EIR-267.txt

EIR-Bericht Nr. 267

EIR-Bericht Nr. 267

Eidg. Institut fur Reaktorforschung Wirenlingen
Schweiz

Chemical state of sulphur obtained by the 35 ¢ (n,p) 35g
reaction during in pile irradiation

E. lanovici, M. Taube

l |

’_:.I

Wdarenlingen, Dezember 1974
EIR -~ 267

CHEMICAL STATE OF SULPHUR OBTATINED BY THE -°Cl(n,p)°°S

REACTION DURING IN PILE IRRADIATION

E. Ianovieci, M Taube

December, 1974
Abstract

The chemical distribution of 358 produced by the 35Cl(n,p)BBS

nuclear reaction was studied. The chemical forms found after

° and higher oxidation form (soug’ + 8052_)

coincide with Maddock's most recent experiments, the prepon-

solution, 82_, S

derent fraction being 82_. The length of the irradiation time
has an important role on the chemical states of radiosulphur.
An oxldation process concerning 82— was observed as the irra-
diation progressed. The effect of post-irradiation heating
above and below the melting point of NaCl was investigated. By

35

high temperature heating an evolution of volatile S was
observed, After melting the preponderent form remains 82—
though an oxidatlon process occurs. The effect of temperature
irradiation on the sulphur distribution was also examined. At
low temperature irradiation the predominance of 82“ and S° was

observed,
The present work gives the preliminary results obtained on
n-irradiated NaCl, the simplest component of the molten
chlorides fast reactor, which project has been described
recently 1in various papers by Tabe et al.(l’z). As the
chlorides of U=-238 and Pu-239 diluted by NaCl are the selec-
ted components for the fused salt reactor, the n-reactions of

chlorine must be taken into account.

In a relatively high neutron flux the most important nuclear

reactions of natural chlcrine are as follows:

2261 (n,y) 3641 & : ER
17 17 5,1+107y 18
31 (n,y) 2%ca b EL
17 17 37,5 m 18

But much more important from the point of view of the chemical

propertlies of the system, are the following reactions:

2201 (n,p) Py B f$01
17 16 88d

and

2261 (n,a) °°p —B 3%
15  14.3d 16

The description of the "chlorine burn-up" given by Taube(g)

gives a good illustration of all these processes.

During reactor operation an important part of the fission pro-
ducts are gaseous and can be removed continuously, others form

chlorides and remain in the salt, while others will precipitate
(1)

as metals . The excess chlorine produced in the system can

react with the strongest reducing agent present UCl, forming

5
UClq which in the pure form is highly corrosive.

At the same time it can be assumed that UClu will react
rapidly with any short-lived oxidising species produced under

the intense fission fragments irradiation of the salt.

If the calculations about the evolution of chlorine from the

(3,4)

1t 1s not the same situation concer-

35 35,
(4)

melt are optimistic
ning tne sulphur. The (n,p) reaction on Cl will produce
at a mean concentration up to a few thousand ppM in the salt
which depends upon the isotopilc concentration of the chlorine
e.g. separated Cl-37. An interaction between UCl3 and sulphur
is expected to take place. It may be supposed that 358 leads
to the precipitation of U as US. An attack of structural

materials by 358 can also take place.

The chemical state of 35S in neutron irradiated sodium chloride

The aim of this’work 1s to give some information about the
sulphur chemical states formed in the NaCl lattice by

3501 (n,p)BSS nuclear reaction. For this reason we have per-
formed experiments concerning the influence of irradiation
time, and of the post-irradiation high temperature heating on

the chemical sulphur distribution.

