Neptunium behaviour in PUREX and GANEX processes

C. Gregson, M. Carrott, D. Woodhead, R. Taylor

SNEC 2014, Manchester

Challenges in Np separations chemistry

•Background

•Neptunium extraction in an Advanced PUREX process

•Neptunium extraction in a GANEX process

Options for future fuel cycles

Homogeneous

Recycling

FR R

U, Pu, MA

FR R

MA

U, Pu

FP

Heterogeneous

Recycling

Advanced PUREX +

DIAMEX-SANEX or EXAm

GANEX

R: Recycle plant

FR: Transmutation reactor

V: Vitrification

V

FP

V

Requirements for advanced reprocessing

Optimise recycling processes so the option to close the fuel

cycle by ~2050 is competitive with other spent fuel

management options

• Reduced costs

• Reduced wastes

• Reduced environmental impact

• Enhanced process safety

• Enhanced proliferation resistance / safeguards

• Enhanced public acceptability

• Flexibility to process wider range of fuel types

• Integrated with fuel fabrication and waste management

PROCESS SIMPLIFICATION / INTENSIFICATION / INNOVATION

Neptunium

Np in the fuel cycle

•Current closed fuel cycles Np in HLW glass

• 237Np radiotoxicity in environment

• Past use for 238Pu production

• Future fuel cycles can burn Np in Pu/TRU fuels

•Current reprocessing by PUREX process Np is split across several streams

•Requires extra SX cycle to decontaminate

•Advanced reprocessing direct Np to single stream

• Reduced costs, wastes

• Enhance proliferation ‘resistance’

Np redox chemistry in HNO3

OHHNONpO

HNONpO

2222

32

2

1

2

1...

...2

3

2

1

NpO2+

NpO22+

Np4+

HNO3 / HNO2

HNO3 / HNO2

Disproportionation

OHNpONp

HNpO

222

4

2

2

42

3 easily inter-convertible oxidation states in HNO3

Np(V) oxidation vs. Np(VI) reduction in HNO3

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30 35 40

time (min.)

Np(V

) R

el. A

bs.

Np(VI) with 1 mM NaNO2 Np(V) with 1mM NaNO2

Np(V) zero nitrite Np(VI) zero nitrite

Addition of 1 mM NaNO2

Reduction of Np(VI) and oxidation of Np(V) in 5 M HNO3 at 50oC with and without NaNO2

No HNO2 to catalyse the redox reaction

No HNO2 to catalyse the redox reaction

HNO2 added catalyses Np(V) oxidation

HNO2 added reduces Np(VI)

Np(V)/Np(VI) equilibrium position

defined by HNO3/HNO2

Np extraction in PUREX

D(NpVI) > D(NpIV) > D(NpV)

Need to stabilise the middle oxidation state

aqueous

organic

NpNp

NpD

][

][

Np(VI) = very extractable

Np(IV) = extractable

Np(V) = not extractable

Conventional PUREX process

~30 % Np

HA Feed

~70 % Np

How can we achieve 100 % extraction in future SX processes? -pulsed columns -centrifugal contactors

Condition U Finishing

Diluent washCondition

HAF

HAR 30% TBP

HA35C

HS50C

HNO3

30% TBP

20% TBP

UIV/N2H4

UIV/N2H4

HNO3

1BX35C

1BS

1BXX 35C 1C 50C

UP1 25C

25C UP2 50C

UP3 40C

PP1S45C

PP1E45C

PP2

HAN

HNO3

30% TBP

Diluent washCondition

Concentrate Finishing

HAN

HNO3

UIV/N

2 H4

HAN

20% TBP

Condition U Finishing

Diluent washCondition

HAF

HAR 30% TBP

HA35C

HS50C

HNO3

30% TBP

20% TBP

UIV/N2H4

UIV/N2H4

HNO3

1BX35C

1BS

1BXX 35C 1C 50C

UP1 25C

25C UP2 50C

UP3 40C

PP1S45C

PP1E45C

PP2

HAN

HNO3

30% TBP

Diluent washCondition

Concentrate Finishing

HAN

HNO3

UIV/N

2 H4

HAN

20% TBP

Advanced PUREX process

Simplified PUREX process •Single cycle SX flowsheet

•Np control

•In a single stream

•With Pu-U product

•With U then separated

Minor actinide (Am,Cm) separations require additional SX cycles on HLW

Extract/scrub

U/Pu separation Np barrier

A1: Pu,Np

A2:Pu,U,Np

B: Np

HLW

(Am,Cm,FPs)

