Radiation-chemical resistance of anion exchangers and safety of sorption processes in nitric acid media: V. Thermochemical degradation dynamics of VP-1AP anion exchanger

Article abstract of DOI:10.1023/A:1022369418615

Thermal degradation of VP-1AP anion exchanger and its mixtures with HNO₃ was studied in open vessels at 40 to 260°C. The anion exchanger ignites only after drying and heating to 220-260°C. Heating VP-1AP mixtures with 3-12 M HNO₃ to 100°C is accompanied by gas evolution at a rate of 0.6 l min^-1 (l_sorb)^-1. Removal of the aqueous phase from the mixtures at 130-170°C initiates intensive oxidation. It is accompanied by a temperature jump in the sorbent phase and by gas evolution acceleration and can be regarded as thermal explosion. Mixture preheating and irradiation reduce the thermal explosion onset temperatures.

Full text of DOI:10.1023/A:1022369418615

Radiochemistry, Vol. 45, No. 1, 2003, pp. 51 55. Translated from Radiokhimiya, Vol. 45, No. 1, 2003, pp. 49 53.
Original Russian Text Copyright 2003 by Bartenev, Zachinyaev, Nazin, Lazarev, Romanovskii, Strelkov, Firsin, Egorov, Hyder.
Radiation-Chemical Resistance of Anion Exchangers and Safety
of Sorption Processes in Nitric Acid Media: V. Thermochemical
Degradation Dynamics of VP-1AP Anion Exchanger
S. A. Bartenev*, G. M. Zachinyaev**, E. R. Nazin**, L. N. Lazarev*,
V. N. Romanovskii*, S. A. Strelkov*, N. G. Firsin*,
G. F. Egorov***, and M. L. Hyder****
*
Khlopin Radium Institute, Research and Production Association, St. Petersburg, Russia
* Timoshenko Military University of Radiation, Chemical, and Biological Protection, Moscow, Russia
** Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Moscow, Russia
*
*
*
*** Sandia National Laboratory, the United States
Received February 6, 2002
Abstract Thermal degradation of VP-1AP anion exchanger and its mixtures with HNO3 was studied
in open vessels at 40 to 260 C. The anion exchanger ignites only after drying and heating to 220 260 C.
Heating VP-1AP mixtures with 3 12 M HNO to 100 C is accompanied by gas evolution at a rate of
3
.6 l min 1 (lsorb) . Removal of the aqueous phase from the mixtures at 130 170 C initiates intensive oxida-
1
0
tion. It is accompanied by a temperature jump in the sorbent phase and by gas evolution acceleration and can
be regarded as thermal explosion. Mixture preheating and irradiation reduce the thermal explosion onset
temperatures.
Ion-exchange sorption processes are widely used in
radiochemistry for spent nuclear fuel processing and
radioactive isotope production, water treatment in
nuclear installations, as well as at radiochemical
enterprises. Ion exchangers typically operate under
fairly severe conditions (ionizing radiation, oxidative
medium, and high temperatures). These conditions
decrease the heat resistance of sorbents when mixed
EXPERIMENTAL
Experiments on gas evolution under heat treatment
of sorption systems up to 100 C were run in a two-
necked flask equipped with a reflux condenser and
placed on a water bath. The temperature of the mix-
tures was controlled accurately to within 2 C; the
volume of the gases evolved was estimated volumetri-
cally, from the volume of the liquid (concentrated
NaCl solution) displaced from the measuring buret.
As the zero (reference) point for measuring the
volumes of the evolving gases we took the instant
when the prepared anion exchanger sample was in-
with HNO solutions and eventually initiate oxida-
3
tion accompanied by heat and gas evolution. The
radiochemical production experience suggests that a
number of emergency situations are associated with
sorption processes. For details on such accidents, as
well as for analysis of published data on the heat
resistance of sorption systems, see [1, 2]. It turned out
that the emergency situations are primarily due to the
ion exchanger degradation in a nitric acid medium
during processing of actinide solutions, with the heat
resistance of the anion exchangers strongly varying
with the ion exchanger type.
troduced into the HNO solution heated to the ap-
3
propriate temperature. The pretreatment of the anion
exchanger sample before experiments consisted in its
swelling in an HNO solution for several hours. Also,
3
air was removed from the anion exchanger pores im-
mediately before the experiment by heating the
swollen anion exchanger samples for 5 min at 100 C
in the acid solution of the appropriate concentration.
