Vitreous matrices for the containment of high-level Purex waste

Annals of Nuclear Energy. Vol. 3, pp. 253 to 274. Pergamon Press 1976. Printed in Northern Ireland

VITREOUS MATRICES FOR THE CONTAINMENT OF HIGH-LEVEL PUREX WASTE J. MUKEPOI and A. S. SANYAL Central Glass and Ceramic Research Institute, Jadavpur, Calcutta-32

(Received 24 April 1975, in revisedform 3 October 1975) Abstract--The paper presents a study of various vitreous matrices for the incorporation of Purex wastes generated in India. The melting behaviour of the matrices, the long time extraction of fission products by various attacking agents, the physical and chemical properties of the matrices are given. The study has been made in such a way that the compositional diagrams presented should help in formulation of vitreous matrices for containment of Purex wastes of wide range of compositions. Both silicate and phosphate glasses have been studied and the factors affecting selection of glass composition for containment of nuclear waste have been discussed.

1. INTRODUCTION The safe and permanent disposal of high-level liquid radioactive waste has engaged the attention of most nuclear countries since the last 20 yr. Various methods of solidifying the liquid wastes such as entrapment in sulphur and pitch, in-tank solidification, exchange with synthetic zeolites, calcination and conversion into glass and ceramics have been tried and their advantages and disadvantages have been discussed in International Symposia [1, 2] and in various reports [3]. It is generally believed that the most desirable process will be that in which the final product is a glass. Glass is a material which is capable of dissolving almost all the elements of the periodic table and in the process makes the elements a part and parcel of its own structure. This results in a permanent and irreversible fixation of the nuclides in the vitreous matrix. Besides, glass is resistant to attack by water, it has long time integrity, it is resistant to alteration by radiation and can be made from cheap raw materials. Work was undertaken at this Institute to develop vitreous matrices for the incorporation of high-level Purex wastes generated in India, and to study their long time properties and other characteristics. After the useful life of a nuclear fuel in a reactor is over it still contains unburnt fuel and potentially useful materials like 2sgPu. The spent fuels are therefore reprocessed in a Fuel Reprocessing plant in order to separate the useful and potential fuels from the fission products. In the case of an aluminium clad Uranium fuel the core, after removal of the alumin-

ium cladding, is dissolved in nitric acid and is subjected to solvent extraction with Tributyl phosphate and kerosene. The Uranium and plutonium passes on to the organic phase. The remaining nitric acid solution containing all the fission products is known as a "Purex waste." 2. COMPOSITION OF THE WASTE Compositions of the Purex wastes generated in India are shown on Table 1. The waste constituents were divided into two groups; one group contained the non-radioactive materials like sodium, aluminium, iron and unrecovered uranium and plutonium and the other contained all the fission products. These two groups have been termed as waste product and fission product respectively. The term waste has been used in this paper to signify Purex waste comprising both the above groups of materials. For the purpose of glass composition formulation the constituents were converted to the corresponding oxides. Those constituents of the waste products of which the concentrations are variable, e.g., of aluminium and uranium the maxima of the ranges were considered. 3. GLASS FORMING SYSTEMS Instead of attempting to find out one particular glass composition for a given waste it was decided that investigation should be carried out to find out a range of glass compositions with desirable properties. This should enable the plant engineer (i) to formulate suitable glass composition in the event of 253

J. MUlO~RJI and A. S. Snt,r~XL

254

Table I. Compositions per litre of CIRUS and TAPP waste solutions and their activities Elements

CIRUS (in molarity)

TAPP (in molarity)

1

2

3

To balance M 2.00 :>< 10-t 3.30 × 10-2 3.70 × 10-3 to 3.70 × 10-3 1.00 × 1 0 -3 1.00 × 10-s 8.50 × 10-3 to 2.12 × 10-a

To balance M 2.00 × 10-t 1.00 × 10-3

2.0 6.60 × 10-~

2.0

Cations 1. H + 2. Na 3. Fe 4. AI 5. Cr 6. Ni 7. U

-1 . 0 0 × 1 0 -3

1.00 x 10-3 1.27 × 10-2

Anions 1. NOB2. SO,-

Fission products 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Rb Sr Y Zr Mo Tc Ru Rh Pd Te Cs Ba La Ce Pr Nd Pm Sm

--

( × 10-4) 4.32 11.50 0.67 37.30 29.20 8.08 13.10 3.30 1.40 2.35 22.00 7.20 6.91 18.50 6.95 25.40 2.51 4.05

( × 10-3) 3.82 9.16 0.146 31.10 24.10 6.67 10.60 2.76 1.17 1.95 18.00 6.09 5.72 13.40 5.72 21.10 1.64 3.59

Activities 1. Decay period --100 days 2. Decay period --1000 days a change in the waste composition due to change of process variables and (ii) to have a wide range in selection of glass compositions, glass durabilities and melting temperatures. Most of the glass forming systems were, therefore, studied by representing them on a triangular coordinate in which the waste composition and the glass former (SiO 2 or P~Os) occupied the two apices and the other apex was occupied by glass modifier. The area of the triangular diagram examined was limited by the following considerations: (a) F o r reason of plant operation difficulties the glasses should have a processing temperature below 1200°C. (b) The glasses should have a leach rate of the order of 10-a to 10-6 gm/cm~/day.

1141 Ci/l.

3641 Ci/l.

97.44 Ci/l.

580 Ci/l.

(c) The upper limit of the waste oxide content should be guided by the expected temperature rise in the glass block [4-6] during storage and the lower limit by the amount of volume reduction achieved. Volume reduction is defined by the ratio of the volume of the liquid waste incorporated in the glass to the volume of the glass formed. (d) The tendency of off-gas release. This is particularly important for phosphate systems in which vapour may be released at certain compositions. 4. METHODS AND PROCEDURES 4.1 Preparation of glass batch and melting F o r the preparation of glass batch the simulated waste solutions were prepared which contained the

Vitreous matrices for the containment of high-level purex waste waste product metals and the fission product metals as their salts in inactive form dissolved in a solution of known nitric acid concentration. Rb, Y, Tc, Ru, Rh, Pd, Te, Pm and Sm were not taken in the simulated waste as either their quantities were insignificant, or they were supposed not to change the glass properties significantly. The chemicals used in the glass batch preparation were of technical grade. Uranium oxide was added as uranyl nitrate solution. Sulphuric acid was added in the glass batch for sulphate ion. Sodium, aluminium and ferric oxides were added in the batch in addition to those coming as waste oxide constituents. The glass formers SiO2 and P~Os, and the modifiers Na~O, MnO, B~Oa, PbO and CaO were added in the glass batch as quartz crystals ( - 2 0 0 mesh), HaPO4, Na2CO3, MnCOa, anhydrous Na2B4OT, PbaO4 and CaCOa respectively. A120a and Fe2Oa were added as the corresponding oxides or nitrates. Glass melting was done in a kanthal A1 wound furnace placed inside a fume cupboard. Temperature was measured with Pt/Pt-13~o rhodium thermocouple. After mixing the glass batch additives to the simulated waste the slurry was dried, and the dried lumps were charged into the melting pot where the total time taken for further dehydration, denitration and glass formation was about 5 hr. For a majority of the compositions the melting temperature was taken as that temperature where the glasses were free from unmelted batch materials, fully vitreous and fluid enough to be poured out of the melting pot. To detect if the glass was free from unmelted batch materials a fibre was drawn and was felt with fingers to detect unmelted particles. Occasionally, in a part of the system AlzOa-PzO~waste oxides, a second solid phase separated out and it was difficult to determine the melting temperature by above method. In some other systems like (Fe20 s + 6 ~ SiOz)-Na20: P2Os-TAPP waste oxides the melting temperature could not be determined by fibre drawing method because the melts were not fully glassy below 1150°C and because of very high fluidity of the melts. In these cases the melts were generally poured out at temperature varying from 1100 to 1200°C, at which they were very fluid. When extraction rates of the individual fission product elements such as Ce, Sr and Cs were required, the glasses were remelted in an airtight furnace after doping with the corresponding gamma emitting radioisotopes Ce-141, Sr-85 + 8 9 and Cs-134. Shorter half-life was the main criterion in the selection of these radioisotopes for doping. Activities 2

255

of 4 to 5 ~tci of each species/g of glass were introduced. 4.2.

