A method of analysis of rare gases released from nuclear fuel in U235 thermal fission

NUCLEAR

INSTRUMENTS

AND

METHODS

33

(t965) 4 5 - 5 2 ;

© NORTH-HOLLAND

PUBLISHING

CO.

A M E T H O D OF ANALYSIS OF RARE GASES RELEASED F R O M N U C L E A R FUEL IN U 23s T H E R M A L F I S S I O N P. A M A D E S I

a n d A. C E R V E L L A T I

C.N.E.N., Divisione di Biologia e Protezione Sanitaria c/o Istituto di Fisica, Universit~t di Bologna Received 5 A u g u s t 1964 U 235 enriched nuclear fuel is irradiated a n d the fission gases released are analyzed for K r a n d Xe whose separation is carried out by g a s c h r o m a t o g r a p h y . Proportional c o u n t i n g allows the determination o f isotopic c o m p o s i t i o n a n d activity of these gases. Therefore it is possible

to determine the concentration in air a n d the release coefficients of nuclear fuel for fission rare gases. Experimental results are then critically discussed a n d s o m e conclusions are drawn in order to i m p r o v e the m e t h o d and to m a k e it m o r e widely applicable.

1. Introduction

reach maximum fluxes of the order of 1 0 7 - 1 0 s neutrons/cmZ-sec. The reactor shape, whose horizontal cross section is shown in fig. 1, is cylindrical and subdivided into three coaxial zones: central zone, utilized for the experiments, multiplying zone and external annular reflector. The core is made up of 92 fuel elements, each containing 105 pellets of UO2, 20% enriched in U 235, alternated with plexiglass pellets, clad in a tubular aluminium sheeth. These tubes, 250 cm long, have an internal diameter of 8 mm and a thickness of 1 mm; the pellets are cylinders of 7 mm diameter, 13 mm long and 5 g in weight, each containing 0.98 g of U 235. The U z35 content of each bar is thus 103 g with a total of 9476 g in the whole reactor. The average thermal flux relating to each bar is monitored by BF3 counters. In this experiment the gases were drawn from an air tight bar, equipped with a vacuum plug at its top, which is linked to an aluminium spiral pipe. A schematic diagram of the apparatus is shown in fig. 2. The bar is evacuated and then filled with He at atmospheric pressure and isolated, through the tap, from the spiral which is in its turn evacuated. The bar and the spiral are positioned in the reactor as indicated by the arrow in fig. 1. The air is replaced with He which, being the gaschromatographic carrier, avoids saturation in the column and noises ordinarly caused by air in the proportional counters. At the end of the irradiation the tap is opened and a known fraction of the active gases is transferred into the spiral which is isolated and connected by the inlet valve to the gaschromatographic column. A scheme of the separating and counting apparatus is shown in fig. 3. The gases, separated in the column, enter the thermal conductivity chromatographic detector. The response is proportional to the mass of each gas and allows control

Among the problems concerning the radiological protection of workers in nuclear plants, the activity determination of rare gases released from nuclear fuel is sometimes important. Unlike other fission or activation products, in gas or aerosol state, trapping of rare gases presents particular problems owing to chemical inactivity. For the same reason they are not retained by human organs, or only to a very slight extent. Thus they do not produce any internal contamination. However, if active gases are diluted in the working site atmosphere, the human body is subjected to radiation doses. The main sources of rare radioactive gases are power and research nuclear reactors, hot laboratories and nuclear tests. The emission of radiactive rare gases may occur both incidentally, owing to fuel cladding failure 1) and, normally, in reactors with unsealed fuel elements2). This work deals with a technique of measuring Kr and Xe isotopes emitted as fragments from U 23s thermal fission and with the evaluation of the fraction of these gases released by a research reactor. As the two gases are released independently of each other, in a way determined by various parameters, such as type of fuel, cladding and working temperature, it is necessary to measure the total activity of Kr and Xe separately. On the other hand, as the isotopes of each gas are released in the same way, the percent aboundance of each isotope can be derived from the overall measured activity and from available fission yield data. The method consists of the gaschromatographic separation of Kr and Xe 3 -s) and of the determination of their specific activity by internal proportional counting of two separated gas fractions 5' 6).