Russian radiochemical enterprises use primarily
VP-1AP anion exchanger. Hence, in this work we
studied the heat resistance of VP-1AP anion ex-
changer in the nitrate form and of VP-1AP HNO3
mixtures with the aim to reveal the process conditions
at which explosion or fire hazards become real.
The heat resistance of the sorption systems above
00 C was studied in stainless steel cells; along with
gas evolution, the temperature variation in the sorbent
phase was monitored.
1
Figure 1 shows schematically the setup for study-
1
066-3622/03/4501-0051$25.00 2003 MAIK Nauka/Interperiodica

5
2
BARTENEV et al.
ing the heat resistance at elevated temperatures. We
3
3
used 6 cm cells; control tests on 50 cm sorbent
3
samples were run in a 300 cm open vessel heated in
an air thermostat. The experiment control and data
recording were computerized.
Based on the temperature vs. time plots for the
samples studied, reference sample (air-dry quartz
sand), or the reaction vessel wall, we elucidated the
conditions at which the sample temperature abruptly
changes.
We tested samples of VP-1AP anion exchanger in
the nitrate form, both moist and air-dry, in particular,
that dried from HNO solutions to the air-dry state,
3
and the sorbent HNO mixture.
3
Fig. 1. Diagram of the differential measurement unit of the
setup for heat resistance studies: (1) system under study,
2, 3) experimental cells, (4) cell cover, (5) internal ther-
mocouples, (6) external thermocouples, (7) gas outlet tubes,
8) thermostat case, (9) electric heating cells, (10) high-
temperature heat carrier, (11) electric and heat insulation,
12) thermocouple of the temperature controller of the
(
RESULTS AND DISCUSSION
(
VP-1AP anion exchanger, air-dry or moist.
Moist VP-1AP anion exchanger does not degrade up
to 100 C. After moisture removal and further heating
(
thermostat, (13) stirrer, and (14) thermostat cover with
a block of experimental cells.
3
of VP-1AP at 200 210 (20 50 cm samples) and
3
2
20 260 C (4 6 cm samples), the temperature in
the sorbent phase changed jumpwise by 50 70 C in
parallel with ejection of charred anion exchanger
granules from the open vessel. At the moment of the
temperature jump, the flame appeared, as recorded
with a photodiode.
Figure 2 shows a typical plot of the temperature
variation with time in the anion exchanger bulk. Im-
mediately before the temperature jump, the tempera-
ture in the anion exchanger ceases to grow for a rela-
tively brief period and may even decrease. It can be
assumed that, immediately before the jump in the
warming-up rate for the anion exchanger samples, the
temperature curve responds to the endothermic de-
composition of HNO . On attaining the thermal ex-
3
plosion onset temperature, the acid decomposition
tends to accelerate, and the amount of NO becomes
2
sufficient for rapid exothermic oxidation of the anion
exchanger.
It should be noted that there was no evidence of the
sorbent degradation at heating of the anion exchanger
before the onset of the exothermic process. This
finding was also supported by our thermogravimetric
studies [3].
Fig. 2. Time dependences of (a) temperature of the air-dry
VP-1AP anion exchanger and of the amount of evolving
gases and (b) sorbent temperature variation rate upon heat-
Our measurements showed that, at the time of the
temperature jump, the bulk anion exchanger exhibits a
significant temperature gradient, as seen from the
temperature curves shown in Fig. 3. Exothermic proc-
ess is initiated in the local zone(s) of the ion ex-
changer characterized by the worst heat removal con-
ing the VP-1AP sorbent in the NO form in an open vessel.
