A novel method of non-violent dissolution of sodium metal in a concentrated aqueous solution of Epsom salt

Article abstract of DOI:10.1016/j.jssc.2004.05.030

A new technique of non-violent and fast dissolution of sodium metal in a concentrated aqueous solution of Epsom salt (MgSO₄.7H₂O) at room temperature (RT) has been developed. The dissolution process is mildly exothermic but could be carried out even in a glass beaker in air under swift stirring condition. The reaction products consist of mixed salts of MgSO ₄ and Na₂SO₄ as well as Mg(OH)₂ which are only mildly alkaline and hence are non-corrosive and non-hazardous unlike NaOH. A 50mL solution having Epsom salt concentration of 2M was found to give the optimal composition for disposal of 1g of sodium. Supersaturated (>2.7M), as well as dilute (<1.1M) solutions, however, cause violent reactions and hence should be avoided. Repeated sodium dissolution in Epsom solution produced a solid waste of 4.7g per g of sodium dissolved which is comparable with the waste (4g) produced in 8M NaOH solution. A 1.4M Epsom solution sprayed with a high-pressure jet cleaner at RT in air easily removed the sodium blocked inside a metal pipe made of mild steel. The above jet also dissolved peacefully residual sodium collected on the metal tray after a sodium fire experiment. No sodium fire or explosion was observed during this campaign. The Epsom solution spray effectively neutralized the minor quantity of sodium aerosol produced during this campaign. This novel technique would hence be quite useful for draining sodium from fast breeder reactor components and bulk processing of sodium as well as for sodium fire fighting.

Full text of DOI:10.1016/j.jssc.2004.05.030

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 3460–3468
A novel method of non-violent dissolution of sodium metal in a
concentrated aqueous solution of Epsom salt
A.R. Lakshmanan,* M.V.R. Prasad, D. Ponraju, and H. Krishnan
Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Safety Group, Kalpakkam, Tamil Nadu 603102, India
Available online 11 August 2004
Abstract
A new technique of non-violent and fast dissolution of sodium metal in a concentrated aqueous solution of Epsom salt
MgSO .7H O) at room temperature (RT) has been developed. The dissolution process is mildly exothermic but could be carried out
even in a glass beaker in air under swift stirring condition. The reaction products consist of mixed salts of MgSO and Na SO as
(
4
2
4
2
4
well as Mg(OH)
having Epsom salt concentration of 2 M was found to give the optimal composition for disposal of 1 g of sodium. Supersaturated
42.7 M), as well as dilute (o1.1 M) solutions, however, cause violent reactions and hence should be avoided. Repeated sodium
2
which are only mildly alkaline and hence are non-corrosive and non-hazardous unlike NaOH. A 50 mL solution
(
dissolution in Epsom solution produced a solid waste of 4.7 g per g of sodium dissolved which is comparable with the waste (4 g)
produced in 8 M NaOH solution. A 1.4 M Epsom solution sprayed with a high-pressure jet cleaner at RT in air easily removed the
sodium blocked inside a metal pipe made of mild steel. The above jet also dissolved peacefully residual sodium collected on the metal
tray after a sodium fire experiment. No sodium fire or explosion was observed during this campaign. The Epsom solution spray
effectively neutralized the minor quantity of sodium aerosol produced during this campaign. This novel technique would hence be
quite useful for draining sodium from fast breeder reactor components and bulk processing of sodium as well as for sodium fire
fighting.
r 2004 Elsevier Inc. All rights reserved.
Keywords: Sodium disposal; Epsom salt; Aqueous solution; pH control; Sodium cleanup; Sodium fire fighting; Fast breeder reactors
1
. Introduction
with water invoke exothermic and violent chemical
reactions. Furthermore, the reaction products are highly
corrosive. Hence there is a search for improved
methods.
In fast breeder reactor (FBR), alkali metals such as
sodium serve as heat transfer media in cooling circuits
which transfer thermal energy to heat water or to
produce steam. In the course of their operation, need
arises for draining sodium from reactor components,
bulk processing of unwanted/leaked sodium and for the
removal of sodium from sodium bonded spent fuels, fast
auxiliary systems such as cold traps, filters, relief valves,
etc. There are several techniques presently used mostly
under inert conditions for sodium removal and disposal,
notable among them being dissolution in NaOH
aqueous solution or ethanol, water vapor-nitrogen
process, water vacuum process, cold steam process or
vacuum distillation. However, each technique is known
to have its limitations. Especially, the reaction of sodium
ꢀ
When heated in dry air, sodium melts at 97.8 C and
ignites. A mixture of dense and highly corrosive sodium
monoxide (Na O) and peroxide (Na O ) fumes is
2
2
2
formed by the reaction of sodium with air or oxygen.
