Fate of Technetium in a UK Intermediate-Level Nuclear Waste Repository

N. Evans, R. Hallam, S. Aldridge and P. Warwick

Overview

99Tc in Magnox wasteRemoval using TPPBDegradation of TPPB and its consequences for PASolubility of Tc(IV) at high pHEffect of cellulose degradation products on solubilityRedox behaviour of Tc in the presence of some organicsSorption of Tc(VII)Sorption of Tc(IV)Conclusions

Magnox Waste

Magnox fuel reprocessing produces wastes containing 99Tc99Tc is a low energy β emitter which is important for performance assessment due to its high yield (6% of fission products) and long half-life (2 x 105 years)Historically, about ⅓ was vitrified, while ⅔ was concentrated in HNO3 and stored as Medium Active Waste (MAC)MAC is stored for many years to allow the short-lived radionuclides, e.g. 106Ru, to decayIt is then treated with alkali to precipitate a Fe(OH)3 flocRemoves Pu, Am & Cm, and some fission and activation productsBut 99Tc remains in solution

Processing MAC

J. H. P. Watson, Nuclear Engineering and Design, 226(3), 2003, 375-385

Zero discharge to sea now

required

Tetraphenyl Phosphonium Bromide (TPPB)

To prevent discharges to sea, TPPBr is added to the MAC

This creates a Tc-bearing precipitate (TPPTcO4) that would be mixed with the ferric flocThe floc will be removed by ultrafiltrationAnd then be encapsulated in cement

P+

Br-

The ILW Repository

One option for the management of ILW is to place it in an underground repositoryThis would then be backfilled with a cementitious materialGenerates a pH of between 13.3 and 12.4Corroding Fe produces a low EhWhat would happen to the Tc as the TPPB is exposed to repository conditions?Both oxidation states of Tc have to be consideredWhat would their solubilities be under these conditions?

Degradation of tetraphenylphosphonium bromide at high pH and its effect on radionuclide solubilityChemosphere, 66(4), 2007, 672-676, S. Aldridge, P. Warwick, N. Evans and S. Vines

Dr Nick Evans - Waste Management 2008 Granada

Radiological Risk against Time

TotalCl-36I-129Tc-99Pb-210Ra-226Th-230U-234U-238Th-229U-233Np-237Risk target

103 104 105 106

Years

1e-4

1e-5

1e-6

1e-7

1e-8

1e-9

1e-10

1e-11

1e-12

Ann

ual I

ndiv

idua

l Ris

k

Performance Assessment for 99Tc

UK inventory of 99Tc is ca. 400 TBqSource term spreading time depends on:

FluxSolubilitySorption

Geosphere spreading time depends on:Travel timeSorption

Worksheet: DataRadionuclide: (0-39) 12 OK Radionuclide is: Tc-99

Near-field solubility: (1-7,X,Y) X OK Near-field solubility: Plotted on the X axisNear-field sorption: (1-7,X,Y) Y OK Near-field sorption: Plotted on the Y axisFar-field sorption: (1-7,X,Y) 5 OK Far-field sorption: 2 OM worseFlux (Q): (1-7,X,Y) 4 OK Flux (Q): 320 (CV)Travel time (T): (1-7,X,Y) 3 OK Travel time (T): 100000 (CV)

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

Peak

Ann

ual I

ndiv

idua

l Ris

k

4 OMworse

3 OMworse

2 OMworse

1 OMworse

Centralvalues

1 OMbetter

2 OMbetter

2 OMbetter

1 OMbetter

Centralvalues

1 OMworse

2 OMworse

3 OMworse

4 OMworse

X Variable

Y Variable

99Tc Sorption v Solubility

Sorption

Solubility

Solubility-limited

Sorption controls risk

Geospherecontrolled

Tc (IV)

Tc (VII)

Degradation of TPPB

TPPB degrades by alkaline hydrolysis at high pHRate of reaction is 3rd order

1st order with respect to TPPB2nd with respect to OH-

It is also prone to radiolytic degradationProducts are generally phenylphosphines, e.g. TPP

Tc(IV) Solubility at High pH

Solubility of Tc(IV) from pH 11 to 14.5

Solubility was measured between pH 11.5 and 14.5To investigate the existence of TcO(OH)3

-

pH 11 – 13.5 the [Tc]solution was independent of pHHowever, above pH 13.5, [Tc]solution increasedFormation constant was calculated from the gradient using the pH range for the solubility increaseThe value was determined to be log K = -14.2

