Mechanisms of immobilization of nuclear waste elements by cement minerals, cement and mortar

CEMENT and CONCRETE RESEARCH. Vol. I I , pp. 789-794, 1981. Printed in the USA. 0008-8846/81/050789-06502.00/0 Copyright (c) 1981 Pergamon Press, Ltd.

MECHANISMS OF IMMOBILIZATION OF NUCLEAR WASTE ELEMENTS BY CEMENT MINERALS, CEMENT AND MORTAR

S. Komarneni and D.M. Roy Materials Research Laboratory The Pennsylvania State University University Park, PA 16802

ABSTRACT The mechanisms of immobilization of nuclear waste elements such as Cs, Sr, Ba, U, La and Nd (the latter two simulating Am and Cm) by three cement minerals, one cement and one mortar were investigated. Cement minerals did not immobilize Cs or Sr or Ba in the absence of C02 but immobilized 62 to 91% of the added Cs and all of the added Sr by forming carbonates when C02 was bubbled through the cement mineral suspensions. The elements La, Nd and U reacted significantly with various cement minerals, cement and mortar and precipitated as hydroxides. For example, C3S* mineral immobilized 92, 73 and 9~.Z% of the added La, Nd and U respectively. Reaction of cement with U resulted in the formation of basic calcium uranyl silicate hydrate or uranophane under simulated repository conditions. These results suggest that cements serve as not only physical barriers but also act as chemical barriers for the migration of especially U and transuranic elements of nuclear waste. Introduction Cements and concretes have played a significant role in the fixation and disposal of low- and intermediate-level commercial nuclear wastes as well as high-level defense wastes (I-3). At Oak Ridge National Laboratory cementitious grout is used for the fixation and isolation of intermediate level waste by the hydrofracturing process in the Conasauga shale formation (4). Cements have also been proposed for the fixation and disposal of specific elements from nuclear waste such as iodine-129 (5,6). Special cements have been suggested (7) for high-level radioactive waste incorporation. Cementitious materials and concrete are also preferred materials for sealing boreholes, shafts and tunnels of any geological nuclear waste repository (8). Cements and concrete not only serve as physical barriers but also act as chemical barriers for the migration of nuclear waste elements. The chemical interaction of cements with radioactive waste elements is, however, not clearly understood. Therefore, the objective *Abbreviations, cement nomenclature: C=CaO, A:AI203, S=Si02, H:H20. 789

790 S. Komarneni, D.M. Roy of t h i s paper is to determine the mechanisms of f i x a t i o n of nuclear waste elements such as Cs, Sr, Ba, U, La, and Nd (the l a t t e r two elements simulating Am and Cm since no stable isotopes e x i s t for these waste elements) by hydrated and unhydrated cement minerals, cement and mortar. Experimental Cement minerals such as C3S, C3A, and ~-C2S, Class H cement with s i l i c a ( s i l i c a adjusted portland cement), and a mortar (grout mix) designated as PSU/WESmixture (68% ClassHcement+22.9% f l y ash [highCaO]+8.34%CaSO4.1/2H20+4.05%NaCl by weight) have been used in the present experiments. Cementminerals were used in hydrated and unhydrated forms. The mineral samples of C3S, C3A, and B-C2S were hydrated for 35 days at I00% r e l a t i v e humidity. This was followed by l week a i r drying in glass tubes. The uptake of Cs or Sr by i n i t i a l l y unhydrated C3S, C3A, and B-C2S minerals was determined by adding 25 ml of lO-~ N Cs or Sr solution to 5g of each cement mineral, e q u i l i b r a t i n g for l and 50-days, in closed polypropylene bottles, centrifuging to separate solid and solution phases and by analyzing for Cs or Sr in solution by atomic absorption spectrophotometry (MS). These are batchtype of experiments. The uptake of Cs or Sr by the above cement minerals in the presence of C02 was also determined by bubbling C02 through the suspensions. In the next series of experiments, 2g each of unhydrated and hydrated cement minerals, Class H cement, and PSU/WES mixture was equilibrated by shaking for l day with a 25 ml of solution containing a mixture of 0.614, 2.63, 2.78, and 0.63 ug/ml of Ba, La, Nd, and U, respectively. In a d i f f e r e n t series of experiments, 2g each of a l l the above samples was equilibrated by shaking with 25 ml of solution containing 19 ~g/ml of U alone to determine the extent of U immobilization. After e q u i l i b r a t i o n , the solid and solution phases were separated by centrifugation, and the solutions were analyzed for the elements remaining in solution to determine the extent of uptake by s a b l e s . All the above ion uptake experiments conducted in duplicate with trace to low concentrations of various elements gave the extent of reaction. Any new reaction products that form would be below detection by conventional bulk x-ray d i f f r a c t i o n . Therefore, another series of experiments was conducted with high concentrations of some of the elements as follows: 200 mg of either Class H cement or PSU/WES mixture was mixed with 200 mg of uranyl n i t r a t e , LaCl~ or NdCl3 in glass v i a l s , 15 ml of d i s t i l l e d water was added, equilibrated for l day by shaking the mixture and the solid and solution phases were separated by centrifugation. Solid reaction products were then subjected to x-ray diffraction analysis (XRD). Because no new reaction products could be detected by XRD, the above solid phases were mixed with t h e i r respective solutions in t i g h t l y capped glass v i a l s and the samples were aged in t h e i r mother liquor at 70°C for 6 days in an oven. The solid and solution phases were then separated by centrifugation. Solid reaction products were washed free of soluble salts by 95% acetone, dried and were characterized by XRD and the solutions were analyzed for the elements remaining. Portions of the above solid reaction products were further treated under mild hydrothermal conditions as follows: Each solid reaction product was loaded into gold capsules mixed with deionized water in a 1:4 solid to water weight r a t i o , the gold capsules were sealed by crimping (9) and were treated in a cold-seal vessel at lO0°C for 7 days under a confining pressure of 300 bars or 30 MPa. The solid samples removed from the gold capsules were subjected to XRD analysis. X-ray d i f f r a c t i o n analysis of a i r dried s l u r r i e s of samples on glass slides was carried out with a Philips x-ray diffractometer using CuK~ radiation. Solutions