Sodium reacts rapidly with water to give NaOH and
hydrogen:
Na þ H2O-NaOH þ 1=2H2 À 35:2 kcal=mol:
ð1Þ
Sufficient heat is liberated by this reaction to melt the
sodium and frequently ignites the evolved hydrogen if
air is present. In an accident of sodium cooled FBR,
sodium fire following the leakage of the primary coolant
can disperse a large quantity of sodium oxide aerosol
2
4
containing radioactive Na and fission products [1].
Effective ways of neutralizing such aerosols are very
*Corresponding author. Fax: +91-4114-280235.
E-mail address: arl@igcar.ernet.in (A.R. Lakshmanan).
0022-4596/$ - see front matter r 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2004.05.030

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A.R. Lakshmanan et al. / Journal of Solid State Chemistry 177 (2004) 3460–3468
3461
much needed for the safety of reactor operators as well
as for the public.
sulfates are reduced to the sulfides by sodium, i.e.,
BaSO4 þ 2Na-BaS þ Na2O2 þ O2:
Experiments with water insoluble salts such as
ð2Þ
In the past, sodium has been disposed off by igniting
it by a fine spray of water and allowing it to burn in a
disposal yard located at a remote place while the
residues were dissolved with a fine spray of water and
the NaOH accumulated neutralized separately [2].
However, such a burning process releases highly
corrosive sodium aerosols into atmosphere. Use of
heavy alcohol like ethyl carbitol is not recommended for
sodium cleanup since explosions in Rapsodie, France in
gypsum, i.e CaSO Á 2H O did really confirm that
4
2
sodium dissolution in them is not feasible. Addition of
sodium into a mixture of gypsum and water resulted in
sodium fire accompanied with aerosol release. However,
the situation changed drastically with water soluble
salts. Among those tried, an aqueous solution of
(NH ) SO was found to dissolve sodium efficiently
4
2
4
1
994 and in FZK research facility in Germany in 1996
resulting in the formation of Na SO . However, a
2 4
during such a campaign indicated that it could decom-
pose to give gaseous products under some conditions
drawback in this case is the intense release of ammonia
gas which is not desirable since sodium is known to react
violently with ammonia. Sodium dissolution in aqueous
solutions of salts like Zn(NO ) and CuCl whose host
[
3,4]. The caustic process which uses a 10 M NaOH
activated aqueous solution is presently used for
destroying large quantities of sodium while decommis-
sioning the FBRs [5]. In this process also sodium
reacts with water but the reaction is made non-violent
due to the presence of concentrated NaOH. However,
to ward off any explosion risk due to hydrogen,
only small quantities of molten sodium are injected
on line at a given time into a strong current of
NaOH solution. The aqueous NaOH content is con-
tinuously adjusted to 10 M while the hydrogen gas is
diluted with nitrogen (acting as a purge gas) and
then released via a stack. The FBRs in Dounreay, UK
also employ a similar process using a 15 M aqueous
solution of sodium and potassium hydroxide for the
disposal of NaK alloy coolant [6]. The NaK feed rate
was a maximum of 18 kg/h while the solution tempera-
3
2
2
metals do not react with water turned very violent at all
salt concentrations. Other salt solutions studied include
NaCl, Na SO , KCl, LiCl, MgCl Á 6H O, CaCl Á 2H O,
2
4
2
2
2
2
CdCl Á 2.5H O, and BaBr Á 2H O. Preliminary studies
2
2
2
2
showed that the dissolution process is not entirely
peaceful in aqueous solutions of KCl, NaCl and BaBr2.
However, CdCl Á 2.5H O was quite effective. On
2
2
sodium dissolution, a black precipitate characteristic
of CdO was observed. CaCl Á 2H O aqueous solution
2
2
which gave rise to Ca(OH) precipitate was also found
2
to be effective. Na SO is another salt in which the
2
4
dissolution was found to be peaceful at high salt
concentrations. However, in this solution, Na SO
2
4
precipitation occurred during sodium dissolution due
to common ion effect. Moreover, the reaction product
consists of NaOH whose alkalinity is high. Despite
Mg(OH) precipitation, MgCl was not as effective as
ꢀ
ture was kept below 85 C. However, such a caustic
process is not suitable for cleanup of sodium in reusable
reactor components as NaOH and KOH are highly
corrosive.
2
2
MgSO . Among the aqueous salt solutions studied,
4
Epsom (MgSO Á 7H O) was found to be very efficient
4
2
Sodium-bonded spent nuclear fuel is distinguished
from other nuclear reactor spent fuel by the presence
of metallic sodium which is a highly chemically
reactive material. When the driver fuel, used mainly in
the center of the reactor core to ‘drive’ and sustain
the fission chain reaction, is irradiated for long
duration, the metallic fuel swells as fission products
are generated until it reaches the cladding wall.
