Fabrication and characterization of fully ceramic microencapsulated fuels

doi:10.1016/j.jnucmat.2012.03.049

Journal of Nuclear Materials

Volume 426, Issues 1–3, July 2012, Pages 268-276

https://doi.org/10.1016/j.jnucmat.2012.03.049 Get rights and content

Abstract

The current generation of fully ceramic microencapsulated fuels, consisting of Tristructural Isotropic fuel particles embedded in a silicon carbide matrix, is fabricated by hot pressing. Matrix powder feedstock is comprised of alumina–yttria additives thoroughly mixed with silicon carbide nanopowder using polyethyleneimine as a dispersing agent. Fuel compacts are fabricated by hot pressing the powder–fuel particle mixture at a temperature of 1800–1900 °C using compaction pressures of 10–20 MPa. Detailed microstructural characterization of the final fuel compacts shows that oxide additives are limited in extent and are distributed uniformly at silicon carbide grain boundaries, at triple joints between silicon carbide grains, and at the fuel particle–matrix interface.

Introduction

Fully ceramic microencapsulated (FCM) fuels consist of Tristructural Isotropic (TRISO) fuel particles embedded in a silicon carbide (SiC) matrix (Fig. 1). A TRISO particle consists of a spherical fuel kernel that is coated with successive layers of porous carbon (buffer layer), a dense inner pyrocarbon (IPyC), SiC, and an outer pyrocarbon (OPyC) layer. Over the past five decades TRISO fuel particle technology has been developed and optimized for gas reactors now capable of delivering remarkable levels of performance with respect to fission product retention, high-temperature performance, and fissile burnup [1], [2]. The newly invigorated and improved TRISO technology has enabled fuel and reactor designers to extend the application of TRISO particles to other fuel and reactor systems. In conventional high-temperature gas-cooled reactor (HTGR) applications, the TRISO particles are dispersed in a graphitic matrix, producing compacts in the form of pebbles or pellets [3], [4]. Under the FCM fuel concept, the graphite matrix is replaced with a SiC matrix that offers the following potential advantages: (i) improved irradiation stability [5], [6], (ii) incorporation of yet another effective barrier to fission product release, (iii) environmental stability under operating (steady state) and transient conditions as well as long-term storage, and (iv) proliferation resistance. The high thermal conductivity of the graphite matrix is also matched by the SiC matrix [5], [7]. Details regarding the advantages and motivations behind the FCM fuel concept are discussed more extensively in a companion paper [8].

In this paper, the materials and processing science that underpin FCM fuel fabrication are discussed. While the majority of this effort pertains to geometries associated with light water reactor (LWR) fuel pellets, fabrication of extended geometries has also been performed. Detailed characterization of the feedstock, study of processing parameters, and investigation of microstructural characteristics have been performed. Specific discussion of TRISO fabrication has been well presented and is available elsewhere [9], [10].

Section snippets

Consolidation of the FCM fuel matrix (NITE process)

The Nano-Infiltration and Transient Eutectic-phase (NITE) processing of SiC is a specific type of liquid phase sintering (LPS) [11] utilizing SiC nanopowder with a limited amount of oxide additives to produce near fully dense SiC [12], [13], [14]. The oxide additives generally consist of alumina–rare earth oxides mixed at eutectic compositions, reducing the typical required processing temperature by about 200–400 °C [15]. The process was originally developed for fiber-reinforced SiC-matrix

FCM precursor material (current feedstock and supply)

The powder mixtures utilized for this particular FCM fuel fabrication consisted of SiC nanopowder (Nanostructured & Amorphous Materials, Inc., lot #4620-123109) with addition of yttria (Nanostructured & Amorphous Materials Inc., lot 5610-091410), alumina (Nano Products Corp., lot 20100905), and silica (Nanostructured & Amorphous Materials Inc, lot 4830-081810). The mean diameter of all the powder particles was reported as roughly 40 nm by the manufacturers. Transmission electron microscope (TEM)

Fabrication by hot pressing

Pellet fabrication was carried out by hot pressing the powder-TRISO particle mixtures in a graphite die at elevated temperatures. The majority of fabrication trials were performed at a maximum temperature of 1850 °C; however, deviations of 50 °C above and below this value were also explored. Higher temperatures are desirable to achieve a higher degree of densification in the matrix while fabrication at temperatures above 1900 °C could potentially compromise the properties of the SiC shell in the

Microstructural characterization

The microstructure of the SiC matrix along with the specific characteristics of the matrix–particle interface could provide extensive information on the expected properties and irradiation behavior of the FCM pellet under LWR operating conditions. Hence a comprehensive set of results generated through scanning and transmission electron microscopy (SEM and TEM) studies are presented in this section. The SEM study was performed with a JEOL 6500 FEG-type microscope operating at 10 kV accelerating

Discussion

In the developmental work presented here, no specific effort was made to achieve uniform fuel particle spacing in the SiC matrix. This is in contrast to the gas reactor fuel compacts where the TRISO particle distribution is tightly controlled prior to compaction by overcoating the particles with graphite–resin (binder) mixtures [28]. Given that the mechanical response and irradiation behavior of the SiC matrix differs greatly from that of graphite, it is unclear whether nonuniform distribution

Conclusions

Fabrication of fully ceramic microencapsulated fuels, consisting of TRISO-coated fuel particles embedded in a SiC matrix, has been demonstrated by hot pressing. SiC nanopowder thoroughly mixed with a limited mass of yittria–alumina additives was used to produce the SiC matrix via the NITE process. Utilization of PEI dispersing agent in alcohol with ultrasonic mixing was shown to be an effective method for achieving uniform dispersion of oxide and SiC powder particles. Hot pressing powder–fuel

Acknowledgments

The work presented in this manuscript was supported by the Advanced Fuels Campaign of the Fuel Cycle R&D program in the Office of Nuclear Energy, US Department of Energy. The HF3300 TEM/STEM and JEOL6500 FEG SEM were supported by ORNL’s Shared Research Equipment (ShaRE) User Facility, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. Efforts by Shawn Reeves and John Henry, ORNL, for TEM specimen preparation and

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Fabrication and characterization of fully ceramic microencapsulated fuels

Abstract

Introduction

Section snippets

Consolidation of the FCM fuel matrix (NITE process)

FCM precursor material (current feedstock and supply)

Fabrication by hot pressing

Microstructural characterization

Discussion

Conclusions

Acknowledgments

Nucl. Eng. Des.

J. Nuc. Mater.

J. Nuc. Mater.

J. Nuc. Mater.

Fusion Eng. Des.

J. Nucl. Mater.

Compos. Sci. Technol.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Eur. Ceram. Soc.

J. Nucl. Mater.

J. Nucl. Mater.