The chemical state of radiosulphur obtained by the reaction
5501 (n,p)BBS in the alkali chlorides has been the object of
many studies(B_lz). However, the most recent studies have

proved that the alkall chlorides are systems of an unexpected

complexity. The complexities are coming from the presence of
a large concentration of hydroxide ions normally found in
alkali chloride crystals as a result of the hydrolysis of

the salt(l“’15).
55

In addition, a large sensitivity of the

(11,12,16,17)

recoil S to experimental conditions was observed

Generally the radiochemical method used, involved solution of
the crystals before analysis, usually in an aqueous solvent.
This means that the relation of the crystal precursors to the
products found after solution depends on the reactions of the
crystals species with the solvent or with point defects formed

by i1rradiation (e.g. V centres) during solution.

Recently interesting results concerning sulphur chemical states

have been published(IB’lg). 3SS—

Using different methods of
specles separation and especially non-aqueous medium it was
possible to identify the 82" and s° precursors but not those

(18). It was shown that the oxi-

of the sulphite and sulphate
dising point defects produced in the crystal during the irra-
diation as V centres or derivatives can oxidise the 558 at the
moment of solutlion. The aerial oxidation can also be very
important in agueous or ammoniacal carrier-free systems but

no oxidation was observed in liquid ammonia-cyanide or agqueous
cyanide systems in the presence at least 82_ carrier. The
solution in an acid medium even in the absence of both air

and carriers invariably lead to complete conversion of all

(18)

active sulphur into sulphate Since sulphide ions are
known to be stable in water 1t is concluded that point defects
produced by lrradiation in the crystal can oxidise all sulphate

at the time of solution.
EXPERIMENTAL

Sodium chloride "Merck" reagent was heated for 60 hrs. at 200 °C

in an oven under the vacuum. The dried samples of 100 mg sealed
in evacuated (J_O_Ll torr) quartz tubes were irradiated near the
core of the "Saphir" reactor (swimming pool) for different

12n cm_zsul and 4,3-1012

Reactor irradiations were carried out at about (estimated only)

2 -1

periods at a neutron flux of 5-10 n cm °s

150 °C and -186 °C. After irradiation the samples were 'cooled!

for 8 days to allow the decay of guNa.

The method of 558~Species separation

The crushing of the irradiated ampoule was made in a special
device from which the air was removed by passing a nitrogen
stream containing oxygen of 10 ppM. After crushing, a gentle
stream of nitrogen was allowed to flow for about 10 minutes.
The gases evolved were collected in cooled traps of containing

0,1 NaOH solution.

The irradiated salt was dissolved in 2 m KCN solution containing
carriers of 82_, CNS 8032_, SOH2—' For the dissolution care
was not taken to exclude the oxygen completely although the
nitrogen gas was passed continuously through the system. The
solution from the traps containing the gases evolved in the
system was oxidised with bromine and nitric acid in the presence

of NaQSOu (5 mg in 3) evaporated and the sulphur was precipi-
tated as barium sulphate.

-
5)S~species separation the chemical method described

(18)

For the
recently by M. Kasrai and A.G. Maddock was used. The barium
sulphate precipitates corresponding to each S-species were

separated on the weighed paper disc in a demountable filter.
The dried separated precipitates were weighed and the activity
of the samples was measured under a thin-window Geiger counter.

All measurements were made in duplicate with and without alu-

55 52

minium absorber for discriminating S from P which was

produced by the 3501 (n,a)BBP reaction.

Post irradiation high temperature heating

The sealed irradiated ampoules were heated in an electric oven
at 77000 for 2 hrs. and 83%0°C for about 5 minutes and then
crushed in a closed system under nitrogen stream., The descrip-

tion of the method used by us can be seen in Fig. 1

RESULTS AND DISCUSSION

A comparison of the S-distribution obtained by us and by other
authors 1s gilven in Table 1. As can be seen the results are
practically the same using the cyanide method even if the con-
ditions of dissolution are different. Unfortunately it was not
possible to make a comparison of the irradiation conditions.

The dissolution in vacuo in an alcoholic cyanide solution gave
exactly the same distribution as the analytical method using

the cyanide and carriers. This shows that the carriers do not
alffect the distribution of the active sulphur, on the contrary
theyprevent the oxidation process that disturb the distribution.
The oxidising agents of sulphur can be Cl and Cl, entities which

2
(20). Chlorine atoms are

results in n-irradiated alkall chlorides
able to create a strong oxidlising environment for the sulphur
at the moment of dissolution., Also possible is an interaction in

crystals between sulphur and chlorine with formation of reactive
Fig.