791 NUCLEAR WASTE IONS, IMMOBILIZATION, MECHANISMS, CEMENT, MORTAR were analyzed for Cs and Sr by atomic absorption spectrophotometry using a Perkin Elmer PE 403 instrument and for Ba, La, and Nd by atomic emission spectroscopy (AES) using a computer interfaced SpectraMetrics SpectraSpan I I I instrument. Uranium in solutions was determined by a recently developed electro-optical technique using a Scintrex UA-3 Uranium Analyzer. Results and Discussion Cs and Sr Immobilization b~ Cement Minerals Neither Cs nor Sr was immobilized by the three cement minerals when treated with Cs or Sr containing solutions up to 50 days (Table l ) . However, when C02 was bubbled through the cement mineral suspensions, most of the added Cs and a l l of the added Sr were immobilized (Table l ) . These results with C02 indicate that Cs and Sr were precipitated as CszCO3 and SrCO3, s t r o n t i a n i t e respectively in the presence of the cement minerals which generated alkaline conditions in TABLE l Immobilization of Cs or Sr by Cement Minerals. % Cs Uptake Cement Mineral

No COz bubbling l day 50 days

% Sr Uptake

COZ bubblin 9 l day

No C0% bubbling l day 50 days

COz bubbling l day

C3S

0

0

91

0

0

lO0

C3A

0

0

69

0

0

lO0

~-C2S

0

0

62

0

0

lO0

the suspensions. The pH values in these suspensions ranged from 11.5 to 12.0. The mechanism of Cs and Sr immobilization with cement minerals is one of carbonation and can be presented as follows: 2CsCl + Ca(OH)2 + C02 ~ Cs2C03 + CaCl2 + H20 SrCl2 + Ca(OH)2 + C02 ~ SrC03 + CaCI2 + H20 The p r e c i p i t a t i o n of CaC03 in cements which are exposed to atmospheric C02 is a well-known reaction. Thus, Cs or Sr immobilization can be greatly enhanced with cement minerals by carbonation. Immobilization of Ba, La, Nd and U b~ Cement Minerals, Cement and Mortar Barium was immobilized by only two samples when a solution containing a mixture of Ba, La, Nd and U was added (Table 2). The behavior of Ba is analogous to that of Sr. The reason for the uptake of Ba by hydrated ~-C2S is not clear. However, the uptake of Ba by PSU/WES mixture may be attributed to the precipitation of BaSO, because CaS04 was present in t h i s mixture. The elements La and Nd simulating Am and Cm were s i g n i f i c a n t l y immobilized by a l l the cement minerals, cement and mortar (Table 2), as has been found previously in commercial cements (lO). The immobilization of La and Nd is probably caused by the p r e c i p i t a t i o n of these rare earth elements as hydroxides. I d e n t i f i c a t i o n of La or Nd hydroxides by XRD would not be possible with these low concentrations of ions used,in order to prove the p r e c i p i t a t i o n reaction mechanism. Therefore, the cement and mortar samples were reacted with LaCl3 or NdCl 3 in a l : l weight r a t i o in water at various temperatures. No new reaction products such as La(OH)3 or Nd(OH)3 could be detected at 25°C after l day of reaction (Table 3). However, after aging at 70°C for 6 days, La(OH)3 c r y s t a l lized (Figure l ) but not Nd(OH)3 (Table 3). La and Nd were almost completely

