes a method for separating pertechnetate from radioactive
waste.
However, the separation is effected using complexing agents such as
calixpyrroles and other
macrocycles.
WO 01/95342 relates to a method for treating radioactive waste, which method
includes the
reduction of oxidic technetium compounds with hydrazine. However, the document
does not
describe the separation of cesium and technetium, let alone the sublimation of
cesium
pertechnetate.
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US 5,185,104 describes a method for treating radioactive waste at temperatures
between
500 C and 3000 C. Various oxidic substances are fractionated using vacuum
distillation. The
sublimation of cesium pertechnetate is not described, nor is the separation of
cesium and
technetium nuclides via the isolated cesium pertechnetate.
There is therefore still a need for improved conditioning and final
disposition of radioactive
waste, which reliably overcomes the problems mentioned above.
The underlying object of the present invention is therefore to provide an
improved method for
reprocessing radioactive waste material.
This object was surprisingly achieved using the method according to the
invention and the
device according to the invention in accordance with the claims below. The
device is
preferably suitable for carrying out the method and is adapted accordingly
thereto.
Surprisingly, it was found that ideal complete separation of the cesium and
technetium
nuclides from radioactive waste allows improved final disposition of the
residual waste. It
was also surprisingly found that the separated nuclides can be reused in
various technical
fields, for example as a source of radiation in medical technology and as
neutron barriers.
Figures
Figure 1 depicts a gas cooling unit of three gas coolers connected in series
according to one
preferred embodiment of the invention.
Figure 2 depicts a particularly preferred gas cooler arrangement according to
the present
invention that contains two gas cooling units, each of which has three gas
coolers (cooling
zones) connected in series. The two gas cooling units are connected in
parallel such that they
are used alternately a) for depositing cesium pertechnetate from the exhaust
gas flow and b)
for obtaining the deposited cesium pertechnetate, thus permitting continuous
operation, e.g.,
in the exhaust gas flow of a vitrification device.
Figure 3 depicts a vessel that can be used for separating cesium pertechnetate
from dry
residues from the reprocessing of nuclear fuels or from the vitrification
process and other
drying residues from HAWC solutions or other solids provided with blasting
agents according
to one preferred embodiment of the present invention.
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Description of the invention
The invention provides an improved method for separating cesium isotopes (in
particular
cesium-137) and technetium isotopes (in particular technetium-99) from the
exhaust gas flow
from nuclear vitrification plants or from reprocessing plants and residues of
such plants in
which radioactive waste and in particular HAWC solutions are processed and
prepared for
final disposition or are used for technical purposes. The aim of the method
according to the
invention is to effectively reduce the waste activity to be disposed of, to
obtain the separated
nuclides, and to use them economically and technically.
The phrase "separation of cesium and technetium" from radioactive waste
encompasses both
obtaining the two nuclides together from the waste (isolation as cesium
pertechnetate) and the
preferred subsequent separation of the two elements from one another
(isolation of the
separated elements). Alternatively, the method can also be referred to as a
method for
obtaining cesium and technetium from radioactive waste, wherein the substance
obtained can
be in the form of cesium pertechnetate or in the form of the two separate
elements, cesium
and technetium. If the two elements are obtained separately, they are
ultimately preferably
present as the solids technetium dioxide and cesium salt (preferably CsC1 or
C52SO4), which
can be reused, for example, in medical technology. In any case, it is
essential to the invention
that the method comprises the targeted sublimation of cesium pertechnetate
(CsTcO4) (i.e.
sustainably separating or obtaining the two elements from radioactive waste
using
sublimation).
One essential step of the method according to the invention is obtaining
cesium pertechnetate
by means of sublimation (step 1).
According to the invention, this sublimation can either occur directly in the
exhaust gas flow
of a reprocessing plant or from solid residues (e.g. in dry, paste, or moist
form) from the
reprocessing of nuclear fuels or from the vitrification process and other
solid residues from
HAWC solutions or other solids, even if these are present in the mixture with
other substances
or in a moist state. Suitable reprocessing plants for mixtures of radioactive
substances
according to the invention are selected, for example, from (conventional)
vitrification plants,
sintering plants, drying plants, combustion plants, cementing plants and
calcinating plants.
