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The solubility of technetium in the near-field environment of a radioactive waste repository

The solubility of technetium in the near-field environment of a radioactive waste repository


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Journal of the Le~-Co~~o~


Metals, 16i ( 1990) 203-2 12



N. J. PILKINGTON AEA Tech~oIo~, Che~~t~

~~v~s~on,Har~e~l Laborata~, Oxfordshire, OX11 ORA @J.K.i

(Received September 4. 1989)

Summary Experimental measurements of the solubility of technetium under conditions similar to those expected to be found in the near-field environment of a radioactive waste repository have been carried out. The measured solubility of technetium under high pH and reducing conditions in contact with hydrated tec~eti~ dioxide has been found to be about 10 - 7 mol dm - 3 and is inde~ndent of pH in the range 8-13 and the type of cement-equilibrated water used. The measured solubility was insensitive to the solid-liquid separation method used, suggesting that signficant quantities of technetium-containing colloidal species are not present. A limited investigation of the effects of organic decomposition products on the solution concentration of reduced technetium has shown that there is a modest enhancement in technetium solubility.

1. Introduction An evaluation of the radiological consequences of the disposal of radioactive waste requires an assessment of the release of radionuclides from the repository. Amongst other aspects, the assessment must take into account the near field of the repository, which comprises the waste itself, the containers, the backfill and the structural materials. One of the factors affecting the release of radionuclides is their solubihty in the near field. The designs for a repository for low and intermediate level wastes in the U.K. include a near field which will contain cementitious material. The waste is immobilized in a steel container, which is placed in the repository and surrounded by a cementitious grout. The presence of such materials will result in a high pH (above 10.5) and reducing conditions, with a typical redox potential (Eh) of - 400 mV. These conditions are likely to persist for a time that is long compared with the structural lifetime of the repository components [ 11. Source-term modelling of the repository has shown which radionuclides will be radiologically significant over long time scales [‘.X1. These include the actinides, long-lived fission products, activation products and actinide daughters, and the Q Elsevier Sequoia/Printed

in The Netherlands


behaviour of a number of these elements has been studied in some detail [3-71. However, the significance of technetium was uncertain. Thermal and fast reactors both produce high yields of y9Tc as one of the fission products. This isotope is a low energy /I emitter (292 keV), but it has a long half-life (2.13 x lo5 years). Information on the solution chemistry of technetium is often contradictory [8], because of complicated redox, complexation and hydrolytic equilibria. Polymerization and colloid formation may also occur under some circumstances. In oxidizing or mildly reducing media, the common technetium species is the fairly stable pertechnetate ion, TcO;. When reduced, various Tc(V), Tc(IV) or Tc(III) species may be formed. The most stable of these oxidation states is generally Tc(IV), although the nature of the species is uncertain, the most common suggestion being TcO,, or TcO,*H,O or TcO,-2H,O. The pertechnetate ion is easily soluble, for example the solubility of sodium pertechnetate is 11 mol dmm3 [9]. In alkaline solution and at low redox potential, the Tc(IV) species produced by the reduction of Tc(VI1) are much less soluble; Meyer et al. [lo, 111 have obtained values between 1 X lo-* and 2 X lo-* mol dm- 3 over a range from pH 4 to 10. The aim of the present experimental programme was to determine the solubility of technetium under typical near-field chemical conditions for three cement types. The types of cement used in this study had been identified as candidate materials for use in a waste repository. 2. Experimental details 2.1. Materials The cementitious materials used in this work were: (i) 10 parts pulverised fuel ash (PFA) to 1 part ordinary Portland cement (OPC) with a water:solids ratio of 0.45; (ii) 6 parts finely ground limestone to 1 part OPC with a water:solids ratio of 0.5; (iii) 3 parts blast furnace slag (BFS) to 1 part OPC with a water:solids ratio of 0.35. All the cements were, sealed into plastic bottles and allowed to cure for at least 28 days at ambient temperatures prior to use. 2.2. Preparation of cement-equilibrated waters The solubility experiments used solutions which had previously been equilibrated with cement. This cement-equilibrated water was prepared in the following way. The cement was crushed and sieved so that material in the size range 250-1000 pm was used. This material was taken into a nitrogen atmosphere glove-box and was shaken with deionized water (1iquid:solid ratio 10: 1 v/m) for a few seconds. The deionized water used in the experiments had been stored for several months in a vessel open to the glove-box atmosphere. The water would therefore have contained a small amount of oxygen. In some experiments this may have led to the oxidation of some Tc(IV). Most experiments, however, contained a