During this process, some metallic sodium enters the
metallic fuel and becomes inseparable from it due to
inter diffusion between the fuel and cladding. Therefore,
mechanical stripping of the driver spent nuclear
fuel cladding is not a practical means of removing
sodium. After careful consideration of various options,
US DOE has decided to treat electro metallurgically the
EBR-II spent nuclear fuel (about 25 metric tons of
heavy metal) [7].
for non-violent dissolution of sodium metal which
results in non-corrosive reaction products. It is also an
inexpensive (50 US $ per metric ton) and eco-friendly
(used as a fertilizer) salt. Moreover, the formation of
Na SO Á MgSO Á 4H O mineral (natural bloedite) is
2
4
4
2
known to occur in nature and the solubility of Epsom in
3
ꢀ
water is quite high (91 g/100 cm at 40 C) [9]. Hence
detailed studies were carried out only with Epsom salt
solution.
The objectives of this work are two-fold. First is the
development of an improved method of non-violent
dissolution of sodium metal in which the reaction
products are non-corrosive and non-hazardous unlike
that of caustic process employing NaOH so that the
same process could be used for large scale sodium
processing as well as for the cleanup of FBR compo-
nents. Second is to develop an inexpensive liquid
hydrant/spray which is effective for sodium fire fighting
and sodium aerosol neutralization since such a devel-
opment would provide a valuable tool in the utility and
safety of sodium cooled FBRs.
In this work, in order to arrive at a non-violent and
eco-friendly method of sodium disposal, a number of
aqueous salt solutions were tried for sodium dissolution.
Literature [8] reports suggest that alkaline earth metal

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3462
Table 1
2
. Experimental
Powder XRD data of water insoluble salt and that of Mg(OH)
reported in JCPDS-file
2
A major complication in sodium disposal is the low
S. no. d-Value (A) Rel. int (%) Mg(OH)
2
density of sodium (d ¼ 0:97) because of which it has a
tendency to float in liquids and once in contact with air,
it burns readily to form dense oxide fumes. But this
difficulty was overcome with vigorous stirring of the
solution. The centrifugal force provided by stirring
prevents the sodium from floating. Once sodium metal is
dropped into the Epsom solution under stirring, it reacts
with water quickly and melts. The molten sodium
readily wets efficiently and reacts fast with the Epsom
salt solution [10]. Sodium dissolution was generally
carried out in a fume hood. A series of experiments with
different concentrations of Epsom salt revealed that in a
concentrated (but slightly under-saturated) aqueous
solution of Epsom salt, sodium dissolves peacefully
without any violent reactions even at room temperature
in air. Both high pure (Ranbaxy make, Laboratory
reagent) as well as fertilizer grade Epsom salts were
used. The rise in solution temperature was continuously
monitored with the help of a chromel–alumel thermo-
couple wire inserted into the solution. An X–Y recorder
was used to plot the temperature of the solution as a
function of time since the sodium drop. The recorder
chart speed was kept at 10 cm/min (X-axis) while the
ordinate showed the temperature measured by the
Exptl [11]
Calculated [12]
Rel. int d(A) Rel. int.
53 4.7400 83.7
d(A)
1
2
3
4
5
6
7
8
9
0
1
2
4.7576
4.5177
4.3852
4.3323
4.1617
3.9865
3.5971
3.4013
3.2667
2.9834
2.9261
2.7925
2.7183
2.5887
2.4232
2.3619
2.0736
1.9391
1.7939
1.5716
1.4946
1.3733
1.3092
58.4
31.6
25.7
16.2
19.4
32.5
12.4
12.5
12.3
14.0
12.8
14.4
16.5
14.7
13.2
100.0
5.8
4.785
1
1
1
13
2.6292 13.7
2.367 100
1
1
1
1
1
4
5
6
7
8
2.3615 99.9
5.3
19
30.7
30.7
10.7
7.6
1.7963 29
1.5723 33
1.4931 13
1.7878 42.5
1.5725 25.1
1.4925 12.2
1.3666 12.0
1.3088 8.0
2
2
2
2
0
1
2
3
8.2
ꢀ
The present results as well as the literature data (experimental as well
as theoretical) were all obtained with CuKa X-rays.
