The chemical effect of alpha particles on uranium hexafluoride

J. Inorg. Nucl. Chem.. 1958, Vol. 7, pp. 210 to 223. Pergamon Press Ltd.. London

THE CHEMICAL EFFECT OF ALPHA PARTICLES ON URANIUM HEXAFLUORIDE* C. H. SHIrLETT,I"M. E. STEIDLITZ,~F. D. ROSEN§and W. DAVIS,JR. Technical Division, Oak Ridge Gaseous Diffusion Plant Union Carbide Nuclear Company, Oak Ridge, Tennessee (Received 3 April 1957)

Abstract--Alpha-particle decomposition of uranium hexafluoride has been studied over the temperature range 26° to 87°C. Radiation was obtained by use of charges of about 0.1 c Rn-222. Products of the decomposition are fluorine and intermediate uranium fluorides; the products react with each other, apparently by a radiation-induced process, to re-form uranium hexafluoride. Reaction rates are independent of temperature over the temperature range studied. Average values of Gu and M / n are, respectively, 0.90 and 0.27, the latter being based on loss of 30 eV by an c,-particle when it produces one ion pair.

ALTHOUGHa great many reactions initiated by 0c-particleradiation have been studied, (1) apparently no such reactions involving uranium compounds have been investigated. The latter are o f particular interest because o f the fact that uranium is itself an 0~particle emitter, and, thus, all o f its c o m p o u n d s are continuously subject to selfirradiation. N o r m a l uranium has a very weak 0~-intensity, the decay constant o f 28su being 4.87 × 10-1S/see. (2) The isotopes 235U and za4U, however, have larger constants, these being 3.09 × 10-17 and 8.8 × 10-14/see, respectively. Nevertheless, the relative abufidances o f these isotopes in normal uranium are so small that, even taking into account their larger decay constants, normal uranium emits only about 25 0~-particles/mg sec. Self-irradiation would, therefore, not produce any large chemical effects in normal uranium compounds. However, uranium which has been enriched 100-fold in 2zsU and 2~U emits about 1230 0~-particles/mg sec, roughly fifty times the n u m b e r f r o m normal uranium. A single 0~-particle from ~ U produces approximately 1.37 X 105 ion pairs over its entire path. Thus, 1 g o f uranium enriched 100-fold will produce about 5.3 × 1018 ion pairs per year. Assuming all 0~-particles are able to complete their paths within the u r a n i u m c o m p o u n d , about 0-2 per cent o f the molecules present would have become ionized in the course of a year by self-0~-irradiation, and the a m o u n t o f chemical reaction as a result o f this over any extended period o f time might become quite appreciable. Decomposition due to self-0~-irradiation has recently been reported in three papers for plutonium hexafluoride. ~8~ C o m p o u n d s o f plutonium provide a m u c h m o r e * T h i s d o c u m e n t is b a s e d o n w o r k p e r f o r m e d f o r the A t o m i c E n e r g y C o m m i s s i o n b y U n i o n C a r b i d e

and Carbon Corporation at Oak Ridge, Tennessee. I" Macalester College, St. Paul, Minnesota. ++International Business Machines, Inc., Poughkeepsie, New York. § North American Aviation, Inc., Downey, California. a~ S. C. LIND, The Chemical Effects o f Alpha Particles and Electrons. American Chemical Society Monograph Series, No. 2, revised edition, Reinho]d, New York (1928). ~2~Nucl. Sci. Abstr. 9, No. 21B, 123 (1955). ta~ C. J. MANDLEBERO, H. K. RAE, R. HURST, G. LONG, D. DAVIES and K. E. FRANCIS, d. Inorg. NucL Chem. 2, 358 (1956); A. E. FLORIN,I. R. TANNENBAUMand I. F. LEMONS, Ibid. 2, 368 (1956); B. W~INSTOCKand J. G. MALM, Ibid. 2, 380 (1956). 210

