ORNL/SPR-2019/1216

Advanced Nuclear Fuel Fabrication: Particle Fuel Concept for TCR

Michael P. Trammell Brian C. Jolly M. Dylan Richardson Austin T. Schumacher Kurt A. Terrani

July 2019

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ORNL/SPR-2019/1216

Transformational Challenge Reactor Program

Advanced Nuclear Fuel Fabrication: Particle Fuel Concept for TCR

Michael P. Trammell

Brian C. Jolly

M. Dylan Richardson

Austin T. Schumacher

Kurt A. Terrani

Date Published:

July 2019

Milestone M3CT-19OR06090130

Prepared by

OAK RIDGE NATIONAL LABORATORY

Oak Ridge, TN 37831-6283

managed by

UT-BATTELLE, LLC

for the

US DEPARTMENT OF ENERGY

under contract DE-AC05-00OR22725

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iii

CONTENTS

CONTENTS .................................................................................................................................................iii LIST OF FIGURES ....................................................................................................................................... v ABSTRACT .................................................................................................................................................. 1 1. INTRODUCTION ................................................................................................................................. 2 2. FABRICATION METHODOLOGY .................................................................................................... 2

2.1 BINDERJET PRINTING OF SiC ............................................................................................... 3 2.2 CHEMICAL VAPOR INFILTRATION (CVI) PROCESS ........................................................ 4

3. CONCLUSION ..................................................................................................................................... 6 4. REFERENCES ...................................................................................................................................... 7

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v

LIST OF FIGURES

Figure 1. Stages of fabrication to produce particle fuel demonstration element. .......................................... 3 Figure 2. X-ray tomography of post-CVI particle fuel element. ................................................................... 3 Figure 3. Illustration of the binderjet printing process. ................................................................................. 4 Figure 4. (a) CVI system. (b) Pre-CVI parts supported by a combination of graphite fixtures and

tungsten mesh. (c) A view inside the furnace during operation. ...................................................... 5 Figure 5. Microstructure of binderjet-printed and CVI-densified test disks.................................................. 5

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1

ABSTRACT

This report presents an assessment of a novel fabrication method to produce a ceramic microencapsulated

fuel element for the Transformational Challenge Reactor (TCR) program. The proposed fuel concept uses

proven materials and fuel technology in combination with readily available advanced manufacturing

systems to further advance nuclear fabrication technology. The concept consists of TRISO-coated fuel

particles embedded in a silicon carbide matrix. The fabrication method begins with a low-density silicon

carbide capsule produced using binderjet additive manufacturing technology that is filled with TRISO

coated fuel particles and then densified with highly crystalline SiC in a chemical vapor infiltration (CVI)

process. The concept, fabrication methodology, and preliminary proof-of-principle demonstration are

described.

2

1. INTRODUCTION

The Transformational Challenge Reactor (TCR) program’s goal is to explore advanced manufacturing

technologies to enhance reactor performance while reducing cost and schedule of deployment for nuclear

power systems. These technologies can include, but are not limited to, any combination of advanced

materials and additive manufacturing methods that enable rapid fabrication of components with complex

geometry, composition, and properties not achievable by traditional manufacturing methods. One specific

area of interest that could significantly benefit from these technologies is the design and fabrication of the

fuel component. This paper describes a ceramic microencapsulated fuel design that builds on recent

developments in this area [1] and evolves the technology by combining the favorable properties of silicon

carbide and the flexibility of additive manufacturing, while continuing to leverage significant

advancements in coated particle fuel technology.

A significant achievement within the nuclear community that is incorporated into this concept is the

development and qualification of tri-structural isotropic (TRISO) coated particle fuel for gas-cooled

reactors [2]. The particle fuel consists of a fuel kernel (historically UO2 or UCO and recently UN [3])

with a diameter of 350-800 µm that is coated with pyrolytic carbon and silicon carbide layers that serve

many functions including structural support for the fuel kernel and fission product retention. For gas

cooled reactors, the coated fuel particles are embedded in a graphite matrix with an overcoating and

pressing process [4]. For the fuel concept described here, these coated fuel particles are directly

embedded in a silicon carbide matrix that is produced by a combination of binderjet printing and then

densification in a chemical vapor infiltration (CVI) process. The work described here is agnostic to the

exact type and geometry of the TRISO fuel particles. The TCR program is targeting TRISO particles with

800 µm UN kernels as the fuel to facilitate criticality with a small core.