holding reductant, so the amount of oxygen in solution would have been lowered, and the oxidation of Tc(IV) would not have occurred. After standing for at least 24 h, the mixture was shaken again prior to filtering through Whatman No. 40 filter paper. The pHs of most (but not all) of these waters were adjusted to cover a range of values, using 0.1 mol dmw3 hydrochloric acid or 0.1 mol dmm3 sodium hydroxide solution made from the deionized water. Approximately 40 ml of cement-equilibrated water was used in each experiment. Samples of all the waters were analysed after an equilibration time of 4 days. The results are given in Table 1. 2.3. Leachates containing organic decomposition products In order to obtain some indication of the effects of organic decomposition products on the solubility of technetium, experiments were also carried out in leachates containing such materials. These leachates were prepared by the methods described in ref. 12. The pHs of these leachates were adjusted to around

TABLE I Analysis of the content of cement-equilibrated waters” Element or ion

Al B Ca Fe K Mg MO Na s Se Si clSO:OHco:Ionic strengthb PH

Concentration (mol dm-“) 3:l BFSIOPCequilibrated water

1O:l PFAIOPCequilibrated water

61 IimestonelOPCequilibrated water

Detection limit

3.7 x <9.3x 1.1 x <1.8x 4.3 x <8.2X < 1.0 x 5.3 x 5.6 x 6.3 x 7.1 x 1.3 x 2.0 x 2.1 x 3.3 x 0.040 12.7

3.3 x 1.4 x 9.2 x < 1.8 x 3.1 x <8.2X 1.6 x 1.3 x 7.5 x 4.4 x 2.8 x 2.4 x 2.6 x 4.1 x 4.0 x 0.014 11.8

8.5 x 9.3 x 1.4 x < 1.8 x 1.5 x 1.6 x 1.0 x 9.1 x 3.7 x 8.2 x 3.0 x 3.7 x 1.2 x 2.8 x 2.1 x 0.044 13.1

3.7 x 9.3 x 2.5 x 1.8 x 2.6 x 8.2 x 1.0x 4.3 x 1.6 x 2.5 x 7.1 x 5.6 x 3.1 x -

10-5 lo-’ 10-J lo-’ 10-j 10-7 10-r 10-X 10-j lo-” 10-d 1o-4 1o-4 10-Z lo-4

1o-4 10-5 10mJ lo-’ 10-X 10-7 lo-’ 10-j 10-d 10-h 10-j lo-“ 1O-3 10-J lo-4

l0-h 10-7 10-I lo-’ 10-3 l0-h 10-7 10-b 10-S 10-h 10-S 10-j 10-j lo-’ 10-b

10-7 lo-’ lo-’ 10-7 l0-h lO-7 10-7 10-r l0--h 10-O 10mh lo-’ 10-7

“The cement-equilibrated waters were prepared by mixing the solid and liquid in a ratio of 1 : 10, and equilibrating for 4 days. The leachate was filtered using Whatman No. 40 filter paper. bIron and sulphur concentrations were not used in ionic strength calculations. The following charges were assumed: A13+, B(OH),, Car’, K+, Mg’+, MOO:-, Na+, SeO:-, [SiO,(OH),]‘-, Cl-, SO:-, OH-,CO*-3



