Chemical effects in fission product recoil

J.Inorgani¢ and Nuclear Chemistry, 1955, Vol. 1, pp. 149-158. Pergamon Press Ltd., London

CHEMICAL EFFECTS IN FISSION PRODUCT RECOIL By G. N. WALTON a n d . I . F . CROALL Atomic Energy Research Establishment, Harwell, Didcot, Berks (Received November 1954)

Abstract--Some measurements of 1131arising after fission recoil from uranium oxide particles into a surrounding medium of solid potassium iodate show that: 1. The fraction of recoils stopping in the iodate phase increases as the relative amount of iodate to oxide increases. 2. The amount of chemical decomposition in the iodate phase is proportional to the number of fission recoils stopping in the iodate phase. The "G" factor under the conditions in question lies between 2 and 6 molecules of iodate reduced per 100 eV of recoil energy absorbed. 3. The specific activity of the reduced forms of iodine (I2, I-) remains fairly constant, whatever the extent of fission, and is always higher than the specific activity of the oxidized form 003-). Approximately 90 per cent of the I T M is retained in the oxidized form. INTRODUCTION SOME studies have been reported on the valency state of elements found after fission (STANLEY and DAVIES, 1951; BURGUS and DAVIES, 1951), but the only observation that appears to have been published on the chemical effect of fission recoil itself, is that large amounts of residual organic matter are decomposed in graphite into which fission fragments recoil (SANGSTER and WRIGHT, 1952). The chemical state of elements formed in fission might be expected to be linked with the radiation decomposition of the medium in which they are absorbed, i.e., the process may be considered as a reaction between the fission product and its surroundings. In default of information by which preliminary experiments could be planned to illustrate this, a system was chosen in which a wide variety of observations could be made by fairly simple and rapid techniques. It was decided to study the fate of fission-product iodine produced after recoil into potassium iodate. The exchange behaviour of iodine amongst its various valency states is known (BuRGUS and DAVlES, 1951; MYERS and KENNEDY, 1950), and its active forms are readily purified and characterized. Uranium oxide, as a source of fission products, was ground with potassium iodate to form a fine powder and irradiated in BEPO for about twelve hours. The resulting mixture of fission products and iodate decomposition products was then extracted with carbon tetrachloride and water. The activity of the iodine occurring as molecular iodine, as iodide, and as unchanged iodate, was measured. In the initial experiments attempts were made to study the proportions of the different short-lived iodine isotopes, but, as pointed out by BURGUS and DAVIES(1951), these grow at different times from their parent telluriums, and cannot readily be compared with each other. In subsequent irradiations the irradiation tubes were therefore not opened until only 8-day 1131 remained of the active isotopes. The only variables in the different experiments were the amounts of potassium iodate and uranium oxide used, and the pile flux. 149 lO

150

G . N . WALTON and I. F. CROALL EXPERIMENTAL

A.R.-grade black uranium oxide and potassium iodate, kept dry in silica gel desiccators, were weighed out and ground up together in a small mortar to a freelyrunning uniform grey powder. The irradiation vessels consisted of silica tubes 15 cm long by 2 cm diameter, with stoppers ground to the external surface so that grease was isolated from the contents.of the tube. The vessels were irradiated inside 10-in. aluminium cans in vertical BEPO experimental holes in a flux of the order 1.6 × 101Zn/cm2/sec. After cooling for three weeks the vessels were unstoppered and immediately closed and inverted behind shielding in the apparatus shown in Fig. 1,

et

rradiation tube

Air ~- Solvents

Sinter"

Suction

CCl 4 trap

FIG. l.--Extraction apparatus.

by means of which the contents could be extracted by solvents without further exposure. Free condensed iodine was visible on the walls of the unopened tubes in which the fission rate had been high, and this was extracted with carbon tetrachloride before the admission of water. On wetting the iodate there was considerable effervescence similar to that observed in the radiation decomposition of nitrates and chlorates (HEANIG et al., 1953). The uranium oxide sludge collected on the sintered glass disc, where it was stirred by the suction of air through the sinter, and the solvents containing the iodate and its decomposition products were run off through the filter. The carbon tetrachloride traps, cooled to 0°C, at which the vapour pressure of iodine is very low, were used to detect any iodine activity carried over. In all cases this was negligible, and the solvents were eventually added to the main fractions. Dissolution was continued until no remaining iodate could be detected in the washings. The carbon tetrachloride layer was separated, washed, and made up to 100 ml, and the aqueous layer to 250 ml.

