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Leach test characterisation of cement-based nuclear waste forms
NUCLEAR
AM)
cmhmxi,
WASTE
MANAGEMENT,
Vol. 6, PP. 24143.1986
Printed in the USA. All rights reserved.
0191-81SW86S3.00+ .oO Copyright0 1986Pergamon Journals Ltd.
LEACH TEST CHARACTERISATION OF CEMENT-BASED NUCLEAR WASTE FORMS A. Atkinson, K. Nelson, and T. M. Valentine Materials Development Division, Building 552, AERE Harwell, Didcot, Oxon OX11 ORA, United Kingdom
ABSTRACT. The release of radionuclides from two cement-based waste forms has been measured experimentally using five
different leach testing procedures. One waste form was a simple OPC matrix whereas the other contained a contaminated zeolite ion exchange material. The tests were also simulated mathematically using only two parameters, Deff and (Y,to characterise each radionuclide/waste form combination. Deff is an effective diffusion coefficient that describes the kinetic behaviour and is most easily measured at high flowrates (e.g., in a Soxhlet test), whereas 01describes the distribution of radionuclide between aqueous and solid phases at equilibrium and is best measured in a static test. It was established that these two parameters could describe the behaviour of each radionuclide/waste form combination in the different tests reasonably well and are therefore convenient parameters for characterising leaching performance. The product cuD,rfis approximately constant and equal to the intrinsic diffusion coefficient in the cement matrix. It is concluded that in these cement-based matrices the main reason for different radionuclides having different leaching characteristics is their different chemical interaction with the solid phases (and expressed by IX).The apparent equilibria reached with monolithic specimens are significantly different from those reached using crushed material.
INTRODUCTION The ability of a particular waste/matrix combination to resist release of radionuclides into water is usually assessed by using some kind of leach test carried out on miniature waste forms in the laboratory. To meet this end, a wide variety of leach testing procedures has been devised by workers throughout the world; all with slightly different objectives or constraints (e.g., simplicity and ease of operations). There are two main questions which arise regarding such leach tests. The first is, “How can the results be used (if at all) to predict the performance of a real waste form in, for example, a flooded repository?” This question will not be considered here despite its obvious importance. The second is, “How do the results of different tests relate one to another, and can they all be rationalised in terms of a few characteristic parameters and, eventually, the physics and chemistry of the important processes?” The work which is presented here addresses this second question by examining the behaviour of two cement-based waste forms (containing up to three ra-
RECEIVED6/2/86; ACCHPTJSD8/6/86. Acknowledgement-This work has been commissioned by the U.K. Department of the Environment as part of its radioactive waste research program. The results will be used in the formulation of Government policy, but at this stage do not necessarily represent Government policy. 241
dionuclides) in a range of five laboratory leach tests. The specific objective is to explore whether each radionuclide/waste form combination can be characterised by just two parameters (one describing equilibrum and one describing kinetics) which will explain behaviour in all five tests. EXPERIMENTS Materials
The two waste forms that were used were a simple OPC (Ordinary Portland Cement) waste form and one comprising a mixture of a zeolite ion exchange medium (clinoptilolite) and OPC. In the case of the OPC waste form, the radionuclides (13’Cs, 85Sr, and 5gFe) were dissolved in the water that was used to mix up the cement (water/cement ratio = 0.3) and no inactive carriers were added. These radionuclides provide a broad cross section of behaviour in that Cs is soluble in the aqueous phase but is not taken into solid solution; Sr is likely to be taken into solid solution substituting for Ca; and Fe is only slightly soluble in the aqueous phase and is likely to be taken into solid solution substituting for Al. In the case of the OPC/clinoptilolite waste form, the radionuclides which are dominant in this waste (13’Cs and g’Sr) were exchanged onto the zeolite prior to mixing with cement (1) and the weights of clinoptilolite, OPC, and free water were in the ratio 1:l:OS.
242
A.ATlUNSON,K.NELSON,ANDT.M.VALKN'IlNE
The approximate composition of clinoptilolite is (Na,K),O*Al,O,*lOSiO~~8H10, and that used in this study was the natural mineral (Mudhills, NV) with a nominal granule size of 420-840 pm. Inactive Cs and Sr carrier had been added to the radioactive tracers such that the effective total loading was approximately 0.01 mole Cs (or Sr) per litre of clinopilolite. All samples were in the form of right cylinders, approximately 25 mm in diameter (geometrical surface area approximately 35 cmz), and, after casting and vibro-compacting, were cured for 2 days at 20 “C followed by 3 days at 55 “C. Leach Tests The leach tests that were used will be referred to as Soxhlet, “ISO”, low flow, static, and crushed equilibrium. They differ mainly in the effective flow-rate of leachant past the specimen. In all tests the leachant was demineralised water (Analar grade) and the temperature was 25 “C. The test configurations are illustrated schematically in Figs. 1 and 2. The radiochemical analysis of solutions was carried out using y-spectrometry for all isotopes except 9oSr,which only emits fl radiation. 90Srwas analysed by adding inactive Sr carrier and then precipitating the carbonate with Na2COJ. Since this is a destructive technique it could not be used in those experiments requiring the return of the leachate after analysis. In the Soxhlet test (Ref. 1 and Fig. la), the specimen was exposed to a continuous stream of freshly
8oxhl.t
Toot
distilled water in a chamber which empties by syphonic action when full. The emptying frequency was approximately once per hour and the effective flow rate approximately 2 liter day-‘. After the passage of a convenient period of time, the leachate in the flask was analysed and replaced with fresh leachant. The “ISO” test (Fig 2a) is a variant of the quasistatic Hespe test (2) in which the volume of leachant (approximately 85 mL in the present experiments) was renewed and analysed at predetermined intervals (every day, for the first 9 days, and every week thereafter). The renewal rates correspond to average flow rates of 85 mL day-‘, for the first 9 days, and 12.1 mL day-’ subsequently. The low flow test (Fig. 1b) was one in which leachant from a reservoir was pumped at a rate of 4.6 mL day-l through a cavity containing the specimen in a PTFE chamber and collected in another container on exit. The volume of liquid in the chamber (i.e., the volume of the chamber minus the volume of the specimen) was approximately 43 mL. The static test (Fig. 2b) was similar in principle to the MCCI test (3). The specimen was immersed in 100 mL of leachant and the leachate was sampled by non-destructive analysis from time to time and then returned after analysis. In the crushed equilibrium test (Fig. 2c) the monolithic waste form was broken in a hydraulic press and then crushed further using steel balls in a vibro-mill. Approximately 10 g of the size fraction below 1.2
Low-flow
Twt
_ Porlrtaltlo
Pump (4.6mld-‘1
8yphon Loachate
3
FIGURE 1.
Schematic diagrams illustrating (a) the Soxhlet and (b) the low-flow leaching tests used in these experiments.
~*1.2WWll
M* =loa SMpkmdrotwn
Samtomdretwn
FIGURE 2. Schematic diagrams illustrating (a) the “ISO” test, (b) static test, and (c) crushed equilibrium test used in these experiments.
mm was immersed in 24 mL of leachant and the approach to equilibrium was monitored as in the static test. The crushed material was contained in a “Visking” dialysis tube so that no fine particulates would be mistaken for ions in aqueous solution. RESULTS
The results of all the experiments (except the equilibria) have been expressed in terms of an equivalent depletion depth, xD, given by the expression
(1) where AL(f)is the total activity which has been leached into the liquid up to time t, A,(O) is the total activity of the solid sample at 1 = 0, V, is the volume of the solid sample (including internal porosity), and Ss is its geometrical surface area. Thus x0 is the depth of sample (from the surface) in which the quantity of radionuclide initially present was equal to that which has been leached into solution. (N.B.: This does not necessarily imply that there is no radionuclide present within this depth after leaching, since this depends on the leaching mechanism.) In the case of the low flow leaching test, only that activity which had emerged from the leaching cell has been included in calculating xD, i.e., the activity in the 43 mL of liquid which resides within the cell has been excluded. The experimental results for leaching of 13’Csand 85Srfrom the OPC waste form (monolithic samples) are shown in Figs. 3 and 4, respectively. No results are given for 59Febecause the concentration of this isotope in the leachates was always below the limit of detection. The loss of Cs from this waste form is always greater than that of Sr and this is consistent with the observations made by other workers on similar systems (4-6).
The analogous results for the OPUclinoptilolite waste form are shown in Figs. 5 and 6. Release from this waste form is much less than for the OPC waste form because of the absorptive properties of the zeolite. Since cement-based waste forms are porous, the kinetics of leaching often approximate to those of a diffusion-controlled process. This is at its simplest in the case of the Soxhlet test which approximates to diffusion from a plane surface into an infinite volume at zero concentration (provided that dissolution of the matrix itself may be neglected). For such a process we expect parabolic leaching kinetics which may be characterised by an effective diffusion coefficient, &, given by (1) Deff =
$- $.