NaCl
~O,l 25

Drying
200°C vacuum
~60 nrs

Quartz
ampoule
sealed

Y

Swimming pool

reactor _

25-1012n cm S
2-99 hrs.

2 -1

£tZ150 ©C

Y

Decaying time

of 2”Na

8 days

] |

vacuum 10"4

No treatment

1

1

Heating, melting

—

Cooling up
to < 100°C

i ]

N | |

N

Crushing of
0 10 pp ampoule

5 min gas-extractign

torr

2
S=yola-
+1l1e

Cold trap
ligquid N

species

after 1o5mip

£ - normal
agueous

solution
2M KCN+Carriers

for SO for Sa- ffor SO

S N

2= 2=
I for SO5

l

1

Radiocactivity measurements Geiger-counter

thin window with and without Al for

discrimination of 2P

Scheme of experimental work

0.1 M NaOH
ageous
Na.SO, as carrier

2
B HNO
r, 3
Table 1 Chemical distribution of

55

S

in n-irradiated NaCl

Sample Solvent Conditicons of Carriers 82— g® SOuz_ + 8032_ References
dissolution ; A %
A liquid vacuum no 63.0 9.4 27.8 18
ammonia
B " air st 61.0 12.2 25.3 "
C agueous solution vacuum no o4.0 12.8 23.2 "
Et-0OH-2 M KCHN
D 4 M KCN air yes 63.0 12.5 24 .4 "
E 2 M KCN air ves c2.3 12.5 25.4 "
F 2 M KCN (as E) N2 ves 64.4 11.9 2%.7 This work 5 -1
®=5-1012n cm S

*Noete sulphite fraction is less than

fraction

5% in our experiments and always lower than sulphate
species which by dissolution give the oxidised form. Dissolution
of the irradiated salt in the presence of a scavenger for Cl or

Cl. should avoid the oxidation. The experiments of Yoshihara(l6)

2
and Maddock(l8)

showed that ethyl alcohcl c¢can reduce but not
eliminate the oxidation. Using this solvent the zero valent
sulphur is lost. It was shown by Maddock that the cyanide
solution was doubly advantageous; to stabilise s° as CNS™ and

to act as a Cl or 012_ scavenger,

Effect of length of irradiation time

In order to find whether the irradiation time has any effect

on the behaviour of the radiosulphur the distribution of sulphur
as a function of the length of irradiation time was studied. The
results are presented in Table 2 and Fig. 2. The irradiation
time was varied between 2 and 99 hrs. As 1s seen in Table 2

S° remains the preponderent fraction independet from the irra-
diation time. This means that in a natural way the preponderent

3501 (n,p)BSS reaction can be 82_.

state of sulphur following
Alternatively it can be supposed that a reduction of sulphur
takes place by capture of electrons arising from the discharge

o' F-centers,.

The results presented in Table 2 show that the sulphur distribu-
tion 1s influenced by the length of irradiation time. Thus the
yield of less than 20 per cent of oxidised forms for 2 hrs. of
irradiation increases to about 30 per cent for a longer time

(99 hrs). The increase of higher oxidation fraction is at the
expense of the sulphide. In the last case the fractlion decreases
from about 70 to 50 per cent at the irradiation time mentioned
above. The fraction corresponding to elementary sulphur, about

10 per cent, seems to be not affected by the length of irradiation

time in the time interval studied by us.
55

Table 2 Chemical distribution of S for different length of irradiation time
S-specles o_ o 5
Number of S s° SO + S0 S-Volatile Conditions of
it . .
parallel fform lrradiation
irradiation runs % % % %
f£ime hrs.
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It 1s remarkable that after 10 hrs. of irradiation the changes in
the chemical distribution of radiosulphur become rapid (Fig. 2).
As is seen the main effect of the irradiation time is the con-
version of part of the sulphide into the higher oxidised fraction.
This may be a consequence of radiation produced defects with oxi-

dising character.