The materials and fabrication processes used to produce this fuel design were selected based on specific

criteria. Silicon carbide is widely accepted as a desirable material for nuclear applications based on its

fission product retention, irradiation stability, and resistance to degradation under accident conditions [5].

SiC in powder form is very suitable for the binderjet printing process. The fabrication process begins with

a low-density silicon carbide capsule that is produced using binderjet technology. Binderjet printing is a

relatively recent additive manufacturing technique that provides design flexibility for complex features

such as fuel free zones and cooling channels that are otherwise difficult, if not impossible, to produce

using traditional fabrication methods. This is especially important for ceramic materials with only a few

fabrication routes available [6, 7]. The capsule is filled with a combination of TRISO-coated fuel particles

and loose SiC powder. At this stage, densification is required. For this concept, a CVI process was chosen

for densification based on several reasons: (a) it provides near theoretical density SiC [5], (b) the resulting

SiC satisfies high purity and irradiation performance requirements for nuclear reactor applications [5],

and (c) the process produces little to no dimensional change. Although the individual materials and

methods described here are well established, the combination presents an opportunity to further advance

fuel fabrication technology1.

2. FABRICATION METHODOLOGY

To assess the viability of the ceramic microencapsulated particle fuel concept, the proposed materials and

fabrication methods were used to produce a proof-of-principle demonstration element using surrogate

TRISO-coated particles. The process begins with SiC feedstock powder and a CAD file with an overly

exaggerated complex geometry capsule design. With binderjet technology, the printer works directly from

1 Patent Pending 62/769,588

3

this CAD file to produce a low-density (~35–45%TD) part comprised of the selected SiC feedstock

material. Once the print is complete, the designated fueled region of the capsule is loaded with a

combination of surrogate TRISO-coated particles and additional SiC powder. The SiC powder is needed

to fill the interstitial spaces between particles and is rolled smooth after filling is complete. The loaded

capsule is then densified in a CVI process to achieve microencapsulation of TRISO particles in a SiC

matrix. The steps to achieve this process are illustrated in Figure 1. The binderjet and CVI processes are

discussed in subsequent sections of this report.

Figure 1. Stages of fabrication to produce particle fuel demonstration element.

Results from x-ray tomography are shown in Figure 2. This image reveals surrogate TRISO fuel particles

in the designated fueled region of the capsule design. The image also showcases the complex network of

interconnected cooling channels that can be achieved with the binderjet/CVI method.

Figure 2. X-ray tomography of post-CVI particle fuel element.

2.1 BINDERJET PRINTING OF SiC

The binderjet process is an additive manufacturing technology that could provide a cost-effective

opportunity for printing ceramic materials with complex geometry for nuclear applications. The process

begins with a built-in software that slices the model of the desired part into sequential 2D layers. The

printhead travels across a powder bed and selectively delivers binder in the shape of the first cross

sectional layer (Figure 3a). The next step, referred to as recoating, consists of several functions (Figure

3b). During the recoating process, the base plate is lowered by the predetermined layer height (typically

50–100 µm), and powder is delivered across the print area. After powder delivery, a counter-rotating

roller travels across the print area to set the layer height and smooth the powder bed in preparation for the

next print layer. The next layer in the sequence is printed, and the process continues to repeat until

4

complete. Once the printing process is complete, the job-box is removed and placed in an air oven at

190°C for ~6 hours. This process fully cures the binder and produces a finished part that is embedded in

loose powder (Figure 3c). The parts are then extracted and cleaned.

Figure 3. Illustration of the binderjet printing process.

At this stage, the printed part has a relatively low density (35–45%TD) and consists of the feedstock

powder bound together with residual binder. To be useful, binderjet-printed parts are typically densified

and/or strengthened by several techniques, including precursor infiltration and pyrolysis (PIP), reactive

melt infiltration (RMI), or direct sintering [7]. The technique used for post processing can vary depending

on the feedstock powder material and the application. For this application and as a novel solution, CVI

was chosen for densification and is described in the subsequent section of this report.

2.2 CHEMICAL VAPOR INFILTRATION (CVI) PROCESS

After the capsule is binderjet printed, it is filled with TRISO particles and loose SiC powder in

preparation for densification. A CVI process is used to densify the capsule by filling the voids in the

printed part and the loose powder while encapsulating the TRISO-coated particles. This process increases

density, provides a hermetic seal, and provides mechanical integrity, all while maintaining dimensional

stability throughout the process.