We have estimated & for each radionuclide/waste form combination by applying Eq. (2) to the Soxhlet data at the longest times, and the results are given in Table 1. Also shown are results obtained in an earlier study using a similar OPC/clinoptilolite waste form (l), and they are seen to be in reasonable agreement with the present results. The equilibria are conveniently expressed as a distribution coefficient, (Y,describing the partitioning of radionulcide between the water-saturated solid sample and the surrounding liquid. In terms of the parameters measured in the test, (;Y,is given by the equation (3) and is identical to the “capacity factor” which is sometimes used to describe diffusion and chemical interactions in porous media (7). For a simple sorption process CY is related to the gravimetric distribution coefficient K., by
244
A. ATKINSON,K. NELSON, AND T. M. VALFBTINE 137cs OPC --Experiment Soxhlet
+-+A
Simulation D cff
= 1.L
x 10s8
0’
s-’
a= 2.5
10
20
30
LO Time
50
60
70
80
(d I
FIGURE 3. Leaching of L3’Csfrom the simple OPC waste form. x0 is the equivalent depth of the matrix which is apparently depleted of Cs. The experimental results are shown in the upper part of the figure and the simulated results (using the indicated values for Deft and or)in the lower part.
a =
E +
eKd,
(4)
E is the volume fraction of porosity and e is the apparent density of the porous medium. If, on the other hand, equilibrium is controlled by a solubility limit, then (ILis the ratio of the average concentration in the waste form to the limiting aqueous solubility (for (Y * 1). The values of (Ydeduced from both the long time limit of the static test and the crushed equilibrium test are given in Table 1. In all cases a! measured in the static test is greater than in the crushed equilibrium test. In the one case where comparison can be made with previous work (“‘Cs from OPC/ clinoptilolite in the static test) the results are in reasonable agreement. The limits of detection for “Fe indicate that for this isotope (Yis greater than 104. The approach to equilibrium for the OPC waste form in the static test, using monolithic samples, and
where
in the crushed equilibrium test is shown in Fig. 7. similar plots for 13’Cs equilibration in the OPWclinoptilolite waste forms indicated that equilibrium was
reached after approximately 10 days for crushed material, but more than 40 days were necessary for the monolithic samples. From Fig. 7 it can be seen that for 13’Csthe leachate had not fully equilibrated with the monolithic OPC waste form even after 100 days.
SIMULATIONS The kinetics of the Soxhlet test and the long-time equilibrium limit of the static test are, in principle, characterised by the parameters Z& and Q[, respectively, as described above. It is not such a simple matter to interpret the “ISO” test, the low flow test, and the approach to equilibrium in the static test. The approach we have taken here is to see whether the parameters Deffand a! can also reproduce the results of the other tests when used in a simple mathematical simulation of the test procedures. The mathematical model used for the simulation is described in the Appendix. We assume release of radionuclide from the
245
=srOPC 0,a-
I
I
I
I
I
I
I
I
LO
50
60
70
a0
Experiment 0, ,6-
-2 ,L-
50
; 0
0
0 .a-
Simulation II cff = 5.2 x lO-‘O cm’ a=150
s-l
,6-
-3 E
$0
0,
0
10
20
30
Time Id1 @‘IcuRE 4.
Leaching Of “Sr from the simple OPC waste form.
porous solid ,waste form into the test volume of wellstirred liquid (i.e., concentration gradients in the free liquid are neglected) which may be unchanged (static test), replenished at intervals (“IW test) of flowing at a constant rate (low flow test). The equations that describe release from the waste form (and hence the eventual simulated leaching behaviour) are different depending on whether the equilibrium with the solid
Waste Form Radianudide Da
(Soxhk)
cm’ set-’
OPC “‘CS
OPC %r
OPCXlino “‘CS
OPUClino POSr
1.4 x lo-’
5.2 x lo-‘0 150
2.8 x lo-” 7800
1.5 x lo-”
a (smic)
2.5
a (crushd arlnifibrium) D,, a (?mtic) ad Xc-I
D&(.%&let) Q (static) Ddf u (static)
cm12Sec-’ cd
xc-’
phase is controlled by sorption or solubility. Consequently, in order to apply the simulation it is necessary to make such a choice. In the applications described here the equilibria were assumed to be co&rolled by sorption. This is probably true for the OPC/clinoptilolite waste form and for Cs in OPC, but not for Sr in OPC. However, the choice is not critical since the depletion depths predicted from the two models
3.5
0.26 x lo-’
-
7.8
25 x lo-.’
Prmious work
(ref. 1) -
2.2
2400 x 10-e
x lo-” 6000 3.5 x 10-a
5.8
510
2.3
x IO-” 1500 3.5 x 10“
A. ATKIN~N, IL NELSON, AND T. M. V“‘Cd
D.ff
= 2.8 x 10”
OPC IClinoptilolite
cm2 s-’
Time FIGURE 5.
(dl
Leach@ of *WQ from the OPC/clinoptilolite waste form.
(sorption or solubility control), using the same values of D,rr and cr, do not differ by more than 10%. The resulting differential equations were solved numerically to give the simulated depletion depth, xD, as a function of time. The input parameters used in the simulations were those of the experimental test procedures, and the values of D,rr and CYwere taken from the Soxhlet and static tests (Table 1). The experimental data and the simulations have thereby been forced to agree at the final points of the Soxhlet and static tests. The simulated leaching kinetics are compared with the corresponding experimental data in Figs. 3 to 6.
In broad terms the simulations are able to give an acceptable representation of the experimental data, thereby demonstrating that all the tests (on the monolithic samples) can be character&d by a single pair of parameters (Deft and or) for each waste form/radionuclide combination. However, there are several features of the experimental data which the simulations are unable to reproduce. The most significant deficiency of the simulated data is that in all cases radionuclide release in the “ISO” test is overestimated. This is illustrated in a slightly different way for OPUclinoptilolite in Fig. 8. In this diagram the concentration in the liquid
“Sr
&peri
OPCI Clinoptilolite
ment
+ +
+ Soxhlet
+ + + + + +
+ +
+ +
+
++
s I
I I
I
I
I I
I
Simulation DIff = 1.5 x 10’” cm* s-1
v
OO
I
20 FIGURE 6.
I
40
I
60
1
80 Time
I
100
I
140
I
I
160
__I
IdI
Leaching of ‘osr from the OPCMaoptilolite
phase, relative to that expected at the eventual equilibrium, is plotted as a function of time in the IS0 test. It can be seen that although the general form of the experimental data is reproduced by the simulations, the actual concentrations are much higher in the simulations than in the experiments (by about a factor of 2 for both lJ7Csand 9oSr).Similarly, there is a general tendency in all the results for the simulation to overestimate release in the low flow test. The reason for this is not clear. It could either be related to an overestimate of DdI in the Soxhlet test (e.g., as a result of significant dissolution of the cement matrix), or to the invalidity of some of the assumptions used
I
120
I I
waste form.
in the simulation (e.g., a well-stirred solution was assumed, even though stirring was not carried out in the experiments). There are other deviations from ideal behaviour, assumed in the simulations, which are specific to each waste form. In the case of the OPC waste form the release at short times is faster than predicted by the simulations. This has also been reported by other workers using tests of the IS0 type (4,6). Such an effect in “ISO” tests can be caused by the change in frequency of leachant renewal and this may be illustrated with the aid of the simulations. In Fig. 9 the simulated data for leaching of 13’Csfrom OPC/clinopti-
248
A.ATKINSON,K.-N,AND I
I
I
I
IIIII
I
OPC Waste
form
I
in
I
static
0
1.0 -
0
T.M.VALJBWmE I
I’“‘~
I
-
tests
000
00
137 Cs
0
-
crushed
00
0 04 0
0
t:
137
l
d
2
0‘6l
l
l
Cs monolithic
.@O
l
04l
Sr crushed
l
0.2 -
0
0
AAAAA
0+ 1
AA
A
A
A
+
I
A
ot1191
A
A
AA A
a5 AAA&AA 1
AI
Ill,,
10
-
Sr monolithic I
I
100 Time
(day)
FIGURE 7. Experimental data showing the approach to equilibrium in the static test, using monolithic specimens, and the crushed equilibrium test. The waste formwas simple OPC. AL(~)is the total activity in the leachate at time t and As(O) is the total activity in the waste form at time zero.
