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INVESTIGATION OF THE VOLATILISATION OF
RECOIL PRODUCTS FROM IONIC CRYSTALS
B. Stojanik, R. Lauer, B. Neidhart, K. Bächmann
To cite this version:
JOURNAL DE PHYSIQUE Colloque C7, supple'ment au no 12,. Tome 37, De'cernbre 1976, page C7-543
INVESTIGATION OF
THE
VOLATILISATION
OF RECOIL PRODUCTS FROM IONIC CRYSTALS
B. STOJANIK, R. LAUER, B. NEIDHART and K. BACHMANN Fachbereich fiir Anorganische Chemie und Kernchemie, Technische HochschuleDarmstadt, FRG
Resum& - Des materiaux polycristallins furent dotes des produits de recul provenant de la
fission de I'uranium-235 par des neutrons. La volatilisation de Xe, I, Tc et Te fut examinee par des experiences isothermes et non isothermes, et le phenomkne de degagement est discute. Les courbes isothermes peuvent dtre approchees par une fonction traCant une reaction du 2 e genre ou de pseudo- 2e genre. Les donnees T m a x , qui peuvent btre derivees du thermogramme, dependent du degre du chadage, de la composition du courant de gaz et sont fortement irduencees par les materiaux ioniques mis en ceuvre comme matikre receptrice. Ces effets sont discutes ph~nom~nologiquement.
Abstract.
-
Polycrystalline materials were doped with recoil products formed in 2 3 5 U (n,f)-reactions. In isothermal and non isothermal experiments the volatilisation of Xe, I, Tc and Te was examined and the release phenomena are discussed. The isothermals can be approached mathema- tically by a function which describes a 2nd order or a pseudo 2nd order reaction. The Tmax values, which can be derived from the thermograms, depend on the heating rate, the composition of the streaming gas, and are strongly influenced by the ionic matrices used as stopping materials. These effects are discussed phenomenologically.
Introduction. - The method of volatilizing recoil products from solid materials can be applied t o a series of technical, analytical, as well as basically scientific problems, e. g. :
1) reprocessing : selective separation of radio- nuclides of technical interest from solid waste,
2) reactor security : release of fission products from defect fuel elements,
3) trace analysis : selective separation of trace elements from an interfering matrix,
4) preparative radiochemistry : preparation of radionuclides with very high specific activity,
5) physical chemistry : activation energies, fre- quency factors, diffusion coefficients.
Depending on each problem the release of volatile elements or compounds must be selective, and/or fast, and/or quantitative. The interpretation of the release phenomena is very difficult, as the systems in question are complex and not very well defined. The complexity results from the overlapping of various effects : a) the materials are mainly poly-
crystalline or amorphe, b) due to the high energies of recoil products, the environment of thermalized atoms or ions is a highly disturbed lattice ; additional damage is caused by high radiation fields. The anneal- ing of these lattice defects is a function of time and temperature, c) for the non noble gas elements chemical reactions with the matrix or a reactive carrier gas must be taken into account. Thus, as usual in solid state reactions, the release phenomena can be determined by diffusion reactions, phase
boundary reactions, and nucleation as well as nuclei growth.
Until now, the point of main effort in many investigations was the diffusion of noble gases in single crystals [l-41. For the interpretation of the experimental results quite a few theories have been developed, none of which is able to describe suffi- ciently the influence of all the different parame- ters [l, 3, 51.
In regard to an even more complex system (poly- crystals, fission products, and other elements than noble gases), it can be expected that in the present state of investigations it is not possible to understand all release phenomena, and to set up an overall theory. This understanding, however, does not alter the fact that it is necessary to examine these complex systems under the aspects mentioned above. Therefore, we started the investigation of volatilizing fission products from ionic crystals with the preli- minary aim to describe the influence of the different parameters phenomenologically, and to give a semi- empirical characterisation of the release pheno- mena [6-81.
Experimental. - Small fractions (25 mg) of the polycrystalline materials are encapsuled in quartz ampoules together with 1 mg of an uranium foil
(- 90
%
U-235). After a three hours irradiation with 7 X 10'' n/cm2.s the samples are allowed tocool down for about one day. Before starting the experiment the U foil is removed from the salt and
the catcher matrix is transferred into a quartz boat which is introduced into the apparatus shown in figure 1. The quartz tube is heated by a furnace. The volatilisation of the radionuclides from the crystals is measured with a Ge(Li)-detector.