thermocouple (4.1 C/cm). The alkalinity of the solution
1
before and after the sodium dissolution was measured
with the help of a pH meter. The effects of water
content, salt content, quantity of solution as well as the
quantity of sodium on the solution temperature were
studied. To reduce solid waste, the effect of repeated
dissolution of sodium on the same solution was also
studied. However when the mass of sodium was higher
than 5 g, additional air ventilation was found essential to
dilute the hydrogen concentration and to prevent the
fire. A comparison of the above Epsom process with the
caustic process has been made under identical experi-
mental conditions. In order to remove the sodium
blocked inside a metal pipe as well as to dispose of
partially burnt sodium (B30 kg) left out on a metal tray,
a portable high pressure (140 bar) cold water jet cleaner
machine (actually a car washer machine) having a flow
rate of 520 L/h was used to spray the Epsom solution.
concentration range 1.4–2.4 M as long as the Epsom salt
to Na weight ratio is X25. Litmus test confirmed that the
vapor released during dissolution is neither acidic nor
basic indicating the absence of sodium aerosol release.
Gas monitors have, however, confirmed the release of
hydrogen during dissolution. After the completion of
sodium dissolution, the water insoluble salt was separated
by filtering the solution and drying the filtered residue
under an infra-red lamp. The remaining solution was
ꢀ
dried at 100 C in a hot plate so that the soluble salts could
be recovered and weighed. When viewed through optical
transmission microscope, both the powder samples were
found to be well grown transparent crystals (37–210 mm)
but the dissolved salts were relatively smaller in size than
that of the insoluble salts. Litmus test revealed that the
water insoluble salt is alkaline. Powder X-ray diffraction
(XRD) data in Table 1 confirmed that the water insoluble
salt (15% by weight) is indeed Mg(OH) . The dissolved
3
. Results and discussion
2
salts (85% by weight) were found to be complex but
indicative of the mixed salts of Na SO and MgSO .
3
.1. Reaction products
2
4
4
Interestingly, no X-ray diffraction lines corresponding to
NaOH was seen in it. In any case, any NaOH formed due
to Na–H O reaction would be readily converted to
A solution having Epsom salt and water to Na weight
ratios of 25 and 33, respectively (B2 M) was found to
give the optimal composition for sodium disposal. This
will be referred to as standard solution. But the sodium
dissolution was found to proceed peacefully in the solute
2
Mg(OH) due to the following reaction:
2
2
NaOH þ MgSO -MgðOHÞ þ Na2SO4:
ð3Þ
4
2

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3463
3
.2. Quantity of sodium
weight ratio was kept same at 1.32, the water to sodium
ratio was increased four-fold from 16.52 to 66.07. The
total quantity of solution also increased four-fold, i.e.,
from 62 to 236 mL. The quantity of sodium added was
kept nearly same, i.e., 2.48–2.67 g when the total
quantity of solution increased. The rise in solution
temperature decreased from 58 C to 17 C, as expected
since for a constant sodium mass, the energy released
during sodium dissolution remain same, while the
quantity of solution to be heated increased. The time
taken to attain the maximum temperature decreased
gradually from about 9 to 5 s when the solution quantity
decreased from 236 to 62 mL. However, in the latter case
intense fire resulted due to the ignition of hydrogen
released at high solution temperature. Epsom salt to Na
ratio and water to Na ratio in this mixture were 12.5 and
16.5, respectively. The recommended ratios for safe
sodium disposal as those are, however, 25 and 33,
respectively.
Since the Epsom technique is proposed to be scaled
up to dissolve larger quantities of sodium, the rise in
temperature of the standard solution during sodium
dissolution as a function of increasing sodium mass was
studied. The solution quantity was increased (from 131
to 957 mL) in proportion to the mass of sodium (2.78–
ꢀ
ꢀ
2
0 g) dissolved. Results (Figs. 1a–c) show that the
maximum temperature is attained in about 10 s irre-
spective of the sodium mass indicating that sodium
dissolution rate is swift in all cases. While 2.78 g of
sodium increases the solution temperature by 30 C, a
seven-fold increase in sodium, i.e, 20 g sodium dissolu-
ꢀ
ꢀ
tion increases the solution temperature only by 38 C.
The rise in solution temperature does not depend
critically on sodium quantity since the total quantity
of solution used for sodium disposal also increased
proportionately. However, for large quantities of
sodium processing inert gas purging will be essential to
dilute the hydrogen gas released during dissolution.
3.4. Epsom salt content
3
.3. Quantity of solution
The effect of Epsom salt content in the aqueous
solution used for sodium dissolution was studied by
increasing the salt concentration from 1.1 to 2.7 M,
while the sodium and water content were kept more or
less the same. Table 2 shows that the dissolution slows
down a bit with increasing solution volume while the
time taken to reach maximum temperature increases
systematically from about 7 to 14 s as the mass of the
salt increased for nearly constant water quantity. The
The dependence of rise in solution temperature during
sodium dissolution on the total quantity of solution
Epsom salt+H O) was studied. While the water to salt
(
2
ꢀ
ꢀ
rise in solution temperature reduces from 26 C to 19 C
with increase in solution volume. However, the dilute
salt solution gives rise to hydrogen fire despite a
ꢀ
moderate temperature rise (26 C). When the solute
concentrations (1.43 and 1.95 M) were within the limits
recommended for safe sodium disposal, the sodium
dissolution has proceeded peacefully without any fire.