Leaching

Durabilities of the glasses were examined by determining the leach rates under different leaching conditions, for example, (i) in fixed amount of boiling distilled water for a given period, (ii) in continuous flow of hot distilled water, and (iii) in fixed amount of boiling simulated sea water for a given period. Leach rates of the glasses in boiling distilled water were determined for screening the durable glasses from the less durable ones and were calculated on weight loss basis. Experimental sample ( - 0.420 mm, + 0.296 mm), generally 2.0-2.5g, after washing with alcohol and drying was taken in a bag of stainless steel wire nets and was suspended in water with the help of a silver wire from a hook at the lower extremity of the condenser which was fitted with a 250 ml round bottomed flask. Onehundred millilitres of boiling distilled water was used for leaching. Extraction was carried out in the water unchanged for a fixed period of 24 hr and then water was renewed. Generally, leaching of the glasses in this way was conducted for 14-21 days. The apparatus, based on soxhlet principle, for the determination of leach rates of individual fission products like Ce, Sr and Cs in continuous flow of hot distilled water was the same as that used by Beattie [7] with the provision of pipetting the leachate from the flask. A known volume of water about 500 ml containing about 0.8 g each of cesium, strontium and cerium nitrates as carriers was taken in the flask. The flask was heated on an electric heater of which the power input could be regulated with a variac, and accordingly the flow rate of water coming through the hole of the capsule after attacking the glass in it could be varied. This flow rate of water was maintained at 6-7 l./week. The amounts of fission products extracted were determined by counting the leachate pipetted out (2 ml). The leach rates of the glasses in boiling simulated sea water (pH 7.9-8.2) were measured in the same type of apparatus as the one used to determine the durability on weight loss basis. Carriers were not added at the start of the experiment as it would change the composition of sea water. Carriers were added to the leachates when the glass samples were removed after leaching for 24 hr, and then boiled for two hours before the leachates were taken for counting. The sea water was renewed everyday. Leach rates were calculated by the formula: Leach

256

J. MUK~t and A. S. SANYAL

rate in gc/m2/day = A/A 1 × S where A ~- activity (or wt) leached out per day, Ax = initial activity (or wt) of the glass, and S = specific surface of the glass particles, 110 cmZ/g. The leach rate in cm/day may be obtained by dividing the leach rate in g/cm2/day by the density of the glass. A gamma ray spectrometer with well type NaI(T1) scintillator was used for counting. Counting was done under the photopeaks of Ce-141 (0.145 MeV), Sr-85 + 89 (0.513 MeV) and the second photopeak of Cs-134 (0.80 MeV). 4.3 Determination of equilibrium p H About 3 g of glass was taken under about 200 m of distilled water in an agate mortar and crushed by means of an agate pestle. After allowing a few minutes for the glass to settle, the pH of the liquid was determined by a pH meter. This procedure was repeated till further crushing produced no change of pH. Further addition of glass to this liquid and crushing did not alter the pH and this was taken to be the 'Equilibrium pH' of the glass [8]. 4.4 Crystallisation temperature and microscopic observation The glasses, during storage may accumulate heat due to absorption of radiation as a result of which they may crystallise and change their physical and chemical properties. The crystallisation temperatures of the glasses were determined by a differential thermal analyser using finely divided powder ( - 2 0 0 mesh) sample. In the case of glasses resistant to crystallisation this temperature was determined by heating sample rods in a gradient furnace. The glass samples were examined in thin sections using a petrographic microscope. 4.50therphysicalproperties Viscosity was measured using a BrookfieldSynchro-lectric viscometer. In this apparatus a platinum cylinder (spindle) is rotated in the molten glass and the torque necessary to overcome the viscous resistance to the induced movement is measured. This is accomplished by driving the immersed spindle, through a beryllium copper spring--the degree to which the spring is wound, indicated by the position of a red pointer on the viscometer's dial, is proportional to the viscosity of the glass for any given speed and spindle. The viscometer is able to measure over a number of

ranges, since, for a given spring deflection, the actual viscosity is proportional to the spindle speed, and is also related to the spindle's size and shape. Calibration of the instrument for speed and size of the spindle, for a given container of the test specimen, etc. is needed for calculation of the viscosity from the dial reading. This calibration was done using fused B~0a as the calibrating fluid using the data given by Shartis, Capps and Spinner [9]. Viscosities at three or four temperatures ranging between 800 and 1250°C were measured. A comparative method was adopted for measurement of thermal conductivity. The apparatus was similar to that described by Sutton [10]. Measurement was done under steady state condition. The glass sample in the form of highly polished disc of diameter 3.5 cm and thickness varying from 0.4 cm to 0.9 cm was sandwiched between two electrolytic copper rods (99.9% pure) of 3.5 cm dia and 17.5 cm length. The test specimen and the copper rods were provided with guard tubes. The thermal expansion was measured with a commercial automatic dilatometer*. The calibration of the instrument was frequently checked by determining the expansion of a platinum and a fused silica rods. Thermal expansion was measured from room temperature to 400°C. Density was measured by specific gravity bottle. 5. RESULTS AND DISCUSSIONS 5.1 Glass systems studied The various glass forming systems studied are represented in the form of triangular compositional diagrams. The diagrams are drawn to scale and may be used to derive the composition of a melt in weight percent. Glass forming regions represented on the triangular coordinates may be divided into sections according to the melting temperatures of the glasses, and their leach rates as measured in fixed amount of boiling distilled water for a given period and calculated on weight loss basis. In some systems the compositions of the glasses are not represented in the triangular coordinates either because this is not possible or because the number of compositions studied in these systems are too few. Both silicate and phosphate glass forming systems have been studied for the containment of high-level nuclear wastes. The important systems are briefly discussed here, the details of which are given elsewhere [11-15]. The silicate glasses of TAPP waste, * Gebruder, Netzsche, Model 404, W. Germany.

Vitreous matrices for the containment of high-level purex waste are termed by " S G " and the phosphate glasses of C I R U S and TAPP wastes are termed by " P G " and ' T G " respectively. The compositions, melting' temperatures, average leach rates and special remarks on individual composition are given in the tables 2a, 2b and 2c.

TA

257

PP WOSte oxides 87

5.1.1 Silicate glass systems for TAPP waste. (a) System I. SiO.z-Na2B4OT-TAPP Waste oxides. The glass forming region, melting temperatures and leach rates of the glasses are shown in the Fig. 1.

., :T'g.',"

/

TAPP WASTE OXIDES

'\

87 NO2 e407 1~4

- 20~

o/

~4'e

u 34"8

¥ 52"2

v 696

87 S[Q2

At 13% Fe203 l e v e l ~M¢lting t e m p e r a t u r e more Lhan 12oo'C --o Low d u r a b l e g l a s s e s

-,~0

Meltin 9 temperatures ~ <~11O0 "C ~ I I O 0 - 1 1 5 0 " C O--TWO LIQUIDS TAPP WCSf.£ Oxides 87

//

17 "4

6g 6

Si 0 2

I;O.B407

- - - ~ Mel{:ing t e m p e r a t u r e s mare thdr~ 12OO'C Low d u r a b l e glasses - ~ increased f o r m c / t i o n of t w o liquids Melting tcmperaLures

- - -

Q~ o I l o o - r 2 o o ' c ~)~IOOO-IPOO°CI,~I--Two liqu;ds Le(3ch r o t e s ( ~ m / c m 2 / d o y )

~

69"

.,

~o"4 [ 2 3 ~ o -3

Fig. 1. System SiO~-Na~B4OT-TAPP waste oxides. Glass forming region, melting temperatures and leach rates. The leach rates decrease and melting temperatures increase with increase in SiO2 content in the glasses. Glass no. SG-8 melts at 1180°C. Glass-forming boundary has, therefore, been restricted roughly up to this glass towards high SiO2 direction in the triangular diagram. Glasses no. SG-6 and 9 are kept outside the glass-forming boundary. They form two liquids during melting and therefore their melting temperatures have not been indicated in the figure. Phase separation increases as the composition moves from SG-9 to SG-6. Melting temperatures decrease, and leach rates increase with increase in borax in glass. Glasses no. SG-20, 21 and 22 melt below ll00°C, but they have little resistance towards water attack. Their extraction rates are of the order of 10-ag/cm~/day. Only

87 NO2B407

17v'4

.,

v 34 8

52v2

v 69"6

~ " ~ 87 Si 02

At 13% Fe203 L•vel Leach rotes (~m !cm2/doy)

r - l ~ [ o 4, glass particles

d~s~ntegrat¢ rapidly; ,~'I ~ 4 - =o~IE~:8S Fig. 2. System SiO=-Na=B4OT-Fe=Os-TAPP waste oxides. (a) Glass forming region and melting temperatures, and

(b) Leach rates. the glass, no. SG-8 has the rate of the order of 10-4 g/cmS/day. (b) System II. SiO2-NazB40--TF%Oa-TAPP waste oxides. In this system of glasses ferric oxide is at 13 percent fixed level. A certain F%Oa level was selected in order to accommodate in the glass the corrosion product, Fe, present in the waste, the amount of of which may vary depending on the time of storage

5

6

PbO 7

8

Fe,2Os MnO 9

CaO 10

11

12

40 34.8

36.5

42.6

29 30

31

32

22.6

26.1

30.5 33.1

30.5

13

13

13 13

13

21.8

24.4

16.5 19.1

26

30.5

28

1150

1090

1140 1060

Could not be determined

1140 1130 Could not be determined

20 13 28.7

38.3 43.5 34.8

25 26 27

13 13 13

1120

13

28.7 30.5 23.5

1100 1040 1080

1180 Poured at 1080

15 33 26

25 30

Poured at 1150

13

Melting temps, (°C)

20 42 43 21 27 40 22 37 37 (System: SiO~-Na3B4Or-Fe303-TAPP waste oxides) 24 36.6 37.4 13

50 40

25 30

4

NaaB407

8 9

3

B303

35

2

1

Na20

Wsate A1303 TiOz oxides

(System: SiOrNa2B4OT-TAPP waste oxides) 6 40 25

SiO3

Glass (SG)

Glass compositions (wt ~ )

3.98 × 10 -~

1.70 x 10 -4

7.78 × 10 -5 4.41 x 10-4

4.32 x 10-4

2.39 x 10 -4 1.35 × 10-4 4.72 × 10 -5

4.74 x 10 -4

9.02 × 10-3 8.12 × 10-3 9.56 × 10-3

1.59 x 10-4 4.39 × 10 -4

Not measured

14

Average leach rates (g/cm3/day) 15

Remarks

Probably formation of two liquids; glass particles disintegrated to powder after two days of leaching Formation ot two liquids; glass particles disintegrated after four days of leaching _ After three days of leaching glass particles disintegrated to powder After four days of leaching glass particles disintegrated

Glass particles disintegrated and came out of the leaching packet after four days of leaching --

White agglomerates present in the glass due to formation of two liquids -Formation of two liquids but much less than the glass, no. SG-6 Swelling during melting Swelling during melting Swelling during melting

Table 2a. Compositions, melting temperatures, leach rates, etc. of silicate glasses incorporating TAPP waste oxides

r,

~" r~ r~

-~

.~

>1200 Poured at 1180 1080 1050 1020

18.9 18.9 13.6 13 12.5

18 19 46

20 20 20

43.6 41.8 40

36 28 21.6

* Waste oxides incorporated was that of CIRUS.