0.6
I
I
I
I
I
I
OPC / Clinoptitolite 0.7
I
I ‘ISO’
I
I
test
-
5 3 ;
0*6--
z Cs (simulation)
ro-
,&,_o+’
-*o--o
‘OSr (experiment)
I
I
I
I
10
20
30
40
I 50 Time
I
I
I
I
I
60 (day)
70
80
90
100
FIG8. The degree of saturation in the liquid phase during “ISO” testing of the OPC/cllnoptllollte waste form. The exwtal data are compared with the predictions of the simulation.
249
lolite in the “ISO” test are plotted against square root of time, i.e., assuming simple diffusion into an infinite reservoir. The curve breaks into two parts, which could be mistaken for straight lines in actual experiments, at the time when the sampling frequency is changed. The apparent diffusion coefficients deduced from these two portions are both lower than the true &. At short times the apparent & is - 75% of the true one, whereas at long times it is only - 28% of the true one. Although it is evident that significant errors can be generated by this means, it cannot account for the experimental observations in OPC waste forms since the effect is also apparent in the experimental data from the Soxhlet test (Figs. 3 and 4). We must therefore conclude that the effect is not an artefact of the test procedure and is a true characteristic of the simple OPC waste form (e.g., related to the leaching of Na and K or a non-uniform initial distribution of radionuclide). In the case of the OPC/clinoptilolite waste forms the opposite effect is seen in that the release is slower than expected at short times. Again this has been observed previously (1) and appears to be a characteristic of these waste forms. DISCUSSION In the analysis described in the Appendix the intrinsic diffusion coefficient and the effective diffusion coefficient are related by Di z (rDeff
(5)
irrespective of whether equilibrium is controlled by sorption or solubility (Eqs. A.3 and A. 11). If this is true then the product oL& should be approximately constant for a given cement matrix. The product of D,ff from the Soxhlet test and (Yfrom the static test is given in Table 1 for the two radionuclides and two waste forms and is seen to be approximately constant (between 2 and 8 x 1Om8 cm2 set-‘) with a mean value of about 4 x lo-* cm’ set-‘. This is compatible with Di measured directly on cement samples (7) which ranges from 0.8 x lo-’ cm2 set-’ for Cs’ to 4 x 10s8 cm2 set-’ for I- ions at a water/cement ratio of 0.4. We may therefore view the “immobilisation” of a radionuclide in a waste form as having two contributions. One is the physical hindrance to out-diffusion of the radionuclide. This is characterised by the intrinsic diffusion coefficient, Di, and is mainly a property of the waste form and not the radionuclide. The second is the chemical “immobilisation” which is characterised by the equilibrium parameter CYand is dependent on the radionuclide and the matrix (and probably also on the leachant composition in some cases). The performance of a radionuclide/waste form combination in a dynamic leaching test, such as the Soxhlet test, is thus determined by both processes expressed as D/CL
From the data in Table 1 it is evident that the equilibrium state (characterised by a) is influenced by the test procedure itself. Considerably smaller values of CY,which correspond to higher concentrations in aqueous solution, were observed using crushed samples rather than monolithic samples. The reason for this is not known, but may be due to better access to occluded porosity in the crushed material. The differences in (Ycan be large (an order of magnitude in the case of Cs in OPC) and therefore the degree of comminution of the waste form should be given some consideration when developing the methodology of equilibrium leach testing. The leaching test which has been most widely used on cement-based waste forms (because of ease of execution) is the IS0 test (often with minor modifications to the original specification). The results of such tests often approximate to simple diffusion-controlled kinetics (xD a fh’t)and are often summarised and reported as a diffusion coefficient, Dlso. In general DIso is not expected to be the same as Deffbecause of the different boundary conditions in the test procedures. The partial approach to equilibrium in the IS0 test (e.g., as illustrated in Fig. 8) leads to the con. . dmon that DIso I Ddf. The two “diffusion coefficients” are most nearly equal when o is small, as is evident from the data from Cs release from OPC in Fig. 3. Conversely, the relative difference between them is greatest when cyis large. This is evident for Cs release from OPC/clinoptilolite from the data in Fig. 5 and is illustrated in more detail in Fig. 9 where both the simulated and experimental data have been plotted accordingly to simple diffusion-controlled kinetics. The simulated data demonstrate the dangers of overinterpreting results from the IS0 test. For example, there appear to be two distinct regions having different diffusion coefficients (2.1 x lo-l2 cm2 see-’ and 0.77 x lo-l2 cm2 set-l), whereas the kinetics were generated from only a single effective coefficient (2.8 x lo-l2 cm2sec-I). The two regions are, in fact, the result of different frequencies of leachant change in the test procedure. Furthermore, the diffusion coefficients deduced from these two regions are both less than Deff as appropriate to true zero concentration boundary conditions. However, this effect is not necessarily serious since in this simulation (i.e., an example in which the effect is large), Dlso is smaller than D,ff by only a factor of 4. The actual experimental data are also shown in Fig. 9. They give a good fit to simple diffusion kinetics over the whole time range, with little indication of the two regions predicted by the simulation. This is probably fortuitious, since the data from the Soxhlet test show significant departure from simple diffusion kinetics (lower release rate) for times up to about 50 days. The measured value of DIso (0.38 x lo-l2 cm2 set-I) is a factor of 2 lower than that deduced from the simulation at
A. ATKINSON,K.lVKLSON, AND I
I
I
I
137
Cs
I
I
I
T.M. VALENTWK
I
I
+/
OPC / Clinoplitolite
‘ISO’
test
/
b
4
-
+’ 0cff = 2.8 x10-‘* cm*,-’
/f
in simulation /+ 4 /‘: /+
Go=
/
0~77xlO”*cm**-’ (simulation)
7
/
+
P +
OQH
PA
FIGURE 9. The results for leaching of lsrCs from OPC/clinoptilollte in the “ISO” test plotted against t* (i.e., assuming diffusion into a large volume of lea&ant). The open circles are experimental data and the crosses are predicted by the simulation using Deff = 2.8 x 1O-Lz cm’ set-’ and Q = 7800.
long times. This again illustrates some deficiency in the assumptions made in the simulation. From the experimental data alone we conclude that &-, from the IS0 test can be almost an order of magnitude smaller than D,rf from the Soxhlet test. In Table 2 are assembled data from the literature (4-6, 8-l 1) of “diffusion coefficients” deduced from “ISO” tests on waste forms similar to the ones used in this study. When comparing these “diffusion coefficients” with D,_r in Table 1 the discussion of the preceding paragraph should be borne in mind. However, it is clear that the range of Diso for the OPC waste forms is far too wide to be accounted for by the features of the test procedure considered here. We conclude that the larger differences (greater than about an order of magnitude) are the result of real differences in the waste forms, but it is not possible to say whether the differences are predominantly physical (i.e., in DJ or chemical (i.e., in (Y)in nature. It is clear that leaching tests of the kind described here are incapable of yielding much insight into the detailed mechanisms of radionuclide release. The best that can be done is to separate out the physical and chemical contributions, and even this requires more than one test. For example, on the basis of the experiments described here, it is not possible to deter-
mine whether the chemical contribution to Sr “immobilisation” in OPC is due to sorption or to incorporation of Sr into the cement hydration products by solid solution. In order to answer this type of question it is necessary to measure additional parameters, and an example of such a study has been reported previously (1).
TABLE 2 Effective Diffusion Cueffklents Measured by Other Workers
in Tests of the %U” Type Dlso
Waste Form
Ref.