The dissolution slowed down a bit in a slightly super-
saturated solution (2.37 M). However, in a highly
supersaturated salt solution (2.72 M) in the absence of
sufficient quantity of water, sodium dissolution in
Epsom salt was found to be incomplete which resulted
in fire and smoke from the un-dissolved sodium on
exposure to air. Similarly, in highly dilute Epsom
solutions (o1 M), sodium-water reaction turned violent
which should hence be avoided.
3.5. Water content
The effect of water content in the salt solution used
for sodium dissolution was studied by keeping the
quantity of sodium and Epsom salt nearly constant
while the water quantity was increased from 89.2 to
168.8 g. The salt concentrations used in this study were
2.13, 1.57 and 1.25 M of solution. Figs. 2a–c show that
Fig. 1. The dependence of solution temperature versus time since the
addition of sodium on the quantity of sodium dissolved. Sodium,
Epsom and water weights, respectively in grams: 2.78, 69.58, 91.84 (a),
9.25, 231, 305 (b) and 19.99, 500, 660 (c). The molarity of the solutions
(M) are: 2.15 (a), 2.1 (b) and 2.1 (c).

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Table 2
The effect of Epsom salt content in the aqueous solution used for sodium dissolution on the rise in solution temperature as a function of salt
concentration from 1.1 to 2.7 M, while the sodium (B2.6 g) and water content (B150 mL) were kept more or less the same
S. no.
Salt content (g)
Solution quantity
mL)
Molarity (M)
Time taken to reach
maximum
temperature (s)
Rise in temperature
ꢀ
(
( C)
1
2
3
4
47.00
99.30
174
207
228
266
1.09
1.95
2.37
2.72
7
9
26
23
20
19
133.10
178.30
12
14
the rise in temperature is more with decreasing water
content because of the decrease in the total quantity of
solution from 207 (Fig. 2c) to 129 ml (Fig. 2a). With
decreasing water content, the time taken to attain the
maximum temperature decreased little, from about 8
(Fig. 2c) to 5 s (Fig. 2a). The sodium dissolution is,
however, fast and peaceful in all the three cases which
indicate the presence of sufficient quantity of Epsom salt
and water.
3.6. Dissolution in aqueous NaOH solution—
a comparison
Figs. 3a–c deal with the effect of water content in the
NaOH salt solution on the sodium dissolution rate and
rise in temperature. The NaOH salt to sodium ratios
were kept constant at 25 while the water content was
increased more than two-fold, i.e., from 85.13 to
1
83 mL. This corresponds to a solution concentration
range of 14–8 M, respectively. Due to the relatively low
molecular weight of NaOH (40) in comparison to
Epsom (246.47), the molar concentrations of the solute
in NaOH solution are nearly an order of magnitude
higher than those used in the Epsom solution. In
general, the quantity of NaOH produced from so-
dium-water reaction, via Eq. (1) is negligible when
compared to the NaOH present already in the solution.
For example, in Fig. 3a, the latter is 14.4 times larger
than the former. Figs. 3a–c show that the rate of sodium
dissolution in the NaOH process decreases drastically
with decreasing water content since the water that is
available for reacting with sodium is not only relatively
Fig. 2. The dependence of solution temperature versus time since the
addition of sodium on the water content in Epsom solution. Sodium,
Epsom and water weights, respectively in grams: 2.70, 67.6, 89.2 (a),
2.53, 63.3, 126.5 (b) and 2.56, 64, 168.8 (c). The molarity of the
solutions (M) are: 2.13 (a), 1.57 (b) and 1.25 (c).
À
less but it is also surrounded by more OH ions.
ꢀ
However, the rise in solution temperature (19–21 C) is
more or less independent of water content since the
sodium dissolution rate decreases drastically with
decreasing water content and hence the energy due to
sodium-water exothermic reaction is released relatively
slowly when water content is less. Therefore, the time
taken to heat the solution to its maximum temperature
increases drastically with decreasing water content (from
decreased solution quantity. In contrast, in the Epsom
solution, as seen earlier in Fig. 2, the sodium dissolution
proceeds fast irrespective of the solute concentration. In
fact unlike the case with NaOH when the quantity of
Epsom solution is relatively less, i.e., with decreasing
water content the sodium dissolution rate proceeds a
little fast and hence the time taken to reach the
maximum temperature is reduced (8–6 s) while the rise
in temperature is relatively high.