The chemicaleffectof alpha particles on uranium hexafluoride

211

striking example of this effect than would be expected of uranium compounds, since the disintegration rates of zagPu and U°Pu are 2000-8000 times as great as uranium enriched 100 times in 2a4U. Thus, reported values of plutonium hexafluoride decomposition by self-0c-irradiation are in the order 1-2 per cent per day instead of a potential 0.2 per cent, or less, per year, for uranium compounds so enriched in 2uU. The present study is concerned with the radiation chemistry of uranium hexafluoride under 0c-particle bombardment, since, in addition to the importance of its reactions on purely theoretical grounds, this compound is used in the gaseous diffusion process; any reactions it undergoes as it becomes enriched and while in storage become of practical significance. EXPERIMENTAL

Apparatus The reaction chamber in most of the experiments was constructed of a nickel cylinder connected by means of small-bore nickel tubing to a Booth-Cromer nickeldiaphragm pressure transmitter, t4) The transmitter chamber had an i.d. of 3 in. and a depth of 0.01 in. giving it a volume of approximately 1.2 cm3. The volume of the connecting tubing was about 0.5 cmS; total volume varied from about 7 to 36 cm3. It is desirable to have the ratio of the total volume to the volume of Booth-Cromer transmitter and the connecting tubing as large as possible so as to reduce the magnitude of the errors of temperature correction. However, large volumes are undesirable with gas reactions in which the total amount of reaction is small, as these proved to be. In a few of the early ~xperiments the reaction chamber was a Pyrex glass bulb connected by a Kovar seal to the pressure transmitter. Fig. 1 shows the arrangement of the various components of a reactor used in several of these experiments.

Materials Radon used for the source of ~-particles was obtained from 230 mg of radium on loan from the Bureau of Mines. The gas was collected and purified in a system previously described.~5~ Uranium hexafluoride contained less than 0.015 weight per cent impurity; further purification was accomplished by pumping the hexafluoride while it was maintained at --78°C. Fluorine was taken from standard fluorine cells, t6~ and had an estimated purity greater than 95 per cent. A lead box with 3 in. thick walls was used as a shield against the y-radiation emitted from the reaction systems. Within this box was contained a second box constructed of ½in. asbestos board. Heaters and a thermal regulator in this air thermostat permitted heating a reactor and maintaining its temperature constant to --b3°C.

Procedure In preparing the system for an experimental run, the apparatus described above was pumped, flamed, and the zero point of the Booth-Cromer transmitter then determined. The system was purged a number of times with fluorine and again c4~ S. CROMER, The Electronic Pressure Transmitter and Self Balancing Relay. Sam Laboratories, Columbia University, MDDC-803 20 June (1944); declassified 20 March (1947). ~5~ C. H. SHIFLETT, M. E. STEIDL[TZand W. DAVIS,JR., Rev. Sci. Instrum. 21, 842 (1950). te~ j. DYKSTRA, S. KATZ et al., Industr. Engng. Chem. 47, 883 (1955).

212

C.H. SHIFLETT,M. E. STEIDL1TZ,F. D. ROSENand W. DAVIS,JR.

pumped and flamed. Finally, fluorine was admitted at just under atmospheric pressure and the system maintained at 80--I00°C for at least 24 hr in order to fluorinate the metal surfaces. When the temperature at which a decomposition experiment was to be run exceeded 45°C, the time of fluorination was increased to several days. BOOTH- CROMER

