February 25, 2015

JEFF KATALENICHNUCLEAR ENGINEERING AND ANALYSIS – NUCLEAR FUELS & MATERIALS TEAMNUCLEAR AND EMERGING TECHNOLOGIES FOR SPACEFEB. 25, 2015

DEMONSTRATION OF SOL-GEL TECHNIQUES FOR DUST-FREE PLUTONIUM-238 HEAT SOURCE FABRICATION

This presentation contains BUSINESS SENSITIVE information, information covered by PENDING PATENT APPLICATIONS, and SUBJECT INVENTIONS defined under 35 USC Section 201. Such information may be  subject to disclosure restrictions including those outlined in 35 USC section 205 and 18 USC 1905. Unauthorized use or public disclosure is prohibited. 

The baseline process for fabricating Pu-238 oxide pellets generates sub-micron powders

Current process: oxalate precipitation of Pu-238

Generates rosette and lath particle morphologiesProduces wide range of particle sizes with large surface areaPrecipitated powders have tendency to agglomerateParticle size/morphology is sensitive to precipitation conditions

After calcination to 238PuO2, powders are ball milled

Normalizes particle size, morphology – necessary to prevent pellet cracking Generates powders with mass median diameters ~ 0.6 μm

February 25, 2015 2238Pu(III) direct-strike precipitates [1]

Pu-238 dust poses a hazard to workers and facilities

Sub-micron 238PuO2 powders pose unique hazards:Long settling times, susceptible to air currents<20 nm particles are subject to alpha-recoil motion

Can penetrate HEPA filtersRequires frequent glove replacement in gloveboxes

Alpha decay leads to fracture (self-dividing powder)<10 μm particles are respirable

Pu-238 powders have caused problems at SRNL and LANL

Corrosion and failure of a can storing Pu-238 residues at LANL [3]Plutonium Fabrication Facility (PuFF) [2]

Surrogate work has motivated follow-on studies

Sol-gel techniques pose a dust-free route to granules for pellet pressing

Pu-238 stays in solution until gelled spheres (100 μm +) are formedSpheres are washed and dried to form 238PuO2 directlyRecent work with cerium oxide surrogates addressed process modifications to prevent cracking & agglomeration

Follow-on work secured at PNNL for sol-gel development & demonstration

3-year LDRD projectBeginning work with gram-quantities of Pu-239 and Pu-238

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Thesis work aimed to reduce dust generation and produce acceptable feed granules

Baseline sol-gel processes can result in dust generation due to particle cracking

Sol-gel spheres undergo significant shrinkage and can crack during drying/heatingImproper gelation conditions can exacerbate microsphere cracking

Pellets produced by powder-baseline processes sometimes crack

Must encapsulate 238PuO2 pellet in iridium shell before cracking prevents use

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Cross section of hot pressed GPHS pellet [4]Sectioned pellet showing cleavage surfaces [1]

Current sol-gel processes required modification for optimal processing

A 2008 INL report raised key questions regarding sol-gel Pu-238 processing [5]:

1) “What is the particle size distribution in the granulate that can be produced by the optimized sol-gel process?”2) “What is the ceramic activity of the final granules produced by the sol-gel method?”3) “Can the required chemical purity be achieved using this method?”

Work at Univ. Michigan answered these questions using cerium oxide

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Cerium oxide microspheres during washing ~30g Dried cerium oxide microspheres ~120 μm

Excellent size uniformities were obtained in the desired size range

Yellow/orange upon drying at 150 °COff-white CeO2 after sintering at 1500 °C

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176 μm ± 5.4 μm

100 μm ± 2.7 μm

72 μm ± 3.9 μm

Sol-gel processing methods impacted ceramic activity

Microspheres produced by baseline processes became strongly agglomerated during heat treatments

Microspheres produced by modified methods:Were free-flowing or weakly agglomerated (easily separated) upon heatingWere agglomerated (separated readily with screening) after heating to 1500 °C

February 25, 2015 8Weakly agglomerated cerium oxide microspheres

Sol-gel processing methods impacted fines generation

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Baseline process led to cracking

Surface spallHairline cracks

Modified processes prevented cracking

Smooth surfacesMinimal cracking or spalling observed

Sol-gel processing meets chemical purity requirements

Performed LECO analysis for carbon content in sintered sol-gel spheres

All samples tested were below 50 ppm

Performed glow discharge mass spectrometry (GDMS)

Impurity levels were well below requirements listed in LANL-26Y-318180 (1992)

Therefore, sol-gel processing doesn’t concentrate any particular elemental impurity to the extent that it impacts purity requirements