It is also possible that either the concentration of the defects
responsible for the reduction of radiosulphur decreases with the
increase of the irradiation time or they are annihilated when new
traps are formed. The oxidation of radiosulphur with increase of
radiation damage concentration may be also due to the increase in
the positive charge on the sulphur as a result of interaction of
recolls with chlorine atoms. The same behaviour was found in the

case of post gamma irradiation K01(15>.

Even 1f the oxidising
process of radiosulphur can be attributed to the V-centers, the
presence of OH in the crystal must not be neglected. It was shown
that the sensitivity to the oxidation is enhanced by the presence
of OH suggesting that the radiolysis of OH can be responsible
for accelerating the oxidising prooess(gl). It must be added also
that 1in the target oxygen containing the product of radiolysis can
be an oxygen atom which acts as a deep electron trap. The electron
traps can be formed either by gamma radiolysis or be intially

present as crystal defects.

35Cl(n,p)jBS in the alkali chlorides can produce

~(22)

The reaction
sulphur as 82_ as well as S For the oxidation up to zero
valency state 1t is possible to imagine only an electron trans-
fer without many changes in the lattice. The precursors of higher
oxidation form may be S+ as a result of an electron loss from a
neutral speciles. However, the interaction of chlorine entities
2_, SC1

may be an important mechanism in forming the precursors of the

formed by irradiation with s to form specles as SCl, SCL 5

higher oxildation states. These entities in an oxidative hydrolysis

will produce sulphate and sulphite fractions.
13

Effect of post-irradiation heating

The effect of post irradiation high temperature heating (including
melting state) can be seen in Table 3 and 4., A comparison between
heated and unheated samples are made for irradiations of 2 hrs.
Table 3, 12 hrs. and 24 hrs. in Table 4. In Table 3 are also presen-
ted results on samples heated at a temperature below the melting

point of NaCl.

The results presented in Table 3 show that the high temperature
heating has only a slight influence on the 82— state, but 1t
affects the S° and higher oxidised forms (SOHE_ + 8032_). A part
of radiosulphur was found under the volatile form. An escape of
radiosulphur from the crystals particularly at higher temperature
(T >400 O) was mentioned earlier(l7).

The proportion of volatile radiosulphur appears at the expense of
s and higher oxidised forms. The results show that with high
temperature heating above the boiling point of sulphur and above
melting point of NaCl the S° and S and/or 8,01, veceive suffi-
cient kinetic energy to migrate to the surface or even to escape

from the crystal and then collected as volatile radiosulphur.

Effect of irradiation time on post-irradiation melted sample

55

However, there are some differences in changes of S-chemical
distribution on heating below and above melting point of NaCl.

1t seems that for a relatively short time of irradiation (2 hrs)
only the sulphate and sulphite precursors account for the volatile

radiosulphur proportion.

The results presented in Table 4 show a change in the distribu-
tion of chemical forms of 358 for longer time of irradiation before
melting. On melting the s® value decreases up to 2% for a longer
time of 1rradiation and this corresponds to an increase of vola-
tile radiosulphur form. A slight influence of the irradiation

time on the yield of SE— and higher oxidation forms may be also

observed.
55

Table 3 Effect of post-irradiaticn heating on the chemical states of S.
Irradiation time 2 hrs; @ = 5‘1012 noem Cs Tk
EXp. Conditions of Post-irradiation 82_ s° (souz_ + 8032_) S-volatile form
irradiation treatment % A % %
1 ~150 ¢ no 7% .4 10.4 15.8 0.01
vacuum
2
" no T2.7 g.2 18.0 0.01
3 ! 770 °¢C 77.4 5.5 2.0 14,7
2 hrs
4 1 " 73.4 5.1 5.5 16.2
5 n 830 ¢ 78.2 12.0 6.3 3.5
5 min
6 n n 748 10.4 9.4 5.3
. " z 78.6 10.7 4.3 6.4