9.1 13 16.7

1180 1000

14.4 12

15 46.4 20 19.2 16 29.6 20 38.4 (System: SiOt-Na~O-PbO-TAPP Waste oxides) 1 56.6 5.6 18.9 2 47.2 5.6 28.3

33.7 32.2 30.8

1000 1160 1150

8 9.6 22.4

36 42.4 36

1100 1100 1100 1100 980 990 970 1080

13 25 21 13.5 21 29.5 14.5 12.2"

12 13 14

24 32 28

1170 1130 1140

24 17 20

1120 1050 1000

20 20 20

1060 1120 1130

23 12 21

16 16 24

Could not be determined 1110

40 32 28

9.6

33.1

7 10 11

13

900

17.4

16

36.5

47 41.8 35.6 13 (System: SiO2-Na~B40~-Fe2Oa-CaO-TAPPWaste oxides) 33 34 25 13 34 41 29 13 35 40 21 13 (System: SiOt-Na~B,OT-Fe20~-AI~O3-TAPPWaste oxide~ 37 35 25 13 39 39 28 13 40 40 24 13 (System: SiO~-Na~B~OT-Na~O-MnO-TAPP waste oxide~ 8 5 50 8 21 8 38 42 8 17 8 41 48.5 8 14.5 8 42 56 8 14.5 8 43 38 8 25 8 44 33.5 8 21 8 45 41 8 28.5 7.4 FAt 47.3 12.6 20.5 (System: SiOz-NatB~OT-PbO-TAPP waste oxides) 4 24 20 40

36 x

1 0 -4

10 -~ 10 -4 10 -5 10 -s

X 10 -t

X 1 0 -4

X 10-'

X 1 0 -4

X × X X

4.45 x 10-* 6.07 × 10 -e 9.10 X 10 -5

Not measured Not measured

3.38 × 1 0 -6 2.67 x 10 -~

4.82 × 10 -e 3.15 x 10-" 1.00 × 10 -4

8.00 × I0 -6 6.32 × 10 -s 3.28 × 1 0 - 4

8.23 x I0 - t

8.89 2.08 5.49 2.71 2.56 2.26 7.10 5.58

3.00 X 10 -s 2.70 x 10 -5 2.15 x 1 0 -5

8.22 X 10 -~ 4.18 X 1 0 `5 4.62 X 10 - t

3.52

Not measured

Two liquid formation; viscous mass.

Glass particles disintegrated after a few days of leaching Highly viscous.

Swelling Glass particles disintegrated after three days of leaching

Swelling; opaque mass; glass particles disintegrated to powder after a few days of leaching.

Swelling.

Swelling.

R

m

Formation of two liquids

~.

PD

e~

~r~

0

5:

l:a

0

P,

<

2

P605

4

3

5

Fe6Os 6

A16Os

11 7 9

19 22 23

47

36

47

11

12

8

11

5.2

46.1

47

6

12

6

30

29

24

14.8

14.1

5

20

12.5

49 44.2 (Other glasses) 46 47

5

6

39.2

48

5

13

12

40.8

42

16.7

16.7

16.7

16.7

18.3

34.2

38

5

9.1

8

PbO

(System: (Fe~Os + 6 ~ SiO2)--PbO-P60~-CIRUS waste oxides) 13 42.7 5.5 21.8

55 60 66

12 12 16 16

7

MnO

+ 6Yo SiO6)--P~O6-CIRUS waste oxides) 6 24 6 15 6 10 6 20 6 17 6 22

NazO

SiOI

7 48 6 29 9 48 6 10 10 53 6 4 (System: AI203-P6Os-CIRUS waste oxides) 15 60 16 48 17 54 18 60

(System: (Fe203 1 47 2 49 3 58 4 58 5 53 6 41

(PG)

Glass

Glass compositions (wt. ~ )

23

18

23

20.9

23

20

19.1

25

25.8

20.9

34 33 25

28 40 30 24

17 36 37

23 30 26 16 24 31

9

Waste oxides

Poured at 1150 Poured at 1100 Poured at 1150 Poured at 1150 1150

Poured at 1150 Poured at 1150 Poured at 1120 Poured at 1150 1120

1150 1150 >1180

1120 >1180 :>1180 --

1150 1100 1150 1150 1140 Poured at 1150 >1180 1100 1080

10

Melting temps, (°C)

x x x x x ×

10 -6 10 -5 10 -4 I0 -~ 10 -5 10 -6

5.10 × 10 -6

8.38 x 10 -6

4.06 / 10 -6

1.40 × 10 -5

2.90 x 10-s

9.13 x 10 -e

1.62 × 10 6

3.72 x 10 -6

9.36 x 10 7

3.42 x 10 6

7.10 x 10 -B 3.31 x 10 -s Not measured

1.12 x 10 -5 N o t measured Not measured Not measured

Not measured 2.40 × 10 -~ 2.16 × 10 -~

6.30 3.38 3.05 1.77 2.33 5.16

11

Average leach rates (g/cm~/day)

Derived from PG-1; little amount of unmelted particles present I5 g. MnO replaced PG-5; little amount of unmelted particles present Opaque mass; SiO6 of PG-1 was replaced by Na~O Partially crystalline mass. Na~O (with that of waste oxides): P 6 0 6 = 1 SiO~ of PG-1 was replaced by Fe6Os

Glass no, PG-1 was replaced by 10g PbO; enough unmelted particles present 20 g, PbO replaced the glass no. PG-6; less glassy than PG-6 20 g PbO replaced the glass no. PG-2, very few agglomerated particles present 20 g PbO replaced the glass no. PG-1; unmelted particles present 20 g. PbO replaced the glass no. PG- 5

Evolution of fumes continued throughout the melting operation

Melting temperature could not be determined. Agglomerated crust found on the glass surface during melting due to phase separation Fine unmelted particles present.

m

--

--

-Heavy evolution of fumes and swelling --Partly crystalline

12

Remarks

Table 2b. Compositions, melting temperatures, leach rates, etc. of phosphate glasses incorporating CIRUS waste oxides

r-

K

O~

10

66

55

55 65

7

13

18 19

16 15

12

12 16

28 30

34 62 6 19 (System: AI2Oa-P~Os-TAPPWaste oxides) 5 60 6 60

6 6

29 20

33

24

28 24

13

8 13

4

18 26

58 51

32

21 tl

14 18

28 33

6

6 6

6 6

58

9

23

8

55 57

7

PbO

16 17

6

MnO

20 28

5

A1203

60 48

4

Fe~O8

4 15

3

Na20

24 32 23

2

1

SiO~

Waste oxides

(System: (Fe~Oa + 69/00SiO2)-P2Os-TAPP waste oxides) 1 53 6 17 2 50 6 12 3 47 6 24

P2Os

Glass (TG)

Glass compositions (wt. ~o)

Poured at 1160 Poured at 1180 > 1200 Poured at 1190

1100 Poured at 1160

Poured at 1200 1180 Poured at 1200 1170

Not measured 1.46 x 10 -5

9.50 x 10 -5

3.80 × 10 -5

2.41 x 10 -5 6.94 x 10 -e

1.20 x 10 -s

4.52 x 10 -5 8.54 × 10 -6

1.02 × 10 -s

1.17 × 10 -4 8.30 x 10 -4

1.81 × 10 -4 3.88 × 10 -5

1.70 x 10 -4 4.82 x 10 -4 2.50 × 10-*

11

10 1100 1160 Poured at 1200 1150 Poured at 1200 1160 1100

Average leach rates (g/emt/day)

Melting temps, (°C)

of

fumes

Swelling appreciable; small amount of agglomerated crust found on the glass surface during melting due to phase separation

Glassy but opaque Small amount of agglomerated crust found o n the glass surface during melting due to phase separation Small amount of agglomerates present; more glassy than TG-5 and TG-6 Unmelted particles present

Unmelted particles present

Opaque mass; evolution throughout melting Unmelted particles present

Hard rock-like substance; unmelted particles present Evolution of fumes and swelling Unmelted particles present

m

12

Remarks

Table 2c. Compositions, melting temperatures, leach rates, etc. of phosphate glasses incorporating T A P P Waste oxides

i

t~ Q

v

O

<

2

6

AlzOs 7

MnO

30

43.4

50.6

23.7 39.8

31

48.5

34.4

26.7

36

37

38

39 40

41

42

43

44

5.3

7.6

16.5

8

6.3 12.2

19.4

15.6

10

11

32

17

7

28

45 18

11

19

40

31

30

35

22

27

19 24

13

16

14

18

Poured at 1150

1150

Poured at 1150 Poured at 1150 Poured at 1150 Poured at 1150 Poured at 1150 Poured at 1150 Poured at 1150 >1200 Poured at 1150 Poured at 1150 Poured at 1150 Poured at