Chemical effects in fission product recoil

151

Estimation of Free Iodine 20 ml of the carbon tetrachloride containing the free iodine were titrated direct with aqueous sodium thiosulphate, using acid potassium iodide (iodine-free), and starch indicator (STONE, 1954). A further aliquot was radiochemically purified by repeatedly extracting the iodine with sulphur dioxide into water and back into carbon tetrachloride with nitrous acid (CAMPBELLand BRADY,1951). After three cycles in which care was taken not to allow the interfaces between solvent and aqueous layers to be carried through, the activity of the iodine showed only an 8-day decay to less than 1 per cent of the initial count. After counting, the iodine was again titrated so as to give the chemical yield of the purification process. The counters used were halogen-filled liquid counters, type M6H CF, made by 20th Century Electronics Ltd. They were calibrated by converting the iodine to silver iodide, which was mounted as a thin film in a 4-7r counter. The counting rates of the liquid samples were corrected for background, chemical yield, and decay from the time of removal of the sample from the pile. Estimation of Iodide The aqueous iodate solution was acidified to release iodine equivalent to that present as iodide or as other reduced forms. This was again extracted with carbon tetrachloride, titrated and counted as described above. Care was taken that the organic layer was washed completely free of traces of iodate. The activity of the original iodide was estimated, knowing (PERLMANet al., 1941) that one-sixth of the iodine measured came from the original iodate according to the equation: 5I- --k IOa- q- 6H + -+ 3I~ q- 3H20. As the specific activity of the iodate was relatively low, this involved only a small correction. The rate of exchange of iodide and free iodine with iodate in solution is mainly dependent on the hydrogen-ion concentration (MYERS and KENNEDY, 1950), and as this was low there would be little opportunity for isotopic exchange. Exchange, however, could occur between iodide and some of the iodine, because not all the latter was extracted with carbon tetrachloride before dissolution of the iodate and iodide in water.

Estimation of lodate 2 ml of the iodate solution were diluted and reduced to the iodide with sulphur dioxide gas. Excess gas was boiled out and the iodine extracted into carbon tetrachloride with nitrous acid. This was titrated and counted as before. Any periodate present was estimated as iodate. The correction involved in the release of iodine in the original acidification was negligible.

Table 1 shows the results. the same pile position for the same neutron dose. In blank the absence of uranium oxide

RESULTS In each batch the tubes were irradiated together in same time and therefore received approximately the runs, potassium iodate irradiated for twelve hours in showed no detectable decomposition by the methods

152

G . N . WALTON and I. F. CROALL TABLE I .--RESULTS

Original contents of tube

Batch

Wt. of U308

Wt. of KI03

(g)

Products in soluble phase I as 12 Wt.

(mg)

Activity dpm × 105

I as I-

Wt. (mg)

I as IO3-

Activity dpm × 10~

Wt. (g)

Iodine

recovery (%)