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Sol-gel microspheres appear to be ideal feed granules for pressing pellets

Decades-old research investigated optimal feed granule properties and preparation methods to reduce pellet cracking

Looked at various permutations of oxalate precipitation conditionsVaried the extent of ball millingRecommended ~85% TD pellets with homogeneous coarse porosityCoarse porosity (inter-granule porosity) obtained by limited pre-densification of feed granules + granule blending

Sol-gel methods are simply a granule production step

Provides control over particle size/shapeProvides consistent size/shapeCapability to tune granule microstructure and porosity without ball milling Sol-gel step replaces precipitation, calcination, ball milling, cold pellet pressing, and slugging/screening with a dust-free processMicrospheres can be thermally seasoned similar to baseline granules for pellet pressingMonodisperse spheres would provide a more homogeneous coarse porosity due to uniformity of feed granules

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Follow-on work has been funded at PNNL

Awarded 3-year LDRD to investigate process modification/demonstration

Use Pu-239 surrogate for process development followed by Pu-238 work

Three main experimental setups to support research:

Commission apparatus in RPL glovebox for fabrication of Pu sol-gel spheresSimplified gelation experiments in RPL glovebox to optimize Pu process parametersConstruct sol-gel apparatus in non-rad laboratory to support research with cerium

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Radiochemical Processing Laboratory (RPL) at PNNL [6]

A sol-gel apparatus for cold work will support process/equipment R&D

Some work can be performed more quickly (and cost-effectively) in non-radiological laboratories

Demonstrate system operation to safety personnelTest/iterate new pieces of hardware more rapidlyInvestigate the effects of process modifications on cerium microspheresValidate new methods/techniques prior to use in radiological spaces

February 25, 2015 13Gelled cerium spheres falling down column

Simplified gelation studies provide initial optimization of feed solution parameters

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Test tube studies of gelation times and gel quality informs microsphere production

Optimize solution concentrationsOptimize reagent ratiosChoose best gel based on quality and desired gelation timeProvides initial conditions for microsphere production trials

Gelation of a cerium feed sample in a hot water bath

A new sol-gel apparatus is being commissioned in a glovebox

Sol-gel equipment has been procured & is being assembledGlovebox modifications are underwayAnticipate microsphere fabrication to begin in spring 2015

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Anticipated sol-gel equipment layout in RPL glovebox

Summary

Current Pu-238 pellet fabrication produces hazardous powders

Sol-gel techniques are expected to largely preclude powder generation

Microspheres appear to be ideal feed granules for pellet pressing

Sol-gel methods have been modified to overcome shortcomings/uncertainties

Demonstration with Pu-238 is funded and in-process

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Acknowledgements

Work performed while at the University of Michigan was supported by:Michigan Space Grant ConsortiumUniversities Space Research Association through the Center for Space Nuclear ResearchThis research was made with Government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a.This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1256260. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Work performed at the Pacific Northwest National Laboratory:The research described in this report was conducted under the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory, a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy. Jeff Katalenich is grateful for the support of the Linus Pauling Distinguished Postdoctoral Fellowship program.

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References

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[1] G. A. Burney, J. W. Congdon, “Direct Fabrication of 238PuO2 Fuel Forms,” Savannah River Laboratory, DP-1621, 1982.[2] D. T. Rankin et al, “Production of Pu-238 Oxide Fuel for Space Exploration,” Westinghouse Savannah River Company, WSRC-MS-2000-00061, 2000.[3] J.D. Lorence. NNSA. “Lessons Learned from Type B Accident Investigation of the August 5, 2003 Plutonium-238 Multiple Uptake Event at the Plutonium Facility, LANL, NM.” [4] R.L. Folger, 238Pu Fuel Form Processes Quarterly Report - January-March 1981,Savannah River Laboratory, 1981.[5] M. Borland et al, “Evaluation of Aqueous and Powder Processing Techniques for Production of Pu-238 Fueled General Purpose Heat Sources,” Idaho National Laboratory, INL/EXT-08-14017, 2008.[6] Pacific Northwest National Laboratory. “PNNL Recognized with DOE Sustainability Awards.” Web. 12 Feb. 2014. <http://sustainable.pnnl.gov/newsletter/second_nature_v2n1.html>.R. A. Kent, “LASL Fabrication Flowsheet for GPHS Pellets,” Los Alamos Scientific Laboratory, LA-7972-MS, 1979.P. K. Smith et al, “Effect of Oxalate Precipitation on PuO2 Microstructures,” Savannah River Laboratory, DP-MS-76-34, 1976.

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QUESTIONS?