905 ,"+let
t h e r m o c ~ u p l e
p r o g r o m c o n t r o l l e d temperolure vnlt
FIG. 1. - Experimental set-up.
matrix is introduced into the apparatus at the time
t = 0. Then the release of the volatile products is measured as a function of time. In our case the variable parameters were the solid matrix, the compo- sition of the streaming gas, and the temperature. In the non isothermal experiments the matrix is introduced into the apparatus at room temperature ;
after this the temperature of the furnace is increased linearly by a program controlled temperature unit. In these experiments instead of the temperature the heating rate was varied. Under the given experi- mental conditions and due to the different intensities of the y-lines and their position in the spectrum Tc, Te, I and Xe have good counting statistics and there- fore have been used for the evaluations.
In general, this type of investigations can be carried Results and discussion. - The influence of the out in an isothermal or a non isothermal mode. In different parameters on the release of Tc, Te, I, the isothermal experiments the quartz tube has been and Xe was investigated in isothermal and non heated up to a defined temperature before the doped isothermal experiments. For the semi empirical
I
A rI K-!l
I I I I I I I 1 I I I 1 I I 1 I 5% .F.\ 1 0 i I ! \. 39% f b Ii
l i I l I I /,' I l I l I l I I I 1 I I I l 2 0 2 b I l I I I I I l I l I 1 1 l l 1 3 0 3 6 / i'",. i,/'
!', 7 % 7 Y . L /'
;
'
\ / \ i i / , ; \., \\;,,,
...' \..'
/ I l I'.
%L
-
600 ' 800 loo0 600 800 K X X ) r [K]-
*
M-
a) b)RG. 2.
-
Thermograms for the release of Tc, Te, I, and Xe from a KC1-matrix (18 l/h). l a : NZ, /? = 24 K/min ; 1 b : NZ, /? = 2.4 K/min ; 2 a : N2/C12, /? = 24 K/min ; 2 b : N2/Cl2, /? = 2.4 K/min ; 3 a : N2/CC14, /? = 24 K/min ;3b:N2/CCI4, B = 2 . 4 K/min Tc ---; Te ; - I
...
; Xe--.---.-- ; % percentage retainedVOLATILISATION OF RECOIL PRODUCTS C7-545
mathematical description of the release phenomena only the KC1 matrix was used whereas for the phe- nomenological characterisation of the systems a considerable number of chlorides and oxides was selected.
The thermograms shown in figure 2 were obtained from the first derivative of the experimental curves. The positions of the maxima
T,,,
(temperature of the maximum rate of release) and the FWHM of the peaks are characteristic for each element under the given experimental conditions (e. g. heating rate, composition of the gas). Corresponding to the theory of the non isothermal experiments [9, 101, the T,,, values are shifted to lower temperatures with decreasing heating rate (Fig. 2, a -+ b). The variation of the chemical composition of the transportation gas clearly changes the thermograms for Tc, Te, and I,whereas Xe behaves like in the inert gas experiments (Fig. 2, 1 -+ 2, 3). This change means a shifting of the T,,, values to lower temperatures in all cases. For the determination of a possible mechanism the data of the non isothermal experiments were treated following the method of gatava V. [l11 and Ses- t8k J. [l21 :
with a g(a) P(X)
Z.
Elog g(a) - log p(x) = log -
R.B (1)
= volatilized fraction (1 - A,/A,),
= function of the rate determining process,
= function depending on the activation energy of this process,
E = activation energy,
Z = preexponential factor, R = gas constant,
p
= heating rate.Under the assumption that the right hand side of eq. (1) does not depend on T, and that log p(x) is approximately a linear function of 1/T, the plot of log g(a) versus l/Ta results in a straight line for the most probable g(a). The different functions g(a) which were tested are listed in table I [13]. First of
kt = a2 one dimensional diffusion
kt = (1 -a) In (1 - a)+a two dimensional diffusion, cylin- drical symmetry
kt = [l - (1 - a)ll3]2 three dimensional diffusion, sphe- rical symmetry, Jander eq.
2 three dimensional diffusion, sphe-
kt=(i---a)-(i-a)2/3
3 rical symmetry, Ginstling-
Brounshtein eq.
1 W
kt = - [l - (1 - - a)1/2]2 three dimensional diffusion, sphe-
W 3 rical symmetry [l61
kt = - In (1 - a) first order. random nucleation kt = [- In (1 - a)] 112 random nucleation, Avrami I kt = [- In (1 - u)]ll3 random nucleation, Avrami I1
kt = 1 - (1 - a)ll2 phase boundary reaction, cylin- drical symmetry
kt = 1 -(l -a)1/3 phase boundary reaction, spherical symmetry
all, for the sake of a simplification, only the Xe data were used for the calculations. The results showed that none of the tested functions g(a) can describe the experimental curves totally and with a satisfactory accuracy. There are several reasons which might be responsible for the deviations : a) within the tempera- ture interval under investigation several processes determine the velocity of release, b) the activation energy E is a function of T, c) the temperature interval is too wide, so that the assumption of the linearity of the function log p(x) vs 1/T is no longer valid [14]. To bypass the latter argument, the intervals were limited from a = 0.1 to a = 0.5. The results of these calculations unequivocally point to a diffusion kinetics, which is fitted best by eq. (2)
For 0.5 G a G 1.0 all theoretical values calculated with eq. (2) are higher than the experimental data. Based on the non isothermal data so far, it is not possible to give a function g(a), which is able to describe the release process totally.