OPC/Na,SO, OPC/Na,SO, OPC/H,B,O, OPC/NalB,O, oPc/so:OPC (30 day cure) OPC (57 day cure) OPC (95 day cure) OPC/clinoptilolite OPC/clinoptilolite
8 6 4 5 9 10 10 10 10 11
(cm’s_*)
Cs
5 5 1.5 1 l-60 Oh-18 0.04-13 1 2-8
x x x x x x x x x
Sr
lo“0 lo-” 10-p 10-8 10-q lo+ 10-g 10-p lo-”
l-4 1 7 2.7 2
x x x x x
lo-” lo-” lo-” lo-” lo-‘0
9 x lo-”
251
CONCLUSIONS
It has been demonstrated that leaching of radionuelides from cement-based monolithic waste forms in a range of four different tests can be characterised approximately by only two parameters for each radionuclide/waste form combination. One is an effective diffusion coefficient. Defr,which describes the kinetics of release in the extreme limiting case when zero concentration is maintained at the surface of the waste form; a condition which is approached in a Soxhlet test. The other is the ratio of average concentration of radionuclide within the waste form to that in a small quantity of water in equilibrium with it, (II,and is measured in the long-time limit of a static test. The kinetic parameter, Deff, is approximately equal to the quotient DJcx, where D, is the intrinsic diffusion coefficient of ions in aqueous solution within the waste form. Thus the two parameters a! and Di are important characteristics of the radionuclide/waste form combination since cx quantifies the chemical contribution to “immobilisation” and Di the physical contribution. Since D, is probably relatively insensitive to the actual radionuclide, to a first approximation, the different observed leaching rates of different radionuclides from the same waste form are mainly caused by chemical differences (i.e., differences in ar). It must be concluded that there is no single leach test procedure that is universally “best.” The choice of the most appropriate test procedure is greatly dependent on the use for which the resulting data are intended. On the basis of these experiments we would make the following recommendations. (i) In equilibrium studies the present results show significant differences between monolithic and crushed samples. Since the reason for this difference is not understood, equilibrium tests should be done using both types of sample. (ii) For mechanistic studies of leaching processes, at least two tests are required; an equilibrium test and a dynamic test. The best dynamic test for this purpose is the Soxhlet test because of its simple boundary condition. However, this test is of no value if the intention is to study groundwater/waste form interactions. Either the IS0 or the low-flow test must then be used and the attendant problems of interpretation accepted. (iii) For comparing waste forms, and the effects of different groundwaters, the IS0 test is the most convenient. Nevertheless, it must be recognised that this test (or a continuous test similar to the low-flow test) does not measure a simple characteristic of the waste form and should not be overinterpreted. For example, the IS0 test can underestimate the true effective diffusion coefficient by up to an order of
magnitude and changes in the frequency of leachant renewal can distort the release kinetics significantly. Appendix: Mathematical Simulation of Leach Tests
The aim of modelbng the leach tests mathematically was to attempt to reproduce the behaviour observed experimentally in the test while using only the two parameters Deffand (;Y to characterise the radionuclide in the waste form. The mathematical analysis is based on the following simplifying assumptions. (i) The transport of radionuclide within the watersaturated waste form is by diffusion. (ii) All chemical reactions are locally at equilibrium and sorption is character&d by a linear isotherm (gravimetric distribution coefficient, Kd). (iii) The diffusion and chemical equilibrium parameters do not change with time. (iv) Transport in the liquid outside the waste form is sufficiently rapid that the concentration of a radionuclide is uniform in the leachate. (v) The effective depletion depth, x0, is sufficiently small in comparison with the size of specimen that diffusion in only one dimension need be considered. The equation which describes radionuclide solubility in the waste form is different from that which describes sorption. Therefore it is convenient to treat these two separately as extreme limiting cases. Radionuciide Chemistry Controlled by Sorption The concentrations in the solid and liquid phases within the waste form of this case are illustrated in Fig. 10a. We assume that the radionuclide is only mobile when in the liquid phase and therefore the sorption process reduces the diffusivity of the radionuclide by partial immobilisation. Transport within the waste form under such conditions may be conveniently described in terms of the average concentration in the water-saturated waste form c C> and characterised by the effective diffusion coefficient
acc> at
D
a*03
(A. 1)
The relationships with other quantities are (7) a! = E + &,
(A.2
Dcff= Di/Q,
(A.31
and
= c&X,
(4.4) where E is the volume fraction of porosity in the waste form, e its density, and 4 its diffusion coefficient for describing transport in terms of concentration in the liquid phase. Earlier modelbng studies (12)
252
A. ATKINSON, K. NELSON, AND T. M. VALENTINE (a)
controlled
Sorption-
release
Waste
Leachate
form
(
H
fusion from the waste form and that being removed, in the general case, by leachate flow. This is expressed by the equation.
-----‘S
I,/’ ;,b / 5oo/
I I
- FLCL.
Cl
(b)
Solubility
- controlled
Lcachate
Waste
In this equation < C> 0 is the average concentration initially in the waste form and < C> f that at time t. FL is the flow rate of the leachate and is required for simulation of the low-flow test. The mathematical procedures developed previously (12) were used to solve numerically the coupled Eq. (A.l) and (A.7) together with boundary conditions (A.5) and (A.6). Equation (1) was used to compute x0 as a function of time which could be compared with the experimentally measured values. Analytical solutions are available which are appropriate to the Soxhlet test and the static test (13), and these were used to check the accuracy of the numerical computations.
release
form
r-_-_---_cs
I
Csol
I CL
X
0
XD
FIGURE 10. Schematic diagram illustrating the spatial variation of radionuclide concentration in the liquid and solid phases when equilibrium between solid and liquid is controlled (a) by sorption and (b) by the aqueous solubiity limit of the radionuclide.
have shown that Eq. (A. 1) not only describes transport in a homogeneous medium but also in a heterogeneous medium (such as clinoptilolite in cement) under most conditions. To describe release from the waste form, Eq. (A. 1) must be solved with appropriate boundary conditions. These are that in the waste form, at sufficiently large distance from its surface, there is no transport, and that at its surface the leachate and waste form are in equilibrium, i.e.,
a ax
-
= 0 at x = X(X $ xD)
-CC> = c&,atx=
0.
(A.7)
(A.9
(A-6)
The temporal boundary condition is that CL = 0 at t = 0 for all the leach tests. In addition for the “ISO” test we have CL = 0 at t = tl, tl . . . t. where t, to t, are the times of leachant renewal. The concentration in the leachate, CL, is determined by mass balance between radionuclide entering by dif-
Radionuclide Chemistry Controlled by Solubility When radionuclide concentrations are solubilitylimited, the concentration distribution within the waste form is different from that of the previous case and is shown schematically in Fig. lob. In this case, it is convenient to assume that a quasisteady state (i.e., concentration gradient varying only with time) exists in the depletion zone, 0 < x < xD. This leads to the following equation for the depletion depth, x0,
du, = D.
xDdt
’
(Gor - CL) C, ’
(A.@
where C,, is the solubility limit. In the Soxhlet test, for which CL 4 Csol,Eq. (A.@ is integrated to give xi (Soxhlet) = 20, !$. s Comparing Eqs. (A.9) and (2) leads to the conclusion that in this case (A. 10) If we again use Eq. (A.4) to define (Y (as the average concentration in the waste form divided by the concentration in a small volume of liquid in equilibrium with it) then we have in this case (cf. A.3) Deff = ; D~/cY.
(A.ll)
The equation for mass balance analagous to (A.7) is j+L Ldt
-_
S$,
(GOI ’
-
XD
CL)
-
FLCL.
(A.12)
CEMENT-BASED NUCLEAR WASTE FORMS
This was solved numerically subject to the boundary conditions x0 = 0 and CL = 0 at t = 0, and that for the “HO” test CL = 0 at t = t,, tz . . . t,. REFERENCES 1. Atkinson, A., Nickerson, A. K., and Valentine, T. M., The mechanism of leaching from some cement-based nuclear waste forms, Rad. WasteManag. Nucl. Fuel Cycle 4: 357 (1984). International Standards Organisation. Long-term leach testing of radioactive waste solidification products, ISO/ TC85/SC5/WG38, Draft IS0 Standard ISO/DIS 6%1(1979). Mendel, J. E. The measurement of leach rates: A review. Nucl. Chem. WasteManag. 3: 117 (1982). Matsuzuru, H. and Moriyama, N. Leaching of radionuclides from a cement composite incorporating evaporator concentrates generated at a PWR nuclear power plant. Nucl. Sci. Eng. 80: 14 (1982). Bernard, A., Nomine, J. C., Cornet, G., Bonnet, A., and Farges, L. Long term leaching tests on full-scale blocks of radioactive wastes. Nucl. Chem. WasteManag. 3: 161 (1982).
253 6.
7.
8. 9.
10
11. 12.
13.
Matsuzuru, H. and Ito, A. Effect of dimension of specimen on amounts of 13’Cs, ‘?Sr and ‘OCo leached from matrix of hardened cement grout. J. Nucl. Sci. Technol. 15: 2% (1978). Atkinson, A. and Nickerson, A. K. The diffusion of ions through water-saturated cement. J. Mater. Sci. 19: 3068 (1984). Matsuzuru, H. and Ito, A. Leaching behaviour of strontium 90 in cement composites. Ann Nucl. Energy 4: 465 (1977). Amarantos, S. G., Papadokostaki, K. G., and Petropoulos, J. H. Comparison of leaching tests and study of leaching mechanisms. CEC report WAS-305-83-15-GR(B) (1984). Glasser, F. P., Rahman, A. A., Crawford, R. W., McCulloch, C. E., and Angus, M. J. Immobilisation and leaching mechanisms of radwaste in cement-based matrices. U.K. Department of the Environment report DOE/RW/83.093 (1983). Mikhail, K. Y. Radioactive waste treatment using zeolites. Ph.D. thesis, University of Salford (1981). Atkinson, A. Mathematical modelling of leaching from porous nuclear waste forms. Rad. W&e Manag. Nuclear Fuel Cycle 3: 371 (1983). Crank, J. The Mathematics of Diffusion. Clarendon Press, Oxford (1975).