When carried out, e.g., in the vitrification plant, the cesium pertechnetate
from the exhaust gas
flow is deposited by means of suitable cooling measures using consolidation
(depositing from
the gas phase) and is then separated off. Performing this method in other
processing plants
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takes place correspondingly. If dry residues are used, they are heated in a
suitable vessel
(preferably under reduced pressure), the sublimated cesium pertechnetate
reconsolidates on
cool surfaces (deposited from the gas phase), and is then separated off
therefrom. In both
cases, the cesium pertechnetate deposits as a very pure solid that can be
easily obtained.
If the sublimation is carried out directly in the exhaust gas flow of a
vitrification plant, i.e., the
method according to the invention is included in a vitrification method, it is
preferred to
remove the cesium pertechnetate present in the hot exhaust gas flow
(preferably at ambient
pressure, but at reduced pressure is also possible) by means of gas coolers
connected
downstream of the exhaust gas flow. For this purpose, the hot vitrification
exhaust gases are
preferably conducted in the air flow over the cooling coils (also known as
"cooling fingers")
of the gas coolers, the cesium pertechnetate diffusing in the exhaust gas flow
and entering the
gas coolers, depositing there in a chemically pure crystalline form. Gas
coolers can be
connected in series (e.g. as multi-stage exhaust gas coolers or gas cooling
units) or in parallel,
which further increases the efficiency of the cesium pertechnetate separation.
One preferred method includes using gas cooling units with two, three, four,
five, or more gas
coolers connected in series. Particularly preferred is using gas cooling units
with three gas
coolers, corresponding to three cooling zones connected in series. Such a gas
cooling unit is
shown in Figure 1. The use of two, three or four (preferably two) such gas
cooling units
connected in parallel with one another is very particularly preferred.
Temperature control of
the zones strongly depends on the vitrification process and vitrification
plant. In one preferred
embodiment, the temperature of the cooling zones ranges from approximately 600
C in the
inlet region to approximately 250 C in the outlet region of the gas cooler.
The most preferred method includes using two gas cooling units, each of which
has three gas
coolers (cooling zones) connected in series, the two gas cooling units being
connected in
parallel such that they are alternately used a) to deposit cesium
pertechnetate from the exhaust
gas flow and b) to obtain the deposited cesium pertechnetate, and thus they
permit continuous
operation. "Alternating" is to be construed to mean that cesium pertechnetate
from the exhaust
gas flow is deposited in one of the two gas cooling units per time unit, while
the cesium
pertechnetate previously deposited in the other gas cooling unit is obtained
there. After
cleaning, the task of the two units is reversed, so that the method is carried
out continuously
overall without downtimes caused by obtaining/cleaning all of the cooling
units. Such a
particularly preferred method is shown in Figure 2.
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As an alternative to the vitrification furnace, the method according to the
invention can also
be used with residues from the reprocessing of nuclear fuels or from other
reprocessing
methods (e.g., sintering, etc.), as well as with general residues from HAWC
solutions or other
solids present in the mixture and containing cesium pertechnetate. In this
case, the residues
are preferably collected in a vessel (e.g. boiler), the CsTcat is converted to
the gas phase by
heating at a suitable temperature (and preferably under reduced pressure), and
is deposited in
a form similar to that in the embodiment in the exhaust gas flow of the
vitrification furnace on
cooled gas cooler surfaces or cooling fingers. A suitable vessel is shown in
Figure 3.
.. The temperature used is preferably in a range of less than 500 C,
preferably in a range from
100 C to 500 C, particularly preferably from 150 C to 450 C, and very
particularly
preferably from 300 C to 400 C. A reduced pressure is preferably applied, e.g.
in the range
from 10-8 to 10-19 bar, preferably from 10-9 to 10-19 bar. This significantly
lowers the
sublimation temperature of cesium pertechnetate, making the process quicker,
easier, and
.. more economical.
According to the invention, the method can be carried out such that either the
deposited
cesium pertechnetate is separated from the vessel used in situ (i.e. from the
cooling
fingers/cooling coils), or the part of the vessel used at which the cesium
pertechnetate has
.. deposited (i.e. cooling fingers/cooling coils) is removed from the vessel
before the cesium
pertechnetate is separated and is transported to another suitable location,
where the cesium
pertechnetate is then separated off and obtained. The separation can be
accomplished in both
cases using mechanical removal or by rinsing with a suitable solvent or water.
This results in
even more efficient and economical separation from the residual waste.