12 before the experiment was begun, using 0.1 mol dme3 sodium hydroxide solution. Approximately 5 ml of leachate was used in each experiment. 2.4. Reducing conditions The reducing conditions (low Eh) which were required for some experiments were achieved by the addition of sodium dithionite (Na2S204) solution to the cement-equilibrated water or leachate. This method has also been used in measurements of the solubility of Np( IV) [ 31. To prepare the solution, 2.6 g of 85% sodium dithionite and 0.2 g of sodium hydroxide were dissolved in 50 ml of deionized water, giving a dithionite concentration of about 0.25 mol dmF3 at pH 12. The solution was kept at high pH because acidification caused rapid decomposition of dithionite. Each sample of cementequilibrated water or leachate was “spiked” with 0.1 ml of this solution, giving a dithionite concentration of about 1O-3 mol dmw3. Some experiments were carried out in the absence of dithionite. 2.5. Stock solutions of ammonium pertechnetate A 0.08 mol dms3 aqueous solution of ammonia containing technetium as ammonium pertechnetate was obtained from Amersham International pk. Four stock solutions were made by successive ~lution of the Amersham solution, using a 0.08 mol dme3 solution of ammonia. These solutions were taken into the glovebox, where they were sampled to obtain values for the initial concentrations used in the experiments. 2.6. Technetium dioxide preparation The following method was used [13]; a 0.1 mol dme3 aqueous solution of ammonia containing technetium as ammonium pertechnetate was obtained from Amersham Intemational plc. After dilution with water, ahquots conta~ng about 1 mg of technetium were placed in glass vials. These were further diluted with water, and a small volume of concentrated hydrochloric acid was added. A few milligrams of zinc metal were added to this solution, whilst maintaining the temperature of the solution at about 40 “C. When all the zinc had reacted, concentrated ammonia solution was added until the pH was between 5.5 and 6 (if the pH rose above 6, zinc hydroxide was precipitated). A black precipitate of hydrated technetium dioxide was produced. The vials cont~~g this precipitate were taken into the glove-box. The supernat~t liquid was decanted and the precipitate washed with water. 2. Z Preparation of experiments Technetium was added to the cement-equilibrated water or leachate as either ammonium pertechnetate solution or solid hydrated technetium dioxide. The method of adding the solution was to spike the water or Ieachate with 0.1 ml of one’ of the stock solutions of ~rno~urn pe~ec~etate; the mixture was then shaken for a few seconds. For those experiments which required technetium dioxide as the starting phase, the material produced in one vial (see Section 2.6) was added to the water or leachate, and the mixture was shaken for a few seconds. Several “blank”


experiments were set up without being spiked with technetium. These were intended to give an estimate of any cross-contamination caused by pH and Eh measurements. The containers used for the experiments were 50 ml polypropylene conical tubes with polyethylene screw caps. The pH measurements were made using a Philips CEl combined electrode with a Philips PW9418 pH meter. Calibrations were made using standard buffer solutions and results quoted to the nearest 0.1 pH unit. The Eh measurements were made using an Orion Research Inc. model 96-78-00 platinum redox electrode containing Orion 90-00-l 1 filling solution. A Philips PW9420 meter was used in conjunction with the redox electrode. Calibrations of the electrode were carried out in accordance with the manufacturer’s instructions. Many redox potential measurements have been made over a wide range of samples, and this has led to an estimate of k 60 mV for the error in the reading; the measurements are expressed to two significant figures. 2.8. Sampling of the experiments The equi~bration time for the experiments using organic leachates was 6 weeks; the other experiments were equilibrated for 7 weeks. The Eh and pH were measured immediately before sampling. Any solid was then separated from the aqueous phase by filtration. Three different methods were used. (1) Centriflo membrane cones. For most experiments, 5 ml samples were filtered through Amicon 25 000 molecular weight cut-off (MWCO) ultrafilters. The filters (which were made of polysulphone) were preconditioned by passing 5 ml of leachate through, before the sample was filtered. The amount of leachate containing organic degradation products was limited, so about 1 ml was used. (2) Centricon ~ispusable microco~ce~trators. Smaller volumes (about 2 ml) were required for this method of ~ltration through Amicon 30 000 MWCO ultrafilters (made of mixed esters of cellulose). These filters were also preconditioned, and only a small volume was used from the organic degradation leachates. (3) Millex-HV. Selected experiments were sampled by filtration through this Millipore 0.45 ,um pore size filter (made of polyvinylidenedifluoride); a volume of about 5 ml was filtered. 2.9. Analysis of samples Samples were removed from the glove-box and analysed by liquid scintillation counting (LSC) using a Beckman LS3800 counter. The counting efficiency and quenc~g ch~acte~stics were similar to those found by Pacer 1141.Under the counting conditions used, the lower limit of detection was 0.4 Bq, with a precision (2o) of about 6%. The lower limit of detection (between 10m8 and lo-’ mol dm-” of technetium) is consistent with that suggested by Meyer et al. [lo]. 3. Results and discussion Four different sets of experiments were carried out, using different starting phases, with or without sodium dithionite. In addition, measurements were made on leachates cont~ning organic decomposition products. The results of the


