9B HLW-glass

WASTE MANAGEMENT. Vol. 10, pp. 23-27, 1990 Printed in the USA. All rights reserved.

00564)53X/90 $3.00 + .00 Copyright © 1990 Pergamon Press plc

DEVITRIFICATION BEHAVIOUR OF GC-12/9B HLW-GLASS Luo Shanggeng,*t Jiang Yaozhong,* and Liu Delu** *Institute of Atomic Energy, Beiling. China, and **University of Science and Technology Beijing, Department of Materials Physics, Beijing, China

ABSTRACT. Crystallization in simulated borosilicate HLW-glass GC-12/9B has been investigated by DAT, XRD, and SEM-XEDS. Three kinds of crystals, which are enriched with U, Ti, and Ca, formed during heating at 500-775°C for varying duration. The temperature range of rapid crystallization is between 600°C and 750°C. No crystals were found in samples heated below 400°C or above 800°C. The volume fraction of the crystals could reach over 30%. Few detectable cracks were observed in the specimens. The crystallization has no remarkable effect on the leachability by Soxhlet tests, but hardness of the glass decreases along with crystallization.

and 5 mm thickness for heat treatments. The specimens were heat treated at 400, 500, 600, 650, 700, 750, 775, 800, and 900°C for varying duration between 4 h and 480 h. The heat-treated specimens were prepared for X-ray diffraction (XRD), differential thermal analysis(DTA), optical and scanning electron microscopy(SEM) with X-ray energy dispersive spectroscopy (XEDS) examinations. The XRD analyses of powder samples were carried out on a PHILIPS APD-10 instrument with Cu Ka radiation. The DTA study of powder samples with particle dimension less than 200 mesh was performed using. A1_,O3 powder as the reference material, heating rate of 10°C per minute in atmosphere. Bulk specimens were etched by 1% HF and coated with carbon layer for SEM observation and XEDS analyses using an TN-5500 system at 20 kV.

INTRODUCTION Various crystalline phases may occur in the waste glass forms as temperature is raised above transformation temperature (1), which could effect the leaching resistance and mechanical properties of the glass forms. Microstructural characterization of the crystalline phases have been reported using various techniques (2-7). Several phases were identified in different glass forms (8). It is important to examine the devitrification behaviour for pouring melting glass and storage conditions, cooling procedure, designing canister and even for designing disposal repository as well as assessing long-term performance. Study on formulation, leachability etc. of GC-12/9B glass has been described in the previous papers (9-11). This work investigated devitrification of the simulated HLWglass GC-12/9B and its subsequent effects.

RESULTS AND DISCUSSION

The Temperature Region and Dynamics of the Crystallization in Glass GC-12/9B

EXPERIMENTAL

The compositions of the Chinese borosilicate glass GC-12/9B containing simulated high-level waste is listed in Table 1. The waste loading capacity was 20

After heating to various temperature between 400 and 900°C for different periods, the glass samples were examined using DTA, XRD, and SEM to determine temperature region, incubation periods and the volume fractions of the crystallization in the glass. The DTA measurements, shown in Fig. 1, indicate that there is a wide exothermic peak in the temperature range 510-660°C, resulting from crystallization in the glass. The optical and scanning electron microscopy showed that bulk crystallization took place during heating between 500°C and 775°C. Micrographs in Fig. 2 showed small crystals formed

(wt)%. Glass samples were melted at ll00°C for 3 h in alumina crucibles, then cut into 10 mm in diameter RECEIVED 16 FEBRUARY 1989; ACCEPTED 6 DECEMBER 1989. tTo whom correspondence may be addressed. Acknowledgements- This work formed one part of IAEA Contract 4282/RB, many thanks to the IAEA's support. The author also thanks Mr. Wang Lian, Mr. Tan Baolong, and Mr. Wang Kei for their help.

2,3

2,4

L. S H A N G G E N G ,

J. Y A O Z H O N G ,

AND

L. D E L U

TABLE 1 Composition of Simulated HLW Glass Product

Base glass Component Wt%

A

B

SiO, B:O3 Na,O AI:O~ MgO CaO TiO: Li:O ZrO.,

B

C C D E

D E

G

G H

H

I

I

200

400

600

800

SrO TiO2 U~O~ ZrO2 mixed RE

0.09 0.12 4.73 0.50 0.75

X R D analyses have indicated that at least three types of crystalline phases formed during the heat treatments. Crystals (type 1) in the specimens heated at 700°C were nucleated inhomogeneously and grow in higher temperature range. Table 3 listed the lattice spacings determined by XRD. The XEDS results of these crystals are given in Table 4. Apart from the above crystals, a second crystalline phase (type 2) formed in the specimens heated at about 600°C. Their diffraction results are listed in Table 3 as well. These type of crystals are numerous and are distributed inhomogeneously in the glass matrix as shown in Fig. 2(b). When the glass sample heat treated at 500°C for 120 h, a large number but with small size particles

4 h; (B) 800"C, 4 h; (C) 700°C, 120 h; (D) 700°C, 5 h; (E) 600°C, 120 h; (F) 600"C, 18 h; (G) 500°C, 240 h; (H) 5000C, 120 h; (I) not previously heated.

at 500, 650, and 750°C, respectively. The average volume fractions from 10 measurements of each sample are given in Table 2. No crystals were found in samples heated at 400°C for 480 h either by X-ray diffraction (Fig. 3) or by microscopic observations. This is also the case for samples heated at 800°C, 4 h and 900°C, 4 h.