Therefore, in addition, isothermal experiments were carried out, which showed as well that a mea- surable release begins above 600 K. As described for the non isothermal experiments, the isothermal data were treated with a linear regression using the inte- grated form of the different kinetic equations g(a) listed in table I. This calculation showed that with none of the used functions a good fit for the experi- mental data was obtained. Only the function given by Damm J. Z. and Tompkins F. C. [l 51 for the anneal- ing of colour centers in KC1 describes the isothermals of Xe as well as I, Te, and Tc sufficiently :
a, and a, are the fractions released at the times t = t and t = co resp
...
An example is given in figure 3, which shows the experimental values for the release of Xe at different
1.0, l , , , , , , l l
,
,,
FIG. 3. - Isothermals for the release of Xe from a KC]-matrix ;
temperatures and the curves calculated with eq. (3). The minor disagreement between the theoretical curve and the experimental data at the lowest tem- perature can be explained by the fact that in this case the experiment did not last long enough to determine the exact value of a,. However, the experi- mental time was not extended, as in that case the radioactive decay of the nuclides would have had to be taken into account.
In eq. (3) k is not really a velocity constant and therefore a physical meaning should not be given unless the mechanism of the release process is known. Qualitatively it was shown that the values of k depend on the time between the irradiation and the experi- ment and are strongly influenced by a thermal treat- ment of the matrix after irradiation and before starting the experiment. Due to the complexity of this pseudo- velocity constant, eq. (3) cannot be converted and transferred to the theory of the non isothermal experiments.
In order to get a general information about the influence of the,polycrystalline material on the release, quite a few different compounds were examined in
non isothermal experiments (temperature range 400 K- 1200 K). The T,,, values and the percentage of the elements retained by the matrices were determined from the thermograms. For a better characterisation of the important parameters, special groups of compounds were selected (Tables I1 and 111), e. g. :
a) constant cation, variable halogen anion : Li/F, Cl, I or K/F, Cl, I (Table 11),
b) variable cation, constant halogen anion : LiC1, RbC1, CsCl (Table 111),
c) NaC1-type structure : halides and oxides (first three blocks in table 11),
a>
other structures and compounds with a higher Cl-content : (4th block in table I1 and 2nd block in table 111), etc.In addition, some of the materials were treated for 1 h at 500-600 OC before the irradiation in order to remove the water. The results for the inert gas experiments with a high flow rate (18 l/h) and the reactive gas experiments with a low flow rate (2 l/h) are listed in table I1 and table I11 resp. The maximum errors of the Tma, values are E 20 K. Although in
T,, values and
%
retained for digerent stopping materials (NZ ; 18 l / h ; 7.4 K/min)Tc Te Xe I
Matrix
-
Xmax -%
-
ret. X-
%
- ret. T,ax%
- ret. Tmax -%
-
ret.LiF (*) 1089 44 100 1 099 5 1 098 2 LiF > 1 120 ? - l00 1 059 23 1 064 20 LiCl (*) - 100 > 1085 ? 727 0 595 75 746 0 LiI > 1100 ? 634 80 < 400 0 1 024 0 891 5 KF (*) 1016 50 - 100 937 5 975 15 KC1 (*) 1013 59 > 1 100 ? 787 5 1 020 45 K1 (*) . . 970 60 - 100 712 0 1 157 30 MgO
(*l
-
100 - 100 ? 60 950 20 CaS (*)-
100 931 80 920 78 956 70 CdO (*) - 100 - 100 ? 80 ? 80 CaO (*)-
100-
100 618 18 903 30 BaO-
100-
100 639 30 1 150 40 831 2 MnO (*) - 100 - 100 ? 70 1110 80 TiO (*) - 100 - l00 ? 85 - 100VOLATILISATION OF RECOIL PRODUCTS
T,,, values and
%
retained for dzfferent chlorides (N2/CCl, ; 2 l/h ; 7-10 K/min)Tc Te Xe
Matrix Tmax
%
ret. Tma,%
ret. Tmax%
ret.- -
-
-
- Tmax - RbCl (*) 765 20 841 2 745 0 575 823 CsCl (*) 701 15 799 2 628 2 581 719 LiCl (*) 902 2 806 2 760 0 726 I%
ret. 80 0 80 5 0 SrCl, (*) 955 0 990 0 927 0 783 0 SrCl, 628 60 784 0 738 10 621 0 848 0 BaC1, 649 55 764 15 743 15 662 10 948 10 YCl, 689 0 783 0 405 65 ? 0 521 40 690 3(*) Pretreated for 1 h at 500-600 OC.
the moment it is not possible to explain all the different effects, the main results are pointed out in the follow- ing :
Xe.