PRESSURE ~ R A N S ~

V/////////////X'/A TO

MANIFOLD

t

'~3/16 HEAVY WALL NICKEL TUBING

t 3 1 4 " NICKEL

V

FIG. 1.--Reaction system. Radon was admitted to the evacuated reactor by direct transfer from the radon purification plant.ts~ The reactor was then connected to a manifold through which the various gases to be studied were admitted. After the experiment had proceeded for a few hours and the rate of pressure change well established manometrically, the reactor was immersed in a water-ice (0°C) or a dry-ice (--78°C) bath. These were contained in Dewar flasks which could be elevated through a door in the bottom of the thermostated box. Pressures at the lower temperatures were observed until thermal equilibrium had been attained. The low-temperature bath was then removed and the reaction allowed to proceed at the experimental temperature. Randon charges used in the individual experiments were measured by 7-ray comparison with radium standards on loan from the Bureau of Mines. A Lind electrometer was used for the measurements. RESULTS Pressures of fluorine and uranium hexafluoride recorded in the figures of this report were obtained by correcting observed pressures to their equivalents at 0°C. These corrections were made by use of the ideal gas laws and may be summarized as

The chemical effect of alpha particles on uranium hexafluoride

213

follows. Let total pressures measured at a reaction temperature T and at a lower temperature, say 0°C, be PT and P0, and the values corrected to 0°C be P ~ and P° o. The conversion from Po to P° o includes a correction for that portion of the reactor volume (mainly the pressure transmitter) that is not immersed in the 0°C (lowtemperature) bath. It has been assumed that uranium hexafluoride exhibits its vapour-pressure tT) of 17.3 mm at 0°C in the presence of fluorine. Thus, Po ° -- 17.3 P°Fs is the pressure of fluorine in the system, corrected to 0°C; P ° r -- po1% = P°UF, is, by difference, the pressure of uranium hexafluoride, also corrected to 0°C. TABLE1. - - R A T I O

OF FLUORINE PRESSURE INCREASE TO

URANIUM HEXAFLUORIDE PRESSURE DECREASE

Expt. no.

[APFJAPuF6 [

3 4 7 16 15 5 24

0.625 0"585 0"29 0"86 0.77 0-24 1.25

Under 0~-particle irradiation there is a depletion of uranium hexafluoride from the gas phase accompanied by an evolution of fluorine. Fig. 2 contains a summary of three typical experiments performed in a vessel originally containing no uranium fluoride deposit. On addition of fluorine to uranium hexafluoride in a reactor containing solid uranium fluorides deposited from the gas phase, there is an initial increase followed by a decrease in the uranium hexafluoride pressure, as seen in Fig. 3. TABLEZ--RATIO OF FLUORINE PRESSURE INCREASE TO

URANIUM HEXAFLUORIDE

PRESSURE DECREASE FOR KNOWN URANIUM FLUORIDES

Compound

[APFJ APuFJ

UF6 UF5 UF4 UFa

0"0 0"5 1 "0 1"5

It has not been possible to establish the exact identity of the solid uranium fluoride product, although some information is obtained from the ratio of fluorine produced to uranium hexafluoride decomposed. Such ratios for seven experiments are summarized in Table 1. Ratios that would be obtained for several possible uranium fluoride products are listed in Table 2. In the presence of solid fluorides and added fluorine, the change in ~ G. D. OLIVER,H. T. MILTONand J. W. GRISARO,

J. Amer. Chem. Soc. 75, 2827 (1953).

214

C.H.

SmFI.ETT, M. E. STEIDLITZ, F. D. ROSEN and W. DAVIS, JR.

20

'

I

'

1

'

I

'

I

'

I

~

I

'

I

i

I. EGEND

A

15

UFB F2 UFs F2

0 •

_

'~

EXR No. 15 EXR No. J6

~i ~x~..~,

1(

n

0

1

2

3

4

5

o

7

8

Time,days FIG. 2,--Time variation o f U F 6 and F z pressures, corrected to 0°C.

32

31

3O

[ 29 ~ 28 e~

D

27

26--

0

4.0

80 120 Trme~hr-

160

200

240

Fla. 3.--Time variation of U F e pressure, corrected t o 0°C. Experiment 9, vessel initially contained UF= deposit.