I

1.70 0.11 5.24 0.02 0.04 5.07 0.08 0.01 1.55

Morphology and Structures

F I G U R E 1. DTA results on glass GC-12/9B following heat treatment ( A - H ) and with no previous heat-treatment (I). (A) 900°C,

I

AI,O~ Cr,O~ Fe:O~ K-O MnO_, Na:O NiO PuO:* SO~**

Component Wt%

Glass-in-glass phase separation took place during heating in temperature range 550°C-775°C, but no obvious effect on the crystallization was detected.

°C

(a) 5000 nm

Component Wt%

*Using U~0. instead of PuO:. **Adding Na2SO,.

F F

46.2 13.4 4.0 2.5 1.5 2.5 5.0 3.4 1.5

High-level waste

I

I

(b) 2500 nm

I

I

(c) 2500 nm

FIGURE 2. SEM micrographs of the specimens heated at 750°C, 4 h (a), 650°C, 10 h (b), and 500°C, 120 h (c).

DEVITRIFICATION BEHAVIOUR OF GC-12/9B HLW-GLASS

25

400 c °, , ~ o h

[

soo c ° 240h

I

L I

800 C ° 4h

900 C ° 4h

100 °

30 °

20

FIGURE 3. XRD patterns of the glass specimens heat-treated at different temperatures and durations.

(see Fig. 2c) crystallized, which only showed a few wide small peaks in X-ray diffraction patterns. Further work is required to clarify the nature of these dispersed particles. Thus, the wide exothermic peaks in DTA results (Fig. 1) are responsible for the crystallizations of above three kinds of crystals. The tem-

perature regions in which they nucleate and grow are different but close to each other. The endothermic peak of DTA curve C in Fig. 1 probably resulted from the solution of large quantity of crystals. Besides, optical microscopy detected a few large crystales grown on the free surface or cracks as shown

26

L. SHANGGENG, J. YAOZHONG, AND L. DELU TABLE 2 Volume Fractions of Crystals in the Heat Treated Glass GC-12/9B Heat treatment 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 775°C,

Volume fraction %*

72 h, 30 h, 18 h, 4 h, 5h, 4 h, 4h,

8 +- 3 5 +- 2 21 -+ 9 6 -+ 1 12-+ 5 7 --+ 3 7_+ 2

"Average value by 10 measurements. TABLE 3 XRD Results of the Crystals Type 1 formed at 700°C

I

2o

d (A)

I (%)

20

d (A)

I (%)

30.4 35.4 38.4 46.5 46.8 50.9 60.4 63.4 63.7 74.9 75.1 82.8 83.1 85.5 85.8 96.2 96.6

2.927 2.534 2.344 1.953 1.941 1.793 1.529 1.467 1.461 1.268 1.265 1.163 1.162 1.134 1.133 1.036 1.033

100 30 20 10 10 70 65 10 10 10 10 25 25 20

31.49 36.53 38.20 47.80 52.62 62.50 62.70 88.90

2.839 2.458 2.356 1.903 1.738 1.486 1.421 1.101

100 30 10 10 50 40 10 10

I 100 ~m

Type 2 formed at 600°C

FIGURE 4. Optical micrograph showing the crystals grown on a free surface of the glass at 70ff'C for 120 h.

deionized water at 95°C for 7 days. After leaching tests the samples were dried and their mass loss was determined by weighting. The results of mass loss (listed in Table 5) indicated that the crystallization of this glass samples has no remarkable effect on its leachability. The color, density, and Vickers Diamond hardness of heat-treated specimens were determined and listed in Table 6. The hardness of the glass samples decreases as their volume fraction of the crystalline phases increases, but little effect on their density was observed. The glass color develops towards brownyellow with crystallization.