T

o

*  The radiocactivity of all measured S-contalining fractions was normalised to 100 per cent.
Table 4 Effect of the post-irradiation heating on the chemical states of S
. . . 2- o - 2 .
Exp. Conditions of Post~1rradiation S S (SOLl + SO3 ) S volatile form
irradiation treatment % % % %
1 EEM,B'lOLCn em “sTH no 66.8 12.1 21.0 0.01
12 hrs.
150 °c
vacuum
2 " 1o 68.1 12.0 19.8 0.01
3 " 830 “c 71.5 1.9 18.9 7.6
5 min
it g=5 1012n cm—zs—l no 66.96 11.4 21.6 0.01
24 hrs
150 °¢
vacuum
5 " no 61.9 12.3% 25.7 0.01
£ n 830 °¢ 70.6 2.0 19.7 7.4
5 min

ST
16

However, the present data are not enough to give a definite picture
of these phenomena and we tried to represent in Fig. 3 the in-

55

fluence of irradiation time on the S-chemical distribution in
the post-irradiation melted NaCl. As is seen in Fig. % a slight
oxidation process concerning Sg_ fraction occurs as the irra-
diation progresses. The increase of higher oxidation forms up

to about 10 hrs. is faster than the decrease of 82_ fraction.

It may be supposed that a part of s© passes to the higher oxi-
dation forms. However, a small and practically constant quantity
of 8° is found in the melt for longer irradiation time. The gas
evolution remains also practically constant. The oxidation process
concerning 82— tends to a pseudo-plateau value as the irradiation

progresses. This behaviour induces a pseudo-plateau in the 1ncrease

of the higher oxidation states yield.

It 1s possible that this arises from a significant decrease of
defects with oxidising character in melting process. Comparison
of" these results and those presented in Fig. 2 shows that in the
melted sample the concentration of radiation damages has not the
same effect as in the non-melted sample. The supplementary infor-
matlon about this can be obtained by studying the effect of high
temperature irradiation on the chemical distribution of radio-

sulphur.

Alternatively it 1s possible to suppose that in the melting state
the active oxidising agents have another nature than those in
heating below melting. The presence of oxygen can have a determinant
role 1n radiosulphur oxidation. At the melting point the formation
of sodium oxides and consequently an oxidising environment may be

assumed.
6 7 8910

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Division Unité §

Irradiation time (hrs.)

18

Effect of reactor irradiation temperature on the chemical distri-

55

bution of S-species

The results obtained on the chemical states of radiosulphur in
n-irradiated NaCl at about 423 K and 77 X are shown in Table 5.
The big proportion of active sulphur as 82" fraction for reactor
"normal'" temperature (~150 OC) was confirmed by the last experi-
52P obtained by 53 (23).

ments on the Cl(n,a)BBP reaction

The pre-
ponderent valence form was found to be phosphine. An evidence for

zero valent phosphorus was also obtained.

The low temperature irradiation (Table 5) leads to a distribution
of 558 petween four valence states with the predominance of 82—
and S°. The results obtained by us for low temperature irradiation

agree with those obtained by J. Paptista and N.S. Marques for

KC1l single crystals(IS).

The comparison of results obtained by irradiation at~423 K and

77 K show that the higher oxidation fraction is lower (3 per cent)

for 77 K. It is remarkable that the sulphide fraction is lower

at 77 K than at 42% K. Tt is clear that the increase of 8° form at

low temperature irradiation is done at the expense of both sulphite

+ sulphate and sulfide fractions.

The low yield of higher oxidation forms may be explained by the
fact that an actilivation energy assoclated with some reaction to

f'orm SXCly presursors can prevent such kind of reaction.