The CVI system consists of a quartz tube inside a clamshell-style furnace that allows the temperature and

pressure to be independently controlled. Figure 4 illustrates the equipment and demonstration capsule

both before (Figure 4b) and during (Figure 4c) the process. In the initial steps of the CVI process, the

desired temperature and pressures are achieved under inert conditions. Note, during this step, the residual

binder from the binderjet printing process is baked out. After heating, the CVI system introduces methyl

trichlorosilane as the precursor, which decomposes on the surface of solid powder particles to yield high purity and crystalline SiC [8]. In this case, the CVI process was initiated at 1000ºC under below-

atmospheric pressure with H2 flowing into the coating chamber. A carrier gas, H2, was routed through a

bubbler to achieve an adequate volume flow rate of methyl trichlorosilane.

5

Figure 4. (a) CVI system. (b) Pre-CVI parts supported by a combination of graphite fixtures and tungsten

mesh. (c) A view inside the furnace during operation.

Results from density and microstructure characterization performed on 5 mm radius by 3 mm thick disks

fabricated using this binderjet/CVI method indicate that a relatively high density (~90%TD) can be

achieved for thin sections. The microstructure of these test disks can be seen in Figure 5 and shows that

some internal porosity still exists. Initially, the CVI process uniformly densifies the green part by

depositing a fully dense coating on all internal and external surfaces. However, as the pores inside the

material close, an outer layer develops (Figure 5b) to create a robust hermetic seal. As the material

thickness increases, it becomes more difficult for precursor gasses to reach the inner porosity, and this

could potentially cause a density gradient. This effect may be reduced by optimizing processing

conditions.

Figure 5. Microstructure of binderjet-printed and CVI-densified test disks.

6

3. CONCLUSION

The TCR program is interested in accessing advanced materials and fabrication methods to further the

advancement of nuclear technology. In this report, a preliminary investigation to assess the possibilities of

producing a ceramic microencapsulated particle fuel concept fabricated using a combination of binderjet

printing and CVI is explored. To determine the viability of this concept, a commercially available

binderjet system was used to produce a fuel capsule from SiC powder. The capsule was filled with a

combination of surrogate TRISO particles and additional SiC powder and then densified in a CVI

process. In conclusion, the results indicate the following:

• Binderjet technology can successfully be used to print complex geometry capsules from SiC

feedstock powder.

• The binderjet-printed capsule will survive the CVI process (binder burnout) even when fully loaded

with particles and SiC powder.

• Densification in a CVI process is achievable and provides suitable density and microstructure.

7

4. REFERENCES

[1] K.A. Terrani, J.O. Kiggans, Y. Katoh, K. Shimoda, F.C. Montgomery, B.L. Armstrong, C.M. Parish,

T. Hinoki, J.D. Hunn, L.L. Snead, Fabrication and characterization of fully ceramic microencapsulated

fuels, J Nucl Mater 426 (2012) 268-276.

[2] D. Petti, J. Maki, J. Hunn, P. Pappano, C. Barnes, J. Saurwein, S. Nagley, J. Kendall, R. Hobbins, The

DOE Advanced Gas Reactor Fuel Development and Qualification Program, Jom-Us 62 (2010) 62-66.

[3] T.B. Lindemer, C.M. Silva, J.J. Henry, J.W. McMurray, S.L. Voit, J.L. Collins, R.D. Hunt,

Quantification of process variables for carbothermic synthesis of UC1-xNx fuel microspheres, J Nucl

Mater 483 (2017) 176-191.

[4] P.J. Pappano, T.D. Burchell, J.D. Hunn, M.P. Trammell, A novel approach to fabricating fuel

compacts for the next generation nuclear plant (NGNP), J Nucl Mater 381 (2008) 25-38.

[5] L.L. Snead, T. Nozawa, Y. Katoh, T.S. Byun, S. Kondo, D.A. Petti, Handbook of SiC properties for

fuel performance modeling, J Nucl Mater 371 (2007) 329-377.

[6] W.C. Du, X.R. Ren, C. Ma, Z.J. Pei, Binder Jetting Additive Manufacturing of Ceramics: A Literature

Review, Proceedings of the Asme International Mechanical Engineering Congress and Exposition, 2017

Vol 14 (2018).

[7] X.Y. Lv, F. Ye, L.F. Cheng, S.W. Fan, Y.S. Liu, Binder jetting of ceramics: Powders, binders,

printing parameters, equipment, and post-treatment, Ceram Int 45 (2019) 12609-12624.

[8] H.O. Pierson, Handbook of Chemical Vapor Deposition, 2nd Edition: Principles, Technology and

Applications, Elsevier Science1999.