1'021

2 0"2 0"8 1'8 3"2

6"75 11 0"81 0'9 2"54 3"5 4"58 11-6 11'3 i 28

0"406 1"996

3'975 4.003

3.3 6-4

17.6 40.3

71.2 275

32.7 117

2-280 2-061

15.7 71.5

99'8 98"4

3

0"349 1"070

2.987 3.518

3-8 3"5

9.71 I 56'5 20'8 168

24-5 65"3

1.670 1'892

17-0 37-6

97-6 98-9

4

1-004 1'002

3"747* 3'743

2.2 8.2

14"0 57"1

27'3 43.5

2.085 2.050

23.8 27-0

98"6 97.7

5

0'519 3"989

8.875 0"505

3.6 5"9

20.6 80

88 59'5

22.6 75"2

5.080 0"217

26-5 19"1

98.3 94"4

6

0'463 0-462

4.089 4"086

9"6 12'8

7"9 8-6

80 84

24-4 25-6

2-341 2"365

23.2 23"5

100-3 101 "6

0"285 0"019 0'067 0"273 0"989

3"940 3"963 2"317 1"354

2

102 111

3'14 2'295 0"47 2'285 0"64 , 1"352 2-03 0"786 3"29 0'552

Activity dpm × 107 3'33 0"47 0'96 3"25 7"00

98"6 97"3 98"6 99"2 96.2

* Components not ground together. described, n o r was a n y I131 activity detected a b o v e b a c k g r o u n d . Mixtures o f p o t a s s i u m i o d a t e a n d u r a n i u m oxide m a i n t a i n e d at 100°C for twelve hours a n d extracted w i t h o u t irradiation, similarly showed no d e c o m p o s i t i o n . A t t e m p t s were m a d e in the initial experiments to devise a m e t h o d o f d r a w i n g an iodine activity balance a n a l o g o u s to the mass balance shown in the last c o l u m n , o f Table 1. Owing to the a b s o r p t i o n o f neutrons in the varying mass o f the p o t a s s i u m iodate, the activity o f c o b a l t foils simultaneously i r r a d i a t e d with the samples d i d n o t give a true measure o f the total a m o u n t o f fission. Analyses for a n o t h e r fission product, Ba 14°, were also carried out, b u t the b a r i u m activity extracted with the aqueous solution o f iodate was relatively less t h a n that c o r r e s p o n d i n g to the 1131, after allowing for the different fission yields. O n the s u p p o s i t i o n that insoluble b a r i u m iodate r e m a i n e d a d s o r b e d on the u r a n i u m oxide, the latter was washed with 0.01N nitric acid. This dissolved off the Ba 14° in a m o u n t s c o r r e s p o n d i n g to the I a31 f o u n d in the iodate, b u t a small a m o u n t o f u r a n i u m was also dissolved, a n d no precision was possible. I n t e r p r e t a t i o n o f the results therefore rests on the s u p p o s i t i o n that all the 1231 that was released f r o m the u r a n i u m oxide exchanged with the various forms o f inactive iodine that were analyzed. DISCUSSION

The Release of Fission Fragments I n a n y given b a t c h there is no p r o p o r t i o n a l i t y between the total a m o u n t o f 1131 f o u n d outside the u r a n i u m oxide a n d the a m o u n t o f u r a n i u m . Table 2 shows qualitatively that for each batch the a m o u n t escaping f r o m the oxide increases with

Chemical effects in fission product recoil

153

increasing concentration of iodate phase, i.e., the more potassium iodate present, the more is a fission fragment likely to come to rest in it. Similar observations have been made by JORDAN for rip-recoil effects in irradiated pastes (JORDAN, 1951). The stopping power of a medium for the major portion of the range of fission fragments is proportional to the electron density (BOHR, 1941), and column 2 of Table 2 is TABLE 2 . ~ F R A C T I O N OF RECOILS STOPPING IN IODATE PHASE

Batch

Concentration of potassium iodate in total contents of tube (corrected for electron density)

Atoms of I TM recoiling into iodate phase per gram of oxide phase

4"6 × 10TM

99% 94 86.5 69.7 32'4

2-6 2.2 2.2 1'3

2

82.0 48"2

8'0 7.0

3

79-8 60.4

9-9 7-0

4

63.4* 63.4

4'5 5 '4

5

88.5 5-5

9'3 1.2

6

80'4 80-4

9'3 9-4

* Components not ground together.