2. Description of the apparatus The experiment has been carried out at the Laboratorio Ingegneria Nucleare dell'UniversitA di Bologna, on RB1, a graphite moderated research reactor that can 45

46

P. A M A D E S 1

AND

A. C E R V E L L A T I

BUFFER

/ REFLECTOR

/(

9

/

o

o,,o

/

/

~.~)

-

/

/

fEEDING ZONE

0 FUEL BARS

Fig. 1. Standard map of RB1 reactor. I. Cadmium flags safety bars; 2. cadmium racks control bars; 3. cadmium nitrate solutions safety bars. o f small quantities o f oxygen, nitrogen a n d other gases i n t r o d u c e d d u r i n g the c o n n e c t i o n o p e r a t i o n . The gas at the outlet o f the c o l u m n is m i x e d with a k n o w n a m o u n t o f C H 4 (or COz) to have g o o d w o r k i n g c o n d i t i o n s in the p r o p o r t i o n a l counters a n d then enters the c o u n t i n g circuit, a set o f 4 cylindrical counters. The characteristics o f the s e p a r a t i o n a n d c o u n t i n g a p p a r a t u s is shown in table 1. The first differential c o u n t e r records the peaks o f K r a n d Xe at their passage. Then, b y means o f a three way tap, the gas is successively conveyed to the filling

TABLE 1

column lenght internal diameter filling carrier gas counting gas internal volume of counters

copper 192 cm 4 mm Linde 5A mol. sieve He He + 30~ CO2 o r C H 4 f Differential type: 10 ml I Filling type: 500 ml

counters where K r a n d Xe are s e p a r a t e l y isolated. These two counters allow decay m e a s u r e m e n t s o f the

A METHOD OF ANALYSIS OF RARE GASES

47

isotope mixture fractions; the second differential counter monitors incidental gas leakages from filling counters. A schematic section o f the two types o f c o u n t e r s is given in fig. 4. 3. Experimental results

/

/

J

One b a r has been i r r a d i a t e d in the r e a c t o r w i t h a flux o f the o r d e r o f 5 x 107 n e u t r o n s / c m 2 . s e c ; the flux d i s t r i b u t i o n a l o n g the b a r axis is given in fig. 5. The spiral is c o n n e c t e d to the g a s c h r o m a t o g r a p h i c c o l u m n 5 to 30 rain after the e n d o f the i r r a d i a t i o n . T h e p r o p o r t i o n a l counters are fed with a m i x t u r e o f H e a n d CH~ ( 3 0 ~ ) , at a flux rate o f 90 ml/min. U n d e r these c o n d i t i o n s the counters exhibit p l a t e a u x in the p r o p o r t i o n a l zone with a fairly wide range o f percentage b o t h o f C H 4 a n d CO2, as shown in fig. 6.

//

/

~ G.S.V.

z//'~

]

jj / J J .

.

.

.

.

.

!

_

T.C.0.[~-

.... ~--]

.............

Y

o o u

__J

........

1'

o

u~

Fig. 2. View of the experimental bar and sampling coil. 1. aluminium sampling coil; 2. vacuum stopcoks; 3. washing by-pass; 4. reflector rods; 5. fuel pellets; 6. Standard arrangement of fuel and moderator pellets.

:

UT

_sc I

Fig. 3. Flow sheet and block diagram of the separation and apparatus. S sampling aluminium coil; GSV gas sampling valve; GCC gaschromatographic column and thermostat; TCD thermal conductivity detector; FC1 proportional gas-flow counter for differential measurements; FC2 monitor gas-flow counter; FC3, FC4 filling proportional counters; FM flow meter; V three ways valve; HPS heating power supply; H.V. high voltage power supply; A amplifier; F pulse shaper; T timer; RM counting ratemeter; R recorders; Sc scalers.