1120 1120 1050 Poured at 1200 1160 1180 Poured at 1200

10

temps, (°C)

Melting

Average

× × × ×

10 -s 10- e 10 -4 10 -6

7.85 × 10 -3

5.12 × 10 -5

1.48 × 10 -a

5.95 × 10 -5

Not measured 2.15 × 10-s

5.21 × 10 -5

2.89 x 10- s

2.05 x 10 -5

9.09 × 10 -6

1.25 x 10 -6

1.03 x 10- 6

2.33 x 10 -5

2.56 × 10 -4 3.10 × 10 -6 4.60 x 10-6

5.88 7.63 7.48 5.46

11

leach rates (g/cm~/day)

Fluid at 1050°C *

Fluid at 1020°C

Fluid at 930°C *

Fluid at 940°C *

Viscous at 1200°C * Fluid at 930°C *

Fluid at 900°C *

Fluid at 950°C *

Fluid at 1000°C *

Fluid at 1000°C *

Fluid at 980°C *

Fluid at 1000°C *

Fluid at 900°C *

Unmelted particles present

Swelling

Swelling Unmelted particles present

12

Remarks

* P,O5 and Na20 including that from waste oxides were in 1:1 molar ratio; mainly crystalline mass; swelling; melting temp. could not be determined.

6

6

6

6

6 6

6

6

6

6

29

10

34

15

34

26

6

13.2

40

6

25

8.4 10 17.5

11

16.7 16.7 16.7

35.8

19.1 25.8 21.6

17.5 13.3 15.8 24.2

9

24

5 5 5

16.7 16.7 16.7 16.7

8

PbO

15

50.8 42.5 39.2

5

Fe2Os

Waste oxides

(System: (FezOs + 6Yo SiO2)-Na~O: P~O6-TAPP waste oxides) 20 37 6 13 29

31 32 35

4

Na~O

+ 6% SiO2)-PbO-P206-TAPP waste oxides) 5 15 5 19.2 5 11.7 5 13.3

3

P ~ O ~ Sio2

(System: (Fe203 22 45.8 27 45.8 29 50.8 30 40.8

1

Glass (TG)

Glass compositions (wt. %)

Table 2c. Compositions, melting temperatures, leach rates, etc. of phosphate glasses incorporating TAPP Waste oxides (contd.)

to

Ix)

Vitreous matrices for the containment of high-level purex waste in tank. The glass-forming region and melting temperatures are shown in the Fig. 2a and the leach rates in the Fig. 2b. The compositions no. SG-27, 28 and 36 form two liquids during melting and are kept outside the glass-forming boundary. The durabilities of the first two glasses were measured. They disintegrated to powder after a few days of leaching. The two liquid phases gave place to a single homogeneous glass at above 1200°C. From the observation made on the three compositions in this region the consolute line seems to be inclined downwards as one moves from the composition no. 36 to composition no. 28. Melting temperatures and durabilities of the glasses increase with increase in SiO.2, and decrease with increase in NazB407. Glass-forming boundary is limited roughly up to glass, no. SG-47 containing approximately I 0 ~ waste oxides in the low waste oxides direction. About 109/o waste oxides in glass may be taken as reasonable minimum limit of incorporation as further reduction will lead to too low a volume reduction. The extraction rates of these glasses vary from l0 ~ to 10-5 g/cm~/day as are shown in the Fig. 2b. The glasses no. SG-24, 30 and 31 disintegrated within two to four days when leached in boiling water. The melting temperatures of the glasses of this system are more or less same as those of the glasses in the system I. Studies of the above system at 5 ~ CaO (glasses no. SG-33, 34 and 35) and at 3 ~ AlzO 8 (glasses no. SG-37, 39 and 40) levels as shown in the Table 2a "TAPP W~TE OXIDES

114

~ 5 0 4 . 1 6 " 1 1b7"2

50"4

N028407

33 6

J6m ))6 SO a AtS~NO20 0t~ds~lwli'lOL£V(] H~aVy sw(lling ~eltin 9 t~mpatraturcs i~)$ o -io $o°c ~z~-~lo 5o- i i oo°c Leach rates (~m/cm2/doy)

~_.

,o- ~

®~,o

67~r

SiO 2

-5

Fig. 3. System SiO=-Na2B4OT-Na20--MnO-TAPP waste oxides. Glass forming region, melting temperatures and leach rates.

263

indicated that CaO decreased the leachabilities of the glasses but had little influence in lowering their melting temperatures, and AI~O3 glasses were durable, viscous and melted around 1150°C. (c) System IlI. SiOz-Na2B4OT-Na20-MnO- TAPP waste oxides. In this system Na20 and MnO are each at 8 ~ constant level. The glass-forming region, melting temperatures and leach rates are shown in the Fig. 3. The glass-forming boundary is limited approximately up to glasses no. SG-43 and 45 towards the high Na2B40 r direction. It may extend towards high waste oxides direction but we have restricted the study up to 30~/o of waste oxides in glass as larger amount of this may cause excessive temperature rise during storage. The glasses of this system are fluid and the fluidity is reached much below the melting temperature. A good number of glasses having melting temperatures below 1000°C have been obtained in this system. The leach rates vary from 10_4 to 10-5 g/cm2/day and are accordingly grouped in the figure. Composition, melting temperature and leach rate of a glass no. FA.~ having the same constituents as this system but incorporating CIRUS waste oxides instead of TAPP are indicated in the Table 2a. (d) System IV. SiO~-Na2B4OT-PbO-TAPP waste oxides. In this system of glasses borax is at a constant level of 20 ~. Glass-forming region, melting temperatures and melting observations are shown in the Fig. 4a. Melting temperatures of the glasses vary from 900 to 1200°C. The glass-forming region is large in this system in comparison to other systems studied. The leach rates in boiling water vary from 10-4 to 10-ng/cm2/day (Fig. 4b). Glasses occupying a large portion of the boundary have leach rates in the order of 10-6 and some of them have melting temperatures near 1000°C. Glass no. SG-11 disintegrated after only three days of attack by water. Glasses no. SG-4 and 14 showed also similar behaviour after a few more days, (e) System V. SiOz-NazO-PbO-TAPP waste oxides. This is similar to the silicate system IV except that Na20 is used in place of Na2BaO 7. The compositions, average leach rates, etc. of glasses no. SG-1, 2, 18, 19 and 46 representing this system are shown in the Table 2a. Glass-forming region of this system cannot be suitably represented in a triangular diagram. SG-1 melts above 1200°C and in the composition no. SG-2 two liquids appear during melting. The compositions no. SG-18, 19 and 46 were formulated from the composition of SG-16. For this purpose the composition of SG-16 without borax was considered and was recalculated on a percentage

264

J.

]V[UKERJIa n d

A . S. SANYAL C LR US waste oxides lad

TAPP WASTE OXIDES

20

6

40

7

\

60

60

40

SO

2O

IC0

4a

~o

16

32

48

80

64

Pbo

60

80

r z203' ~ gm•sio 2

100

P20S

sio 2 ueltin~

At ~O'~ NO2B40? LeV,Zl Leach rates (,cjrn/cm21doy) ~

164-16 s, glass

disintegrate

temperatures .

~ ~oso:uaa°c [~.4o-.so°c

B-~,lao=c

CIRUS WASTE OXIDES IOO

particles

rapidly; I--I-~;~~

TAPP WASTE OXl DES

/ & 40

7 ,oo

80/

7

~ I6

v 32

• 48

~ 64

Pbo

\

20%

Na28407

/o

;o

io

';a

f:¢203~" ,~ gins 5i02 ::~: ......

\ 8Q si

At

60

,oo P20 s

increased Increased

evolut.ion of fumes tendency towards

• sw¢11in.g crystallisation

~ Evolution of fume• • •welling C:]--Fumes lesslswellin 9 Ittle ClRUS WASTE" OXIDES

02

LEVEL

LOW d u r a b i l i t y -~ Highlyviscous Meltin~ temperatures I¢¢~I -- 9 o o*-, o o o" I--l--, ooo'-,, oo" ~ - - , ,

oo'-,2oo"

Fig. 4. System SiOrNa~B4OT-PbO--TAPP waste oxides. (a) Glass forming region and melting temperatures, and (b) Leach rates.

basis. This composition was replaced by 10, 15 and 20% Na~O for SG-18, 19 and 46 respectively. The glasses no. SG-18 and 19 have the same order of extraction rates 10-e g/cm2]day as that of the corresponding composition containing borax (glass no. SG-16) but their respective melting temperatures 1080 and 1050°C are somewhat more than that of

/

?

\

,007.,. ,...Y.. \ ;IO

40

60

Fe~O3e6gins SiO z

Leach rates

80

ioa

P205

(gin / c m 2 / d a y )

ml _ ,6 6 f-'l_,~ s ~ - i o - " Fig. 5. System (Fe:Os + 6% SiO~)-P~Os-CIRUS waste oxides. (a) Melting temperatures, (b) Melting observations and glass forming region, and (c) Leach rates.