The chemical effectof alpha particles on uranium hexafluoride

215

fluorine pressure with uranium hexafluoride decomposition exhibits different values before and after the time of maximum uranium hexafluoride pressure, Fig. 3. These values are illustrated in Table 3. TABLE 3.~-CoMPARISONOF THE CHANGE IN FLUORINE PRESSURE TO THE CHANGE IN URANIUM HEXAFLUORIDE PRESSURE IN THE PRESENCE OF THE SOLID FLUORIDES

lae~,/AP~F,I Expt. no.

8 9

Before max. UF, press Aftermax. UF. press 0"88 (F~ depleted) 0.67 (F2 depleted)

0.13 (F~ formed) 0.28 (F2 formed)

DISCUSSION From the data obtained, it is obvious that the uranium hexafluoride is ionized by 0c-particles and dissociated to produce solid uranium fluorides. Chemical analyses have failed to aid in the identification of this material because the quantities available are so small (1-2 nag). X-ray diffraction patterns of the compound are not very clear due to the radiation background level and to small particle size, estimated to be less than 100 A. However, halos have appeared in the region where fl-uranium pentafluoride exhibits strong lines, ts) Such evidence is indicative but certainly not conclusive of the presence of some pentafluoride. As shown by the ratios of fluorine pressure change to uranium hexafluoride pressure change (Tables 1,2 and 3), the solid compound may have a composition intermediate between uranium tetrafluoride and hexafluoride. Because further identification is not possible, the solid products are designated below as UFx. If the reaction of uranium hexafluoride under 0c-particle irradiation were to be dependent on the gas pressure and the radiation intensity alone, equation (1) could be used to represent the whole course of the decomposition. UFs ---~---~ UF~ + ( ~ - f - )

F2

Rate constant = k t

(1)

The rate expression corresponding to equation (1) is d(UF.) dt -----kt (UF6)E° e-at,

(2)

where E o is the initial radon intensity, in curies, and 2 is the decay constant of radon. This equation may then be integrated and solved according to equation (3), where E/E o = e -at. klE° e-at -t- C log (UFa) -----2.3032

(3)

If log (UFe) or log PvF, is plotted against exp (--2 0 = E/E o, and the reaction is a simple decomposition, the data should follow a straight line with an intercept of C (8}

W . H . ZACHARIASEN,

Acta Cryst.

2, 296 (1949).

216

C . H . SnlELETr, M. E. STEIDLITZ, F. D. ROSEN and W. DAVIS, JR.

equal to log (UFe) o or log (Pvr,)o. Fig. 4 shows that this plot is not a straight line, but a curve of decreasing slope. In view of the decreasing slope, a back reaction of the solid product with fluorine has been assumed. This reaction has been indicated to be dependent upon radiation, since, at the reaction temperatures employed, it is doubtful that measurable thermal

I

I

I

-~ 0 . 9 5 ] ~

1

I

EXPERIMENT 4 • EXPERIMENT 5 a EXPERIMENT 15 o EXPERIMENT 1 6 -

-",

""

0~0

I

1.0

"-o...

o-g

--~... 0-8

0.7

0-6

I

0.5

I

0"4

I

0"3

0-2

E/Eo FIo. 4.--Variation of log PoF6, corrected to 0°C, with ~-activity. reaction would occur. (9) Further, when fluorine was placed and maintained at room temperature in a vessel that had previously been used in a radiation experiment, uranium hexafluoride was not formed in measurable quantities. (6 -- x) UF~ + ~ F~ --~---~ UF 6

Rate constant = k 2

(4)

The rate equation for the overall reaction of equations (1) and (4) may be written as d(UF6)

dt

= klE° (UF6) e - n -- k~E0(F2) (S) e-at,

(5)

where (S) is some function of the surface of solid UF=. The fluorine concentration has been found experimentally to be a linear function of the uranium hexafluoride pressure, which is expressed by equation (6): (F~) = a -- b(UF6)

(6)

Typical correlations indicated by equation (6) may be seen in Fig. 5. Values of the constant b have been summarized in Tables 1 and 3. The rate equation may be rewritten following substitution of equation (6) into (5). d(UFe) -- [{kz + k~(S)b) (UF 6) -- kz(S)a] Eo e-at

dt

(7)