The defects with oxidising and reducing character formed by low
temperature irradiation become important factors in determining the
sulphur precursors and thus the chemical distribution by dissolution.
It 1s possible to suppose that the low yield of higher oxidation

states 1s determined by the following reaction:
Table 5 The influence of irradiation temperature on the chemical states

of radiosulphur

Exp. Conditions of Temp. 82— g° SOqC_ + SOEB_ S-volatile form
irradiation
flux-time % % % %

1 7=5°107°n em °s”t 73,4 10,4 15,8 0,01
2 hrs. ~423% K
vacuum

2 " 72,7 9,2 18,0 0,01

_ 1 -2 -

3 ¢=5+10 2n cm 23 L 60,7 32,3 3,0 0,01
2 hrs. 77 K
vacuum

4 i 66,1 30,8 7,1 0,01

61

High temperatur irradiations (T >500 OC} are being looked at.
20

As in the low temperature irradiation as the F center concentra-
tion is higher a bigger fraction of higher oxidation states will
be reduced. It was shown that the thermal stability of F centers
(24)

1s markedly decreased in oxygen containing samples It may
be postulated that the bleaching of F centers leads to formation

of some oxygen centers with an oxidising action for sulphur.

The decrease of 82— fraction can be also explained by the pre-

sence of V centers as follows:

In conclusion the results obtained on the chemical distribution
of radicsulphur by dissolution may be explained by the existence
of crystal entities as 82—, SO, s and/or SXCly and their inter-
action with oxidising and reducing defects formed by irradiation

or 1ntially present in the crystals,
21

LITERATURE
1. M. Taube and J. Ligou, Ann. Nucl, Sci., Eng. 1, 277 (1974)
2. M, Taube, Ann. Nucl., Sci. Eng. 1, 283 (1974)
5. B.H. Harder, G. Long and W.P. Stanaway, Symposium on Repro-
cessing of Nuclear Fuels, 25 Aug. Iowa, p. 405, (1969)
. S. Smith and W.E. Simmons, Winfrith Private communication
5. W.S. Koski, J. Amer. Chem. Soc. 71, 4042 (1949)
6. U. Croatto and A.G. Maddock, Nature 164, 614 (1949)
7. W.S., Koski, J. Chem. Phys. 17, 582 (1949)
6. U. Croatto and A.G. Maddock, J. Chem. Soc. Suppl. 351 (1949)
9. M. Chemla, Compt. rend. 262, 1553, 2424 (1951)
10 T.A. Carlson and W.3., Koski, J. Chem. Phys. 23, 1596 (1955)
11. R.H. Herber, J. Inorg. Nucl. Chem. 16, 361 (1960)
12 R.H. Herber, Chem. Effect of Nuclear Transformations vol.ITI
251  (1961)
15. A.G. Maddock and P.M. Pearson, Proc. Chem. Soec. 275, 1962
14 J. Rolfe, Phys. Rev. Letters, 1, 56 (1958)
15. J.L. Baptista and N.S. Marques, J. Inorg. Nucl. Chem. 36,
1683 (1974)
16 K. Yoshinara, Ting Chia Hwang, E. Ebihara and N. Shibata
Radiochim. Acta 3, 18% (1964)
17. C.Chiotan, M. Szabo, I, Zamfir and T. Costa, J. Inorg. Nucl.
Chem. 30, 1377 (1968)
18. M. Kasrai and A.G. Maddock, J. Chem. Soc. A, 1105 (1970)
19. C.N. Turcanu, Radiochem. Radiocanal. Letters 5/6/287 (1570)
20 J.G. Rabe, R. Rabe and A. Ohlen, J. Phys. Chem. 70, 1068 (1966)
2l. J.H. Growford, Adv. Phys. 17, 93 (1968)
22. F. Fischer and H. Grundig, 7. Phys. 184, 299 (1965)
25. A.G. Maddock and A.J. Mahmood, Inorg. Nucl. Chem. Letters 9,
509 (1973)
24, J.L. Baptista, T. Andersen, Phys. Stat. Soc. (b) 44, 29 (1971)