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. . 12 -2 2 2 73.1 + 0.4 9.8 + 0.8 16.9 + 0.8 0.01 @=5-10""n cm ~150 OC vacuum r~ ' n ~ 12 = 12 2 67.5 + 0.7 12.1 = 0.1 20.4 + 0.6 ¢=4.%3-10""n cm 24 2 64.4 £ 0.5 11.9 + 0.5 235.7 + 2.0 " " 99 1 50.47 15.90 33.62 " " 0T ~t (&N} - (o9 eV . . 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 7 89108 3 4 5 6 7 8910 100 i { ] % 150 S é ; ‘ I | 1 2 3 4 5 6 7 8 91t 2 3 4 5 6 7 83100 6 7 8B g108 3 4 5 6 7 8910 Logar. Teilung | 4 40000 Einheitl o5 0 Ed. Aerni-Leuch, Bern Nr. 526 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. 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