Integration of equation (7) leads to the following relationship: In I (UFe)

a Eo k 1 + k~(S)b J ~___{k I Au k2(S)b ) -~ e-~ + C

1

(8)

When values o f k 1 and k2(S ) are determined and the results plotted according to Fig. 6, tg) j. j. KATZ and E. RAmNOWITCH, The Chemistry of Uranium. Part I, NNES, MPTS, Div. VIII, Vol. 5, McGraw-Hill, New York (1951).

The chemical effect of alpha particles on uranium hexafluoride

217

5

IIIll

I= • m

0

-5 0

I--t.-I--t-zzZZ

m

~o

-5

6F~F~ O/ I

I

I

/

c~

o

X X X X

,~

I

0<]@13

_

--~>

,.-, 0

-

I. 0

0

0

~"

~

-~D

~

-O~,00r.-

~0

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~

q.

(,')

6

6

6

0

q (s)~#+% 2~na o ¢s)¢~

,

0

I [ I I I I I I

I III

III

I I I

I

[II

I I

t~ z z z z z z w w ~ w w w

n

w ~ w w w w

x x x x x x

•

o

Z l l l l l l W I I I I I I

~IIIIII

,,

~8 0

-

6

I °

218

C . H . SHIFLETT,M. E. STEIDLITZ,F. D. ROSEN and W. DAVIS, JR.

a straight line results. Calculated values o f k 1 and ks(S), determined by least-squares procedures, are listed in Table 4. TABLE 4.--RATE CONSTANTS

Expt. no.

fOR U R A N I U M

Initial UFe Temp. (°C) Radon charge press* (ram Hg) (c)

3t,++ 4t,¢ 5 ,+

24 16 15 7 31 33 36 34

HEXAFLUORIDE DECOMPOSITION

30 30 30 26 30 33 48 52 84 85 87

(o.I) (o.i) 0"042 0"078 0"105 0.099 0'042 0'091 0"112 0'036 0'034

120 123 105 78"0 90"5 125"0 185'0 51 '5 62"7 71"8 70'0

UF, decomposition kl X 105 (c-I see-l)

UFe formation k~(S) x 105 (c-1 see-0

(3 "95) (2.04) 1 '06 4"15 2"68 3"26 3.02 2'10 4"03 2"08

(23"05) (13'49) 23"08 6'83 9"09 11.37 16"13 1"59 3"11

1"64

3"70

1.43

* Corrected to 0°C. 1"The radon charge was not measured. The activity was estimated to be I00 inc. ++Experiments 3, 4, and 5 were carried out in spherical glass bulbs of radii 1.94, 1.96 and 1.98 cm, respectively.

Calculation of E a, G, and M/n U r a n i u m hexafluoride can be ionized and decomposed to non-volatile uranium fluorides. These ionization processes are readily observable in a mass spectrometer; appearance potentials o f the various ions, UFs+, UF4 +, etc., have been reported by CAMERON,(10) The extent o f a second decomposition process, namely electronic excitation o f uranium hexafluoride molecules and their subsequent decomposition to free radicals (such as U F 5 and F) or molecules (such as U F 4 and F~), is unknown. Since the relative importance o f ionization and electronic excitation is not known, the quantity M/n, the n u m b e r o f molecules o f uranium hexafluoride decomposed per ion pair, can be calculated but with more uncertainty than the quantity G, the n u m b e r o f molecules o f uranium hexafluoride decomposed per 100 eV o f energy absorbed. The latter term is independent o f the mechanism o f dissociation. Terms used in calculating G, dN/dt, the n u m b e r o f molecules o f u r a n i u m hexafluoride decomposed per see, and dEa/dt, the quantity o f energy absorbed per see (in eV/sec), are as follows. ea, ez, ez = energies o f 0c-particles f r o m SS2Rn, ~lSpo, and 214po, 5.486, 5.998 and 7.680 MeV, respectively; e = average 0~-particle energy, (e I + ez + e3)/3 = 6.388 M e V ; Sa, $2, $3 = stopping powers o f uranium hexafluoride for the ~22Rn, ~lSpo, and ~l~Po 0c-particles, respectively, cm -1 at STP; S = average stopping power, ($1 + $2 + $3)/3; c10)A. E. CAMERON, Determinationof the Isotopic Compositionof Uranium. Technical Information Service, Oak Ridge, Tenn.; January, 1950; declassified February 1955 (TID-5213).