The pure cesium pertechnetate deposited according to any of the above-
mentioned
embodiments of the method according to the invention is preferably separated
by dissolving
with an inorganic or organic solvent comprising water. Suitable inorganic
solvents are liquid
ammonia and carbon dioxide. Suitable organic solvents are inert solvents, such
as halogenated
hydrocarbons. Pure water is preferably used. The basis for this is the
solubility of the cesium
pertechnetate in water of 8.79 g/1 at 40 C. The temperature of the solvent is
preferably 20 C
to 60 C, preferably 30 C to 50 C (step 2). The gas coolers (i.e. cooling
fingers) can then be
reused for the separating process.
In this way an aqueous cesium pertechnetate solution is obtained that can
undergo further
processing. Alternatively, the separated cesium pertechnetate is not isolated
as an aqueous
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cesium pertechnetate solution, but instead further processing with the
separated cesium
pertechnetate takes place directly (e.g. on the cooling finger).
The preferably performed processing of the cesium pertechnetate according to
the present
invention comprises the two optional steps below (step 3 and step 4).
Technetium is
chemically separated from cesium in step 3, which is preferably carried out
according to the
present invention. For this purpose, cesium pertechnetate is reduced either in
aqueous solution
or in non-aqueous solution (preferably in aqueous solution), in which
reduction the
pertechnetate is converted to technetium dioxide (Tc02 = 1-2 H20), which
precipitates out of
the solution as a solid while cesium remains in aqueous solution. Suitable
reducing agents are
in principle all substances that are able to reduce pertechnetate, such as,
e.g., LiA1H4, NaBH4,
and alkali metal hydrides. Particularly suitable are reducing agents whose
products do not
introduce any additional elements into the solution following the reaction,
such as hydrazine,
carbon monoxide, and organic reducing agents such as formaldehyde,
acetaldehyde, formic
acid, oxalic acid. Hydrazine is particularly preferred.
The technetium dioxide precipitated as a solid can then be separated off,
preferably by
filtration. After the filtrate has been neutralized (e.g. with sulfuric acid
or hydrochloric acid),
cesium is present in the aqueous phase as a dissolved cesium salt (e.g. as
cesium sulfate or
cesium chloride). In this way, cesium and technetium are efficiently separated
using the wet
chemical method. Cesium-technetium separations based on extractions from the
aqueous
phase with organic solvents are conventionally carried out using tri-n-
octylphosphorus oxide
in cyclohexanone. However, these reactions always require additional
separation of the
solvent or re-extraction of the organic phase with an aqueous solution in
order to return the
technetium to the aqueous phase from the organic phase. This re-extraction
from organic
solvents is very complex.
In an optional subsequent step 4, the aqueous phase is concentrated by
evaporation (for
example under vacuum at temperatures in the range of 30 C to 50 C) and cesium
is obtained
in the form of solid cesium salts that can undergo further purification and
processing. The
salts can preferably be converted to anhydrous salts at temperatures in the
range of 80 C to
100 C (for example at 90 C) in a vacuum. Particularly preferred are cesium
sulfate or cesium
chloride, which - in anhydrous form - can be used in medicine directly for the
production of
cesium-137 radiation sources following the characterization and activity
measurement.
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The technetium dioxide filtered off is cleaned of adhering cesium salt and
dried. It can be
converted to its anhydrous form in a vacuum at elevated temperatures (greater
than 200 C,
e.g. at 300 C). The powdery anhydrous technetium dioxide can then be pressed
into pellets
and, e.g., sent for transmutation in the reactor or released into a nuclear
repository.
The method according to the invention has the following advantages in
particular compared to
conventional methods from the prior art:
The separation of cesium pertechnetate makes it possible to minimize the entry
of cesium
pertechnetate into the exhaust system of vitrification plants and therefore
solves a
longstanding technical problem of many vitrification plants. This makes it
easier to maintain
and operate running vitrification plants, and less radioactive residue remains
in the exhaust
system. The method according to the invention also allows dried or solid
residue to be freed
of, for example, HAWC or rinsing solutions of cesium pertechnetate. This makes
the
processing and final disposition of this waste more economical and
environmentally friendly.
Furthermore, the method according to the invention allows cesium isotopes to
be put to
economic use in the production of cesium radiation sources for medical or
technical
applications, since the cesium obtained is chemically pure (e.g., as anhydrous
cesium sulfate
or cesium chloride).