AM)
cmhmxi,
WASTE
MANAGEMENT,
Vol. 6, PP. 24143.1986
Printed in the USA. All rights reserved.
0191-81SW86S3.00+ .oO Copyright0 1986Pergamon Journals Ltd.
LEACH TEST CHARACTERISATION OF CEMENT-BASED NUCLEAR WASTE FORMS A. Atkinson, K. Nelson, and T. M. Valentine Materials Development Division, Building 552, AERE Harwell, Didcot, Oxon OX11 ORA, United Kingdom
ABSTRACT. The release of radionuclides from two cement-based waste forms has been measured experimentally using five
different leach testing procedures. One waste form was a simple OPC matrix whereas the other contained a contaminated zeolite ion exchange material. The tests were also simulated mathematically using only two parameters, Deff and (Y,to characterise each radionuclide/waste form combination. Deff is an effective diffusion coefficient that describes the kinetic behaviour and is most easily measured at high flowrates (e.g., in a Soxhlet test), whereas 01describes the distribution of radionuclide between aqueous and solid phases at equilibrium and is best measured in a static test. It was established that these two parameters could describe the behaviour of each radionuclide/waste form combination in the different tests reasonably well and are therefore convenient parameters for characterising leaching performance. The product cuD,rfis approximately constant and equal to the intrinsic diffusion coefficient in the cement matrix. It is concluded that in these cement-based matrices the main reason for different radionuclides having different leaching characteristics is their different chemical interaction with the solid phases (and expressed by IX).The apparent equilibria reached with monolithic specimens are significantly different from those reached using crushed material.
INTRODUCTION The ability of a particular waste/matrix combination to resist release of radionuclides into water is usually assessed by using some kind of leach test carried out on miniature waste forms in the laboratory. To meet this end, a wide variety of leach testing procedures has been devised by workers throughout the world; all with slightly different objectives or constraints (e.g., simplicity and ease of operations). There are two main questions which arise regarding such leach tests. The first is, “How can the results be used (if at all) to predict the performance of a real waste form in, for example, a flooded repository?” This question will not be considered here despite its obvious importance. The second is, “How do the results of different tests relate one to another, and can they all be rationalised in terms of a few characteristic parameters and, eventually, the physics and chemistry of the important processes?” The work which is presented here addresses this second question by examining the behaviour of two cement-based waste forms (containing up to three ra-
RECEIVED6/2/86; ACCHPTJSD8/6/86. Acknowledgement-This work has been commissioned by the U.K. Department of the Environment as part of its radioactive waste research program. The results will be used in the formulation of Government policy, but at this stage do not necessarily represent Government policy. 241
dionuclides) in a range of five laboratory leach tests. The specific objective is to explore whether each radionuclide/waste form combination can be characterised by just two parameters (one describing equilibrum and one describing kinetics) which will explain behaviour in all five tests. EXPERIMENTS Materials
The two waste forms that were used were a simple OPC (Ordinary Portland Cement) waste form and one comprising a mixture of a zeolite ion exchange medium (clinoptilolite) and OPC. In the case of the OPC waste form, the radionuclides (13’Cs, 85Sr, and 5gFe) were dissolved in the water that was used to mix up the cement (water/cement ratio = 0.3) and no inactive carriers were added. These radionuclides provide a broad cross section of behaviour in that Cs is soluble in the aqueous phase but is not taken into solid solution; Sr is likely to be taken into solid solution substituting for Ca; and Fe is only slightly soluble in the aqueous phase and is likely to be taken into solid solution substituting for Al. In the case of the OPC/clinoptilolite waste form, the radionuclides which are dominant in this waste (13’Cs and g’Sr) were exchanged onto the zeolite prior to mixing with cement (1) and the weights of clinoptilolite, OPC, and free water were in the ratio 1:l:OS.
242
A.ATlUNSON,K.NELSON,ANDT.M.VALKN'IlNE
The approximate composition of clinoptilolite is (Na,K),O*Al,O,*lOSiO~~8H10, and that used in this study was the natural mineral (Mudhills, NV) with a nominal granule size of 420-840 pm. Inactive Cs and Sr carrier had been added to the radioactive tracers such that the effective total loading was approximately 0.01 mole Cs (or Sr) per litre of clinopilolite. All samples were in the form of right cylinders, approximately 25 mm in diameter (geometrical surface area approximately 35 cmz), and, after casting and vibro-compacting, were cured for 2 days at 20 “C followed by 3 days at 55 “C. Leach Tests The leach tests that were used will be referred to as Soxhlet, “ISO”, low flow, static, and crushed equilibrium. They differ mainly in the effective flow-rate of leachant past the specimen. In all tests the leachant was demineralised water (Analar grade) and the temperature was 25 “C. The test configurations are illustrated schematically in Figs. 1 and 2. The radiochemical analysis of solutions was carried out using y-spectrometry for all isotopes except 9oSr,which only emits fl radiation. 90Srwas analysed by adding inactive Sr carrier and then precipitating the carbonate with Na2COJ. Since this is a destructive technique it could not be used in those experiments requiring the return of the leachate after analysis. In the Soxhlet test (Ref. 1 and Fig. la), the specimen was exposed to a continuous stream of freshly
8oxhl.t
Toot
distilled water in a chamber which empties by syphonic action when full. The emptying frequency was approximately once per hour and the effective flow rate approximately 2 liter day-‘. After the passage of a convenient period of time, the leachate in the flask was analysed and replaced with fresh leachant. The “ISO” test (Fig 2a) is a variant of the quasistatic Hespe test (2) in which the volume of leachant (approximately 85 mL in the present experiments) was renewed and analysed at predetermined intervals (every day, for the first 9 days, and every week thereafter). The renewal rates correspond to average flow rates of 85 mL day-‘, for the first 9 days, and 12.1 mL day-’ subsequently. The low flow test (Fig. 1b) was one in which leachant from a reservoir was pumped at a rate of 4.6 mL day-l through a cavity containing the specimen in a PTFE chamber and collected in another container on exit. The volume of liquid in the chamber (i.e., the volume of the chamber minus the volume of the specimen) was approximately 43 mL. The static test (Fig. 2b) was similar in principle to the MCCI test (3). The specimen was immersed in 100 mL of leachant and the leachate was sampled by non-destructive analysis from time to time and then returned after analysis. In the crushed equilibrium test (Fig. 2c) the monolithic waste form was broken in a hydraulic press and then crushed further using steel balls in a vibro-mill. Approximately 10 g of the size fraction below 1.2
Low-flow
Twt
_ Porlrtaltlo
Pump (4.6mld-‘1
8yphon Loachate
3
FIGURE 1.
Schematic diagrams illustrating (a) the Soxhlet and (b) the low-flow leaching tests used in these experiments.
~*1.2WWll
M* =loa SMpkmdrotwn
Samtomdretwn
FIGURE 2. Schematic diagrams illustrating (a) the “ISO” test, (b) static test, and (c) crushed equilibrium test used in these experiments.
mm was immersed in 24 mL of leachant and the approach to equilibrium was monitored as in the static test. The crushed material was contained in a “Visking” dialysis tube so that no fine particulates would be mistaken for ions in aqueous solution. RESULTS
The results of all the experiments (except the equilibria) have been expressed in terms of an equivalent depletion depth, xD, given by the expression
(1) where AL(f)is the total activity which has been leached into the liquid up to time t, A,(O) is the total activity of the solid sample at 1 = 0, V, is the volume of the solid sample (including internal porosity), and Ss is its geometrical surface area. Thus x0 is the depth of sample (from the surface) in which the quantity of radionuclide initially present was equal to that which has been leached into solution. (N.B.: This does not necessarily imply that there is no radionuclide present within this depth after leaching, since this depends on the leaching mechanism.) In the case of the low flow leaching test, only that activity which had emerged from the leaching cell has been included in calculating xD, i.e., the activity in the 43 mL of liquid which resides within the cell has been excluded. The experimental results for leaching of 13’Csand 85Srfrom the OPC waste form (monolithic samples) are shown in Figs. 3 and 4, respectively. No results are given for 59Febecause the concentration of this isotope in the leachates was always below the limit of detection. The loss of Cs from this waste form is always greater than that of Sr and this is consistent with the observations made by other workers on similar systems (4-6).
The analogous results for the OPUclinoptilolite waste form are shown in Figs. 5 and 6. Release from this waste form is much less than for the OPC waste form because of the absorptive properties of the zeolite. Since cement-based waste forms are porous, the kinetics of leaching often approximate to those of a diffusion-controlled process. This is at its simplest in the case of the Soxhlet test which approximates to diffusion from a plane surface into an infinite volume at zero concentration (provided that dissolution of the matrix itself may be neglected). For such a process we expect parabolic leaching kinetics which may be characterised by an effective diffusion coefficient, &, given by (1) Deff =
$- $.
