Qualitative and quantitative analysis of neutron irradiation effects in SiC/SiC composites using X-ray computed tomography

Abstract

Silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composites are candidate materials for cladding of light water reactor (LWR) fuels. Loss of fission product gas retention due to the formation of microcrack networks is considered a potential failure mechanism for SiC/SiC-cladded fuels. In this study, a variety of SiC/SiC composite tubes were irradiated with and without an LWR-relevant radial heat flux in the High Flux Isotope Reactor, followed by detailed characterization with X-ray computed tomography (XCT). This first set of XCT data for neutron-irradiated samples confirmed that the internal stresses arising from a combination of temperature gradients and irradiation-induced swelling act as the primary driver for cracking. While the observed cracking patterns varied depending on the tube architectures, the sharp edges of relatively large pores were found to be the common stress concentrator. These findings are useful to help improve the design and manufacturing of SiC/SiC fuel claddings for reduced failure probability.

Introduction

In recent times, silicon carbide (SiC) materials have gained prominent attention for nuclear fission and fusion applications [1]. In particular, continuous SiC fiber–reinforced SiC matrix composites (SiC/SiC composites) are being considered for a wide range of nuclear energy applications, such as the enhanced accident tolerance fuel cladding and channel boxes in light water reactors (LWRs) [[2], [3], [4]]. Desirable characteristics of SiC as a nuclear material include low neutron absorption, exceptional radiation tolerance, excellent high-temperature strength, and steam oxidation resistance [5]. Taking advantage of all of these intrinsic characteristics of SiC, the SiC/SiC composites offer additional benefits including damage tolerance and predictable failure behaviors. An additional key benefit of the continuous fiber composites is their flexibility, as the various mechanical and physical properties can be tailored to meet the design requirements by using different architectures, architectural parameters, and combined selections of the matrix, fibers, and fiber–matrix interfaces [[6], [7], [8]].

Despite these advantages, there remain challenges for SiC/SiC composites that must be resolved before they can be considered technically feasible as nuclear fuel cladding. In particular, there is a mechanism that can drive a loss of leak-tightness of the composite cladding during normal reactor operation: neutron-induced dimensional changes under a temperature gradient [9]. The heat flow from the fuel to the coolant creates a steep, radial temperature gradient across the cladding wall thickness that creates significant internal stress across the component. Additional loads also arise from internal pressure as fission product (FP) gases build up, likely in advance of the onset of pellet–cladding mechanical interactions resulting from fuel swelling. Previous fuel performance modeling of the UO₂–SiC fuel-cladding system predicted that these stresses might lead to cracking of the cladding [[10], [11], [12], [13], [14], [15]]. Cracking will not immediately lead to structural failure, thanks to the damage tolerance of continuous-fiber composites; however, extensive microcracking through the SiC/SiC composite can create crack networks that cause loss of hermeticity and release of FP gases to the coolant [5]. The loss of FP gas containment is considered a failure for an LWR fuel. The failure rate of the current state-of-the-art zirconium alloy cladded fuels is on the order of parts-per-million.

Based upon extensive studies previously conducted on unirradiated and irradiated properties [6,9,16,17], this potential failure mode, which is unique to SiC-cladded fuels, has been predicted using a multi-physics computational model [10,18]. To validate the model prediction experimentally and to examine the impact of complex micro-/meso-structures in the real SiC/SiC composites, a series of unique reactor irradiation experiments were recently designed and conducted in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) [19]. The early results from this experiment indicated the first evidence for differential swelling-induced cracking in SiC/SiC composites under LWR-relevant cladding operating conditions [17]. In this study, the authors obtained a more complete picture of these microcracking behaviors using X-ray computed tomography (XCT) to gain a detailed understanding of the phenomena and explore the effects of cladding design, fiber architecture, and manufacturing imperfections.