8
to 80 s as one goes from Figs. 3a–c) which increases
heat dissipation losses. Thus the maximum temperature
reached in Fig. 3a is less than what is expected with

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3465
ꢀ
ꢀ
the two processes is minimal (21 C and 19 C, respec-
tively).
3.7. Sodium solubility limits in Epsom and NaOH
solutions
In Epsom solution, for a given water content
B100 mL), the amount of sodium that could be
(
dissolved peacefully increased from 6 to 10.5 g when
the salt content was increased from 1.4 to 1.9 M. The
above solubility limits were arrived at on the basis of
observation of sodium fire initiated by sodium-air
reaction when further sodium was added. In contrast
in the NaOH solution, for a given water content
(
B120 mL), the sodium solubility has been found to
decrease from about 30 to 15 g when the salt content was
increased from 8 to 12 M as expected from the reduced
availability as well as reduced thermodynamic activity of
water at such high salt concentrations. The sodium
solubility limit in this case was arrived at on the basis of
NaOH salt reaching its solubility limit in water, i.e.,
further sodium addition resulted in NaOH precipitation.
Another notable difference between the two processes is
seen in their super-saturated solutions. While sodium
addition in super-saturated Epsom solution results in
sodium fire and aerosol release, super-saturated NaOH
solution was found to dissolve sodium peacefully though
very slowly.
Fig. 3. The dependence of solution temperature versus time since the
addition of sodium on the water content in NaOH solution. Na,
NaOH and water weights, respectively in grams: 2.58, 64.49, 85.13 (a),
2.88, 72.16, 144.3 (b) and 2.77, 69.32, 183 (c). The molarity of the
solutions (M) are: 14 (a), 10.1 (b) and 8.1 (c).
Sodium is a highly reactive metal. On exposure to
moist air, a film of sodium monoxide (Na O) is formed
2
around it, which reacts first with water exothermically to
produce NaOH. In pure water, sodium was found to
explode within 3 s which indicates that the Na O outer
2
layer is removed instantly in pure water under stirring
condition whereas in NaOH solution it is possibly
getting removed slowly. However, even with pure
sodium metal obtained after the removal of the sodium
oxide surface layer, the dissolution was found to
proceed relatively slowly in concentrated (14 M) sodium
hydroxide solution when compared to 8 M solution.
This confirms that the major reason for slowing down of
sodium-water reaction in 14 M solution is the presence
of excess of Na and OH ions which form a sort of
envelop around the water molecules since they are
highly polar. In fact, studies with different salt solutions
have shown that the slowing down of the sodium
dissolution rate with increased salt content is observed
only in the case of NaOH and possibly in KOH
solutions. It is one parameter which is indicative of the
free ion character of these two salt solutions which
distinguishes them from nearly all other salt solutions.
A comparison of Figs. 2 and 3 indicate that for
sodium dissolution carried out under nearly identical
conditions, the rise in temperature in the Epsom process
3.8. Repeated addition of sodium and production of solid
waste
For radioactive sodium disposal, it is necessary to
reduce the quantity of waste generated. This means it is
necessary to find ways to reduce the presently used
Epsom salt solution quantity per unit quantity of
sodium dissolved (B50 mL/g). However, we have seen
earlier that since the dissolution process is quite fast, a
reduction in the quantity of Epsom solution by 50%
boosts the rise in temperature nearly by a factor of 2
+
À
ꢀ
(i.e., 29–58 C) which results in the ignition of hydrogen
in air. A way to overcome this problem is to add sodium
repeatedly in the standard Epsom salt solution so that
the rise in solution temperature is within a limit. After
each dissolution, the beaker containing the standard
solution was cooled to RT quickly by immersing it in an
ice bath before the commencement of the next dissolu-
tion. The standard Epsom solution produced a solid
waste of about 7.8 g/g of sodium dissolved. This figure
was obtained by dissolving sodium repeatedly 7 times in
the same solution and evaporating the water. Eighth
addition resulted in sodium fire thereby indicating that
the safe solubility limit has been crossed. The decreased
molarity of Epsom solution on sodium dissolution as a
result of the conversion of MgSO to Mg(OH) and
ꢀ ꢀ ꢀ
(
34 C, 25 C or 21 C) is slightly higher than those
ꢀ
ꢀ
ꢀ
obtained in NaOH process (21 C, 21 C or 19 C). This
again is a consequence of the difference in reaction rate
in the two processes. When the sodium dissolution
proceeds fast in both dissolution processes, e.g., Figs. 2c
and 3c, the difference in rise in temperature observed in
4
2
Na SO is responsible for the above solubility limit.