We have estimated & for each radionuclide/waste form combination by applying Eq. (2) to the Soxhlet data at the longest times, and the results are given in Table 1. Also shown are results obtained in an earlier study using a similar OPC/clinoptilolite waste form (l), and they are seen to be in reasonable agreement with the present results. The equilibria are conveniently expressed as a distribution coefficient, (Y,describing the partitioning of radionulcide between the water-saturated solid sample and the surrounding liquid. In terms of the parameters measured in the test, (;Y,is given by the equation (3) and is identical to the “capacity factor” which is sometimes used to describe diffusion and chemical interactions in porous media (7). For a simple sorption process CY is related to the gravimetric distribution coefficient K., by
244
A. ATKINSON,K. NELSON, AND T. M. VALFBTINE 137cs OPC --Experiment Soxhlet
+-+A
Simulation D cff
= 1.L
x 10s8
0’
s-’
a= 2.5
10
20
30
LO Time
50
60
70
80
(d I
FIGURE 3. Leaching of L3’Csfrom the simple OPC waste form. x0 is the equivalent depth of the matrix which is apparently depleted of Cs. The experimental results are shown in the upper part of the figure and the simulated results (using the indicated values for Deft and or)in the lower part.
a =
E +
eKd,
(4)
E is the volume fraction of porosity and e is the apparent density of the porous medium. If, on the other hand, equilibrium is controlled by a solubility limit, then (ILis the ratio of the average concentration in the waste form to the limiting aqueous solubility (for (Y * 1). The values of (Ydeduced from both the long time limit of the static test and the crushed equilibrium test are given in Table 1. In all cases a! measured in the static test is greater than in the crushed equilibrium test. In the one case where comparison can be made with previous work (“‘Cs from OPC/ clinoptilolite in the static test) the results are in reasonable agreement. The limits of detection for “Fe indicate that for this isotope (Yis greater than 104. The approach to equilibrium for the OPC waste form in the static test, using monolithic samples, and
where
in the crushed equilibrium test is shown in Fig. 7. similar plots for 13’Cs equilibration in the OPWclinoptilolite waste forms indicated that equilibrium was
reached after approximately 10 days for crushed material, but more than 40 days were necessary for the monolithic samples. From Fig. 7 it can be seen that for 13’Csthe leachate had not fully equilibrated with the monolithic OPC waste form even after 100 days.
SIMULATIONS The kinetics of the Soxhlet test and the long-time equilibrium limit of the static test are, in principle, characterised by the parameters Z& and Q[, respectively, as described above. It is not such a simple matter to interpret the “ISO” test, the low flow test, and the approach to equilibrium in the static test. The approach we have taken here is to see whether the parameters Deffand a! can also reproduce the results of the other tests when used in a simple mathematical simulation of the test procedures. The mathematical model used for the simulation is described in the Appendix. We assume release of radionuclide from the
245
=srOPC 0,a-
I
I
I
I
I
I
I
I
LO
50
60
70
a0
Experiment 0, ,6-
-2 ,L-
50
; 0
0
0 .a-
Simulation II cff = 5.2 x lO-‘O cm’ a=150
s-l
,6-
-3 E
$0
0,
0
10
20
30
Time Id1 @‘IcuRE 4.
Leaching Of “Sr from the simple OPC waste form.
porous solid ,waste form into the test volume of wellstirred liquid (i.e., concentration gradients in the free liquid are neglected) which may be unchanged (static test), replenished at intervals (“IW test) of flowing at a constant rate (low flow test). The equations that describe release from the waste form (and hence the eventual simulated leaching behaviour) are different depending on whether the equilibrium with the solid
Waste Form Radianudide Da
(Soxhk)
cm’ set-’
OPC “‘CS
OPC %r
OPCXlino “‘CS
OPUClino POSr
1.4 x lo-’
5.2 x lo-‘0 150
2.8 x lo-” 7800
1.5 x lo-”
a (smic)
2.5
a (crushd arlnifibrium) D,, a (?mtic) ad Xc-I
D&(.%&let) Q (static) Ddf u (static)
cm12Sec-’ cd
xc-’
phase is controlled by sorption or solubility. Consequently, in order to apply the simulation it is necessary to make such a choice. In the applications described here the equilibria were assumed to be co&rolled by sorption. This is probably true for the OPC/clinoptilolite waste form and for Cs in OPC, but not for Sr in OPC. However, the choice is not critical since the depletion depths predicted from the two models
3.5
0.26 x lo-’
-
7.8
25 x lo-.’
Prmious work
(ref. 1) -
2.2
2400 x 10-e
x lo-” 6000 3.5 x 10-a
5.8
510
2.3
x IO-” 1500 3.5 x 10“
A. ATKIN~N, IL NELSON, AND T. M. V“‘Cd
D.ff
= 2.8 x 10”
OPC IClinoptilolite
cm2 s-’
Time FIGURE 5.
(dl
Leach@ of *WQ from the OPC/clinoptilolite waste form.
(sorption or solubility control), using the same values of D,rr and cr, do not differ by more than 10%. The resulting differential equations were solved numerically to give the simulated depletion depth, xD, as a function of time. The input parameters used in the simulations were those of the experimental test procedures, and the values of D,rr and CYwere taken from the Soxhlet and static tests (Table 1). The experimental data and the simulations have thereby been forced to agree at the final points of the Soxhlet and static tests. The simulated leaching kinetics are compared with the corresponding experimental data in Figs. 3 to 6.
In broad terms the simulations are able to give an acceptable representation of the experimental data, thereby demonstrating that all the tests (on the monolithic samples) can be character&d by a single pair of parameters (Deft and or) for each waste form/radionuclide combination. However, there are several features of the experimental data which the simulations are unable to reproduce. The most significant deficiency of the simulated data is that in all cases radionuclide release in the “ISO” test is overestimated. This is illustrated in a slightly different way for OPUclinoptilolite in Fig. 8. In this diagram the concentration in the liquid
“Sr
&peri
OPCI Clinoptilolite
ment
+ +
+ Soxhlet
+ + + + + +
+ +
+ +
+
++
s I
I I
I
I
I I
I
Simulation DIff = 1.5 x 10’” cm* s-1
v
OO
I
20 FIGURE 6.
I
40
I
60
1
80 Time
I
100
I
140
I
I
160
__I
IdI
Leaching of ‘osr from the OPCMaoptilolite
phase, relative to that expected at the eventual equilibrium, is plotted as a function of time in the IS0 test. It can be seen that although the general form of the experimental data is reproduced by the simulations, the actual concentrations are much higher in the simulations than in the experiments (by about a factor of 2 for both lJ7Csand 9oSr).Similarly, there is a general tendency in all the results for the simulation to overestimate release in the low flow test. The reason for this is not clear. It could either be related to an overestimate of DdI in the Soxhlet test (e.g., as a result of significant dissolution of the cement matrix), or to the invalidity of some of the assumptions used
I
120
I I
waste form.
in the simulation (e.g., a well-stirred solution was assumed, even though stirring was not carried out in the experiments). There are other deviations from ideal behaviour, assumed in the simulations, which are specific to each waste form. In the case of the OPC waste form the release at short times is faster than predicted by the simulations. This has also been reported by other workers using tests of the IS0 type (4,6). Such an effect in “ISO” tests can be caused by the change in frequency of leachant renewal and this may be illustrated with the aid of the simulations. In Fig. 9 the simulated data for leaching of 13’Csfrom OPC/clinopti-
248
A.ATKINSON,K.-N,AND I
I
I
I
IIIII
I
OPC Waste
form
I
in
I
static
0
1.0 -
0
T.M.VALJBWmE I
I’“‘~
I
-
tests
000
00
137 Cs
0
-
crushed
00
0 04 0
0
t:
137
l
d
2
0‘6l
l
l
Cs monolithic
.@O
l
04l
Sr crushed
l
0.2 -
0
0
AAAAA
0+ 1
AA
A
A
A
+
I
A
ot1191
A
A
AA A
a5 AAA&AA 1
AI
Ill,,
10
-
Sr monolithic I
I
100 Time
(day)
FIGURE 7. Experimental data showing the approach to equilibrium in the static test, using monolithic specimens, and the crushed equilibrium test. The waste formwas simple OPC. AL(~)is the total activity in the leachate at time t and As(O) is the total activity in the waste form at time zero.
