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Chemical Separations of Pu-238 from Irradiated Neptunium Targets

David DePaoli, Dennis Benker, Kevin Felker

Nuclear and Emerging Technologies for Space 2015 (NETS)

February 23, 2015

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Pu-238 Supply Project: Integrated Process

Np Oxide

Target Processing

Recovery Recycled NpO2

ORNL Building 7920

ORNL HFIR/INL ATR

INL

Neptunium Feed Stock Chemical Processing

Chemical Processing

Target Fabrication

Dissolution for Removal of 233Pa

Conversion to Oxide Powder

Shipment to ORNL

Target Irradiation

LANL

Delivery of New 238Pu to LANL

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Process chemistry of Np and 238Pu needs to be developed and demonstrated

• Both Np and Pu have multiple oxidation states – Typical oxidation states: Np(IV), Np(V), and Np(VI); Pu(III),

Pu(IV), and Pu(VI) – Selection and control of oxidation states is key to effective

separations – Some demonstration of redox control in used fuel processing at

much lower concentrations

• Pu-238 is a high-specific-activity alpha emitter Isotope Specific Activity Half-life Specific Heat Ci/g y W/g Pu-238 17.1 87.7 0.570 Pu-239 0.062 24110 0.0019 Pu-240 0.227 6561 0.0071 Pu-241 103.8 14.29 0.0041 Pu-242 0.0039 375000 0.0001

– Above properties make Pu-238 attractive option for radioisotope heat sources but difficult for process chemistry and material handling

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Pu-238 Supply Project includes targeted technology development

• Adaptation of existing methods to new circumstances

• Separation methods known – Savannah River Pu-238

production – Radiochemical Engineering

Development Center (REDC) history (e.g., Pu-242 recovery; fuel reprocessing)

– Current Cf-252 production practices at ORNL

• Incorporation of process improvements when needed

Aluminum Decladding in Caustic Nitrate

Actinide and Fission Product Dissolution in Nitric Acid

Neptunium Extraction Solvent Extraction

Irradiated Targets

Np Pu

Neptunium Purification (Pa-233 Removal)

Neptunium Product Oxide Conversion

(Modified Direct Denitration)

Plutonium Purification Anion Exchange

Plutonium Product Oxalate Precipitation

Plutonium Product Oxygen-16 Exchange

Plutonium Product Shipment

Target Fabrication Neptunium Targets

Target Irradiation

Fission Products

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Several separations are needed • Separate Pu and Np from fission products

– Maximize recovery

• Recover Pu-238 product – Meet product specs

• Recover Np-237 for recycle – Low fission product content for use in shielded glove boxes – Pu-238 content < 300 ppm

• Approach: – Solvent extraction for first-cycle separations – Purification of Np and Pu products by solvent extraction or ion exchange

• Processes developed within constraints: – Waste minimization and disposition pathways for all wastes – Use the existing hot cell equipment and stay within the current safety basis

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Process development begins with small quantities in limited tests up to target-processing tests

Aluminum Decladding in Caustic Nitrate

Actinide and Fission Product Dissolution in Nitric Acid

Neptunium Extraction Solvent Extraction

Irradiated Targets

Np Pu

Neptunium Purification

Neptunium Product Oxide Conversion

(Modified Direct Denitration)

Plutonium Purification

Plutonium Product Oxalate Precipitation

Plutonium Product Oxygen-16 Exchange

Plutonium Product Shipment

Target Fabrication Neptunium Targets

Aluminum dissolution is exothermic. Tested process controls and safe operation at full loading of Al. Investigated solids formation in lab-scale studies with successful follow-up hot cell test

Developing computer models from used fuel processing for Np and Pu extraction and separation. Completed hot cell test with Np only to investigate valence control. Completed mixed Np and Pu test in hot cells.

Previous tests have shown that high-fired Np was difficult to dissolve. The pellets from the target irradiations are being dissolved in small batch tests in the hot cell. Nitric acid dissolution is looking promising and more aggressive methods may not be necessary

Testing purification of Pu products using anion exchange with Pu-238 materials. Three purification runs completed.

Testing O-16 exchange in collaboration with University of Dayton Research Institute (UDRI). Initial testing comparable with surrogate testing at UDRI

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Process to utilize available equipment • Solvent extraction operations to

be performed using mixer-settler system at Radiochemical Engineering Development Center (REDC) – Originally designed and used for flow

sheet testing with used nuclear fuel – 3 banks of mixer-settler contactors – Throughput: up to 5L/h total flow – Design offers flexibility in operations

• Issue: – Need to demonstrate separations

with feed solutions at significantly higher concentrations of Np and Pu-238 than previous operations and experiments

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Currently pursued approach is similar to traditional processing • Based on differences in extractability of actinides in various

valence states. For tri-n-butyl phosphate (TBP): – Pu(IV)>Pu(VI)>>Pu(III) – Np(VI)>Np(IV)>>Np(V)

• Extraction depends on rate and extent of redox reactions • Np(V) in nitric acid solution is reversibly oxidized to Np(VI):

• For full recovery, adjust Np oxidation state by concentrations of nitric and nitrous acid