48

P. A M A D E S I A N D A. C E R V E L L A T I

mn

/ "

n

<1[ iiii

/

Fig. 4. Cross sections of the gas-flow (a) and filling (b) proportional counters.

The column is kept at 50cC and the retaining times of Kr and Xe are 5 and 21 min respectively, these being also the counting delays starting from the injection time. The activities of the gas flowing through the first counter are displayed by means o f a ratemeter-operated chart recorder and are simultaneously measured by two

The pile-up and decay functions following irradiation of each Kr and Xe mixture component vs irradiation duration T and the delay time t, elapsing between the end of irradiation and the measurements, have been calculated. As the active isotopes of Kr and Xe are produced in the U 235 fission according to the following scheme:

BAR

,f

,

\

/

90

~ 80

i %< \~ 70

\

.=: o"

B---

2c

C

~ D (stable)

\

/

6O

2B

" A--4

where C is Kr and Xe isotope, the pile-up and the decay of the activity 7c(T) of the isotope C is given by the formula:

o

x D

=:

2A

U 23s

~oo°£

d

2

1

3

'°/f

SO

87 2.1

40

~

2.3

2.4

2,5

2.5

23

2N

kg

g 30

d u] a:

~c E

I

c~

2O

d

cm

SO

100

1 SO

2;0

z~o

3~o

(b)

16

Fig. 5. Axial map of thermal neutron flux in RBI reactor. 14

timer-operated scalers that record the number of pulses relative to a 5 sec interval. Such digital differential measurement is more precise than the analog response of the ratemeter. As such measurements are carried out with irradiation times and measurement delays that are both variable it is necessary to refer the experimental data to a particular condition. This allows the calculation of quantities of radioactive fission gases released by the reactor after any irradiation period.

12~

2.4

2:s

2,~

2[7

~v

Fig. 6. Plateaux obtained with FCI gas-flow proportional counters. (a) He with: (1) C O 2 - 15%, (3) C O 2 - 2 5 / ,o, /, (2) CO2 - 20 ~ , (4) CO2 - 30 ~ . (b) He with C H 4 - 3 3 % .

A METHOD OF ANALYSIS OF RARE GASES _ [1+ eAff~G'sN -~C(*&B - - - 7 =-~- ~A) e - J"B T + 2AP25 _ (2B-- 2A) (ZB-- 2c)

~c(Y) -

'/~B(2C

--

~'A)

e - ;'c r

49

(b)

~c/

):B

]

(l) where eg = fission yield o f the element A ar = fission microscopical cross section o f U 2 3 5 = 582 b a r n , v) q~ = n e u t r o n flux in n e u t r o n s / c m 2" sec. G25 = 9476 g o f U 23s in whole r e a c t o r NA = A v o g a d r o ' s n u m b e r P25 = a t o m i c weight o f U 235. The f u n c t i o n ( l ) has been c o m p u t e d for ~b = 1 neut r o n / c m Z . s e c a n d for Gzs = 1 g at v a r i o u s T a n d t values. T h e d i a g r a m s o f fig. 7 show, with thin d a s h e d curves, the b e h a v i o u r o f the function y(T,t)= ec(T, t)/chG25for all the considered K r a n d Xe isotopes. The solid curves show the b e h a v i o u r o f the i s o t o p i c sum o f each element, Y(T,t). The values of the fission yields a n d o f the decay

161 _

1 0- 2

o7

sl

z\~

",, \

.....-

\ \,

i

~

\", X e 137

ld 5

_

(a)

\ o

10-~ 4

S~

5.5

8

~ 10

T+t (hourS)

~q .c?.