Vitreous matrices for the containment of high-level purex waste

265

TAPP WASTE OXIDES I00

CIRUS WASTE OXIDES

20

II0

40

60

60

40

/

/

'~,~/

,ooL__

#o

#o

\

~o

;o

,~o

F~Z03 ~6 gl'~S 5102

~--

P2OS

T A P P WASTE

/

I©G

~ 20

A~

X

~ O~T.Oi~__ ~

~',ncr¢ased ~Iting

OX!DES

',=. "\

!

//

A 1203

Fumes & sw~lling --~LOW d u r a b i l i t y Melting temper0tur.~s ;,~ound r~oo'C- L - ~ - - , , 5 o ' - , 2 o o ' c ~ ) , ~ o o ' c

II

40

60

Cvoiution

of

80

lO©

['205

fumes ~ SW~H]n]

temperatures

~I!~O'C

{}~

Lee, oh r a t e s

-.-,o-~ ~ "

1120--1150"C

(£m /Era 2

~o-S

Iday)

Fig. 7. System AIsOa-P~OrCIRUS waste oxides. Glass forming region, melting temperatures and leach rates. 0o

I00~ F

" '"2~0

~'20~ + ~ gins Si 02 Leech

~

40

~O~T

60

fails below 41% (composition PG-6). Release of off-gases accelerates at F%Oa content below 10% and P~O5 content above 60 % in the glasses. The leach rates of the glasses studied vary from

.o

TAPP Waste o x i d e s io o

ICO

80 P208

r a l : e s (gin t crab/day) 20

Fig. 6. System (Fe~O~ + 6% SiO2)--P~Os-TAPP waste oxides. (a) Glass forming region and melting temperatures and (b) Leach rates. SG-16. The average leach rate of the composition no. SG-46 is in the border of 10..4 and 10-s g/cm~/day but its melting temperature 1020°C is near to that of SG-16. However, there is room for further study of glass compositions in this system if investigation is done at fixed Na~O level.

SO

(a) Systems l(a) (Fe~Os + 6% S i O ~ ) - P~O~CIRUS waste oxides, and l(b) (FezOs + 6% SiOz)P2On-TAPP waste oxides. Melting temperatures of the glasses increase and evolution of fumes decrease with increase in FezOs in the glasses (Figs. 5a, 5b and 6a). The melts start crystallising as the PzOn content

40

I

iii

" IOO

5.1.2 Phosphate glass systems for CIRUS and TAPP wastes.

BO

2O

AI203

4O

T

I 6O

---I. FUmeS & Swelling ~ M e l t i n g Melting t e m p e r a t u r e s & ~ > 12OO" C

~_

META~

BO

~00

P205

/~1200 "C

8~11001--J 2OOIC

L¢{}ch r a t £ s ,o-~ EEl., o-S

(gmlcm2lday.)

Fig. 8. System AI~OrP~Os-TAPP waste oxides. Glass forming region, melting temperatures and leach rates.

266

J. MUKERJIand A. S. SANYAL

10-a to 10~g/cmZ/day for system l(a) and from 10-8 to 10-~g/cm2/day for system l(b) (Figs. 5c and 6b). Most of the durable glasses in either system having leach rates of 10-5 to 10-e g/cm~/day were found to lie in the area between the lines joining the points of ortho and meta ferric phosphate compositions to the waste oxides apex in the compositional triangular diagram. Durability and melting temperature increase as the glass composition moves from the meta to the ortho line. Glass no. PG-46 (Table 2b) was derived by replacing half the amount of Fe2Oa in the glass PG-1 by MnO and glass no. PG-47 by adding 15 g MnO to the glass composition PG-5 and recalculating on percentage basis. The glasses have more or less same leach rates as those of the original glasses but are more fluid. (b) Systems II(a) AI2Oa-PaOs-CIR US wasteoxides, and II(b) AI203-P~Os-TAPP waste oxides. Glassforming region, leach rates, melting temperatures, etc. of the above two systems are shown in the Figs. 7 and 8 respectively. Release of off-gases increase with increase in P~O5 and/or with decrease in AI~O3. The glass-forming regions are small in comparison to the systems I(a) and I(b). The promising glasses are found more or less in the vicinity of the join between the meta phosphate composition and the waste oxides apex in the triangular diagram. The durabilities of the glasses vary between 10-5 and lO-ag/cmZ/day. Melting trials in 304 L stainless steel pots indicated that amongst phosphate compositions glasses in this system were least corrosive and the pot withstood the molten glass for 48 hr without appreciable corrosion. When alumina was added as glass batch additive instead of aluminium nitrate a solid phase having the X-ray diffraction pattern of crystobalite was found to separate out when the percentage of alumina was more than 12 ~. (c) Systems III(a) (Fe20 a + 6~o SiO2)-PbOP2Os-CIRUS waste oxides, and IH(b) (FezOa + 6% SiO~)--PbO-PzO.~-TAPP waste oxides. Lead oxide was added to examine if it had any favourable effect on melting temperatures and durabilities of the glasses of the systems 1(a) and 1(b). Presence of PbO was found to decrease the leachability of the glasses but no lowering of melting temperature was observed. The compositions of the glasses PG-13, 38, 42, 48 and 49 of the system III(a) and their characteristics are shown in the Table 2b. In the system HI(b) PbO is at 16.7 ~ fixed level. Glass-forming region, melting observations and melting temperatures are shown in the Fig. 9a and the extraction rates in the Fig. 9b. The extraction

TAPP WASTE OXIDES B3"3

I6'6

(>'~4

3~';~

498

49 6

332

*66

J66

Fe2O3+ 6 gm$ 51 02

P2 '35

swelling t.4elfin cj t e m p e r a t u r e s _,

o s o-,,

so'c [~--,,

At z6"7%

pbo

so -,2oo'c

I m ~,=oo "C

Level

TAPP WASTE OXIDES 83'3

16'6

b6"4

33"2

49"8

49"6

33"2

.,,/ . . 16"6

33"~

W

,,\,,.

49"8

66"4

Fe 2034" 6 ~mS SiO 2 Leoch

t~m. ,66

P205 rotes

(gmlcm=lday).

[Z~--I 5 s

~

64

At =6"7% p b o L e v e l

Fig. 9. System (FesOa + 6~0 SiOs)-PbO-PsOs-TAPP waste oxides. (a) Glass forming region and melting temperatures, and (b) Leach rates. rates of the glasses vary from 10-4 t o 10-e g/cm~/day. The rate decreases with increase in Fe,,Oa in the glasses. The rates of the glasses no. TG-30 and 35 are of inhomogeneous samples, as the melts could not be made homogeneous within 1200°C. The minimum limit of waste oxides incorporation is restricted to approximately 10 wt ~o. (d) System IV (Fe20 a + 6 ~ SiO2)--Iqa20: PzOsTAPP waste oxides. In this system, Na20 (including that of waste oxides) and PaO5 are in 1:1 molar ratio. The products are semi-glassy or non-glassy in appearance. They are generally fluid between 900 and 1000°C. The melting temperatures could not be determined as fibres could not be drawn due to

Vitreous matrices for the containment of high-level purex waste ,,p.

267

indicate a gradual build up of activity in the leachate with time. Count rate of the leachate suddenly jumped up when the inactive cations of Sr, Ce and Cs were added to the leachate because of the release of adsorbed or exchanged active cations from the inner surface of the glass flask. The count rate due to the accumulation of the extracted cations in the leachate increased gradually thereafter (Table 3). The inactive

,,AYE° OXIOES

Table 3. Effect of addition of inactive cations on the quantitative estimation of extracted fission products from glass Day

oo/

20 F¢203+6grns si 02 ~LOW

40

60

80

\oo NQ 20 : P205

durability

Pourinq t~mpcratur~s I-~ ~ 9oo-,o5o'c

_>,2oo'c

Fig. 10. System (Fe,O8 + 6~, SiOs)-Na,O: P,Os-TAPP waste oxides. Pouring temperatures.