therefore expressed in electron density units. If m I is the mass, and Z 1 the average atomic number per unit mass of component 1, then the electron concentration C of component 1 in a mixture of components 1 and 2 is given by: C--

mtZ1 m l Z 1 -~- m2Z 2

The average atomic number per unit mass, Z, of a compound is given by: nAZ A

z=Y

M

where n is the number of atoms of atomic weight A and atomic number Z A in a molecule of molecular weight M. Chemical Effect of Fission Recoils In Fig. 2 the total number of reduced iodine atoms found in each tube (Ie @ I-) is plotted against the total number of I T M atoms found outside the oxide phase, to show an approximately linear relationship over a wide range. This holds for very different conditions. For instance, in Batch 4 the iodate phase in the tube in which

154

G.N.

WALTON and I. F. CROALL

the components were ground together had more activity, and also showed correspondingly more decomposition than the tube in which the same weights of components were merely mixed. In the two tubes of Batch 5 the ratio of uranium to iodate varied by a factor of 134, and yet the decomposition per fission fragment only differs by a factor 1.3, being low for the tube containing most uranium. From Fig. 2 the number of atoms of reduced iodine formed per atom of 1lal is 1.0 × 108. This enables an estimate to be made of the " G " factor for iodate decomposition by fission-fragment energy, assuming one iodate ion decomposes for every reduced iodine atom found. The range in air (KATCOFF et al., 1948), and in aluminium (SUZOR, 1948; FINKLE et al., 1951), for fission fragments shows extensive straggling according to the different masses, but mass number 131, being near the

,/

IO

E o

I@

o/

C

h5 o

16

o BatrJn

0/ /

OIJ

E£

8

~ L ld' .~7

I

X

n

A

"

4

v a

" •

5 6

-

10 1011 1012 10 '3 ' 11 74 Fission recoils, Atoms of I m in Iodof,e phase FIG. 2.--Iodate decomposition and iodine activity.

middle of the fission-product series, may be taken as representative of the others as an approximation. The fission yield of mass number 131 is 3.0 per cent (YAFF~ et al., 1953), and the molecular decomposition of iodate per fission is therefore 3 × 106 molecules/fission. The mean range of fission recoils in uranium metal has been reported to be 4.3 microns (OZEROFF, 1949). The uranium oxide used in these experiments consisted of irregular masses 10 to 50 microns in diameter, and there is therefore much uncertainty in considering how the recoil energy is distributed between the oxide and iodate phases. However, this may be calculated for limiting cases. According to the Bong theory (BoHR, 1941), the energy of fission recoils is lost over the major part of the range by ionization, and only at the end of the range by nuclear collisions. For the first part of the track the ionization per unit-length of track d E / d R is linearly related to the residual range R. For the purposes of our calculation a satisfactory approximation is dE/dR = 2aR

where a is a constant, whence E ~ aR 2

(1)

Chemical effectsin fission product recoil

155

Using this approximation, the energy of monoenergetic particles escaping from a flat uranium oxide surface in all directions from the interior (neglecting scattering) is distributed in such a way that one-third is absorbed in the surroundings (see Appendix). If the exponent of R in equation 1 is less than 2, as suggested by the work on tracks in photographic emulsions by MATHIEU and DEmERS (1953), this fraction is greater. For recoil atoms escaping from the convex surfaces of small particles the fraction would also be greater, but cannot be greater than unity. If each fission which produces fragments coming to rest in the iodate phase releases 162 MeV of kinetic energy, one-third of which is absorbed in the iodate to decompose 3 × l0 s molecules, the " G " factor would be 5.6 molecules per 100 eV absorbed. If all the 162 MeV is absorbed in the iodate phase, the " G " factor is 1.9 molecules/ 100 eV, and the true value will be expected to lie between these extremes.

lo' +

+

+.,'//

÷

-.

E

+

-.