The chemical effect of alpha particles on uranium hexafluoride

219

R1, R2, Ra ranges of 2~ZRn, 21Spo, and 214Po a-particles in uranium hexafluoride, cm at STP; R = average a-particle range, (R x + R2 q- R3)/3; B = the number of a-particles emitted in 1 see from a system containing 1 c of 2ZZRn in equilibrium with its decay products, 3 × 3.72 × 10x° ~/see e; IP = the average amount of energy required to produce one ion pair, eV, ----

Elo I I I I I I I I • u - - AREA A= AREA B+AREA C / ~ x

I

,,AVERAGE I O N I Z A T I O N / --/-T= 5"5 X IO5 ION / A

o a.

x

23 RANGE R= l c m ~

--

._N

1

1

1

1

1

1

1

0-5

O

Distance~cm

1

~ l 1-0

AT 1 a t m o s pressure

FIG. 7.--Schematic

ionization curve.

L1 = average path of an ~-particle emitted from gaseous 2Z2Rn that is homogeneously distributed in a sphere of internal radius r; L 2 = L z = average path of an ~-particle emitted from solid zlaPo or solid 214Po that is uniformly distributed on the inner surface of a sphere of internal radius r; A = Loschimdt number, 2.69 × 1019 molecules/cm3 at STP; 1 = decay constant of 22~Rn, 2.10 × 10-6/see; ni = number of ~-particles per see from nucleus i(i = Z2ZRn, ~18po, 21~Po); Pj = pressure of gas fi atm; V~ = principal volume of reactor (Fig. 1), cruZ; V, = total reactor volume, cm z LINDtl~ has shown that the average paths of a-particles are related to the radius, r, of a spherical container by equation (9). L x = 0.75r Lz = L 3 = 0.5r

(9)

An accurate calculation of the rate of absorption of energy by a mixture of gases in a spherical container of volume 11, radius r, at pressure P, containing initially E o c of radon can be made only if the ionization curve of each ~-particle from the radon decay scheme in each pure gas is known. Such ionization curves, schematically shown in Fig. 7, have not been measured for uranium hexafluoride. Thus, estimates of G are based on an estimate of the average range, R, of the radon scheme 0c-particles in this gas, or on the number of ion pairs produced per cm of ~-particle path. If the only assumption involved in calculating the energy absorption rate, dEa/dt,

220

C . H . SHrFL~anr, M. E. SrEmUTZ, F. D. ROSEN and W. Davis, JR.

were that ionization be constant over the path, then equation (10) could be written if the path length is less than the range (i.e. every ~-particle reaches the wall).

d(E,,)~. = L~___ 2 etPjni dt Ri~

(10)

Subscript i refers to the 0~-particle, j to the gas. The quantity n~ may be expressed as B

n, = ~ E0 e -at.

