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Q: "Won't ceramic cladding be more brittle than metal cladding?"

A: SiC/SiC composites are damage tolerant; that is, they are capable of accommodating a high degree of deformation because of the crack arrest phenomena driven by the interface between fiber and matrix. SiC/SiC composites are one of the leading candidates for fusion reactor blanket materials.

Zircalloy cladding becomes very brittle when oxidized. The following image shows the results of load testing two types of zircalloy cladding after various amounts of oxidation. The current allowable level of oxidation is 17% which occurs very quickly in accident conditions. That is a primary reason for current reactor power limits.

Q: �What about the lower thermal conductivity of the ceramic materials?�

A: While SiC composite cladding does have a lower thermal conductivity than zirconium alloys and the thermal conductivity will decrease further with irradiation and temperature, there are many more contributors to heat transfer in a fuel rod than just the cladding. For example, oxidation of the outer surface of zirconium based cladding can significantly reduce the effective thermal conductivity of the system. This phenomenon will not occur with SiC composite cladding.

Fuel rod designs utilizing SiC composite cladding will need to take advantage of its favorable characteristics while minimizing the impact of its less favorable characteristics. There will not likely be a one-to-one substitution of the new cladding for the old.

Q: �What are the high temperature creep characteristics of SiC cladding?�

A: Irradiation enhanced creep rates at 1000�C are very low. Thermal creep rate at reactor operating conditions is effectively zero. Therefore, the SiC composite cladding can withstand much higher internal pressures without the "ballooning" phenomenon which occurs in zircalloy cladding if operated with internal pressure above primary system pressure.

Q: "Won't fission products leak out because a composite is porous?"

A: CTP's unique approach is to use a monolithic internal layer that is gas tight to prevent any fission product release during normal and operating conditions. This inner layer provides both the gas retention and most of the pressure containment capability of the cladding. The outer composite layer reinforces the inner layer, carrying up to about 30% of the pressure load and allows the cladding to fail gracefully in severe accident conditions out to high strains while maintaining the fuel in a coolable geometry.

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