1st 2nd 4th 5th 6th 7th 8th 9th 11 th 12th 13th 14th

Counts Ce 43 58 59 194 102 77 92 96 58 1211 1633 1884

Sr 32 70 69 172 106 68 40 85 60 847 1531 1846

Cs 38 61 107 316 136 117 95 128 68 1252 1391 1554

high fluidity of the melt. Consequently, the melts (FP, glass, 11 1.10gin, attacked by continuous flow of all compositions except no. TG-39 were poured at 1150°C. The boundary drawn in the Fig. 10 cannot of hot distilled water. Initial volume of water in the leaching flask, 350 ml; leachate drawn each day, 20 ml; be called glass-forming region as the products are period of counting in the gamma ray spectrometer, not glassy. The lower limit of waste oxide incorpor- 3 min; additional inactive cations added after the 11th ation, as in other systems, is taken at approximately day.) 10~. Melts having the above ratio of Na~O: P~O5 was observed to have higher volatilisation loss. salts to be used as carriers, were therefore, added to Average leach rates of the solids except TG-42 water during experimental runs to minimise the loss within the boundary are of the order of 10-~ g/cm ~/ of Ce-141, Sr-85 + 89 and Cs-134 from the leachate. 5.2.2 Criticism of extraction extTeriments with day and some are very near to 10-e. Composition fixed amount of waterfor a fixed time. Many workers no. TG-42 has a leach rate in 10-4 g/cm2/day region. [16, 17] in the field of nuclear waste fixation have Solids no. TG-41, 43 and 44 have leach rates of the order of 10-4 g/cm~/day on the first day but decrease determined extraction rates of glasses in a fixed amount of distilled water unchanged for a period to 10-5 on the succeeding days. of one week. This procedure is subject to the follow5.2 Extraction studies: Effect of various factors on ing criticism. the extraction vahws. Every silicate and phosphate glass when in con5.2.1 Use of carriers in counting of extracted tact with a fixed amount of water attains, in course fission products. The amounts of Ce, Sr and Cs, of time, a ptI which is known as equilibrium pH of both active and inactive leached out of the experi- the glass (see Section 4.3). At this pH no more alkali mental glass samples doped with radioisotopes or P20 5 is removed from the glass and extraction would be very small and a substantial part of these virtually stops. Besides the attack is likely to slow would get adsorbed on the inner surface of the flask down due to common ion effect. The leach rates for or would be lost due to exchange with other cations one day as well as for one week period of few glasses of the glass flask. It was observed that when the are compared in this connection. Water was renewed experimental samples were attacked continuously each day in the former and each week in the latter with distilled water based on soxhlet principle, with- case. It has been found that within experimental out using carriers samples of leachate drawn out error the daily leach rates amount to 5 0 - 1 0 0 ~ of periodically gave very erratic readings and did not the weekly rates (Table 4). It is therefore evident

J. MUKEP~n and A. S. SANYAL

268

Table 4. Daily and weekly leach rates of FPz glass

Day

Leach rate (g/cmS/day) (× 10-5)

Day

Leach rat¢ (g/em~/day) ( x 1 0 -6)

1st 2nd 3rd 4th 5th 6th 7th

5.33 7.42 11.18 9.19 9.67 7.56 5.20

8th 9th 10th 11th 12th 13th 14th

2.82 3.61 7.91 6.97 8.70 6.45 8.70

Total for 1st 7 days

55.55

Total for 2nd 7 days

1st week

11.45

2nd week

5.3 Leach rates 5.3.1 Screening test. All the glass compositions studied were made to undergo a durability screening test. The durability values quoted in Tables 2a, 2b and 2c are average for 14 to 21 days run. Glassforming regions depicted in the triangular diagrams were divided into different segments according to the order of these leach rates. Glasses which indicated a durability of 10-Sg/cm2/day and better were selected for more elaborate studies.

45.16

5.3.2 Leach rates of the samples taken out at different temperatures from glass melts.

8.06

In some glasses having high durability as is evident from the durability screening test samples of melts were withdrawn at different temperatures to investigate how the properties of the melts evolved with the rise of temperature. Selection of composition for such study was made on following criteria: (1) Compositions of which the major part was glassy at a lower temperature but contained small amount of unmelted particle and are fluid enough to be pourable out of the melting pot. Dissolution of these solids required a rise in temperature of 100-150°C e.g. PG-38, 42, 47, TG-5 and 27.

that in a weekly extraction run with water unchanged for one week the glass was rapidly attacked on the first day and in some cases on the second day but no attack t o o k place on successive days of boiling. Therefore, for screening test a one day extraction experiment was adopted and for long time extraction runs of individual fission products an apparatus based on the soxhlet principle was used where always fresh distilled water attacked the glass at a given flow rate.

Table5. Leach rates of samples taken at different temperatures fromglassmelts Glass PG-42 Leach rates, g/cm~/day ( x 10-6) of samples taken at Days

920°C

Ist-2nd 3rd 4th-5th 6th-7th 8th-9th 10th-12th

7.25 (av) 23.20 3.25 (av) 1.8 (av) 2.65 (av) 10.40 (av)

Mean

7.03 × 10-6

970°C 6.10 8.90 3.95 3.75 14.40 5.07

1020°C

(av) (av) (av) (av) (av)

6.71 x 10-e

7.55 6.80 6.15 3.25 40.50 8.64

(av) (av) (av) (av) (av)

12.30 × 10-6

1070°C

1120°C

8.50 (av) 7.60 7.80 (av) 10.75 (av) 25.40 (av) 13.84 (av)

6.65 3.80 5.45 3.15 2.10 6.33

12.83 x 10-6

(av) (av) (av) (av) (av)

4.79 x 10-6

Glass TG-25

Days

Leach rates, g/cm2/day ( x 10-s) of the sample taken at 980°C

lst-2nd 3rd-4th 5th 6th-7th 8th 9th 10th

1.76 (av) 0.85 (av) 1,33 0,42 (av) 1,40 1,17 1.26

Mean

1.12 x 10-5

Days lst-2nd 3rd 4th 5th-6th 7th-8th 9th-12th Mean

Leach rates, g/cm2/day ( x 10-5) of the sample taken at 1150°C 2.35 2.09 2.04 0.96 0.86 0.62

(av) (av) (av) (av)

1.24 × 10-5

Vitreous matrices for the containment of high-level purex waste (2) Compositions which formed non-glassy, fluid melts at low temperatures and did not show any significant improvement in melt characteristics with rise in temperature by 100-150°C e.g. TG-25. The temperature where the melt was just fluid to be pourable was the lowest temperature of sampling. This temperature varied from 900 to 1000°C for the glasses investigated. Maximum temperature of sampling was 1150°C. Temperature was maintained constant for about 1½ hrs before the sample was withdrawn at each temperature. Durabilities of such samples were compared by measuring their extraction rates in fixed amount of boiling distilled water on weight loss basis. Almost identical extraction values were obtained for samples withdrawn at different temperatures from each melt. Table 5 gives the data for two such compositions. The data for the others may be obtained elsewhere [14]. Microscopic observations of the glasses no. PG-38, 42 and 47 containing 5-6 % silica showed that the samples at lowest temperatures of pouring comprised of predominantly glassy matrix with some fine crystals and quartz grains. As the temperature was raised the fine crystals dissolved in glass leaving behind the quartz grains. Quartz grains were found to react (Figs. 1 la and 1 lb) with the glassy matrix but did not dissolve completely in the matrix even at the highest temperature of pouring (1150°C). It may be said to be existing in the matrix as an inert substance at the temperature of study. This was corroborated by the fact that the glasses studied in the phosphate systems with and without silica, for example, PG-1, 8 and 11 had almost the same leach rate (Table 2b). This observation is contrary to the observation made previously that 5-6 ~o silica helps in producing better glasses [18, 19]. From the above discussion it appears that many glasses can be processed at temperatures much below their homogeneous glass-forming temperatures, and depending on their compositions the processing temperatures may be in the range of 900--1000°C and yet they may show the same extraction rates (10-5 to 10-e g/cmU/day) in boiling water as those given by their homogeneous samples made at relatively higher temperatures. The products at these processing temperatures may be semi-glassy or nonglassy or may contain some undissolved particles but, since there is no improvement in durability by making these products homogeneous at higher temperatures, such a higher temperature is unnecessary and is not recommended, as this may bring in problems like selection material for melter pot, increased vaporation, etc.

269

5.3.3 Individual leach rates of Ce, Sr and Cs of some of the selected glasses in continuousflow of hot distilled water. Glasses which had desirable processing temperatures ( < 1100°C) and passed the durability screening test were subjected to more elaborate leaching by continuous flow of hot distilled water. Amongst many of the glasses which may be selected for this study only FA2; PG-42 and 47; TG-5 and 27, and SG-16, 18, 33, 40 and 43 were examined after doping with the radioisotopes Ce-141, Sr-85 + 89 and Cs-134. The individual extraction rates of fission products Ce, Sr and Cs were determined by counting technique. Detailed information may be found elsewhere [15]. The glasses were generally leached for 50--90 days. In the glass no. FA2 the loss of activity follows the order Sr > Cs > Ce, while in the other glasses the order is Cs > Sr > Ce. The rates do not differ much among themselves and are of the order of 10-5 g/cm2/day. The average values of the leach rates for Ce, Sr and Cs computed over the entire period of leaching are shown in the Table 6. Table 6. Average leach rates of Ce, Sr and Cs of some selected glasses in continuous flow of hot distilled water and in boiling simulated sea water Average leach rates (× 10-s) (g/cm2/day) of Glass

Ce

Sr

Cs

FA~ PG..42 PG-47 TG-5 TG-27 SG-16 SG-18 SG-33 SG-40 SG-43

4.07 1.71 1.61 2.94 (6.44) 5.73 (14.00) 4.17 3.33 (4.22) 6.39 (1.57) 2.34 (0.92) 3.24 (2.68)

7.95 3.66 2.36 5.97 (6.97) 6.05 (14.50) 9.46 4.21 (5.53) 7.03 (1.65) 501 (1.57) 8.73 (7.71)

7.39 5.10 3.32 7.00 (7.46) 6.62 (16.10) 9.87 6.09 (6.97) 7.73 (2.21) 5.52 (2.83) 9.89(13.80)

* The values within the brackets are the leach rates in boiling simulated sea water. In the Figs. 12(a,b,c,d) the individual extraction rates of Ce, Sr and Cs from the glasses no. PG-47, TG-5, FA2 and SG-40 are plotted against the number of days when the glasses are attacked by continuous flow of hot distilled water. The points in the plots cannot be suitably connected with any curve following a mathematical equation. Similar plots were obtained by other authors [20, 21]. The points have therefore been left as such and no attempt has been made to join them together. The leach rates increase and decrease to our estimation quite randomly, probably around a mean value. The statistical error of counting was much less than the difference in values between a pair of neighbouring points and