Dissolved SNF

Np follows U,Pu

Option A. Np follows Pu Option B. Np follows U

U backwash

U

GANEX Process

Dissolved

SNF

U

extraction

U

GANEX 1st cycle

An + Ln

extraction

FPs

An stripping Ln stripping

Ln

GANEX 2nd cycle

Pu, Np, Am, Cm

Co-separation of all TRU actinides

PUREX vs. GANEX processes

• SX from nitric acid into an organic kerosene phase

• Different extracting ligands

N

O

C2H

4

N

O

C8H

17

CH3

CH3

C8H

17

O

C6H

13

OP

O

O

O

TODGA DMDOHEMA

TBP

ON

O

N

O

C8H17

C8H17C8H17

C8H17

ON

O

N

O

C8H17

C8H17C8H17

C8H17

U Pu Np

U Pu Np Am Cm

PUREX

EURO-GANEX

Np extraction depends on

• HNO3

• HNO2

• Extraction of HNO2 and Np(VI) into solvent

• U saturation of the solvent

• Chemical kinetics in aqueous and organic phases

• Temperature

• Any other redox active species

• Residence time in contactor

• Mixer-settlers > pulsed columns > centrifugal contactors

Common problem to Advanced PUREX & GANEX processes

Np extraction in Advanced PUREX

• Experiments increasing in complexity

• Single phase, 2-phase, single centrifugal contactor stage

• Flowsheet trial

• Modelling

• …See talk by Hongyan Chen!

• The Np(V) oxidation reaction is promoted by

• Conditions in aqueous solution

• High [HNO3], [HNO2]/[Np](aq) ~ 2, T = 50C

• U loading reduces D(HNO2)

• Within a flowsheet need to consider:

• Sufficient [HNO2] in regions of low [U] during extraction

• Increase [HNO3] across the extract section

• Elevated temperature around the extract and scrub section

NNL Np extraction flowsheet test

1 HS1 4 5 HA1 8 9 HA2 12 13 HA3 14

HEATER HEATER

HA

F1 F2

SP1

A2

FC1

AR1

S1

A1

1 HS1 4 5 HA1 8 9 HA2 12 13 HA3 14

HEATER HEATER

HAF

F1 F2

SP1

A2

FC1

AR1

S1

A1

1 HS1 4 5 HA1 8 9 HA2 12 13 HA3 14

HEATER HEATER

HA

F1 F2

SP1

A2

FC1

AR1

S1

A1

1 HS1 4 5 HA1 8 9 HA2 12 13 HA3 14

HEATER HEATER

HAF

F1 F2

SP1

A2

FC1

AR1

S1

A1

Miniature centrifugal contactors

Simulant feed U+Np

No hot test yet

CEA Np extraction test

HAF

HAR <0.11 % Np

30% TBP

E1

E2

HNO2

HNO3

30% TBP

X

S HNO3

SP U,Np,Pu

AR <0.25 % Np

•Spent fuel test

•LWR UOx fuel

•Pulsed columns

Dinh, B., et al. in Solvent

extraction: fundamentals to

industrial applications.

Proceedings ISEC 2008

International Solvent

Extraction conference

pp.581-586 (2008).

Key results – 2 successful tests

NNL

• >99 % Np extracted

• U/Np test

• HNO2 added

• No radiation field

• HM ~ 250 g/L

• Centrifugal contactors

• Lab scale

• Heated to 50 C (stages 1-8)

• 4.5 mol/L HNO3 feed

CEA

• >99.6 % Np extracted

• Spent fuel test

• No HNO2 feed required

• Radiolytically generated HNO2

• HM ~250 g/L

• Pulsed columns

• Pilot scale

• Ambient temperature

• 4.5 mol/L HNO3 feed

Np extraction in GANEX

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

0.1 1 10

[HNO3]aq,ini (M)