Solubility of Tc(IV) from pH 11 to 14.5

Effect of Organic Ligands on Tc(IV) Solubility at High pH

Effect of Ligands on Tc(IV) Solubility

Complexing ligands will be present, e.g. EDTA and NTAIsosaccharinic acid (ISA) will be formed by cellulose degradation

TcO2 was prepared by the reduction of TcO4- electrochemically

and then contacted with anaerobic solutions of the ligandsMean Tc solubility in their absence was 4.6 ×10-10 mol dm-3

Supports the view that Tc(VII) had been completely reduced to the 4+ state

OH OH

O

OH OH

OH

Electrochemical Cell

Electrochemical cell used for reduction of Tc(VII) to Tc(IV)

Platinum Electrode

Stability Constant for Tc(IV)-Gluconate

Stability constant for Tc(IV) with gluconic acid was measured using the solubility product approachSolubility product for the dissolution of TcO2.2H2O was calculated to be log Ksp = -33.6 ± 0.3 at pH 13.3Conditional stability constant was determined to be:

Tc(IV)-Gl, log β = 26.6 ± 0.2

Redox in the Presence of Organics

Tc Solubilities from TcO2 and Reduced TcO4

- at High pH in Presence of Ligands

Repository will not be homogenousLikely to be both reducing and oxidising areasCould mean that both TcO4

- and TcO2 are presentIf TcO4

- migrates into area of reducing conditions, organics may complex with it during reduction to form complexes|Approach used to make 99mTc radiopharmaceuticalsDoes it make a difference to the final solubility if you start with Tc(IV) or Tc(VII)?

Results

EDTA and NTANo difference is observedConclude that Tc(IV) complexes are formed with these ligandsWhether or not Tc(VII) or Tc(IV) is the starting point

ISA and gluconic acidTc solubility in systems with reduction in the presence of the ligands was higher than when TcO2 was the starting pointSuggests that TcO4

- was not reduced to Tc(IV)Maybe an intermediate oxidation state complex was formedProbably Tc(V)Tc(V) complexes are known from formation of 99mTc radiopharmaceuticals

Tc Solubilities from TcO2 and from Reduced TcO4

- in Presence of Ligands

Tc(V)

Coordination is controlled by square pyramidal geometry of TcO3+

Complexes with polyhydric alcohols and 2-hydroxy carboxylatesare stable in solution due to chelating effectsMost complexes have not been characterised due to the difficultyof obtaining pure crystalline formsTc glycolato complex has been identified as [TcO(OCH2CH2O)2]-

OO

HO

HO

O

OO

O

OH

OH

HOOH

O

Tc

Sorption of Tc(VII) and Tc(IV) to Geologic Materials

Materials Used for Tc(VII) SorptionMineral

Milky QuartzHornfelsBaritiesHaematite GoethiteGypsumPlagioclase Feldspar PyritePurified Sand

Clay and clay mineralBentoniteIlliteIllite - Smectite mixed layer MontmorilloniteOxford ClayLondon clay

Sedimentary RocksSandstone

ShaleIronstone

Crystaline rockBlue GraniteMicro GraniteGraphic granite Biotite GneissGabbroTownite BasaltRhyoliteRapakivi GraniteAndesiteBiotite Granite Grey Granite Adamelite Granite

CarbonatesCalciteDolomiteLimestone

Tc (VII) Sorption

Tc (VII) Sorption (2)

0

200

400

600

800

1000

1200

1400

1600

1800

Rd

(ml g

-1)

pH 4pH 7pH10

Illite Bentonite Gypsum Shale

95mTc – Reason for Use

Specific activity of 99Tc too lowHalf life of 99mTc too short95mTc - half life = 61 daysHigher specific activity and a gamma emitterExtracted from a molybdenum target95Mo(p,n)95mTcUsing 11 MeV protons

95mTc Purity

• Problem of 96Tc "contamination“• Unavoidable when shooting at natural molybdenum• Produced the 4.2 day 96Tc at about 1.1 mCi per

hour at 10 microamps of beam on natural Mo• Ca. 22 mCi at end of 20 hour bombardment• For this 22 mCi to decay to same order of activity

as the 300 microCi of 95mTc, 7 half-lives or 29 days was needed for cooling

• And another 29 days to get the "impurity" down to 1% level

Gamma Spectrum

95mTc Production

•

Thanks to Jerry Nickles, University of Wisconsin

95mTc Extraction

Target refluxed in H2SO4 (50 cm3, conc.)NaOH added (15 mol dm-3, 100 cm3)H2O2 added (50 cm3)Passed through DOWEX resinResin washed with K2(COO)2 (1 mol dm-3, 50 cm3)95mTc removed with HClO4 (100 cm3 - 1 mol dm-3)