Glass and Crystal

Glass and Crystal

Glass and Crystal

Glass and Crystal

Glass and Crystal

Glass

Glass

Glass Glass

Glass

PG-38

PG-42

PG-47

TG-5

TG-27

SG-16

SG-18

SG-33 SG-40

SG-43

1.000 (126 °) 0.853 (287 °) 1.268 (93 °) 1.052 (276 °) 1.327 (82 °) 1.177 (202 °) 1.066 (395 °)

2.317 (106 °) 1.061 (278 °) 2.936 (424 °) 0.812 (128 °) 0.825 (140 °) 0.874 (274 °) 0.791 (357 °) 1.152 (142 °) 1.031 (328 °) 0.971 (423 °)

1.048 (117 °) 1.182 (308 °) 1.214 (433 °)

3

2

Glass

1

Nature of sample

FA~

Sample

Thermal conductivity at different temperatures (°C) ( × 10-3) (CGS uni0 4

(796 °) (901 °) (1000 °) (I098 o) (900 °) (996 °) (1091 °)

(988 °) (1071 °) (1134 °) (1199 °)

1037.0 (953 °) 281.8 (1060 °) 84.4 (1152 °) 35.5 (1253 °) 660.7 (848 °) 134.9 (936 °) 32.4 (1050 °)

588.9 87.1 22.9 10.0 690.2 174.6 63.4

549.5 213.8 107.2 56.2

Viscosity at different temperatures (°C) (poise)

11.50

8.00

3.84

10.75

2.68

2.33

--

3.70

3.42

3.05

2.98

3.35

2.44

6

Density (g/crn 3)

9.75

8.25

7.00

10.25

10.00

--

5

Coeff. of linear expansmn per °C ( × I0 -e) (0-400°C)

Table 7. Physical properties of some selected glasses

10.1

m

9.6

10.5

9.0

--

3.6

6.7

5.3

--

7

Equilibrium pH

No peak up to 800°C

800

No peak up to 800 °C

No peak up to 800°C

No peak up to 800°C

760

725

620

620

8

Crystallization temperature (°C)

ca.

Fig. I1 (a) Glass W-38 afta pouring at I 150°C (200 1. th) Glass PC-42 after pouring at II10 c (X0 ).

i ,

*d

..

Fig. 13. Typical crust of mancontaining silicate ganese glasses (60 j.

Vitreous matrices for the containment of high-level purex waste

271

12 o o

O

ooo oi% o°,o°%, oo ha

O

op

o

o

a~

4

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0

o

o 0

o

0

0

0

I

0

I

f

I

I

I

!

I

|

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1

gO

.412

% ~ l

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oo

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12

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It

12

8

T

l~nt

20

30

40 SO DAYS-----

60

70

80 O

L.R.-Leach rate in gm/cm}t6ay of g l a s s No. PG-47 Extraction of Ce [o], S,[,] and cs[-]

~

It

/, It

4

IO

It It

S

S

~

[, IO

I

I

I

20

30

40

I

L.R.-Leach r o t e

I

60

70

80

--.~ I

in g m / c m / d a y

of gloss

No. F A e

EXtraction

(a)

,~

:50

D A yS

~t

of C,[o£SrE.]an d C,[-] (c)

8F 0 o

41.

T

o

o

o °

o

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-

-

•

.'~ io°

l

Q"

l

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12

o,

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n

o I

I

o i

o

o i

•

1

. •

•

•

. I

"0/

x

x

]o

.~4[

•

x

°it

• •

~./

°

c~ 4]-J

~08r

•

e e

o

e--xl21

•

21 B~

o

o

o

f

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°°°

o o

I

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ir

le

x

I

GI •

.~.

Ii

ir

x x x

x

x

x

~e

x

x O" i

O

20

i

x

40 DAYS --4-

i

i x

60

i

Ix

60

L• R = .Leach r a t e in gm I cm~/day _ _ of glass £ x t r a c t l o n o f ce [o], sr [ . ] , a n d cs [x]

(b)

IO,

wl 20

L.R-Leach NoTG

Extraction

IO

3

,

14.1

I

40

50

60

__

DAYS ~ 2 r a t e in g m / c m T d a y of glass No. S G - 4 0 Of C e [ o ] , S r [ o ] a n d c s [ - ]

(d)

Fig. 12. Plots of individual leach rates of Ce, Sr and Cs against days. (a) Glass No. PG-47(b) Glass No. TG-5 (c) Glass/40. F A , and (d) Glass No. SG-40.

272

J. Mur~RJt and A. S. SANYAL

the rise and fall in the leach rates may probably be attributed to the nature of extraction mechanism ot the individual components of glass. 5.3.4 Leach rates in boiling simulated sea water. Leaching in boiling sea water was performed with some of the selected glasses (TG-5 and 27; and SG-18, 33, 40 and 43) doped with Ce-141, Sr-85 + 89 and Cs-134. The individual leach rates of Ce, Sr and Cs in sea water are shown in the Table 6 along with their leach rates obtained in continuous flow of distilled water. The release of fission products in these glasses follows the same order Cs > Sr > Ce as above and is of the order of 10-Sg/cm~]day in most of the glasses.

and 750°C. The silicate glasses e.g. SG-16, 18, 33 and 43 and FAz did not show any crystallisation peak in D T A runs up to 800°C and might be more resistant to crystallisation. The crystallisation temperature of FA 2 glass could not be determined from gradient furnace experiment. The glasses no. SG-16, 18 and 33 were subjected to heat treatment in muffle furnace at 600°C for 24 hr. and at 660°C for 17 hr. The temperatures were above the transformation temperatures of these glasses. But no crystallisation took place as was evident from microscopic observation. It may be interesting to note that the glasses in the system containing MnO produced on heat treatment a coherent crust of braunite (3 MnzO3. MnO. SiOz) on the surface (Fig. 13) which may increase resistance to attack by water.

5.4 Equilibrium p H Equilibrium p H of glass may be taken as one of the guiding factors in selecting the final glass composition for the fixation of the waste or in locating the sites of burial. Generally the rate of extraction during leaching will gradually become slower as the p H of the leachate tends towards the equilibrium p H of the glass or in other words a solution having the equilibrium p H will not attack the glass. It is therefore safer to bury the glass at a site where the p H of the underground water is near the equilibrium pH of the glass. The equilibrium pH's of the glasses measured are given in the Table 7. The silicate glasses have p H values above 7 and the phosphate glasses below 7 indicating that the basic constituents of a silicate glass and the acidic constituents of a phosphate glass are extracted preferentially. In phosphate glass therefore, attack should be more in sea or in salt water than in distilled water and this is actually found in the case of glasses no. TG-5 and 27. In the case of silicate glass, the compositions SG-33 and 40 are attacked more in distilled water as required by the theory of equilibrium pH. The extraction rates of Ce and Sr of the silicate glass SG-43 are less in sea water but that of Cs is more. Silicate glass SG-18 has almost same extraction rates in both the waters. 5.5 Crystallisation temperature The crystallisation temperatures of some of the selected glasses obtained from D T A runs as shown in the Table 7 should be a guide regarding permissible temperature rise during storage. It is that temperature at which the glass nucleates and the crystals formed grow rapidly. The temperatures measured in some of our phosphate glasses were between 600

5.60therphysicaIproperties The thermal conductivity, viscosity, coefficient of linear expansion and density of some of the promising glasses are given in the Table 7. In general conductivities of our glasses are lower than those of commercial glasses. The values for commercial sodalime and pyrex glasses including photon conduction, are 0.0035 and 0-0025 in CGS unit respectively at 200°C. The opaque nature of the glasses and the presence of non-glassy substances as inclusion in them may be the cause of lower thermal conductivity. Viscosity of 100 P for the experimental glasses is found between 900 and 1150°C depending on the nature of the glass composition. At this viscosity commercial glasses are normally free-flowing. The coefficients of linear expansion of the glasses remain constant from 0-400°C and are almost same as that of commercial sodalime glasses (8.5 × 10-°). 5.7 Waste oxide incorporation and volume reduction Depending on the type of glass system 10-30~ waste oxides can be incorporated in the durable glasses. Corresponding volume reduction for CIRUS and TAPP wastes are 10--30 and 7.4--22.2 respectively. The respective activity incorporation will be of the order of 392-1176 and 1718-5154 Ci/kg of glass. The above activity values have been arrived at by assuming that the fuel will be reprocessed after 1000 days of cooling. If incorporation of the waste in glass is done after shorter periods of cooling the amount of solid waste to be incorporated will remain roughly the same but the activity incorporated will increase.