measurements made in 3: 1 BFS/OPC-, 10: 1 PFA/OPC- and 6: 1 limestone/OPCequilibrated waters are given to two significant figures in Tables 2-4. The results for the leachates containing organic degradation products are given in Table 5. Measurement of the technetium concentration in the “blank” samples indicated that the highest level of cross-contamination was about 1.5 X 10e8 mol dme3. The’ TABLE 2 Technetium solution concentrations dithionite Cement-equilibrated water


using ammonium pertechnetate

Eh (mv)

Initial concentration (mol dm-3)

starting phase and sodium

Final technetium concentration (mol dmm3) 0.4.5,um filter

3oooOM WC0filter

3: 1 BFS/OPC

12.4 12.4 12.3 12.3

-260 - 300 - 290 +60

2.1 9.2 8.1 7.9

x 10-7 x lo-’ X 10-h X 10-s

4.9 x 10-x 4.0 x 10-x 8.6 X 10-g 2.9x 1O-5

5.2 5.7 2.1 3.1

x 10-x x 10-s X 10-7 x10-5

10 : 1 PFA/OPC

11.5 11.5 11.5 11.3

-510 -440 - 260 -70

2.1 9.2 8.1 7.9

x 10-r X lo-’ X lo-” X 10-s

1.2 X 10-K 4.0X 10-s 2.5 X lo-” 2.0X 10-s

2.5 4.2 2.6 2.0

x 1O-x X lo-’ x 10-h X 10-s

6 : 1 limestone/OPC

11.9 12.2 12.5 12.6

+60 -420 -330 -30

2.1 9.2 8.1 7.9

X 10-7 x lo-’ x 10-h x 10-5

6.8 X 10-s 6.1 x lO-x 1.2 x 10-7 3.8 X lO-5

7.2 X 1O-x 6.1 X lo-” 1.4 X 10-r 4.0 x 10-s

TABLE 3 Technetium solution concentrations dithionite Cement-equilibrated water

using technetium dioxide starting phase, without sodium

Technetium concentration (mol drK3)


0.45pm fdter

3oooO M WC0 fdter

3 : 1 BFS/OPC

12.2 12.2

+40 +80

5.1 X 10-c 9.8 X 10-h

9.4X 10-h 1.3 x 10-s

10 : 1 PFA/OPC

11.6 11.1

+40 +50

2.3 x 10-h 2.3 X 10-h

2.3 x lo-” 2.4X lo-’

6 : 1 limestone/OPC

12.6 12.6

-40 -60

6.3 X lo-(’ 1.0 X 10-5

6.9 X lo-” 1.1 X 10-S

The initial concentration of technetium would have been about 2.5 X lo-“ mol dme3, if ah the technetium dioxide had dissolved.