Fig. 7 (b). Diagrams of pile-up (dashed-lines) and decay (solid lines) functions for Xe.

b¸

ore~

~

10-1

Kca7

~.\

-

....-~2[

-<\

---- --

! 10 3 T*t

(hours)

Fig. 7(a). Diagrams of pile-up (dashed lines) and decay (solid lines) functions for Kr.

c o n s t a n t have been r e p o r t e d in K a t c o f f ' s paperS). W i t h such curves it is possible to calculate the released activities o f K r a n d Xe at the end o f an i r r a d i a t i o n o f given d u r a t i o n To, the activities m e a s u r e d after time t from the end o f an i r r a d i a t i o n T being known. In fact, one puts: a ( T , t ) : K r a n d Xe activities o f the sample measured after a time t since the e n d o f the i r r a d i a t i o n d u r a t i o n t i m e T at a flux ~b. a(To,0): A c t i v i t y o f the same s a m p l e as m e a s u r e d at the end o f an i r r a d i a t i o n o f d u r a t i o n time T o at a flux qS. Vs, VB: i n t e r n a l free volumes o f the spiral a n d bar, equal to 45 cm 3 a n d 31 cm 3 respectively. g25 : weight in g o f U 235 actually releasing K r or Xe in the whole reactor. NB: n u m b e r o f fuel elements equal to 92.

50

P. A M A D E S I A N D A. C E R V E L L A T I

One has:

A(To, 0) = a(T, t)"Y(T°'O) 5 Vs+VBNI3 Vs

(2)

7!2

'

'l

A(To, O) 1 a(T,t) I/s+VB NB g25 = Y(To, O)'-~ = Y(T,t)" k s "--~-" I f the release coefficient o f K r and Xe is C = one has again:

a(T, t) Vs + l~; C =

y(T~t)

°

VS

,o

g25/Gzs

Nn

9 e

625(/) .

(3)

The measurements have been carried out with T = 5.5 h, t = 4 5 ' , ~b=4.8 x 107 neutrons/cm2-sec. A paper strip record of the counting rate, as measured while K r and Xe were flowing t h r o u g h the counter, is shown in fig. 8.

½r Tr.5'24"

7 6 ] s-

2xe

1

1-10 eT#

rr.~a' 24'

18'24" 5'0

lOtO

1+o

2o'o

zgo

ado

a~0

4~

Fig. 9. Activity of separated gas fractions measured with the timer-scaler assembly.

-e~.-3 Fig. 8. Radiochromatogram obtained with FCI proportional gas-flow counter. Fig. 9 shows the same peaks counted with the coupled electronic scalers. The overall counting rate o f the two gases, equal to the areas under the activity peaks, has the value: aKr(T, t) = axe(T, t) = YR,(T, t) = Yxe(T,t) =

4.3 X 104 0.775 x 104 3.45 X 10- 2 0.96 x 10 -2

dps (t = 50 minutes, T = 5.5 h) dps (t = 66 minutes, T = 5.5 h) d p s ' cm 2. sec/neutron, g dps.cm2.sec/neutron.g. (4)

Putting the obtained values into (3), one has CK~ = 4.3 x 10 - 4 and Cxe = 2.8 x 10 -4.

(5)

It is to be pointed out that the values (4) represent only a part o f the activity released by the fuel o f a bar.

In fact, part o f the fission fragments have enough energy to get out o f the fuel element, to penetrate into the aluminium cladding and to be absorbed in it. F o r this reason, the values o f the release coefficient obtained by (3) are smaller than the actual. It is worth while to make a comparison with the theoretical values that one obtains assuming that the fuel of one bar is made up of a long cylinder o f UO2 whose diameter is 2r = 7 ram. In this case, if R is the average o f the fission fragments, the weight fraction of U 235 active for the release is (2R/r)" ¼, where the ratio ¼ is the probability that a fission fragment, originated at a distance smaller than R from the surface, leaves the fuel element. Since from the literature 9) R . . . . xo = 7p and R . . . . Kf = 11 /2 and since the energy distribution o f fission fragments and therefore their range, is approximately gaussian, it can be assumed that the mean range is the average of the m i n i m u m and m a x i m u m range, and so Rxe=3.5/2; RKf=9/2, under the hypothesis that the m a x i m u m range o f heavy fragments coincides with the m i n i m u m range o f light fragments.

