Fabrication and characterization of driver-fuel particles, designed-to-fail fuel particles, and fuel compacts for the US AGR-3/4 irradiation test

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Abstract

Fuel compacts have been fabricated for the third in a series of irradiation tests designed to study tri-structural isotropic (TRISO) coated particle fuel performance in support of advanced gas-cooled reactor (AGR) development. The purpose of this third irradiation test, designated as AGR-3/4, is to measure fission product release and transport by irradiating compacts containing a small fraction of fuel particles that are intentionally designed to fail (DTF) early in the irradiation test. Transport of fission products released by the mixed uranium carbide/uranium oxide kernels within the DTF particles will be studied in the compact's carbon matrix and in cylindrical rings surrounding the compacts, which were made from either compact matrix material or structural graphite. Results will be used to refine fission product transport models. Coating of the 20-μm-thick pyrocarbon-coated DTF and standard TRISO driver-fuel particles, fabrication of the fuel compacts containing these particles, and characterization of the key fuel properties are discussed in this paper.

Introduction

The fuel elements fabricated for the AGR-3/4 irradiation test (Grover and Petti, 2012) were cylindrical, graphite-matrix compacts containing two types of fuel particles. Most of the particles were standard TRISO-coated driver-fuel particles expected to perform normally and generate typical irradiation conditions within the fuel compacts. However, 1% of the uranium in each compact was contained within designed-to-fail (DTF) particles, which will release fission products during the irradiation. Each compact contained 20 DTF particles distributed along the center axis surrounded by ∼1900 driver-fuel particles. Both particle types contained kernels from a 19.7% 235U low-enriched uranium carbide/uranium oxide (UCO) kernel composite supplied to ORNL by the Babcock and Wilcox (B&W) Nuclear Operation Group in Lynchburg, Virginia.

Driver-fuel particles were fabricated with the same parameters used to produce the baseline fuel particles for the AGR-1 irradiation test. The AGR-1 irradiation test was completed in November 2009 and demonstrated excellent fuel performance, with no gas release associated with TRISO-coating failure observed from any of the ∼300,000 particles irradiated to a peak burnup of 19.5% fissions per initial metal atom under High Temperature Reactor (HTR) conditions (Grover et al., 2010). The TRISO-coated driver-fuel particles consisted of a 350-μm-diameter spherical kernel coated with an ∼50% dense carbon buffer layer (∼100 μm thick), followed by a dense inner pyrocarbon (IPyC) layer (∼40 μm thick), followed by a SiC layer (∼35 μm thick), followed by another dense outer pyrocarbon (OPyC) layer (∼40 μm thick).

For the DTF particles, the TRISO coating layers were replaced by an ∼20-μm-thick pyrocarbon layer that was developed to survive compacting but crack during the first several irradiation cycles. This results in a controlled release of fission products that migrate out through the compact matrix and into the surrounding matrix or graphite rings during the experiment. The AGR-3/4 irradiation test began at the end of 2011, and gamma-emitting gaseous fission products detected in the capsule sweep gas indicate that the DTF layers are failing as expected (Grover and Petti, 2012).

Section snippets

Driver fuel fabrication

The driver-fuel TRISO coatings were deposited by use of a lab-scale fluidized-bed chemical vapor deposition furnace that had a 50-mm inner diameter cylindrical graphite coating chamber with a conical bottom through which the gas entered (Fig. 1). Buffer was deposited at 1450 °C with a deposition rate of ∼22 μm/min using 61 vol % acetylene (C2H2) mixed in argon. The IPyC layer was deposited at 1265 °C with a deposition rate of ∼3.3 μm/min using 16.4 vol % acetylene (C2H2) and 14 vol % propylene (C3H6)

Designed-to-fail fuel particles

The DTF particle concept of depositing a thin, high-density, high-anisotropy pyrocarbon layer directly on a fuel kernel for temporary containment of the kernel material was originally used by General Atomics (GA) for the HRB-17/18 (Ketterer and Myers, 1986), HFR-B1 (Burnette et al., 1994), and COMEDIE BD-1 irradiation tests (Medwid and Gillespie, 1993). PARFUME calculations have indicated that the DTF layer will fracture when the irradiation-induced kernel swelling and pyrocarbon shrinkage

AGR-3/4 compact fabrication

Fabrication of the AGR-3/4 fuel compacts was achieved by using a process similar to that used for the high-performance AGR-1 fuel compacts (Pappano et al., 2008), with added complications related to overcoating the smaller DTF particles and locating them along the centerline of the compact. As described below, the DTF and driver fuel particles were embedded in a graphite matrix formed by overcoating each particle with a graphite/resin blend, pressing the overcoated particles in a cylindrical

Optical and X-ray imaging

Fig. 9 shows images of one of the AGR-3/4 compacts. The compacts were well-formed, with no apparent cracks or chips. However, the OPyC surface of several of the TRISO-coated driver-fuel particles could be seen on the compact ends, where the matrix was too thin to be retained.

The X-ray radiograph in Fig. 10 shows that the driver-fuel particles were uniformly distributed throughout the compact. To better image the DTF particles, a 2.5-mm-thick section was cut from the center of a compact using a

Summary

DTF particles were fabricated by coating UCO kernels with high-density, high-anisotropy, ∼20-μm-thick pyrocarbon layers. These DTF particles were put into cylindrical fuel compacts by using a procedure that located the DTF particles along the center axis of each compact, surrounded by low-defect driver-fuel particles. Twenty DTF particles were placed in each compact, such that 1% of the total uranium content in the fuel compacts was in DTF particles. Characterization of the final fuel compacts,

Acknowledgment

This work was supported by the U.S. Department of Energy, Office of Nuclear Energy, under the Next Generation Nuclear Plant Program.

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