The method according to the invention also makes it possible to separate
cesium isotopes and
technetium isotopes from the other fission products of the nuclear fission of
uranium and
plutonium. As a result, the residual activity of the radioactive waste
generated in the nuclear
fission, and thus the amount of waste in the nuclear fission, is significantly
reduced. Thus, up
to 41% of the medium-term activity of waste from peaceful or military use of
nuclear fission
can be returned for commercial use. Finally, the method according to the
invention also
makes it possible to separate technetium-99, which represents up to 81% of the
activity of the
long-lived fission materials, from the radioactive waste (e.g. HAWC). The
chemically stable,
anhydrous technetium dioxide can either be made available for transmutation
or, due to its
water-insoluble properties, can be sent directly into deep geological rock
formations for final
disposition. The method is suitable for all cesium and technetium isotopes in
the manner
described. Overall, according to the invention, the ability to subject
technetium-99 to final
disposition is significantly improved, since technetium dioxide is chemically
and thermally
stable and water-insoluble and is not volatile under the expected ambient
conditions. Further
immobilization measures are therefore not required for final disposition.
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The invention furthermore provides a device for separating cesium and
technetium from
radioactive waste. The device is preferably adapted to the method according to
the invention
and is suitable for carrying out the method according to the invention. This
applies in
particular to the embodiment for implementation in the exhaust gas flow of a
vitrification
plant. The device according to the invention makes it possible to obtain and
isolate cesium
pertechnetate, in pure form, that is either disposed in gaseous form in the
exhaust gas flow of
a vitrification plant or has been obtained using sublimation from dry residues
from
reprocessing nuclear fuels or from the vitrification process or other residues
from HAWC
solutions. The crystalline cesium pertechnetate obtained can then be separated
into the two
elements cesium and technetium as described above in the method according to
the invention.
The device according to the invention has a plurality of gas coolers (i.e.
cooling zones/zone
gas coolers) that in principle are commercially available and contain cooling
fingers or
cooling coils for separating sublimated cesium pertechnetate disposed therein.
The gas coolers
can be connected in parallel and/or in series, which results in gas cooling
units. Two, three,
four, five, or more gas coolers are preferably connected in series one after
the other. A gas
cooling unit with three gas coolers is particularly preferred. The device
according to the
invention is preferably connected upstream of the exhaust gas flow from a
vitrification plant.
In other words, the exhaust gases from a vitrification plant, which contain
cesium
pertechnetate in the gas phase, are introduced into the device according to
the invention so
that cesium pertechnetate can be deposited there in pure form and with high
efficiency. A
device according to the invention (gas cooling unit) is shown in Figure 1.
Very particularly
preferred is a device in which two, three or four (preferably two) such gas
cooling units are
connected in parallel with one another in series.
The most preferred device according to the invention comprises two gas cooling
units, each
having three gas coolers (cooling zones) connected in series, the two gas
cooling units being
connected in parallel such that they can be used alternately a) for depositing
cesium
pertechnetate from the exhaust gas flow and b) for obtaining the deposited
cesium
pertechnetate, thus enabling continuous operation. "Alternating" is to be
construed to mean
that cesium pertechnetate from the exhaust gas flow is deposited in one of the
two gas cooling
units per time unit, while the cesium pertechnetate previously deposited in
the other gas
cooling unit is obtained there. After cleaning, the task of the two units is
reversed, so that the
method is carried out continuously overall without downtimes caused by
obtaining/cleaning
all of the cooling units. Such a particularly preferred device is shown in
Figure 2. This device
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is particularly suitable for use in the exhaust gas flow of a vitrification
plant for obtaining
crystalline cesium pertechnetate.
Preferred embodiments for obtaining cesium pertechnetate and its further
processing, with
separation of cesium and technetium, according to the inventive method and the
inventive
device are described in more detail below:
1. Integration of the method according to the invention in the exhaust system
of a
vitrification plant
A system as indicated in Figure 2 is used for this. The centerpiece is the gas
cooling unit
(zone gas cooler), shown in detail in Figure 1, two of which are provided in
the system. This
gas cooling unit is connected to the exhaust air flange of the vitrification
furnace (not shown)
via valve V11 (V21). The connection between the exhaust air flange of the
vitrification
furnace and V11/V21 is thermally insulated and electrically heated to
approximately 600 C.
The system is designed redundantly. During operation, one cooler is operated
while the other
is being cleaned.
