Behavior of triplex silicon carbide fuel cladding designs tested under simulated PWR conditions

[摘要] A silicon carbide (SiC) fuel cladding for LWRs may allow a number of advances, including: increased safety margins under transients and accident scenarios, such as loss of coolant accidents; improved resource utilization via a higher burnup beyond the present limit of 62 GWd/MTU; and improved waste management. The proposed design, referred to as Triplex, consists of three layers: an inner monolith, a central composite, and an outer environmental barrier coating (EBC). The inner monolith consists of dense SiC which provides strength and hermeticity to contain fission products. The composite layer is made of SiC fibers, woven around the monolith, and then infiltrated with a SiC matrix. The composite layer adds strength to the monolith and provides a pseudo-ductile failure mode. The EBC is a thin coating of SiC applied to the outside of the composite to protect it against corrosion. The ends of the tubes may be sealed via the bonding of SiC end caps to the SiC tube. Triplex tube samples, monolith-only samples, and SiC/SiC bonding samples (consisting of two blocks bonded together) were tested in three phases as part of an evaluation of the SiC cladding system. A number of samples were exposed to PWR coolant and neutronic conditions using an incore loop in the MIT research reactor (MITR-II). Other samples remained in their as-fabricated states for comparison. First, mechanical testing revealed significant strength reduction in the Triplex samples due to irradiation-induced point defects, corrosive pitting of the monolith, and possible differences in the behavior of the Triplex components. Some manufacturing abnormalities were also discovered which could have compromised strength. The Triplex samples tested here were not as strong as reported in a previous study. SEM analysis was able to follow the propagation of cracks from initiation, at the monolith inner surface, to termination, upon breaching the EBC. The composite layer was found to be key in dissipating the energy driving the crack formation. Second, three SiC/SiC bonding methods (six samples total) were tested in the MITR-II to 0.2 dpa, and five of the six samples failed. SEM analysis indicates radiation induced degradation of the bond material. Dimensional and volume measurements established the anisotropic swelling of the two SiC blocks in each bond sample, which would have caused shear stresses on the bonds, contributing to their failure. Finally, thermal diffusivity measurements of the Triplex samples show substantial decreases with irradiation (saturating at about 1 dpa) due to the accumulation of phonon-scattering defects and corrosion of SiC. By 1 dpa, the thermal diffusivity/conductivity of this SiC cladding design is diminished to a value lower than that of Zircaloy. In the as-fabricated state, a large difference exists between the monolith-only and Triplex samples due to the phonon scattering centers at the interfaces of the layers. With irradiation this difference decreases, suggesting that similar corrosion and radiation damage effects exist in both the monolith and Triplex samples.

[发布机构] Massachusetts Institute of Technology