DN

p

Np(IV) 0.2M TODGA

Np(VI) 0.2M TODGA

Np(IV) 0.5M DMDOHEMA

Np(VI) 0.5M DMDOHEMA

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

0.1 1 10

[HNO3]aq,ini (M)

DN

p

Np(IV)

Np(VI)

Np(V)

GANEX: Np(IV)>>Np(VI)>>Np(V) Np(IV): TODGA>DMDOHEMA Np(VI): DMDOHEMA>TODGA Np(V) more extractable than PUREX

Optimising Np extraction

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

600 700 800 900 1000 1100 1200 1300

Wavelength (nm)

Ab

so

rba

nc

e

10 minutes following extraction

17 hours following extraction at RT

Np(IV) Np(V)+(IV)

Np(IV)+(VI) Np(VI) in DMODHEMA

Initial aq. = 5 M HNO3

Stability Np(IV)>Np(VI)>>Np(V)

Promote extraction by promoting Np(V) disproportionation in flowsheet

Np(V) disproportionation

• Redox reaction residence time dependent

• Faster in organic phase than aqueous phase

• Faster in GANEX solvent than PUREX solvent

• Rate increases with increased nitric acid concentration and temperature

• Kinetic equation derived in 30 % TBP

• Products Np(IV,VI) are both extractable

• Promoting disproportionation should promote extraction

“EURO-GANEX” process tests

13 Scrub 16

32 Ln Strip 29

Solvent Feed

0.5M HNO3 Active feed 5.9 M HNO3 + 0.055 M CDTA SF solution (10 g/L Pu)

FPs

0.01M HNO3

Actinides Lanthanides

Lanthanides

Pu, Np, Am, Cm

Extract

Solvent Feed

Solvent Raffinate

0.5M HNO3 1M AHA 0.055M BTP

1 12

An Strip 28 17

23 22

Loaded Solvent Feed

Flowsheet results

Glovebox test (NNL)

Hot cell test

(ITU)

Contactors Centrifugals Centrifugals

Feed Surrogate Fast reactor spent fuel

Pu content in feed (g/L) 10 10

Pu recovery in E/S (%) > 99.9995 100.0

Am recovery in E/S (%) > 99.99 100.0

Np recovery in E/S (%) ~71 99.93

Np in HAR (%) ~ 29

Np in TRU product (%) ~69 99.9

Conclusions & outlook

• Np control is one of main chemistry challenges for advanced

reprocessing

• Affects plant layout, economics, waste management, proliferation

resistance, corrosion

• Complex chemistry in nitric acid and organic phases

• Extraction is residence time dependent

• Now demonstrated that complete extraction is achievable by

adjustment of acidity and temperature

• In advanced PUREX flowsheets

• In EURO-GANEX flowsheet

• & with short residence time centrifugal contactors

• Enhanced extraction under HA conditions (with SNF)

• Predictive & mechanistic based models now required…

Acknowledgments

NNL Signature Research programme in Spent Fuel and Nuclear Materials

EU Framework VII project “ACSEPT”

Sellafield Ltd.

UK Nuclear Decommissioning Authority

Chris Maher, Chris Mason, Katie Bell, Jamie Brown, Mark Sarsfield, Zara

Hodgson (NNL), Helen Steele (NNL/CEA)

Danny Fox, Cécile Roube, Gill Crooks, Chris Jones, Eddie Birkett (ex-NNL)

ACSEPT project – Andreas Geist (KIT), Rikard Malmbeck (ITU), Giuseppe

Modolo, Andreas Wilden (FZJ), Xavier Hères, Stéphane Bourg, Manuel

Miguirditchian (CEA)

Colin Boxall, Scott Edwards (Lancaster University)

Megan Jobson, Hongyan Chen, Andrew Masters (University of Manchester)

Key references 1. M. J. Carrott, et al. Neptunium extraction and stability in the GANEX solvent (0.2 M TODGA / 0.5 M DMDOHEMA /

kerosene), Solv. Extr. Ion Exch. 31, 463-482 (2013).

2. R. J. Taylor et al. Progress towards the full recovery of neptunium in an Advanced PUREX process, Solv. Extr. Ion

Exch. 31(4) 442-462 (2013).

3. C. Gregson et al. Neptunium (V) oxidation by nitrous acid in nitric acid, Procedia Chemistry, 7, 2012, 398-403