A METHOD

OF ANALYSIS

Therefore the theoretical release coefficients are: C~r = 12.9 x 10 -4 and C~, = 5.0 x 10 -4 .

(6)

By comparing (5) and ( 6 ) i t can be concluded that about 6 7 ~ of the K r and 45% of the Xe emitted by the fuel are trapped in the cladding material. Moreover, from (2) it can be deduced that for To = 8 h, usual operating time of the reactor, and for fluxes of 4.8 x 10 v n/cm 2. sec, the activities released by all the fuel at the end of irradiation have the following values: AKr = 283 pC; YKr(To,0) = 5.4 x 10 -z dps cm 2.sec/neutron-g

(1o = 8 h)

Ax~ = 117 #C; Yxo(T0,0) = 3.45 x 10 -z dps cm 2"sec/neutron'g

(TO = 8 h)

Assuming that such activities are diluted in a volume of 1000 m 3, approximately equal to the volume of the reactor room, in which there is no air replacement, the concentration reaches the following values: FKr = 2.83 x 1 0 - 7 / / C / c m 3 Fx~ = 1.17 x 10 -7/~C/cm 3. The following table gives the concentrations of each radioisotope of K r and Xe here considered, deducing the percent isotopic composition for the two gases from the functions shown in fig. 7. Such values are compared with the MPC in table 2. TABLE 2 --]

Experimental data*

M.P.C. (40 h/week)

(/~C/ml)

Radionuclide [ concentration KrSSm Kr87

KrSS Kr total Xe135m Xe135 X e 133 Me 137

i i

A

1 . 8 6 x 10 - 7 0 . 7 4 x 10 - 7 0 . 2 3 x 10 7 2.83 x 10 7

1 x 10-6 2 x 10 - 7

5.42x 4.80x 1.36x 0.07 x

Xe 138 Xe total

(/~C/ml)

10 - s 10 - s 10 - s 10-8

C

B

2 × 10 - 6 3.2 x 1 0 - 6

6 x 10-6 1 × 10 - 6

3x 10-6 4

xl0

7

0.06 x 10 8 11.71 x 10 . 8 - -

7 x 10 - 7 1 . I x 10 5

* A f t e r 8 h r s o p e r a t i o n . F l u x = 4.8 x 107 n e u t r o n s / c m 2. sec. A=EURATOM,

ref. t % B = N . B . S .

ref. 11), C = I . C . R . P . ref. 12).

4. Conclusions

The method previously described allowed the separation of fission K r and Xe from each other and, under certain assumptions, the determination of the activity of the two gases, released by the fuel of the whole reactor in particular operating conditions and to

OF RARE

GASES

51

evaluate their concentration in the environment. Moreover, with the help of some hyphotesis on the range of the fission products in sinterized UO2, it has been possible to evaluate the percentage of the gases that are released by the fuel and trapped inside the cladding material. To check experimentally the trapped fractions, it is necessary to perform a similar experiment, collecting and measuring all fission fragments leaving the fuel surface. This may be accomplished by leaving a gasfilled space between the cladding envelope and the fuel elements. A few remarks can be made on the method employed. a) The coupling of gaschromatographic separation with radioactivity measurement allows the evaluation of K r and Xe activities by separating them from other gases, active and inactive, produced in U 235 fission, such as A, Br, I. In fact, during the setting up of the equipment, Kr and Xe have been isolated from mixtures containing H2, A, 0 2, N2, CH4; b) The separation facilitates both the decay measurements on each isotopic mixture of Kr and Xe and the g a m m a spectrometry necessary to obtain further informations on isotopic compositions; c) a factor that affects the decay is the retention time, which is maximum for Xe (20 min). This can be reduced by increasing the operating temperature of the gaschromatographic column from 50 ° C to 100 ° C without worsening the resolution of the two elements; d) the vector gas, helium, must be mixed with about 30% in volume of CO2 or CH4. Since this mixture is not easily reproducible it is difficult to fit the working voltage of the proportional counter. CH4 is a very good counting gas in the proportional region: it could be of some interest to investigate its behaviour as a chromatographic carrier gas 13): a number of tests seems to indicate that something of this sort is feasible. The sensitivity obtained with the method described is 0.2 nano Curie for each gas, in 20 cm 3 samples, thus allowing one to detect fission gases released by the nuclear fuel of the reactor even after only a few minutes of operation. However, since such sensitivity corresponds to maxim u m detectable concentrations of 10 -s/~C/cm 3, it is not sufficient to detect directly fission gas concentrations in the air of the order of M.P.C. In fact, the order of magnitude of M.P.C. for Kr and Xe is 10 - 7 / t C / c m 3 and this value is 100 times smaller than the sensitivity of our method. I f Kr and Xe activities are to be measured directly in the air it is necessary to improve the sensitivity. To this purpose it can be remarked that