X

X

10 E

i+ , / ~+~cific .d"

activity of molecular iodine

x

. . . . . .

o

,,

io+I,/

i

10 ~

10"

,,

iodine as i o d i d e . . . . . .

~ I0 'i

icx::Iote

L 10 '3

Relative fission.Atoms of Imper gram of 14~ FIG. 3.--Relative specific activity.

" G " values for the decomposition of iodate by photons or light particles do not appear to have been measured. The value for solid potassium chlorate decomposition by pile-neutron irradiation has been reported to lie between 2.0 to 3-0 molecules O2/100 eV (HEANIGet al., 1953).

The Specific Activity of the Iodine If the 1lm were equally distributed throughout all the inactive iodine in the tube, the specific activity of each iodine species would be the same. In Fig. 3 the specific activity observed for each species is plotted against that expected if such complete exchange had taken place. This shows that while the specific activity of the iodine in the iodate varies over a wide range (according to the relative amounts of I TM and inactive iodine present), the specific activities of the molecular iodine and iodide remain within a relatively small range. Similar behaviour was observed by BOLD (1952) in his experiments on the neutron irradiation of solid potassium bromate,

156

G.N. WALTONand I. F. CROALL

where the rate of formation of decomposition products was proportional to the rate of radiobromine formation. Fission product 113~ is the daughter of Te 1'~1. STANLEYand DAVIES (1951) show that all tellurium formed in fission appears as TeO~ or TeO~ after dissolution in water, and these might be expected to be the forms in an iodate medium. WILLIAMS (1951) gives evidence to suggest that in the isomeric transitions that occur in tellurium decay, the atoms which undergo internal conversion of the 7-ray leave the molecule in which they are combined, while those which are not internally converted stay unchanged. This process could result in a constant proportion p of the X atoms of 1131 present being released from the oxidized state. If, as has been shown above, the number of molecules n of iodate reduced is proportional to X, i.e., n = kX

(2)

the specific activity s of the reduced forms will be pX S-1l

therefore, from equation 2, P k which is independent of X as observed (to a first-order approximation). From Fig. 2, k = 1.0 × l0 s, and from Fig. 3 (converting from disintegrations/min/gram to atoms o f 1lal per atom of inactive iodine) s ~ 1.1 × 10-9, whence p = 11 per cent. This value is too small for the internal conversion coefficient, and it is concluded that at least some of the 1131 in the iodate must have arrived by exchange with the reduced forms. A similar situation has been observed by BOYD (1952) in the solid bromates, where the active bromine turned out of bromate by the nT-reaction was induced to return to the bromate ion form by heating and by 7-irradiation. WILLIAMS (1952) has also observed the same effect for solid sodium iodate irradiated by neutrons. The maximum retention observed in the latter case was 90 per cent, and in the experiments reported here the retention is also about 90 per cent. If the specific activity of the reduced forms s R is greater than that of the oxidized form So, any effect which promotes isotopic exchange will cause the specific activity of the oxidized form to increase and give rise to apparent retention, i.e., after the exchange of a fraction x of the atoms between the reduced and oxidized forms, the new specific activity of the oxidized form s o' is given by: So' = so(1 - - x ) + x s R and if

sR > s o

then

s o' > s o

No special oxidizing back reaction therefore need be postulated. The dislocating effect of fission recoil at track ends may be expected to create the conditions for isotopic exchange, and may have contributed to the high apparent retention observed. In Fig. 3 the specific activity of the iodide is seen to lie in nearly all cases between those of the iodine, which is always the highest, and the iodate. This suggests that