Operation is explained below using the gas cooling unit (zone gas cooler) WT
10. The WT 20
unit works analogously. When the gas cooling unit (zone gas cooler) WT 10 is
in operation,
the valves V11, V12 are open and the valves V13, V14 are closed. Exhaust gas
from the
vitrification furnace flows via the heated line via V11 from the furnace into
the zone gas
cooler WT10 and is cooled incrementally on the cooling coils, i.e. in the
lower region of the
first cooling coil bundle, from approx. 600 C to 500 C at the upper edge of
the first cooling
coil, in the middle region from 500 C to approx. 350 C at the upper edge of
the second
bundle of cooling coils, and in the upper region to approx. 250 C at the third
bundle of
cooling coils. During this process, cesium pertechnetate is deposited on the
cooling coils in a
pure (crystalline) form. Due to the temperature control, crystallization
begins starting at the
upper edge of the second bundle of cooling coils. Over the course of the
second and third
bundles, all of the cesium pertechnetate disposed in the exhaust air
crystallizes out on the
cooling coils. Since the diameter of the WT10 is reduced through continuous
operation,
pressure measurements Pll and P12 can determine a pressure difference P11-P12,
starting
from which the zone gas cooler is to be cleaned. Alternatively, it is possible
to use the
metering rate measured on the metering rate sensor DL1 to decide the time of
cleaning of the
zone gas cooler.
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The valves V21 and V22 are opened for cleaning and the zone gas cooler WT 20
is put into
operation. V11 is then closed. Now the cooler WT 10 is operated with coolant
at 40 C on all
cooling coils. As soon as all cooling temperatures are stable at 40 C, the
cooler WT 10 is
filled with water as a solvent via V40 and V13 (a higher temperature up to 100
C is possible
and also increases the solubility of the pertechnetate, but care must be taken
to ensure that the
solution does not cool down during further processing so that cesium
pertechnetate then
precipitates again). The fill level can be checked using fill level sensor Li.
The uppermost
cooling coil bundle must also be completely covered with solvent. The
crystallized cesium
pertechnetate is dissolved from the cooling coils at approx. 40 C using
circulation with pump
1 via V13 and V14. The process can be monitored by sampling. The cesium
pertechnetate is
considered to be completely dissolved once no further increase in activity
concentration can
be found in the solution. WT 10 is then emptied via the valve V30. The
resulting solution can
be provided for further processing (i.e., the separation of cesium and
technetium, as described
below). The cooler WT 10 is again available for operation. Both coolers WT 10
and WT 20
are alternately operated and cleaned during the vitrification so that cesium
pertechnetate can
be continuously separated in aqueous solution during the vitrification
process.
2. Application of the method according to the invention for processing solid
residues
from reprocessing or vitrification, which residues primarily comprise
radioactive
cesium-137-pertechnetate-99
The method according to the invention is applied to residues that are disposed
directly in a
container. The method according to the invention can also be used for residues
that result,
e.g., from sandblasting or similar methods, while the cesium pertechnetate is
detached from
component surfaces, for example with blasting sand, and is collected in a
collection container
together or separately from the blasting material. These residues are
collected in a specially
constructed radiologically shielded container that can be heated externally
(e.g., electrically),
as shown in Figure 3. This container has a vacuum-tight flange lid with a
cooling finger. The
container is closed in a vacuum-tight manner and a reduced pressure is applied
via the lid
connection. A pressure of 10-9 to 10-10 bar is preferably set (high vacuum).
The residues are
then heated to the sublimation temperature for the cesium pertechnetate at the
respective
pressure + 50-100 K. Pure (crystalline) cesium pertechnetate is deposited on
the cooling
finger. The cooling finger temperature can be approximately 20 C (return
temperature of the
cooling water).
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Following sublimation, the system is cooled and then the container is
ventilated, and the lid,
with the cooling finger, is moved onto a shieldable container and screwed
tightly with its
flange. This container is already shielded or can be shielded for upcoming
transport.
The cesium pertechnetate is dissolved with pure water at about 40 C via the
rinsing
connection of the container and the connection of the lid. The solution is
preferably circulated
with a pump (connected on the pressure side to the lower container connection)
via the
connection in the lid. The dissolution process is considered to be complete
when the cesium-
137 activity in samples of the aqueous solution no longer increases. The
aqueous cesium
pertechnetate solution produced is separated off and sent for further
processing (i.e.,
separation of cesium and technetium, as described below). The cleaned lid
flange can be used
for the next sublimation. The sublimated cesium pertechnetate produced in this
way is
chemically pure and can be further processed without any problem.