792 S. Komarneni, D.M. Roy TABLE 2 Immobilization of Various Elements by Cement Minerals, Cement and Mortar.

Cement minerals, cement and mortar

pH of equilibrated supernatant solution

% uptake of added,elements Ba

La

Nd

U*

U÷

C3S

12.6

0

92

73

99.2

99.99

C;A

11.7

0

92

88

95.7

99.95

~-C2S

12.5

0

93

78

99.0

99.84

C3S, hydrated

12.3

0

93

74

99.6

96.14

C3A, hydrated

11.4

0

99

97

97.1

99.74

B-C2S, hydrated

11.7

62

93

78

98.7

99.89

Class H cement with s i l i c a #

12.6

0

93

77

93.8

99.97

PSU/WES mixture

12.7

15

90

72

96.2

99.92

*0.63 ug/ml of U was added along with 0.614, 2.63 and 2.78 ~g/ml of Ba, La and Nd respectively. ~19.0 ug/ml of U only was added. # s i l i c a = fine quartz

[

URANOPHANE

~

SILICA

immobilized at 70°C as indicated by t h e i r trace concentrations remaining in solution (Table 3). Nd(OH)3 did not c r y s t a l l i z e even a f t e r hydrothermal t r e a t ment at lO0°C/300 bars for 7 days in the presence of cement or mortar. Crystallization of Nd(OH) 3 probably requires either a longer duration or a higher temperature of treatment. Thus, there seems to be a s l i g h t difference in the hydrolysis behavior of these two elements. A higher fraction of La was immobilized than that of Nd (Table 2) and is another indication of a possible difference between the two rare earth elements. The immobilization reactions of La and Nd with cement samples may be represented as follows:

]

[~Lo

(OH}3 CALCITE

PSU/WES MIXTURE 4- Lo 3÷ AT 70%

CLASS H CEMENT + IJ6÷

[CLASS H CEMENt ~- J6+'~F ?O°C IO 14 r8 22 26 30 DEGREES TWO THETA (CaKe)

34

38

3Ca(OH)2 + 2LaCl3 2La(OH) 3 + 3CaCl2

FIG. 1 X-ray diffractograms of cement or mortar interactions with La and U.

3Ca(OH)2 + 2NdCl3 = 2Nd(OH) 3 + 3CaCl2

793 NUCLEAR WASTE IONS, IMMOBILIZATION, MECHANISMS, CEMENT, MORTAR TABLE 3 Analyses of U, La, and Nd Remaining in Solutions and XRD Analysis of Reaction Products TABLE 3 Analyses of U, La, and Nd Remaining in Solutions

and CRD Analysis of Reaction Products. I d e n t i f i c a t i o n of new reaction products by XRD at different temperatures~

Concentration (~g/ml) of added elements* remaining in solution a f t e r 70°C treatment

Mixtures of I : I by weight in 15 ml d i s t i l l e d water

U

La

Nd

70°C/6 d a y s

i O 0 ° C / 3 0 0bars/7 days

Class H cement with s i l i c a + U n i t r a t e

0.016

.

.

.

.

None detected

CaCO: • uranophane

PSU/WES mixture + U n i t r a t e

0.016

.

.

.

.

CaCO~

CaCO~ + Unidentified phase

Class H cement with s i l i c a + LaCI~

--

0.27

--

CaCO~ + La(OH)~

CaCO~ ÷ La(OH)~

PSU/WES mixture + LaCl}

--

0.25

--

CaCO~ + La(OH)~

CaCO~ + La(OH)~

Class H cement with s i l i c a + NdCl~

. . . .

0.71

CaCO~

CaCO~

PSU/WES mixture + NdCl?

. . . .

0.85

CaCO~

CaCO~

"Initially

added concentrations

of U, La and Nd are 6752, 5097, and 5363 ug/ml respectively.