(11)

Then

dE~ _ B Eo e -~ Y L~ e,P~ dt 3 ~ Rij or

dEadt- - BE°e-~t3 j~ [t0"75 rRl~ e I

@ 0"5 r

(12)

( e2~ -~ ~o.)} Pj

(13)

3j/J

Quantities Rij have not been determined experimentally, but have been calculated as described below. The necessity of estimating R makes the use of specific values of ei superfluous; hence, ei has been replaced by the average value e in equation (14).

d E ~ B e E o e - a t l ', 7 5yP -r - dt 3 T Rj

= 0-5833 BeE o e - %

~

(14)

The quantity 0"5833r is the calculated average path of ~t-particles from the three nuclei; however, LIND and BARDWELLml experimentally determined a value of 0.61 r, which is used in subsequent discussion and calculation in this report. Reactors used in the present work, with the exception of the first few, were not spherical; instead they were composed (Fig. 1) of a cylindrical section that constituted the main volume, a section of small-bore tubing, and the Booth-Cromer pressure transmitter. LIND{II has quoted LYNN to the effect that the average path in a cylinder of diameter nearly equal to its length is about the same as that in a sphere of the same volume. Thus, designating as Vr the main reaction volume, the effective radius of this unit is F 3 V r l 1/3

r =

L'-4-~J

'

(15)

and the average path of an 0~-particle may be written as L = 0.61 r = 0.379[V,] l/a.

(16)

Dissociation of uranium hexafluoride also occurs in the small bore lines and in the pressure transmitter. However, the ~-particle paths in these sections are considerably smaller than the paths in the volume V~. A reasonable estimate is given by assuming 0~-particle utilization in these short-path sections to be only 15 per cent as efficient as it is in the volume Vr. The effective value of the randon charge, E't, is then given by equation (17). E"=E~E'~+0"15

V~--V'7]=[O'15+O'85-g~ E ' V .

(11~S. C. LIND and D. C. BARDW]ELL, J. Amer. Chem. Soc. 45, 2585 (1923).

(17)

The chemical effectof alpha particles on uranium hexafluoride

221

By inserting the right-hand term of equation (17) and equation (16) into (14), we obtain the rate of energy absorption for the reactors actually used.

dE~dt= (0"379 VII3) eB ( 0"15 + 0.85 ~V~)Eoe-at~P~ ~

(18)

As described previously, the effects of ~-particles on uranium hexafluoride can be described in terms of reactions (1) and (4), the first of these summarizing decomposition, the second, reformation of the hexafluoride. It is apparent that kl of equation (2) actually contains the efficiency factor of equation (17), i.e.

ki=k'l

(0 " 1 5 + 0 " 8 5 ~

19,

.

The number rate of decomposition of uranium hexafluoride by reaction (1) is

dN_~_= A V, dP___Ur6dt-- A V~klPur E o e-~.

(20)

Then GT, given by 100 times the ratio of quantities in equations (18) and (20), is as follows:

GT =

IOOAVsklPuF6 0"379V~/3 eB 0.15 -t- 0.85

GT =

~

100AVsk'lPuF6 0.379V~/3 eB Z Pj j R~.

or

(22)

The summation in the denominator contains a term for uranium hexafluoride and one for fluorine. Since the pressure of fluorine was usually less than one-tenth that of the uranium hexafluoride, and since RF~ is in the order of seven times R u t e, this summation may be replaced by PvrJRur¢ whence equation (21) reduces to Gu =

100A V~klRur e

(23)

0"379Vr1'3 eB(0.15 ,4- 0.85-~) As indicated previously, the absence of experimental range data has necessitated estimation of such quantities. For this purpose the correlation of STEIDLITZet aL, (m equation (24), has been used. R = ~ Voa/~ n~Z~~13 (24)

J

=6.75x

10-26mm""[secj 3

\cm/ Vo = initial velocity of :c-particle nj = number of atoms of atomic number Z~ From equation (24) the average range of the three :~-particles in uranium hexafluoride is 5.72 mm and in fluorine, 42.54 mm. ta~)M. E. STEIDLITZ,F. D. ROSEN,C. H. SHIFLETTand W. DAws,JR., Y.phys• Chem.56, 803 (1952)• 3

C . H . SHIFLETT,M. E. STEIDLITZ,F. D. RosEN and W. DAvis, JR.