3. Processing of the cesium pertechnetate solution/separation of the elements
technetium
and cesium
The resulting cesium pertechnetate solution is heated to a temperature between
30 C and
40 C, and a 20% solution of hydrazine in water is added dropwise while
stifling. Gray
coloration develops, and a brown-black solid separates due to further addition
of hydrazine
solution.
The following reaction takes place:
4 Cs+ + 4 TcO4¨ + 3 N2H4 ¨> 4 Tc02 (prec.* 1 ¨ 2 H20) + 4 Cs+ + 4 OH¨ -I- 4
H20 + 3 N2
C
The hydrazine added by drops reacts with the pertechnetates to create
technetium dioxide,
which is insoluble in water and precipitates as a dihydrate. An aqueous cesium
hydroxide
solution remains. Careful metering of hydrazine continues until no more
technetium dioxide
30 precipitates. Upon complete reduction and completion of the metering, a
hydrazine excess of
10 to 20 mg/kg should verifiably remain in the solution for 10 mm at 30 C to
ensure the
absence of pertechnetate. At the end of the reaction, the pH is in the
alkaline range (greater
than pH 8). The solution is then left to stand for about 1 hour so that the
technetium dioxide
dihydrate formed can settle. The precipitate is filtered off and washed with
cold water until no
significant cesium concentration can be measured in the precipitate using a
gamma sensor
measurement.
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Date Recue/Date Received 2020-07-13
CA 03088286 2020-07-13
The technetium dioxide dihydrate that was filtered off can be dried and freed
of crystallization
water at 300 C and reduced pressure:
TcO2x 2 H20 __________ > 300 C, TCO2 2 H20
Vacuum
Technetium dioxide can then either be sent directly to final disposition or
pressed into pellets
and sintered into fuel rod-like structures for transmutation.
Processing the remaining cesium:
The wash water and filtrate are combined and neutralized to pH 7 with a
suitable acid.
Suitable acids are inorganic acids, such as sulfuric acid, hydrochloric acid,
and phosphoric
acid. Hydrochloric acid, with which the reaction proceeds as follows, is
particularly preferred:
CsOH HC1 ¨ CsC1+ 1120
Neutralization produces cesium chloride, which can be crystallized by
evaporating the water
at 40 C in a vacuum and can be dried at 90 C in a vacuum to form the anhydrous
salt. The
anhydrous cesium chloride can be used as a starting material for medical
radiators. Weighed
out tablets compressed by means of a press can be used, e.g., in medical or
technical radiation
sources.
Example:
The example below further illustrates the method according to the invention:
1. Sublimation
A mixture of 5 g solid blasting material (e.g. garnet blasting sand) and 3.5 g
cesium
pertechnetate (CsTc04) is added to a sublimation apparatus and sublimated at a
reduced
pressure of approximately 8.5 = 10-5 mbar and 390 C. About 3.1 g pure white
CsTcat is
deposited on the surface of the cooling finger in fine crystalline form. The
product can be
removed with 1 L distilled water at approx. 50 C and, after concentration by
evaporation in a
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Date Recue/Date Received 2020-07-13
CA 03088286 2020-07-13
vacuum at 60 C, provides about 3.1 g CsTcat. Alternatively, the sublimated
CsTcat can
remain on the cooling finger and be used directly for the following reductive
separation.
2. Reductive separation
For reductive separation, 3.1 g CsTcat is used, obtained either following
concentration by
evaporation from the sublimation or as a fine crystalline solid adhering to
the cooling finger.
70 mL of a 20% aqueous hydrazine solution is added to the solid. While
stifling, black
technetium dioxide deposits as dihydrate (Tc02 = 1-2 H20). The reaction is
completed by
careful concentration by evaporation. The moist crystal slurry obtained is
taken up in distilled
water and suctioned off through a 0.5 [tm Teflon filter. The filter cake is
washed with water
until a desired residual cesium-137 activity in the filter cake is reached.
The washed filter
cake is dried on the filter and then removed mechanically. The Tc02 = 1-2 H20
obtained can
be freed from crystallization water at 300 C in a high vacuum.
The combined filtrates and eluates of the resulting cesium hydroxide solution
are neutralized
to pH 7 with dilute hydrochloric acid and then concentrated by evaporation in
a vacuum. The
precipitating cesium chloride is dried at 100 C in a drying cabinet under
vacuum to form the
anhydrous salt.
14
Date Recue/Date Received 2020-07-13
