Silicon carbide (SiC) fiber–reinforced SiC matrix (SiC/SiC) composites continue to undergo development for fission applications worldwide because of the inherent advantages of the material, including low activation, high-temperature capability, relatively low neutron absorption, and radiation resistance. SiC/SiC composites are being considered for use in current light water reactors (LWRs), most Gen IV reactor concepts, and future-generation nuclear power reactors. Recent research highlighted that SiC/SiC composites showed limited mechanical degradation following neutron irradiation at 800°C to 70 dpa. On the other hand, recent research has also found degradation phenomena: irradiation at 320°C to 92 dpa significantly degraded the strength of a specific SiC/SiC composite—a chemical vapor–infiltrated (CVI) SiC matrix composite consisting of Hi-Nicalon Type-S (HNS) SiC fibers coated with an SiC/PyC (pyrolytic carbon) multilayer interphase. Such degradation is associated with interphase damage.

To improve the irradiation resistance of SiC/SiC composites at an LWR temperature of ~300°C, SiC/SiC composites with a modified interphase have been neutron-irradiated to 12 dpa and tested. The mechanical tests revealed that there was no notable irradiation effect on the strength of the material. The proposed work will extend the post-irradiation examination activities to a higher neutron dose.

The focus of this proposal is to test CVI SiC/SiC composites reinforced with single-layer PyC-coated HNS fibers—which are currently being explored for LWR cladding and channel box applications—following neutron irradiation at ~300°C to 30 dpa. Five composite specimens will be evaluated. The irradiation resistance of the composites will be investigated based on the dynamic Young’s modulus, flexural behavior, and fracture appearance. The dynamic Young’s moduli of the SiC composites will be determined using the impulse excitation of vibration method. Four-point flexural tests using a 4-point-1/4-point fixture will be conducted. Analysis of the flexural behavior will provide the proportional limit stress, ultimate flexural strength, and apparent strain of failure. The fracture surfaces will be characterized by scanning electron microscopy. In addition, two SiC passive temperature monitors will be evaluated using a dilatometer to investigate actual irradiation temperatures. The samples are currently in the LAMDA facility at Oak Ridge National Laboratory and ready for testing. Each experiment in the LAMDA laboratory will take up to 2 days (8 days total), without analysis.

This work will provide critical experimental data on how high-dose neutron irradiation at LWR-relevant temperatures affects the mechanical properties of recent nuclear-grade SiC composites—a topic that has not been explored previously. Comparisons of irradiation resistance among SiC/SiC composites have used previous studies; this study will show how modification of the interphase affects the irradiation resistance and consequently will guide better design of composites for use in high-dose radiation environments. It is anticipated that the duration of the testing will be up to 4 months.

Silicon carbide (SiC) fiber–reinforced SiC matrix (SiC/SiC) composites continue to undergo development for fission applications worldwide because of the inherent advantages of the material, including low activation, accident resistance, relatively low neutron absorption, and radiation resistance. SiC/SiC composites are considered for use in accident-tolerant core structures of current light water reactors (LWRs) and in components of most Gen IV reactors and future-generation nuclear power reactors. Therefore, successful development of SiC/SiC composites will support the DOE Office of Nuclear Energy mission to develop technologies that can improve the reliability and safety of current reactors and new reactors.

The proposed work will evaluate SiC/SiC composites neutron-irradiated at an LWR-relevant temperature of 300°C to 30 dpa. High-dose irradiation is an important topic because possible core structures such as boiling water reactor channel boxes made of SiC/SiC composites will offer a competitive advantage if it is demonstrated that the core components can last substantially longer in their operating environments. However, high-dose irradiation effects have been insufficiently investigated because of the limited opportunities for neutron radiation. The outcomes from this project will improve understanding of the high-dose irradiation resistance of recently developed nuclear-grade SiC/SiC composites at relatively low temperatures and consequently guide improvements in the design of composites (especially for interphase structures) and lead to longer lifetimes for current and future reactors.