2
4

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3466
In 2 M Epsom solution, the stirrer speed had to be
be removed only with subsequent steps of reaction with
steam and washing with water [13]. But for active and
large components this process has been reported to give
rise to problems of inflammability of alcohol and the
issue of organic active waste management. Another
process used for sodium cleaning is steam or dispersed
increased on repeated sodium dissolution due to the
increased viscosity of the solution as a result of the
formation of Mg(OH) which is milky in nature. This
2
difficulty was overcome by employing a larger quantity
of dilute Epsom solution. For example 700 mL of 1.2 M
solution produced the same solid waste per unit weight
of sodium dissolved as 160 mL of 2 M Epsom solution.
However, the viscosity of the dilute solution did not
pose a problem during repeated sodium dissolution. Re-
water in inert gas (nitrogen or CO ). While nitrogen gas
2
involves a risk of caustic corrosion, CO transforms
2
NaOH into a non-corrosive carbonate, which can also
limit the efficiency and may stop the reaction by creating
a protective layer of carbonate between the sodium and
water [14].
conversion of Mg(OH) product obtained after sodium
2
dissolution to MgSO using a little amount of H SO
4
4
2
(
3 mL/4 g of sodium) pushed up the sodium solubility
The real advantage of the Epsom process, namely the
pH control would be fully realized in sodium cleanup.
This process could easily replace the presently used
organic cleaning. When the standard Epsom salt
solution was poured into a 3-necked cylindrical glass
flask containing 2 g of sodium in air, fire and smoke
were generated due to floating sodium coming in contact
with air. However, when the experiment was repeated
with argon purge gas, the sodium got dissolved peace-
fully without fire or smoke in about 3 min, the time
taken to pour the solution into the flask. The quantity of
Epsom salt solution needed for sodium dissolution used
in this case was approximately twice that used in sodium
disposal described earlier. Argon purging causes the
solution to bump which is akin to the rotation process
adopted for sodium dissolution and hence the sodium
limit by at least 50% which brings down the solid waste
produced to 4.7 g per 1 g of sodium dissolved. In 8 M of
NaOH solution, sodium was dissolved similarly. The
solid waste produced in this case was 4 g of NaOH/1 g of
Na which compares reasonably well with the figure
obtained with Epsom process. Stirring speed was not
found to be critical in this case as the solution viscosity
did not increase significantly unlike that in Epsom
solution. Therefore from radioactive solid waste con-
siderations, the two processes are comparable. However,
NaOH has to be converted to NaCl or Na CO before
2
3
storage/disposal while in the Epsom process the waste
could be stored/disposed off directly. So Epsom process
could be a favorable option in the disposal of radio-
active as well as non-radioactive sodium.
dissolved swiftly. Mg(OH) and MgO suspensions could
2
3.9. Preliminary studies on sodium cleanup and sodium
disposal
be dissolved easily with 0.1 N HNO . Therefore,
3
subsequent to sodium cleaning, washing with water/
steam and then with 0.1 N HNO are recommended for
3
Unlike the disposal studies discussed above, the
removing any undissolved salts sticking with reactor
components. As the above preliminary result is highly
encouraging, it is proposed to repeat this experiment
simulating the actual cleanup situations described
above. The salient features of the NaOH and Epsom
processes are summarized in Table 3.