222

On the basis of equation (23), values of Gv have been calculated for those experiments in which the radon charge was measured. These are listed in Table 5. In addition to Gtr there are listed values of M/n. The latter have been calculated by replacing the 100 eV term in equation (23) by 30 eV. This choice is somewhat arbitrary, but is consistent with the average energy required to produce one ion pair in a number of fluorocarbons.t12} In any system in which radon is intimately mixed with the reacting gas, the recoil TABLE5.--VALUESOF Gir ANDM/n

V,

V~

Glr

M/n

molecules/100eV

molecules/ion pair

0.76 1"40 0"96 1"13 1'04 0"59

0"23 0"42 0"29 0"34 0"32 0"18 0"34 0"18 0"14 0"27 q- 0"22

Expt. No. cm3

5 24 16 15 7 31 33 36 34

35"9 7"4 11"4 10"5 10"5 7"1 7"1 7"1 7"1

cm8

31"4 6"7 9"5 8"6 8'6 5"5 5"5 5'5 5"5 95Vo C.I

1"14

0'59 0"47 0"90 -4- 0"73

atoms cause some ionization. However, this effect amounts only to a 3 per cent reduction in the M/n values with the conditions of geometry and pressure employed in these studies. Hence, these effects may well be neglected.

Re-formation of uranium hexafluoride In order to facilitate an understanding of the back reaction, two of the reactors which had been used in studies~of the decomposition of uranium hexafluoride were re-used with a mixture of fluorine and uranium hexafluoride, experiments 8 and 9. Under these conditions the pressure of uranium hexafluoride increased, and later decreased, as shown in Fig. 3. While the uranium hexafluoride pressure passed through a maximum, the fluorine pressure went through a minimum. It is noteworthy that the maximum uranium hexafluoride pressure occurred when the solid fluoride had been converted almost 100 per cent to uranium hexafluoride. Data of experiments 8 and 9 were divided as follows: group 1, taken before the maximum uranium hexafluoride pressure was attained; group 2, taken after this maximum. Values of the constants so calculated are listed in Table 6. It is apparent from this table that, within experimental error, k 1 is constant throughout the course of reaction. However, the group 1 value of ks(S ) is larger in both experiments than the group 2 value. The data thus suggest that during the interim between the completion of a decomposition experiment and subsequent re-use of a vessel, the UF~ deposit becomes more reactive toward

The chemical effect of alpha particles on uranium hexafluoride

223

TABLE 6.--RATE CONSTANTSOF EXPERIMENTS 8 AND 9

UFe decomposition Run No.

8---Group 8---Group 9---Group 9---Group

(°C)

kl X 10 5 (C-1 seC-1)

63 63 65 65

3"16 2'82 3"96 4706

Temp.

1 2 1 2

UF6 formation k~(S) x 105 (C-I S ~ -1)

16"52 10'49 53"94 21 "66

Initial F~ press. (cm)

10.4 5.1

0c-particle induced fluorination to UFr. Two factors that may be involved in this activation are crystal growth and evacuation of the reactor.

Effect of temperature Experiments were performed at temperatures ranging from 26 to 87°C. Although there is a considerable spread in the data, Tables 4 and 5, values of Gu or M/n do not show any increase with temperature. This lack of temperature dependence is expected, since temperature, within limits, does not have any appreciable effects on ionization processes and, in addition, would have little effect on the efficiency of any electronic excitation process over this small temperature range. It is apparent, however, that as the temperature is increased toward 200-300°C, thermal reaction of fluorine and uranium fluorides would become important. (9)

Acknowledgements--Theauthors wish to express their appreciation to Dr. S. C. LINDfor his interest and helpful suggestions during the course of this work; to Miss VmGINIAFEASTER,formerly of the Technical Division, for the X-ray analyses of the solid products; to A. DELAGARZA,M. S. Glmmmo, and M. DAvis of the Operations Analysis Division for the mathematical analyses of the data; and to the Bureau of Mines for the loan of the radium used in this work.