• Complicated by – Radiolysis – Complex role of nitrous acid as catalyst for oxidation and reactant for reduction

NpO2+ +

32

H+ + 12

NO3–

𝑘𝑟

𝑘𝑓 NpO2

2+ + 12

HNO2 + 12

H2O

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First-cycle solvent extraction separations are focused on Pu and Np recovery

1. Coextraction • Remove fission

products (FPs) • Oxidize Np(V) • Recover Np and Pu

2. Partitioning • Reduce Np and strip

in aqueous phase • Retain Pu in organic

phase

3. Stripping • Reduce Pu and

recover in aqueous phase

Feed: Np, Pu

Strip

Stripped Organic Extractant

A-Bank Np, Pu

Reductant

Np(IV), Np(VI), Pu(VI), Pu(IV)

Np(V)

Np(VI)

Np(V), Pu(VI) Np(VI), Pu(IV) Np(VI), Pu(IV))

Pu(IV) Pu(III)

Organic Extractant

Scrub

Scrub

Raffinate

Np product to 2nd cycle

Pu product to 2nd cycle

B-Bank

C-Bank

Pu

FPs

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Coextraction conditions follow recent success reported in literature • >99% Np recovery was demonstrated

by Taylor and coworkers (2013) using: – Higher acid feed and scrub

– Acceleration of oxidation • Addition of HNO2 to catalyze reaction • Heating of stages near feed

extract

• Issues for our application: – Np concentration in feed >100X higher

• Need to demonstrate sufficient Np oxidation rate • Nitrous and nitric acid concentrations will vary more significantly with reaction

– No Pu or FPs in previously reported tests • Need to demonstrate Pu recovery and FP removal • Evaluate Pu-238 radiolysis effects on chemistry

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Predicting neptunium behavior is challenging but essential

• Model developed to guide testing incorporates information from open literature: – Np(V)/(VI) reaction kinetic expressions

• Tochiyama (1995) and Tachimori (1997) • Temperature dependence included by activation energy from Precek and

Paulenova (2010)

– Np distribution extraction from Kumar and Koganti (2001) – Nitrous acid extraction prediction from Marin et al. (1973) – Legacy solvent extraction code for countercurrent solvent

extraction calculations

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Kinetic model for redox reactions shows reasonable agreement with published data

• Comparison with experiments of Gregson et al. (2012)*:

Conditions: Np(V) initially; 3 to 5 M nitric acid, 1 mM nitrous acid, 50°C

Conditions: 5 M nitric acid, 1 mM nitrous acid, 50°C

Oxidation Oxidation and reduction

*Procedia Chemistry 7, 398 – 403

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Excellent recovery of Np and Pu achieved in coextraction during initial hot test

Np Pu Expt 0.020% 0.008%

Model 0.001% 0.001%

Fraction not extracted

Test with tracer-level fission products (FPs) indicates good decontamination for FPs other than Zr

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Partitioning of Np and Pu is inadequate with insufficient nitrite

Concentration profiles under low nitrite conditions

Np product stream has acceptable Pu level, but low Np yield

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Partitioning of Np and Pu is better with higher nitrite addition

Np product stream has acceptable Pu level and good Np yield

Concentration profiles under higher nitrite conditions

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Good progress is being made in solvent extraction testing • Excellent recovery of Pu and Np in coextraction

– Oxidation works with high Np concentration and presence of Pu-238 – Model predicts oxidation performance reasonably well

• Good partitioning performance with sufficient nitrite – Np product has low Pu concentration – Up to 97% recovery of Np in first-cycle Np product

• There is opportunity for improvement of kinetic models for Np reduction

• Next testing will focus on – Fission-product decontamination factors – Second-cycle purification of Np and Pu products

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Anion exchange is being evaluated for purification of Pu

Unloaded anion exchange resin

Initial stages of loading – upflow

Loaded bed being washed - upflow

Stripping Pu from column – downflow

Stripped bed

Anion Exchange Run PXAX-2, September 17, 2014

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T, °C

Thermal modeling of anion exchange column

• Use adapted version of SRNL code (Laurinat, WSRC-TR-2006-00123) to estimate temperature profile in bed under normal and no-flow conditions

• Good agreement with limited run results

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0

5

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5

0

Rel

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stan

ce a

long

bed

Radial position Loading

1/3 loading 2/3 loading Loading complete During wash

Elution 1 Elution 2 Elution 3

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Summary

• Good progress is being made in development of chemical separations processes for Pu-238 production

• Promising results in first-cycle solvent extraction testing with hot feed materials

• Testing in support of second-cycle separations is in progress

• First integrated demonstration with irradiated target materials to start late summer 2015

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Acknowledgments

• NASA, Planetary Science Directorate • DOE Office of Nuclear Energy, NE-75 • Multiple contributors at ORNL, including:

– J. D. Burns, E. D. Collins, L. H. Delmau, M. Du, C. L. Jensen, J. McFarlane, C. E. Phelps, J. R. Spahr, K. Wilson

– Nuclear Analytical Chemistry and Isotopics Laboratory – Hot Cell Operations staff