G. E. Boyd, Q. V. Larson, E. E. Motta, JACS 82, 809 (1960)

Tc(IV) Sorption Experiments

Three different concentrations of technetium used:1 x 10-9, 1 x 10-10, 1 x 10-11 mol dm-3

Four different initial pH values, 7, 10, 12.5, 13.319 different solids0.1 g solid20 cm3 99Tc solution0.3 cm3 spike 95mTcNitrogen atmosphereSodium dithionite as holding reductant

Materials used for Tc(IV) Sorption

MineralMilky QuartzHaematite GoethitePlagioclase Feldspar Purified Sand

Clay and clay mineralBentoniteIlliteIllite - Smectite mixed layer Montmorillonite

Sedimentary RocksSandstoneShale

Crystalline rockMicro GraniteGraphic granite Townite BasaltRapakivi Granite

Carbonates/CementsLimestoneNRVBPCMOPC

Minerals

1

10

100

1000

10000

100000

3 4 5 6 7 8 9 10 11 12 13 14pH

Rd

(ml g

-1)

Milky QuartzHaematiteGoethitePlagioclase FeldsparSand

Clays

1

10

100

1000

10000

100000

3 4 5 6 7 8 9 10 11 12 13 14pH

Rd (

ml g

-1)

BentoniteIlliteIllite-SmectiteMontmorillonite

Sedimentary Rocks

1

10

100

1000

10000

100000

3 4 5 6 7 8 9 10 11 12 13 14pH

Rd

(ml g

-1)

SandstoneShale

Crystalline Rocks

1

10

100

1000

10000

100000

3 4 5 6 7 8 9 10 11 12 13 14pH

Rd (

ml g

-1)

Micro GraniteGraphic GraniteTownite BasaltRapakivi Granite

Limestone

1

10

100

1000

10000

100000

6 7 8 9 10 11 12 13 14pH

Rd

(ml g

-1)

limestone

Cements

1

10

100

1000

10000

100000

6 7 8 9 10 11 12 13 14pH

Rd

(ml g

-1)

NRVBPCMOPCLimestone

Material Rd below 6 (ml g-1)

pH of Peak

Rd atPeak

Rd at pH 12.5 - 13.3

Minerals <100 72000 –20 000

20 - 1200

Clays 10 - 1000 87000 –20 000

100 - 400

Sedimentary 4 - 400 73000 –15 000

70 - 600

Crystalline 20 - 1400 812 000 –30 000

70 - 5000

Limestone N/A 8.5 17 000 100 - 200

Cements N/A 91200 –17 000

100 - 7000

Tc(IV) Speciation

Interim Conclusions

Sorption is greatest at near neutral pH valuesAt high Na+ concentrations maybe causing competition at high pHAlso high stability of Tc(OH)4 (aq)>S-O-TcO(OH) + 3OH- ↔ >S-O- + Tc(OH)4

Surface Complexation Modelling

Scientific Conclusions

Gradual increase in Tc solubility over time as Tc was being released by degradation of TPPTcO4

-

Solubility of Tc(IV) increases at high pH, specifically > pH 13.5This increase may be due to the formation of TcO(OH)3

-

The formation constant was estimated to be log K = -14.2Conditional stability constant for Tc(IV)-Gl, log β = 26.6 ± 0.2Tc solubility in systems with reduction in the presence of ISA was higher than when TcO2 was the starting pointSuggests a Tc(V)-ISA complex was formedSorption of Tc(VII) Rd = up to 2 000 ml g-1 on a few materialsSorption of Tc(IV) Rd = up to 30 000 ml g-1

Significant on all materials tested

PA Conclusions

TPPB degradation will have no deleterious effectSolubility of Tc(IV) at repository pH is acceptably low

Need to look at effect of carbonateOrganic ligands could be important at high concentrations

More research neededSorption of Tc(VII) measurable but probably insignificantSorption of Tc(IV) significant at near-neutral pH

Further Work

XRD of solid samples at relevant pHsNeed to know surface groupsSelected other solids?

Acknowledgements

PhD students Stuart Aldridge and Ricky Hallamwho did most of the work

Sarah Vines, Mike Poole, Steve Williams and Tim Heath of the NDAProfessor Peter Warwick