Vitreous matrices for the containment of high-level purex waste 6. SELECTION OF GLASS COMPOSITION FOR HIGH-LEVEL PUREX WASTE

6.1 Glass composition, processing temperature and chemical durability A low melting temperature of 1000-1100°C and a durability of 10-6 to 10-6 g/cm2/day are desirable. Glasses meeting these primary requirements may be found in many systems presented earlier. If a durability of the order of 10-4 to 10-Sg/cm2/day, equivalent to that of commercial sodalime glass is considered sufficient for long time storage a very wide choice of such glass compositions may be made from the systems presented. Selection of glasses from the systems with SiO2 or P~O5 as glass former and ferric oxide as glass modifier has the advantage that they can accommodate in them the corrosion product, iron, present in the waste, the amount of which may vary. PbO increases the chemical durability in the system (FezO3 ÷ 6 ~ SiOz)-P2Os-waste oxides. Presence of 6 Yo silica in phosphate glasses was reported by other workers to help in producing better glasses [18, 19]. But this could not be corroborated by us. The best glasses in the phosphate systems were found in the vicinity of the join between the metaphosphate composition and the waste oxides apex. In the aluminophosphate system the region where acceptable glasses can be found is small. Only non-glassy products are obtained in the system where Na20 and P205 are in 1:1 molar ratio and ferric oxide is glass modifier. The products are fluid in the range 900-1000°C and have good durabilities. Phosphate glasses can make remote control operation easier, than do silicate glasses, by allowing an all-liquid batch feeding [22]. Silicate glasses containing a high percentage of borax such as SiO2-Na2B4OT-PbO--waste oxides and SiO2-Na~B4OT-Na20-MnO- waste oxides are low melting and can be processed at temperatures of 950-1000°C. Most glasses in the above systems have long time stability against attacking agents. In selecting glass composition one should be careful to avoid regions in compositional diagrams where phase separation occurs. Such regions have been indicated in Figs. 1 and 2. Addition of CaO or A120s in the system SiO2-Na2B4Ov-Fe2Oa-TAPP waste oxides increases durabilities of the glasses. About 10-30~o waste oxides can be incorporated in the systems studied. The amount of waste to be incorporated will evidently depend on the activity contained in the waste, the heat generated during storage and the volume reduction to be achieved. Taking the above systems as a guide it is possible to improve a given composition in so far as melting

273

temperature and durability are concerned, with the help of minor additives. Suitable silicate glass compositions for CIRUS waste may be formulated from the compositional diagrams presented for TAPP waste in the Figs. 1-4.

6.2 Reactions of the glass batch to form stable compounds at below glass melting temperatures In a silicate glass batch containing Purex waste the main reactions below 700°C are the reactions of alkali metal nitrates NaNOa, CsNO a and RbNO a with the silica and B20 a to form essentially soluble silicates and borates [23]. As the temperature is raised the silicates and borates formed gradually dissolve in the matrix, other batch ingredients which are mainly the nitrates or oxides of the other fission products like Ba, Sr, La, Ce, Nd, Pr, Ru, Rh, Pd, Mo, Zr, etc. A sufficiently high temperature, generally more than 1000°C and long time of melting are needed to get a silicate glass which has dissolved in it all the fission product elements. On the other hand, the reaction of fission product nitrates with phosphoric acid is quick and takes place at room temperature, producing metal phosphates, which except the alkali phosphates, are all insoluble in water. Thus in a phosphate glass fixation of the fission products except the alkali metals can be achieved at room temperature. It is possible to convert this mixed fission product phosphates with the help of excess P2Os into an insoluble rock by heating it to 800-900°C. It may be noted that in the phosphate glass-forming systems, mentioned earlier, samples having unmelted partides, and semi-glassy and non-glassy products obtained by processing the melts between 900 and 1000°C showed the same extraction rates of 10-5 to 10--e g/cm~/day in boiling water as those for homogeneous glasses made at relatively higher temperaatures of 1100 to 1200°C. The same is not true for a silicate glass which when withdrawn before complete glass formation may give a mixture of alkali silicate or borate glass and fission product oxides in the form of a sister or semi-vitreous mass. The matrix, in such a case, is essentially a soluble product [23]. If a silicate glass composition is selected for waste incorporation it should therefore be processed until a more or less homogeneous glass is obtained. In the case of a phosphate composition, processing may be stopped at much below the true glass-forming temperature and yet yields an insoluble rock.

274 6.3 Incorporation o f sulphate

J. M ~

and A. S. SANYAL

the course of the work. They are also thankful to Shri A. K. Basu, A. K. De and P. B. Kayal for experimental assistance. Thanks are also due to all other colleagues of the Institute who have helped in the execution of the work.

It is difficult for a silicate glass to assimilate sulphate present in some of the Purex wastes. Sulphates will be present as a soluble layer on the REFERENCES top of the molten glass and will consist mainly of 1. Proceedings of the Symposium on Treatment and sodium sulphate, mixed with sulphates of fission Storage of High-Level Radioactive Wastes HeM by products, the amount of the latter present will the International Atomic Energy Agency in Vienna, depend on their partition coefficients between the 8-12 October 1962. glass and the sulphate layer. Segregation of fission 2. Proceedings of the Symposium on Solidification and product constituents in the salt layer may occur in Long Term Storage of Highly Radioactive Wastes. Division of Technical Information, Atomic Energy sufficient quantity, and this was actually found Commission, United States of America, 14-18 during bulk melting of a manganese borosilicate February, 1966. glass no. FA2 for CIRUS waste. The phosphate glass 3. Pleat G. B. and Lennemann W. L. (1966) Consideramelts, on the other hand, are capable of dissolving tions for Long Term Waste Storage and Disposal at USAEC Sites. U.S. Atomic Energy Commission, an appreciable amount of sulphates [24]. This Washington D.C. 20545. February 3. criterion of glass composition selection should, 4. Kotewale D. A. and Ganguly A. K. (1960) Disposal therefore, be considered when a sulphate-containing of Radioactive Wastes, pp. 213-224. Vol-I IAEA, waste is to be fixed in glass. Vienna. 5. Sundaram V. K., Gopinath D. V. and Ganguly A. K. 6.4 Corrosion (1965) Nucl. Struct. Eng., 1, 151-158. Phosphate glasses are more corrosive than silicate 6. Ravindranath V., Keshava Chandra, Sehgal J. D. glasses. Borosilicate and aluminophosphate glasses and Thomas K. T. (1967) BARC-288. 7. Beattie I. R. (1953) J. Soc. Glass Technol. 37, were kept in molten condition in 304 L stainless 240T-248T. steel pots for 48 hr. The glasses did not corrode the 8. Budd S. M. and Frackiewicz, J. (1961) Phys. Chem. pot appreciably. Amongst the phosphate systems Glasses 2(4) 111-114; 115-118. the aluminophosphate glasses were found to be 9. Shartsis L., Capps W. and Spinner S. (1953) J. Am. least corrosive. Ceram. Soc. 36(10) 319-326. 10. Sutton W. H. (1960) J. Am. ceram. Soc. 43 (2) 6.50ff-gasproblem 81-86. The off-gases released during glass melting are 11. Sanyal A. S. and Mukerji J. (1968) CGCRI report No. 5 on Fixation of High-Level Atomic Waste in likely to be nitric oxide vapour, water vapour, Glass. sulphates and oxides of some of the fission products 12. Sanyal A. S. and Mukerji J. (1969) CGCRI report as Cs and Ru. In phosphate glasses, P205 either No. 6 on Fixation of High-Level Atomic Waste in alone or as compound with other oxides may Glass. volatilize. This volatility is not high if the P20 5 13. Sanyal A. S. and Mukerji J. (1970) BARC/I-270. content is less than 60 wt. ~o in the glass and glasses 14. Sanyal A. S. and Mukerji J. (1971) BARC/I-283. having about 40--45yo P~O5 give off decidedly less 15. Sanyal A. S. and Mukerji J. (1972) BARC/I-285. 16. Grover J. R. and Chidley B. E. (1960) AERE-R 3178. off-gases as was evident during our melting of 17. Tuthill E. J., Weth G. C., Emma L. C., Strickland G. phosphate glasses. Volatilisation was found to be and Hatch L. P. (1966) BNL-10109. very high in glasses where the NazO and P20 s were 18. Elliot M. N., Grover J. R. and Hardwick W. H. (1963) Symposium on Treatment and Storage of in the molar ratio 1:1. On the other hand in phosHigh-Level Radioactive Wastes, pp. 381-396. IAEA, phate glasses ruthenium volatility may be suppressed Vienna, Austria. by the use of reducing agents like phosphites and 19. Barton G. B. (1967) BNWL-373. hypophosphites as glass additives [25]. 20. Elliot M. N. and Auty D. B. (1968) Glass TechnoL 9, 5-13. High thermal conductivity, low thermal expansion and lowviscosity should be among the other desirable 21. Bonniaud R., Sombret C. and Laude F. (1963) Symposium on Treatment and Storage of High-Level properties of a matrix selected for high-level waste Radioactive Wastes, pp. 355-379. IAEA, Vienna, containment. Equilibrium pH of glass may be a Austria. guiding factor in choosing the final glass composition 22. Hatch L. P., Weth G. C. and Tuthill E. J., ibid., pp. 531-542. or in locating the sites of burial. 23. Mukerji J. and Kayal P. B. (1973) BARC/691. Acknowledgement The authors are thankful to Shri 24. Van Wazer J. R. (1966) Phosphorous and Its ComK. D. Sharma, Director, Central Glass and Ceramic pounds, Vol. 1, p. 793. Interscience, New York. Research Institute and to Shri K. T. Thomas, Director, 25. Clarke W. E. and Godbee W. H. (1963) Symposium Engineering Services Group, Bhabha Atomic Research on Treatment and Storage of High-Level Radioactive Centre, Trombay for encouragement received during Wastes, pp. 411-439. IAEA, Vienna, Austria.