TABLE 4 Technetium solution concentrations using technetium dioxide starting phase and sodium dithionite

Cement-equilibrated wuter



Technetium concentration (mol dm- 3,

(mv) 0.45~mfilter

3 : 1 BFS/OPC

10 : 1 PFA/OPC

6 : 1 limestone/OPC

7.0 7.2 7.8 8.0 8.0 10.7 12.5 12.7”

- 240 - 220 -220 - 260 -310 -400 -530 - 520

7.6 8.0 10.6 11.1 11.4 11.7 11.8” 12.3

- 300 -320 -470 -470 -450 -460 -450 -530

6.2 7.6 7.9 9.8 11.0 11.9 12.7 12.7”

- 160

-180 - 210 -360 -450 -450 -500 - 500

1.1 x IO-7 2.4x 10-H 8.9x 10mx 9.5 x 10-x

1.5x10-’ 9.3x 10-x 4.3x lo-” 1.2 x 10-7 2.4x lo-’ 6.2 x 10-s 1.3 x lo-’

7.8x 10-K


M WC0 filter

25ooO MWCOfilter

2.2 x 5.6 x 1.5 x 2.0 x 6.4x 2.0 x 1.7x

10-7 lO-x 10-7 10-7 1O-x 10-7 lo-’

2.2 2.9 1.1 1.5 2.1 8.6 9.3 1.7

x x x x x x x x

10-7 1O-x 10-7 lo-’ 10-x 10-s lo-% 10-7

9.1 x 1.5 x 1.5 x 1.0 x 8.4x 1.0 x 3.2 x 1.2 x

10-x 10-7 10-7 10-7 1O-8 10-7 10-n 10-7

1.1 1.6 1.4 1.0 8.3 5.3 3.8 1.3

x x x x x x x x

10-7 lo-’ 10-7 10-7 1O-x 10-x 1O-x 10-7

2.1 x 7.0 x 5.1 x 1.2 x 1.5 x 2.4x 8.6 x 1.5 x

10-7 10-s 10-x 10-7 10-7 lo-’ 10-n 10-7

2.5 x 1.5 x 5.9 x 6.4 x 1.5 x 2.5 x 1.5 x 2.4x

lo-’ 10-7 10-7 1O-x 10-7 lo-’ 10-7 lo-’

“The unadjusted pH for this leachate.

overall analytical accuracy of the results was estimated to be * 10% relative, although this value may be larger near to the lower limit of detection. Those experiments which contained sodium dithionite as a holding reductant gave low values of redox potential, showing that there was little oxygen present in solution. (1) Ammonium pertechnate starting phase; no sodium dithionite. The results of these experiments showed that the final solution concentration was close to the inventory in all cases. All of these experiments were at redox potentials of - 30 to + 60 mV, so the technetium was presumably soluble as pertechnetate up to the initial concentration. (2) Ammonium pertechnetate starting phase; sodium dithionite present. These results are given in Table 2; the highest technetium initial concentrations had comparatively high redox potentials. This may have been because the technetium initial concentration (about 8 x lo- 5 mol dm- 3, was close to the concentration of dithionite (about 10e3 mol dmm3). If substantial amounts of the dithionite had been oxidized, then the redox potential would be high. There was also some black solid


TABLE 5 Technetium solution concentration organic materials Organic material”

Nylon Poly(methyl methacrylate) Poly(vinyl chloride) Cellulose Major mix” Minor mix ’ -


12.0 12.3 12.1 12.2 12.1 12.4


Eh (mv)

-570 - 530 - 560 - 530 -470 - 540

after accelerated chemical decomposition

Technetium concentrationC (mol dm-‘) 3~ MWCO filter

25ooO MW~U filter

2.0 X 10-h 1.1 x 10-h 2.6 x 10-O 7.3 x 10-7 5.4X 10-7 6.8 X 10-7 10-7

2.4 X 10-h 1.8 x lo-” 2.7 x 10-h 8.6 X lo-’ 8.3 X lo-’ 7.2 x lo-’


PIutonium concentrationd (mol dme3) 2.5ooOMWCO fiber 6.6 X 10-s 5.5 X 10-x 8.3 X 10-s 7.0 X 10-e 1.5 X 10-7 1.2 X 10-h 5 x lo- r0