Chemical effects in fission product recoil

157

I TM m u s t first arise as iodine, a n d subsequently distribute itself between a small a m o u n t o f i o d i d e a n d a large a m o u n t o f iodate. I f the I TM arose initially as i o d a t e which then chemically d e c o m p o s e d , there is n o way b y which the p r o d u c t s could b e c o m e enriched in I TM. REFERENCES BOHR, N. (1941) Phys. Rev., 59, 272. BOYD, G. E., COBBLE,J. W., and WEXLER,S. (1952) J. Amer. Chem. Sot., 74, 237. BtmGUS, W. H., and DAVIES, T. H. (1951) N.N.E.S. Vol. IV, 9, The Fission Products, Paper 19 (McGraw-Hill, New York). CAMPaELL, G. W., and BRADY, E, L. (1951) N.N.E.S Vol IV, 9, The Fission Products, Paper 277 (McGraw-Hill, New York) CLEARY,R E., HAMILL,W. H., and WiLLiAMS,R. R. (1952) J. Amer. Chem. Sot., 74, 4675. FINKLE, B., HOAGLAND,E. J., KATCOFF,S., and SUGARMAN,N. (1951) N.N.E.S. Vol. IV, 9, The Fission Products, Papers 45 and 46 (McGraw-Hill, New York). HrANIG, G., LEES, R., and MATHESON,M. S. (1953) J. Chem. Phys., 21(4), 665. JORDAn, P. (1951) J. Chim. Phys., 48, 179. KATCOEE,S., MIRKEL,J. A., and SVANLE¥,C. W. (1948)Phys. Rev., 74, 631. MATHIEU, R., and DEMERS,P. (1953) Can. J. Phys., 31, 97. MYERS, O. E., and KENNEDY,J. W. (1950) Y. Amer. Chem. Sot., 72, 897. OZEROEF, L. (1949) AECD, 2973. PERLMAN,I., MORTON, M. E., and CHAIKOEr,I. L. (1941) J. Biol. Chem., 139, 433. SANGSTER,D. F., and WRIGHX,J. (1952) Nature, 30, 368. STANLEY, C. W., and DAVmS, T. H. (1951) N.N.E.S. Vol. IV, 9, The Fission Products, Paper 18 (McGraw-Hill, New York). SXONE, K. G. (1954) Analyt. Chem., 26, 396. SUZOR, F. (1948) C.R. Aead. Sci., Paris, 226, 1081. WILLIAMS,R. R. (1951) N.N.E.S. Vol. IV, 9, The Fission Products, Paper 20 (McGraw-Hill, New York). YAFEE, L. et al. (1953) Can. J. Chem., 31, 120. APPENDIX Distribution q f Recoil Energy between Fissile Material and Surroundings F r o m any p o i n t in the fissile m a t e r i a l the chance o f a recoil a t o m m o v i n g off in the angle i n c r e m e n t 8~ in the plane o f the surface, a n d 60 f r o m the n o r m a l to the surface, is sin 0 6~ 80/47r. The n u m b e r o f fissions in a thin layer o f unit area 6x c m t h i c k parallel to the surface is cSx, where c is the n u m b e r o f fissions/cm 3 in the material. The n u m b e r o f recoils 6 N m o v i n g in the solid angle increment is therefore 8 N -~ c sin 0 8x 80 8qJ/4zr

(1)

F r o m Fig. A1 only the recoils o f range R o within the angle cos -1 x / R o to the n o r m a l will escape f r o m the surface. The total n u m b e r N o f the recoils escaping f r o m the surface is then N = ~,~. Jx=o

=

=o

go

.J~=o

Roc £

sin 0 dx dO dq~

(2)

The residual range R when a recoil a t o m escapes f r o m the surface is f r o m Fig. A1, R ~ R o - - x/cos 0

158

G.N. WALTONand I. F. CROALL

Using the approximation for the relation between energy E and residual range for each recoil atom E = aR ~ (3) where a is a constant for the material in which the recoil originates, the total energy

,..

//otx

,

""/Z,.~I

.-"Surface

Fro. A1.

E, expended by all the recoils after escaping from the surface into the surroundings is derived as for equation 1 :

f0:oo -

I x

E~ = ~

=o

Jo=o

J~=o

ca R o z

12 The total original energy E 0 of the recoils leaving the surface is Eo:

NaRo 2

Hence from (2) E s / E o = ½.