+No new reaction products were detected at l day of reaction at 25°C.

I f La and Nd simulate the reactions of Am and Cm, these results with cement samples suggest that cement can be a chemical barrier for the migration of these transuranic elements. L:ranium was immobilized by the cement samples irrespective of i t s concentration in the added solutions (Tables 2 and 3). The mechanism of U reaction probably is precipitation resulting in the formation of UO2(OH)2 or UO3.H20. The mechanism of U immobilization was investigated by mixing cement or mortar with an equal weight of uranium n i t r a t e and reacting in water at 25° and 70°C. The solution in the above reaction mixtures was yellow i n i t i a l l y but turned clear upon reaction with cement or mortar leaving a yellow precipitate as a sediment. XRD characterization of the reaction products formed at 25°C did not reveal any new phases. Aging the cement or mortar mixtures with U solution at 70°C did not result in the c r y s t a l l i z a t i o n of any uranium phases but resulted in almost complete removal of U from solutions (Table 3). The reaction products of cement or mortar with U were treated hydrothermally at lO0°C in order to c r y s t a l l i z e the phases for XRD i d e n t i f i c a t i o n . Cement reacted with U at lO0°C to form basic calcium uranyl s i l i c a t e hydrate or uranophane, Ca(UO2)2(SiO3)2(OH)2-5H20 (Table 3 and Fig. l ) and the mechanism may be represented as follows: CaO+2UO2(OH)2 + 2SiO2 + 4H20 ~ Ca(UO2)2(SiO3)2(OH)2.5H20 The reaction of mortar with U at lO0°C did not result in any well crystallized phase but resulted in a poorly defined unidentified phase which is probably (UO2)(OH)2. These reactions of U with cement samples are very significant because U is taken out of solution almost completely, and similar types of cement reactions can be predicted for Pu which is one of the most toxic and feared elements in the nuclear waste. The results at 70° and lO0°C/300 bars are not only useful in understanding the mechanisms of reactions but are also relevant because cements and concrete are prime candidates for repository borehole, shaft and tunnel plugging and these materials w i l l be exposed to higher temperatures because of geothermal gradient and/or because of the heat generated by the decaying waste. Cements and concrete used for borehole plugging of nuclear waste repositories may serve to immobilize U and transuranic elements by forming new phases. Conclusions Cements or cementitious materials immobilize nuclear waste elements such as Cs, Sr and Ba upon carbonation by forming carbonates. Thesematerials are capable of immobilizing U and probably transuranic elements by forming mainly hydroxides

794 S. Komarneni, D.M. Roy These hydroxides may combine with Ca and s i l i c a to form phases such as calcium uranyl (or plutonyl) silicates under radwaste repository conditions ( l l ) . References I.

"Alternatives for Long-Term Management of Defense High-Level Radioactive Waste," ERDA 77-42/I, Savannah River Plant (May 1977).

2.

R.O. Lokken, "A Review of Radioactive Waste Immobilization in Concrete," PNL-2654 (June 1978).

3.

A.H. Kibbey and H.W. Godbee, "A Critical Review of Solid Radioactive Waste Practices at Nuclear Power Plants," ORNL-4924 (March 1974).

4.

H.O. Weeren, Nucl. Eng. Des., 44, 291 (1977).

5.

W.E. Clark, Nucl. Tech., 36, 215 (1977).

6.

M.T. Morgan, J.G. Moore, H.E. Devaney, G.C. Rogers, C. Williams and E. Newman, in "Scientific Basis for Nuclear Waste Management," (Ed. G.J. McCarthy) Vol. l, pp. 453-459, Plenum Press, New York (1979).

7. 8.

D.M. Roy and G.R. Gouda, Nucl. Tech. 4_9_0,214 (1978). D.M. Roy, ONWI-212, Proceedings, 1980 Natl. Waste Terminal Storage Program Information Meeting, Columbus, OH, 'Dec. 9-11, 1980, pp. 165-169. S. Komarneni, W.P. Freeborn, and C.A. Smith, Am. Mineral., 6_4_4,650 (1979).

9. lO.

llo

D.M. Roy, B.E. Scheetz, L.D. Wakeley and M.W. Barnes, Leach Characterization of Cement Encapsulated Wastes, Conference on Leachability of Radioactive Solids, Gatlinburg, TN, Dec. 9-11, 1980, Nuclear and Chemical Waste Management (in press). This research was supported by the U.S. Department of Energy.