15 10

TABLE 4 XEDS Results of the Crystals (wt%)

CONCLUSIONS

Crystal

U

Ca

Ti

Fe

Type 1 Rod-like

56 64

14 3

29 32

1 2

1. Three types of crystals with different morphology and crystalline structure formed in simulated HLW-glass of GC-12/9B during heat treatments at temperature from 500°C to 775°C for varying durations. No crystalline phases were detected in specimens heated at 400°C for 480h, 800°C for 4 h, and 900°C for 4 h by XRD, DTA, and SEM examinations. 2. XRD and XEDS results showed that the crystals formed at 700°C are U and Ti rich. Further study is necessary to identify the crystalline structure and the distribution of the elements between vitreous and crystalline phases.

in Fig. 4. The crystals A (grey plates) are a kind of kirschsteinite containing Si, Ca, Mg, and Fe. The crystals B (dark rod-like) are consist of Ti, U, Ca, and Fe. Their XEDS results are shown in Table 4.

Effects of Crystallization on Glass Properties The leaching experiments of heat-treated specimens were performed with Soxhlet method in 200 ml

TABLE 5 Results of Soxhlet Leaching Tests Mass Loss (10-' g/cm z - d) Heat °C Treat.. h

Non

Mass loss

1 . 6 8 2.84

400 48(1

550 48

600 120

650 10

700 120

750 10

1 . 9 1 1 . 2 4 1 . 1 5 1 . 3 6 2.45

800 4

900 4

1 . 6 3 1.96

DEVITRIFICATION BEHAVIOUR OF GC-12/9B HLW-GLASS

27

TABLE 6 Density and V.Hardness of Heat Treated Glass

Heat treatment 400°C. 500°C 500°C 500°C. 500°C 600°C 600°C 600°C

360 h 5h 18 h 72 h 120 h 18 h 72 h 120 h

Hardness kg/mm-" 933 920 907 858 853 894 835 813

g/cm ~ Heat treatment 2.74 2.71 2.71 2.70 2.71 2.70 2.71 2.70

3. Usually the stresses induced by crystallization and subsequent cooling of the waste form would produce cracking, that increases leaching rate. However, in all specimens examined few detectable cracks were observed. The crystallization of this kind of glass form has no remarkable effect on the leachability, a little effect on the density. The hardness of the glass specimens decreases with crystallization. REFERENCES I. Turcotte, R. P. and Wald J. W. Devitrification behavior in a zinc borosilicate nuclear waste glass. PNL-2247, Pacific Northwest Laboratories, Richland, WA (1978). 2. Hcaly, J. T., Headley, T. J., Hlava, P. E, Strachan, D. M., and Kupfer, M. J. Microstructural characterization of solidified simulated reactor waste forms. Sci. Basis Nuc/. Waste Manag. 1:83 (1978). 3. Kahl, L., Ruiz-Lopez, M. C., Saidl, J., and Dippel Th. Herstellung and Charakterisierung eines Verbesserten Borosilikatglases zur Verfestigung von hochradioaktven Spaltproduktlosungen (HAW) KfK 3251, Kernforschurgszentrum, Karlsruhe (1982). 4. Van Iseghem, P., Timmermans, W., and De Batist, R. Char-

600"C, 700°C, 700°C, 700°C, 700°C, 800°C, 900°C,

5.

6.

7.

8. 9.

10.

11.

240 h 5h 18 h 72 h 120 h 4h 4h

Hardness kg/mm:

g/cm ~

782 835 813 782 752 808 782

2.70 2.72 2.70 2.68 2.67 2.72 2.71

acterization of high active waste forms and their interaction with clay. Final Report 1981-1984, Mol, Studicentrum Voor Kernenergie (1985). Spilman, D. B., Hench, L. L., and Clark, D. E. Devitrification and subsequent effects of the leach behavior of a simulated borosilicate nuclear waste glass. Nucl. Chem. Waste Manag. 6:107 (1986). Beeve, K. D., Levins, D. M., Seatonberry, B. W., Ryan, R. K., Hart, K. P., and Stevens G. T. Final Report on fabrication and study of synroc containing radioactive waste elements. AAEC/C60 Lucas Height Research Laboratories, Sydney (1987). Liu, Delu, Piercy, G. R., Purdy, G. R., and Hayward, P. J. Nucleation of sphene in glass ceramics for the immobilization of nuclear waste. Phys. Chem. Glasses 26t6): 197 (1985). Clarke, D. R. Nuclear waste ceramics. Annu. Rev. Mater. Sci. 13:191-218 (1983). Luo Shanggeng, Pu Shigang, Jiang Yaozhong, Wang Lian, Xue Qinhua, Li Xiufang, Qin Renwei, and Ou Zhimin. A study of vitrification formulation for HLW with high sulphur content. Chinese J. Nucl. Sci. Eng. 3(4): 238 (1987). Luo Shanggeng, Xue Qinhua, Yan Jiade, and Li Weiyin. Study on the leachability of simulated HLW-glass form../. Nucl. Radiochem. 10(1): 30 (1988). Luo Shanggeng, Xue Qinhua, Li Weiyin, Shi Suolang, and Yan Jiade. Study on surface structure of leached HLW-glass forms. Chinese J. NucL Sci. Eng. 8(1): 39 (1988).