“The organic material (10 g) was i~obiiized in a mixture of OPC (22.5 g), PFA (47.5 g) and water (45 g) and held under water saturated conditions. Experimental details are given in ref. 15. bThe pH of the solutions were adjusted to this value prior to measuring solution concentration. “The immobilized organic material had been held at 80 “C for 400 days. dThe immobilized organic material had been held at 80 “C for 350 days. “Major mix-50% PVC, 10% cellulose (paper, tissue), 10% white Hypalon, 10% black Hypalon, 10% polythene, 10% neoprene. ‘Minor mix-12.5% poly(metby1 methacrylate), 12.5% polytetratluoroethylene, 12.5% polystyrene, 12.5% Nylon, 12.5% bakelite, 12.5% latex, 12.5% polypropylene, 12.5% polymethylpentene.

visible in the experiments of highest technetium initial concentration. This may have been tec~etium dioxide formed by the reduction of some pe~e~~etate. The two lowest technetium initial concentration gave rise to final solution concentrations in the range 1 X lo-* to 7 X low8 mol dmm3. (3) Technetium dioxide starting phase; no sodium dithionite. The solid technetium dioxide was visible in all these experiments. The excess of solid oxide should result in minimal wall sorption effects. The results covered the range 2.3 X 10F6 to 1.3 X low5 mol dme3 (Table 3). They were higher than those for the experiments containing dithionite, and this may have been caused by small amounts of dissolved oxygen, which could have oxidized some technetium dioxide to the more soluble pertechnetate. (4) Tec~eti~ dioxide starting phase; sodium dithionite present. The solid technetium dioxide was visible in all these experiments. The range of these results was from 2.1 X 10e8 to 5.9X lo-’ mol dmm3 (Table 4), which is similar to the range for the experiments where ammonium pertechnetate was used as the starting phase. There was little difference in the results for different leachates, nor did there seem to be any particularly significant trend with the pH and this is consistent with the findings of Meyer et al. [ 10, 1 I]. There was also no significant change in the solution concentration on changing the pore size of the filter, which indicates that


colloids did not contribute significantly to the concentration in solution. The results are close to that (2 x 10 -s mol dmP3) predicted by thermodynamic modelling [ 141 and a little higher than those of Meyer which are in the range 1 x 10 - 8 to 2 X 10e8 mol dm- 3. A best estimate of 10 -’ mol dm- 3 is suggested for the solubility of hydrated technetium dioxide under reducing conditions. The solution concentrations described above were measured in the standard cement-equilibrated waters. There has been some concern recently that organic waste materials may decompose and produce water soluble complexing agents which could increase the solubility of long-lived radionuclides. Experimental studies to date have concentrated on the actinides, plutonium and americium [ 151. A limited number of experiments have been carried out with technetium under reducing conditions with leachates obtained from organic materials immobilized in a cement matrix. The cemented organic materials had been stored at above ambient temperature to accelerate any chemical decomposition reactions. The results (Table 5) showed that there was a modest enhancement in solution concentration in each case compared with the typical value of about lo-’ mol dmm3, suggesting that there was some complexing between the organic decomposition products and the technetium. The “major mix” used in the experiments represents a typical mix of organic materials found in intermediate level wastes. It is interesting to compare these results with those obtained in comparable experiments with plutonium [ 151. The enhancements in solution concentration of plutonium were much greater, particularly in the cellulose experiments, suggesting that plutonium formed stronger complexes with the organic ligands. However, the pattern of enhancement for technetium was different from that for plutonium.

4. Conclusions (1) The solubility of technetium in contact with hydrated technetium dioxide under high pH and reducing conditions was found to be about 10 - ’ mol dm - 3. (2) Changing the pH of the solution in the range 7-12.5 or changing the solid-liquid separation method had little effect on the measured solubility of technetium. (3) The presence of organic degradation products increased the measured solubility of technetium by about a factor of 10.

Acknowledgments I thank United Kingdom Nirex Ltd. and British Nuclear Fuels plc the work. I also thank staff of Actinide Chemistry and Analysis Group on technetium analysis, Mr. M. W. Spindler for the supply of leachates organic decomposition products and Dr. D. R. Woodwark and Dr. J. D. their advice and encouragement.

for funding for advice containing Wilkins for


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