0.6
I
I
I
I
I
I
OPC / Clinoptitolite 0.7
I
I ‘ISO’
I
I
test
-
5 3 ;
0*6--
z Cs (simulation)
ro-
,&,_o+’
-*o--o
‘OSr (experiment)
I
I
I
I
10
20
30
40
I 50 Time
I
I
I
I
I
60 (day)
70
80
90
100
FIG8. The degree of saturation in the liquid phase during “ISO” testing of the OPC/cllnoptllollte waste form. The exwtal data are compared with the predictions of the simulation.
249
lolite in the “ISO” test are plotted against square root of time, i.e., assuming simple diffusion into an infinite reservoir. The curve breaks into two parts, which could be mistaken for straight lines in actual experiments, at the time when the sampling frequency is changed. The apparent diffusion coefficients deduced from these two portions are both lower than the true &. At short times the apparent & is - 75% of the true one, whereas at long times it is only - 28% of the true one. Although it is evident that significant errors can be generated by this means, it cannot account for the experimental observations in OPC waste forms since the effect is also apparent in the experimental data from the Soxhlet test (Figs. 3 and 4). We must therefore conclude that the effect is not an artefact of the test procedure and is a true characteristic of the simple OPC waste form (e.g., related to the leaching of Na and K or a non-uniform initial distribution of radionuclide). In the case of the OPC/clinoptilolite waste forms the opposite effect is seen in that the release is slower than expected at short times. Again this has been observed previously (1) and appears to be a characteristic of these waste forms. DISCUSSION In the analysis described in the Appendix the intrinsic diffusion coefficient and the effective diffusion coefficient are related by Di z (rDeff
(5)
irrespective of whether equilibrium is controlled by sorption or solubility (Eqs. A.3 and A. 11). If this is true then the product oL& should be approximately constant for a given cement matrix. The product of D,ff from the Soxhlet test and (Yfrom the static test is given in Table 1 for the two radionuclides and two waste forms and is seen to be approximately constant (between 2 and 8 x 1Om8 cm2 set-‘) with a mean value of about 4 x lo-* cm’ set-‘. This is compatible with Di measured directly on cement samples (7) which ranges from 0.8 x lo-’ cm2 set-’ for Cs’ to 4 x 10s8 cm2 set-’ for I- ions at a water/cement ratio of 0.4. We may therefore view the “immobilisation” of a radionuclide in a waste form as having two contributions. One is the physical hindrance to out-diffusion of the radionuclide. This is characterised by the intrinsic diffusion coefficient, Di, and is mainly a property of the waste form and not the radionuclide. The second is the chemical “immobilisation” which is characterised by the equilibrium parameter CYand is dependent on the radionuclide and the matrix (and probably also on the leachant composition in some cases). The performance of a radionuclide/waste form combination in a dynamic leaching test, such as the Soxhlet test, is thus determined by both processes expressed as D/CL
From the data in Table 1 it is evident that the equilibrium state (characterised by a) is influenced by the test procedure itself. Considerably smaller values of CY,which correspond to higher concentrations in aqueous solution, were observed using crushed samples rather than monolithic samples. The reason for this is not known, but may be due to better access to occluded porosity in the crushed material. The differences in (Ycan be large (an order of magnitude in the case of Cs in OPC) and therefore the degree of comminution of the waste form should be given some consideration when developing the methodology of equilibrium leach testing. The leaching test which has been most widely used on cement-based waste forms (because of ease of execution) is the IS0 test (often with minor modifications to the original specification). The results of such tests often approximate to simple diffusion-controlled kinetics (xD a fh’t)and are often summarised and reported as a diffusion coefficient, Dlso. In general DIso is not expected to be the same as Deffbecause of the different boundary conditions in the test procedures. The partial approach to equilibrium in the IS0 test (e.g., as illustrated in Fig. 8) leads to the con. . dmon that DIso I Ddf. The two “diffusion coefficients” are most nearly equal when o is small, as is evident from the data from Cs release from OPC in Fig. 3. Conversely, the relative difference between them is greatest when cyis large. This is evident for Cs release from OPC/clinoptilolite from the data in Fig. 5 and is illustrated in more detail in Fig. 9 where both the simulated and experimental data have been plotted accordingly to simple diffusion-controlled kinetics. The simulated data demonstrate the dangers of overinterpreting results from the IS0 test. For example, there appear to be two distinct regions having different diffusion coefficients (2.1 x lo-l2 cm2 see-’ and 0.77 x lo-l2 cm2 set-l), whereas the kinetics were generated from only a single effective coefficient (2.8 x lo-l2 cm2sec-I). The two regions are, in fact, the result of different frequencies of leachant change in the test procedure. Furthermore, the diffusion coefficients deduced from these two regions are both less than Deff as appropriate to true zero concentration boundary conditions. However, this effect is not necessarily serious since in this simulation (i.e., an example in which the effect is large), Dlso is smaller than D,ff by only a factor of 4. The actual experimental data are also shown in Fig. 9. They give a good fit to simple diffusion kinetics over the whole time range, with little indication of the two regions predicted by the simulation. This is probably fortuitious, since the data from the Soxhlet test show significant departure from simple diffusion kinetics (lower release rate) for times up to about 50 days. The measured value of DIso (0.38 x lo-l2 cm2 set-I) is a factor of 2 lower than that deduced from the simulation at
A. ATKINSON,K.lVKLSON, AND I
I
I
I
137
Cs
I
I
I
T.M. VALENTWK
I
I
+/
OPC / Clinoplitolite
‘ISO’
test
/
b
4
-
+’ 0cff = 2.8 x10-‘* cm*,-’
/f
in simulation /+ 4 /‘: /+
Go=
/
0~77xlO”*cm**-’ (simulation)
7
/
+
P +
OQH
PA
FIGURE 9. The results for leaching of lsrCs from OPC/clinoptilollte in the “ISO” test plotted against t* (i.e., assuming diffusion into a large volume of lea&ant). The open circles are experimental data and the crosses are predicted by the simulation using Deff = 2.8 x 1O-Lz cm’ set-’ and Q = 7800.
long times. This again illustrates some deficiency in the assumptions made in the simulation. From the experimental data alone we conclude that &-, from the IS0 test can be almost an order of magnitude smaller than D,rf from the Soxhlet test. In Table 2 are assembled data from the literature (4-6, 8-l 1) of “diffusion coefficients” deduced from “ISO” tests on waste forms similar to the ones used in this study. When comparing these “diffusion coefficients” with D,_r in Table 1 the discussion of the preceding paragraph should be borne in mind. However, it is clear that the range of Diso for the OPC waste forms is far too wide to be accounted for by the features of the test procedure considered here. We conclude that the larger differences (greater than about an order of magnitude) are the result of real differences in the waste forms, but it is not possible to say whether the differences are predominantly physical (i.e., in DJ or chemical (i.e., in (Y)in nature. It is clear that leaching tests of the kind described here are incapable of yielding much insight into the detailed mechanisms of radionuclide release. The best that can be done is to separate out the physical and chemical contributions, and even this requires more than one test. For example, on the basis of the experiments described here, it is not possible to deter-
mine whether the chemical contribution to Sr “immobilisation” in OPC is due to sorption or to incorporation of Sr into the cement hydration products by solid solution. In order to answer this type of question it is necessary to measure additional parameters, and an example of such a study has been reported previously (1).
TABLE 2 Effective Diffusion Cueffklents Measured by Other Workers
in Tests of the %U” Type Dlso
Waste Form
Ref.