-
In the case of Xe chemical reactions can beexcluded. Therefore there is no difference between the inert gas- and the reactive gas experiments (LiCl, YCI,). The Tmax values spread over a wide range (370 K-1 099 K) which indicates that the influence of the crystal structure or the chemical composition of the matrix is ambiguous. It is obvious that the amount retained by the matrices is smaller for most of the halogen systems than for the oxigen systems. The most remarkable results are the threeLax
values for YC1, and, of course, the two T,,,, values for BaO. This effect leads to the assumption that the release behaviour is determined by different diffusion and/or annealing processes, which have discrete activation energies.I, Te, Tc. - For these elements the release behaviour is even more complex, as chemical reactions must be taken into account as well. The reaction partner of a recoil product can be either the reactive gas or the matrix itself. In the reactive gas experi-
ments almost 100
%
of each element is volatilized before the melting point of the matrix is reached. As was expected Tc and Te are not released from the oxide systems in the inert gas experiments. The vola- tilisation of I from these systems indicates that iodine can be released as element or radical as well. Some of the thermograms show two Tm,-values as was already mentioned for Xe.The results of the experiments described reveal the whole complexity of the systems under investigation. The fact, however, that it is possible now to describe the isothermals mathematically represents the first step to a clearer characterisation and with this a .much better understanding of the different processes. There are also first indications of a relationship between the release phenomena and some crystallo- graphic data of the polycristalline materials. Therefore we expect that further experiments will enable us to present a satisfactory description of most release phenomena. From a qualitative point of view, the vola- tilisation of recoil products from ionic crystals surely is a separation technique, which may be used efficiently in solving many technical and analytical problems.
References
[ l ] FELIX, F. W., HMI-B 93 Juni (1970).
[2] ONG, A. S., ELLEMANN, T. S., J. Nucl. Mater. 42 (1972) 191.
[3] ELLEMANN, T. S., FOX, C. H., MEARS, L. D., J. Nucl. Mater. 30 (1969) 89.
[4] KELLY, R., MATZKE, Hj., J. Nucl. Mater. 17 (1965) 179.
[5] ONO, A. S., ELLEMANN, T. S., Nucl. Znstr. Meth. 86 (1970) 117.
[6] NEIDHART, B., BXCHMANN, K., J. Radioanal. Chem. 13 (1973) 53.
[7] NEIDHART, B., BXCHMANN, K., KRXMER, S., LINK, I.,
Radiochem. Radioanal. Lett. 12 (1972) 59.
[g] STOJANIK, B., LAUER, R., NEIDHART, B., BXCHMANN, K.,
Radiochem. Radioanal. Lett. 25 (1976) 317.
[9] SESTAK, J., SATAVA, V., WENDLANDT, W. W., Thermochim. Acta 7 (1973) 335.
[l01 VAN HEEK, K. H., J ~ ~ N T G E N , H., Ber. Bunsenges. P h y ~ . Chem. 72 (1968) 1223.
[l11 SATAVA, V., Thermochinz. Acta 2 (1971) 423. [l21 SESTAK, J., Thermochim. Acta 3 (1971) 475.
[l31 SHARP, H. J., BRINDLEY, G. W., NARAKARI ACHAR, B. M.,
J. Amer. Ceram. Soc. 49 (1966) 379. [l41 SESTAK, J., Thermochim. Acta 3 (1971) 150.
[l51 DAMM, J. Z., TOMPKIN, F. C., Disc. Faraday Soc. 31 (1961) 184.
DISCUSSION K. R~~SSLER. - IS there any influence of the carrier
gas (CCl, etc.) on bulk diffusion via interaction with dopent-gas (Cl, C etc.) or on surface reactions ?
B. NEIDHARDT. - There is no influence of the reac- tive carrier gas on bulk diffusion ; neither in the iso- thermal nor in the non-isothermal experiments. See figure 3 : same T,,, values of Xe for the different gas mixtures. However, there is a significant shift of the
T,,, values of I, Tc, and Te when using different carrier gases. This effect is due to chemical surface reactions.
Hj. MATZKE. - Your experiments might turn out
to be very useful in bridging the gap between two
precious types of work on the same problems, i. e. those performed with homogeneously labelled samples (mostly done in Berlin by Felix et al. at the HMI) and with ion bombarded samples (mostly done by Kelly, Kornelsen, Jech, myself et al.). In the first case, one deels with long diffusion distance ( sample dimension,