cleanup experiments are more delicate and ideal condi-
tions such as stirring are not feasible. For example, in
one case study of sodium removal from irradiated core
sub-assemblies, ethanol was pumped from one tank into
another tank through the core-assembly using com-
pressed nitrogen during which the ethanol cleans only
the outer surface of the sub-assembly since sodium
reacts only gently with alcohol, liberating hydrogen
through the following reaction:
A 1.3 M Epsom solution sprayed with a high-pressure
jet cleaner at RT in air in a open yard easily removed the
sodium (B1 kg) blocked inside a mild steel metal pipe
0
0
00
(length B24 and diameter B2 ) in 5 min. In this
campaign, a dilute solution (1.3 M) was found to be
more effective than the standard (2 M) solution for
sodium dissolution and its removal as well as for
containing the sodium aerosol release. Interestingly,
the above sodium blockage could not be removed
despite prolonged heating with an oxy-acetylene torch
accompanied by repeated hammering since sodium
probably formed inter-metallic compounds with mild
2
Na þ 2C2H5OH-2C2H5ONa þ H2:
For each such cycle it took about 15 min and a total
ð4Þ
of 5–6 cycles of cleaning were found essential to remove
the sodium completely. In one such experiment, about
0 L of ethanol was used to remove about 100 g of
8
sodium from the sub-assembly. When compared to this,
the Epsom salt solution needed for 100 g of sodium
dissolution (5 L) is quite less. For sodium removal from
fuel sub-assembly, a similar cleaning procedure has been
followed as steam cleaning could not be adopted since
the fuel was pyrophoric in nature. At the end of the
process, all sodium and sodium hydroxide were removed
leaving behind oxides and other impurities which could
ꢀ
steel. The solution when sprayed at a 45 inclination was
found to be effective in sodium removal as it provided
an easy pathway for the hydrogen released. When the
solution was sprayed directly into the pipe, the hydrogen
trapped inside could not find an easy pathway. As a
result, the hydrogen gas pressure has built-up inside the

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3467
pipe and after a while the entire quantity of sodium
trapped inside the metal pipe was thrown out suddenly
which resulted in hydrogen ignition as well as sodium
aerosol release since a large quantity of molten sodium
suddenly came in contact with the atmosphere which
was beyond the capacity to cope-up with a single
solution jet. However, the above solution jet dissolved
peacefully residual sodium (B50 kg) collected on a
metal tray after a sodium fire experiment within a few
minutes. The solution spray effectively acted as an
efficient neutralizing agent to the sodium aerosol
produced during this operation. This novel technique
would hence be quite useful for draining sodium from
sodium metal cooled fast breeder reactor components,
bulk processing of sodium as well as for sodium fire
fighting. The only precaution to be taken in this
campaign in a closed environment such as reactor
containment building is to provide a pathway for the
hydrogen released during sodium dissolution. This
could be easily achieved by controlling the solution
spraying rate as well as by providing sufficient ventila-
tion to reduce the hydrogen concentration in air. In the
high-pressure car washing jet used in this experiment,
the air accompanying the solution jet effectively diluted
the hydrogen released during sodium dissolution there-
by preventing hydrogen ignition or explosion.
4. Conclusions
A non-violent and eco-friendly method of sodium
processing with potential applications in the course of
sodium cooled fast breeder reactor operations as well as
during their decommissioning has just been developed.
While 2 M Epsom solution is recommended for peaceful
sodium dissolution, dilute (o1 M) and super-saturated
(
42.7 M) solutions can cause violent reactions and
hence must be avoided. The solid waste produced in the
Epsom and NaOH processes are comparable provided
the Mg(OH) produced on sodium dissolution in Epsom
2
is converted back to MgSO with H SO . The waste
4
2
4
produced being only mildly alkaline do not cause skin
burns and can be disposed off/stored without any
further processing unlike the case in caustic process. A
1
.3 M Epsom solution sprayed with a high-pressure jet
cleaner in a open yard easily removed the sodium
B1 kg) blocked inside a mild steel metal pipe. The
(
above solution jet also dissolved peacefully residual
sodium (B50 kg) collected on a metal tray after a
sodium fire experiment within a few minutes. The real
advantage of the Epsom process, namely the pH control
would be fully realized in the sodium cleanup of reusable
reactor components. This is much simpler, safer and envi-
ronmental friendly technique than the presently used
sodium clean up techniques such as alcohol/steam or
dispersed water in inert gas or the electrometallurgical

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A.R. Lakshmanan et al. / Journal of Solid State Chemistry 177 (2004) 3460–3468
3468
technique recommended in the case of sodium-bonded
spent nuclear fuel.
research facility, same as in [2], Session 3 on ‘Cleaning with
Alcohol’.
5] E. de Magny, M. Berte, Fast reactors bulk sodium coolant
[
disposal NOAH process application, same as in [2], Session 4 on
‘Sodium Bulk Removal’.
Acknowledgments
[
[
6] A.W. McIntyre, PFR liquid metals disposal at Dounreay, same as
in [2] Session 4 on ‘Sodium Bulk Removal’.
7] W.D. Magwood IV, Record of decision for the treatment and
management of sodium-bonded spent nuclear fuel, US DOE
Report, Federal Register Notices/Vol. 65, No. 182, September 19,
The authors are thankful to Sri S.E. Kannan, Head,
Engineering Safety Division, IGCAR for support
rendered during sodium disposal work.
2
000, pp. 56565–56570.
[
[
8] M. Sittig, Sodium: its Manufacture, Properties and Uses,
Reinhold Pub Corp, New York, Chapman & Hall, Ltd., London,
1
956.
References
9] R.C. Weast, Handbook of Physics and Chemistry, CRC Press,
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[
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[
10] C.H. Lemke, Encyclopedia of Chemical technology, in: Kirk-
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2] K.K. Rajan, K. Gurumoorthy, M. Rajan, R.D. Kale, Sodium
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[
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[
[
3] P. Marmonier, R. Del Negro Cea, Information about the accident
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[
14] C. Latge, F. Masse, Recommendations (Session 8), same
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