3
The insert in Fig. 2b shows the scaled plot for the sorbent
temperature variation rate in the same coordinates as the
main plot. (1) Reaction vessel wall temperature; (2) air-dry
VP-1AP sample temperature, and (3) volume of the evolv-
ing gases.
RADIOCHEMISTRY Vol. 45 No. 1 2003

RADIATION-CHEMICAL RESISTANCE OF ANION EXCHANGERS... : V.
ditions (in deep-lying layers). As the heat conductivity
53
of the dry ion exchanger is low, the temperature
gradients are significant even for small volumes of
the ion exchanger.
Condensed reaction products are charred anion
exchanger granules sintered into black caked frag-
ments, plus a minor amount of a black tarry liquid.
The appearance of a flame, as well as the character
of the exothermic processes initiated by heating the
NO form of VP-1AP anion exchanger, suggest that
3
these processes can be regarded as thermal explosions.
The primary prerequisites to their initiation are the
drying and heating of the anion exchanger to the
appropriate onset temperatures.
Fig. 3. Temperature gradient in the bulk of the VP-1AP
Thermal explosion of the NO form of VP-1AP
3
sorbent in the NO3 form upon heating in an open vessel:
(1) cell wall temperature, (2) VP-1AP sample temperature
measured at the cell center (separated from the bottom by
7 mm), and (3) VP-1AP sample temperature measured at
a distance of 3 4 mm from the cell wall (separated from
the bottom by 7 mm).
anion exchanger occurs at very high rates; at the ther-
mal explosion moment, the anion exchanger tempera-
1
ture increases at a rate of 7 10 deg s . The fact that
the thermal explosion products contain nonoxidized
and partly oxidized anion exchanger granules even
3
with 4 cm samples suggests that, because of a high
degradation rate, the reaction vessel content is blown
out at a time when the bulk of the anion-exchanger
has not yet fully reacted.
Mixtures of the NO form of VP-1AP anion ex-
3
changer with HNO solutions. Heat treatment of
3
VP-1AP anion exchanger samples in 4 and 7 M HNO3
solutions at 40 and 60 C is not accompanied by gas
evolution. Noticeable amounts of gaseous products
appear only at heating temperatures as high as 95
Fig. 4. Variation of the gas evolution rate in heat treatment
1
00 C. Studies of gas evolution upon heating the
of VP-1AP anion exchanger in HNO solution of the con-
centration (1) 4, (2) 7, and (3) 12 M. Volume of the
swollen anion exchanger 10 ml.
3
VP-1AP anion exchanger samples (Fig. 4) show that
the gas evolution rates are significant [on the 0.2
0
ment period (first 20 30 min). Further heating de-
celerates evolution of gaseous products to 0.01
0
virtually ceases. At the HNO concentrations within
4
increase with increasing HNO concentration in the
aqueous phase. Active gas evolution can be due
either to oxidation of the products of the anion ex-
changer synthesis or impurities contained in the
sorbent that are more reactive than the sorbent matrix,
or to air release from the resin pores. The sorbent
itself is very stable under these conditions.
.6 l min 1 (lsorb
)
1
level] in the initial heat-treat-
products and the gas evolution rates are significantly
lower in this case [under 0.01 l min 1 (lsorb) ]. Thus,
the heat resistance of the anion exchanger increases
after washing off its degradation products.
1
.02 l min 1 (lsorb) , and within 3 4 h gas evolution
1
3
12 M, the maximum gas evolution rates linearly
At temperatures above 100 C, the exothermic
processes initiated by heating the anion exchanger
3
HNO mixtures proceed similarly to air-dry and moist
3
samples.
Figure 5 presents typical plots of the temperature
variation on heating the anion exchanger mixtures
with 3 12 M HNO3.