OPC/Na,SO, OPC/Na,SO, OPC/H,B,O, OPC/NalB,O, oPc/so:OPC (30 day cure) OPC (57 day cure) OPC (95 day cure) OPC/clinoptilolite OPC/clinoptilolite
8 6 4 5 9 10 10 10 10 11
(cm’s_*)
Cs
5 5 1.5 1 l-60 Oh-18 0.04-13 1 2-8
x x x x x x x x x
Sr
lo“0 lo-” 10-p 10-8 10-q lo+ 10-g 10-p lo-”
l-4 1 7 2.7 2
x x x x x
lo-” lo-” lo-” lo-” lo-‘0
9 x lo-”
251
CONCLUSIONS
It has been demonstrated that leaching of radionuelides from cement-based monolithic waste forms in a range of four different tests can be characterised approximately by only two parameters for each radionuclide/waste form combination. One is an effective diffusion coefficient. Defr,which describes the kinetics of release in the extreme limiting case when zero concentration is maintained at the surface of the waste form; a condition which is approached in a Soxhlet test. The other is the ratio of average concentration of radionuclide within the waste form to that in a small quantity of water in equilibrium with it, (II,and is measured in the long-time limit of a static test. The kinetic parameter, Deff, is approximately equal to the quotient DJcx, where D, is the intrinsic diffusion coefficient of ions in aqueous solution within the waste form. Thus the two parameters a! and Di are important characteristics of the radionuclide/waste form combination since cx quantifies the chemical contribution to “immobilisation” and Di the physical contribution. Since D, is probably relatively insensitive to the actual radionuclide, to a first approximation, the different observed leaching rates of different radionuclides from the same waste form are mainly caused by chemical differences (i.e., differences in ar). It must be concluded that there is no single leach test procedure that is universally “best.” The choice of the most appropriate test procedure is greatly dependent on the use for which the resulting data are intended. On the basis of these experiments we would make the following recommendations. (i) In equilibrium studies the present results show significant differences between monolithic and crushed samples. Since the reason for this difference is not understood, equilibrium tests should be done using both types of sample. (ii) For mechanistic studies of leaching processes, at least two tests are required; an equilibrium test and a dynamic test. The best dynamic test for this purpose is the Soxhlet test because of its simple boundary condition. However, this test is of no value if the intention is to study groundwater/waste form interactions. Either the IS0 or the low-flow test must then be used and the attendant problems of interpretation accepted. (iii) For comparing waste forms, and the effects of different groundwaters, the IS0 test is the most convenient. Nevertheless, it must be recognised that this test (or a continuous test similar to the low-flow test) does not measure a simple characteristic of the waste form and should not be overinterpreted. For example, the IS0 test can underestimate the true effective diffusion coefficient by up to an order of
magnitude and changes in the frequency of leachant renewal can distort the release kinetics significantly. Appendix: Mathematical Simulation of Leach Tests
The aim of modelbng the leach tests mathematically was to attempt to reproduce the behaviour observed experimentally in the test while using only the two parameters Deffand (;Y to characterise the radionuclide in the waste form. The mathematical analysis is based on the following simplifying assumptions. (i) The transport of radionuclide within the watersaturated waste form is by diffusion. (ii) All chemical reactions are locally at equilibrium and sorption is character&d by a linear isotherm (gravimetric distribution coefficient, Kd). (iii) The diffusion and chemical equilibrium parameters do not change with time. (iv) Transport in the liquid outside the waste form is sufficiently rapid that the concentration of a radionuclide is uniform in the leachate. (v) The effective depletion depth, x0, is sufficiently small in comparison with the size of specimen that diffusion in only one dimension need be considered. The equation which describes radionuclide solubility in the waste form is different from that which describes sorption. Therefore it is convenient to treat these two separately as extreme limiting cases. Radionuciide Chemistry Controlled by Sorption The concentrations in the solid and liquid phases within the waste form of this case are illustrated in Fig. 10a. We assume that the radionuclide is only mobile when in the liquid phase and therefore the sorption process reduces the diffusivity of the radionuclide by partial immobilisation. Transport within the waste form under such conditions may be conveniently described in terms of the average concentration in the water-saturated waste form c C> and characterised by the effective diffusion coefficient
acc> at
D
a*03
(A. 1)
The relationships with other quantities are (7) a! = E + &,
(A.2
Dcff= Di/Q,
(A.31
and
= c&X,
(4.4) where E is the volume fraction of porosity in the waste form, e its density, and 4 its diffusion coefficient for describing transport in terms of concentration in the liquid phase. Earlier modelbng studies (12)
252
A. ATKINSON, K. NELSON, AND T. M. VALENTINE (a)
controlled
Sorption-
release
Waste
Leachate
form
(
H
fusion from the waste form and that being removed, in the general case, by leachate flow. This is expressed by the equation.
-----‘S
I,/’ ;,b / 5oo/
I I
- FLCL.
Cl
(b)
Solubility
- controlled
Lcachate
Waste
In this equation < C> 0 is the average concentration initially in the waste form and < C> f that at time t. FL is the flow rate of the leachate and is required for simulation of the low-flow test. The mathematical procedures developed previously (12) were used to solve numerically the coupled Eq. (A.l) and (A.7) together with boundary conditions (A.5) and (A.6). Equation (1) was used to compute x0 as a function of time which could be compared with the experimentally measured values. Analytical solutions are available which are appropriate to the Soxhlet test and the static test (13), and these were used to check the accuracy of the numerical computations.
release
form
r-_-_---_cs
I
Csol
I CL
X
0
XD
FIGURE 10. Schematic diagram illustrating the spatial variation of radionuclide concentration in the liquid and solid phases when equilibrium between solid and liquid is controlled (a) by sorption and (b) by the aqueous solubiity limit of the radionuclide.
have shown that Eq. (A. 1) not only describes transport in a homogeneous medium but also in a heterogeneous medium (such as clinoptilolite in cement) under most conditions. To describe release from the waste form, Eq. (A. 1) must be solved with appropriate boundary conditions. These are that in the waste form, at sufficiently large distance from its surface, there is no transport, and that at its surface the leachate and waste form are in equilibrium, i.e.,
a
-
= 0 at x = X(X $ xD)
-CC> = c&,atx=
0.
(A.7)
(A.9
(A-6)
The temporal boundary condition is that CL = 0 at t = 0 for all the leach tests. In addition for the “ISO” test we have CL = 0 at t = tl, tl . . . t. where t, to t, are the times of leachant renewal. The concentration in the leachate, CL, is determined by mass balance between radionuclide entering by dif-
Radionuclide Chemistry Controlled by Solubility When radionuclide concentrations are solubilitylimited, the concentration distribution within the waste form is different from that of the previous case and is shown schematically in Fig. lob. In this case, it is convenient to assume that a quasisteady state (i.e., concentration gradient varying only with time) exists in the depletion zone, 0 < x < xD. This leads to the following equation for the depletion depth, x0,
du, = D.
xDdt
’
(Gor - CL) C, ’
(A.@
where C,, is the solubility limit. In the Soxhlet test, for which CL 4 Csol,Eq. (A.@ is integrated to give xi (Soxhlet) = 20, !$. s Comparing Eqs. (A.9) and (2) leads to the conclusion that in this case (A. 10) If we again use Eq. (A.4) to define (Y (as the average concentration in the waste form divided by the concentration in a small volume of liquid in equilibrium with it) then we have in this case (cf. A.3) Deff = ; D~/cY.
(A.ll)
The equation for mass balance analagous to (A.7) is j+L Ldt
-_
S$,
(GOI ’
-
XD
CL)
-
FLCL.
(A.12)
CEMENT-BASED NUCLEAR WASTE FORMS
This was solved numerically subject to the boundary conditions x0 = 0 and CL = 0 at t = 0, and that for the “HO” test CL = 0 at t = t,, tz . . . t,. REFERENCES 1. Atkinson, A., Nickerson, A. K., and Valentine, T. M., The mechanism of leaching from some cement-based nuclear waste forms, Rad. WasteManag. Nucl. Fuel Cycle 4: 357 (1984). International Standards Organisation. Long-term leach testing of radioactive waste solidification products, ISO/ TC85/SC5/WG38, Draft IS0 Standard ISO/DIS 6%1(1979). Mendel, J. E. The measurement of leach rates: A review. Nucl. Chem. WasteManag. 3: 117 (1982). Matsuzuru, H. and Moriyama, N. Leaching of radionuclides from a cement composite incorporating evaporator concentrates generated at a PWR nuclear power plant. Nucl. Sci. Eng. 80: 14 (1982). Bernard, A., Nomine, J. C., Cornet, G., Bonnet, A., and Farges, L. Long term leaching tests on full-scale blocks of radioactive wastes. Nucl. Chem. WasteManag. 3: 161 (1982).
253 6.
7.
8. 9.
10
11. 12.
13.
Matsuzuru, H. and Ito, A. Effect of dimension of specimen on amounts of 13’Cs, ‘?Sr and ‘OCo leached from matrix of hardened cement grout. J. Nucl. Sci. Technol. 15: 2% (1978). Atkinson, A. and Nickerson, A. K. The diffusion of ions through water-saturated cement. J. Mater. Sci. 19: 3068 (1984). Matsuzuru, H. and Ito, A. Leaching behaviour of strontium 90 in cement composites. Ann Nucl. Energy 4: 465 (1977). Amarantos, S. G., Papadokostaki, K. G., and Petropoulos, J. H. Comparison of leaching tests and study of leaching mechanisms. CEC report WAS-305-83-15-GR(B) (1984). Glasser, F. P., Rahman, A. A., Crawford, R. W., McCulloch, C. E., and Angus, M. J. Immobilisation and leaching mechanisms of radwaste in cement-based matrices. U.K. Department of the Environment report DOE/RW/83.093 (1983). Mikhail, K. Y. Radioactive waste treatment using zeolites. Ph.D. thesis, University of Salford (1981). Atkinson, A. Mathematical modelling of leaching from porous nuclear waste forms. Rad. W&e Manag. Nuclear Fuel Cycle 3: 371 (1983). Crank, J. The Mathematics of Diffusion. Clarendon Press, Oxford (1975).