52

P. A M A D E S I A N D A. C E R V E L L A T I

the m e a n c o u n t i n g rate R in the differential measurement o f the activity o f a gas f r a c t i o n 14) is R = AV/Vp

(7)

where R = m e a n c o u n t i n g rate in cps A = c o m p o n e n t activity in dps Vp = p e a k v o l u m e in ml V = c o u n t e r v o l u m e in ml. By increasing V f r o m 10 to 70 ml, the s a m p l e volume f r o m 20 to 100 ml, b y decreasing the p e a k v o l u m e Vp, ( t h r o u g h the increase o f the c o l u m n t e m p e r a t u r e f r o m 50°C to 100°C) a n d the gas flow f r o m 90 to 50 ml/min, the analysis t i m e can be decreased o f 1 0 ~ a n d the sensitivity can be i m p r o v e d at least by a factor o f 50. By increasing the shielding thickness o f the p r o p o r t i o n a l counters the b a c k g r o u n d can be easily halved, p e r h a p s m a k i n g it possible to m e a s u r e c o n c e n t r a t i o n s o f the o r d e r o f M.P.C., i.e. 10 - 7 ,//C/crn 3. The a u t h o r s wish to t h a n k Dr. Ing. Z a p p e l l i n i o f the C . N . E . N . C e n t r o di Calcolo, for the helping in the w o r k carried o u t on the RB1 r e a c t o r a n d Mr. G o n d o n i for his helpful technical assistance.

The a u t h o r s wish also to t h a n k Prof. O. R i m o n d i for his c o n t i n u o u s interest, guidance a n d criticism while this research was being carried out.

References x) W. R. Kritz, Nucleonics (1961) 106. 2) p. Amadesi, L. Bruzzi and G. Cicognani, C.N.E.N. Centro di Calcolo, Internal Report 34-CH-OS (Jan. 1963). 3) R. Aubeau, L. Champeix and J. Reiss, J. of Chromatography. 6 (1961) 2O9. 4) R. C. Koch and G. L. Grandy, Nucleonics (1960) 76. 5) p. Amadesi and A. Cervellati, C.N.E.N. Divisione di Biologia e di Protezione Sanitaria, Internal Report (March 1963). ~) L. Bruzzi, A. Cervellati and A. Castelli, Nucl. Instr. and Meth., 26 (1964) 305. 7) H. H. Baucom, Nucleonics, 18 (1960) 198. 8) S. Katcoff, Nucleonics, 18 (1960) 201. 9) A. M. Weinberg and E. P. Wigner, The physical theory of neutron chain reactors (The University of Chicago Press, 1958). 10) EURATOM Doc. 17i/59 ga final (15.1.1959). H) N.B.S. Handbook, 69 (1959). 12) Recommendation of 1.C.R.P. Brit. J. Rad. supl., 6 (1955). 13) A. Alberigi Quaranta, B. Righini, V. Prodi and O. Rimondi, Nucl. Instr. and Meth., I4 (1961) 13. 14) R. Wolfgang and F. S. Rowland, Anal. Chem., 30 (1958) 903.