Upon drying, a portion of HNO and NO remains
3
2
The anion exchanger samples pretreated with 4
in the micropores of the anion exchanger, and the
latter gets enriched in the oxidant. This is respons-
ible for certain distinctions in the initiation of the
exothermic processes in mixtures compared to the
12 M HNO solutions for 100 h at 100 C and washed
3
to remove water-soluble degradation products exhibit
the same gas evolution trends as the mixtures with the
initial sorbent. However, the volumes of the gaseous
air-dry anion exchanger in the NO form.
3
RADIOCHEMISTRY Vol. 45 No. 1 2003

5
4
BARTENEV et al.
12 M HNO . It should be noted that increase of the
3
3
HNO concentration in the mixture tends to reduce
3
the onset temperature of exothermic processes. The
minimal onset temperature of thermal explosion
recorded for a mixture with 12 M HNO was 127 C.
3
It should be noted that keeping this mixture at 120
5
C for 25 h was not accompanied by self-heating.
The influence exerted by the HNO concentration
3
can be explained by increase in the amount of the
oxidant occurring in the sorbent phase, which results
in a more profound oxidation of the sorbent matrix
and formation of a larger amount of degradation prod-
ucts and active centers in the sorbent matrix; this is
responsible for the reduced onset temperature of exo-
thermic reactions (130 150 C).
Fig. 5. Temperature variation with time upon heating the
VP-1AP anion exchanger HNO mixtures in open vessels:
3
(
(
1) temperature of the anion exchanger HNO mixture and
2) temperature variation rate (for curve 1).
3
The condensed reaction products are both variously
colored (from the initial color to black) granules and
caked charred lumps.
Preheating the anion exchanger in HNO solutions
3
tends to reduce the onset temperature of exothermic
processes in mixtures, which initiates thermal ex-
plosions. In particular, for mixtures with 7 M HNO3
preheated for 100 h at 40, 60, and 100 C the onset
temperatures of exothermic processes are estimated
at 185, 180, and 130 C, respectively. In mixtures with
1
2 M HNO preheated for 100 h at 100 C, thermal
3
explosion occurs at 120 C.
Fig. 6. Temperature variation with time in the irradiated
5 MGy) and reference samples of VP-1AP anion ex-
changer when heated in open vessels: (1) temperature of the
irradiated sample, (2) temperature of the reference sample,
and (3) temperature of the cell wall.
Figure 6 illustrates how irradiation affects the
thermochemical degradation dynamics, with VP-1AP
(
7
M HNO mixture irradiated at a 5 MGy dose as an
3
example. It is seen that, in the irradiated mixture,
thermal explosion occurs at a lower temperature com-
pared to nonirradiated mixture. It was found that, after
irradiation with an external -source at a dose of up to
5 MGy, thermal explosion in mixtures occurs at tem-
peratures as low as 114 116 C. This is preceded by
exothermic processes initiated in the irradiated mix-
ture at 80 90 C, which are accompanied by gas
evolution. The condensed products of degradation of
irradiated and nonirradiated mixtures differ in the
external appearance as well. In the former case, the
products are, essentially, black powders with different
particle size (in particular, finely divided powder,
dust), and the anion-exchanger granules degrade vir-
tually completely. In the case of nonirradiated mix-
ture, these are black, charred granules which preserved
their form plus partially caked lumps.
Above 100 C, the endothermic vaporization of the
aqueous phase in mixtures occurs in parallel with
oxidation of the anion exchanger with HNO and
3
NO , which is accompanied by evolution of gaseous
2
products. Evidently, oxidation is accompanied by heat
release, but the endothermic process prevails as long
as the sorbent granules are separated by the aqueous
phase. At a certain moment at 130 150 C, the amount
of the heat released begins to exceed that of the heat
lost by the aqueous phase vaporization. The tempera-
ture of the mixture tends to rise to the onset tem-
perature of the thermal explosion, which by 20 50 C
exceeds that of the exothermic processes. The exo-
thermic process initially proceeds in a local volume of
the mixture, at a considerable distance from the ex-
ternal heating source, whereupon it spreads through-
out the mixture bulk.
The fact that the heat resistance of the irradiated
mixtures decreased to such a significant extent can be
attributed to the radiolysis-induced formation of easily
oxidizable degradation products. Also, additional
Thermal explosion was recorded upon heating the
mixtures of the NO form of the anion exchanger with
3
RADIOCHEMISTRY Vol. 45 No. 1 2003

RADIATION-CHEMICAL RESISTANCE OF ANION EXCHANGERS... : V.
55
enrichment of the mixture with NO yielded by
of the sorbent to 109 C and its conversion from the
moist to air-dry state within 1.5 2 h and further
warming-up to 160 200 C (within 30 40 min, as
suggested by thermophysical calculations) and thermal
explosion.
2
HNO radiolysis cannot be ruled out.
3
However, after washing the irradiated sorbent with
water, its mixtures with HNO ignite at the same tem-
3
peratures as the nonirradiated sorbent. Hence, the heat
resistance of the ion exchanger is strongly affected by
the radiolysis products.
Our previous studies [2 4] showed that exothermic
oxidative processes can start below 160 C, and in the
case of sorbents irradiated to 5 MGy, even below
Thus, studies on thermal degradation dynamics of
1
00 C. The sorbent temperature sensor can give no
the NO form of the VP-1AP anion exchanger and its
3
response because of a large distance separating it from
the warming-up site and low heat conductivity of the
dry anion exchanger. To ensure fire and explosion
safety of the processes utilizing VP-1AP anion ex-
changer, the highest priority in preventive measures
should be assigned to monitoring of the aqueous
phase level above the sorbent.
mixtures with HNO suggest that heating them to
3
temperatures close to 100 C in open vessels in the
presence of the aqueous phase presents no hazard,
provided that the forming gases are removed.
The NO form of the VP-1AP anion exchanger can
3
ignite if two conditions are met simultaneously: the
anion exchanger granules are not separated by the
aqueous phase (drying) and the ignition temperature is
In the presence of sorbed plutonium hexanitrate
complexes, the anion exchanger should get enriched
with the oxidant, thus substantially complicating the
situation. The influence of the sorbed hexanitrate
complexes on the safety of the anion-exchange proc-
esses is beyond the scope of this study, though, in our
opinion, this is a very urgent problem.
attained. In the anion exchanger HNO mixtures, oxi-
3
dative processes occur even without drying, as sug-
gested by evolution of gaseous products, but these
processes acquire the character of a thermal explosion
only when the two above-mentioned conditions are
met.
It is highly probable that these conditions will be
met when the sorbent phase contains radionuclides
characterized by significant heat evolution capacity. In
this case, even without external heating, the anion ex-
ACKNOWLEDGMENTS
This work was financially supported by the US
Department of Energy.
changer HNO mixtures within a certain period could
3
get dried and heated to thermal explosion onset
temperatures, which are not so high. This does not
require drying and warming-up of the entire sorbent
bulk; for ignition and development of exothermic oxi-
dation processes, it is sufficient that a fairly small
layer (or zone) of the sorption mixture get dried.
REFERENCES
1
2
. Milest, F.W., Isot. Radiat. Technol., 1969, vol. 6, no. 4,
pp. 428 440.
. Bartenev, S.A., Zachinyaev, G.M., Nazin, E.R., et al.,
Radiokhimiya, 1998, vol. 40, no. 4, pp. 380 384.
This is exactly the situation dealt with at the
Mayak Production Association, where the plutonium
column was damaged. We suggested in [2] that drying
3. Bartenev, S.A., Zachinyaev, G.M., Nazin, E.R., et al.,
Radiokhimiya, 2002, vol. 44, no. 2, pp. 146 149.
of the upper layer of the column, containing about
1
4
. Bartenev, S.A., Zachinyaev, G.M., Nazin, E.R., et al.,
Radiokhimiya, 2002, vol. 44, no. 4, pp. 150 153.
00 g of 2 Pu, was responsible for the warming-up
38
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