A place for composites in tomorrow's reactors

One of the qualities of silicon-carbide composites is their heat resistance, which could make them compatible for use in the fuel cladding of future reactors. Carbon oxidizes and deteriorates above 500°C, so it is replaced by silicon carbide (SiC) in composite materials. SiC/SiCf composites are more expensive to produce than carbon fibers, so their use is restricted to specific applications involving very high temperatures and oxidizing environments, such as the jet nozzles of certain fighter aircraft or the tiles making up the thermal protection system of the US space shuttle.

Inventing materials

SiC/SiCf composites have yet to find an application in the unique environment of a nuclear reactor. As temperatures in today’s reactors do not exceed 300°C, metals can be used for the most exposed mechanical structures. In the future, however, some reactors, like the gas-cooled, fast neutron reactor, will have to operate at much higher temperatures, in the region of 500 to 850°C. Materials used in these reactors will have to withstand mechanical forces, very high temperatures and a particular type of irradiation from fast neutrons. This is why new reactor designers now see SiC/SiCf composites as a potential solution for fuel cladding, which is the first barrier against nuclear reaction products.

Thermomechanical protection

Bulk silicon carbide is a ceramic material which, though heat resistant, is not very shockproof. Fibers can be assembled to strengthen the material. In this process, the fibers, each of which has a maximum diameter of one hundredth of a millimeter, are compacted together to form strands half a millimeter in diameter. These strands are woven, then embedded in an SiC “matrix”. How does the material respond to impacts? A crack runs through the matrix until it comes up against a kind of shield surrounding each fiber. The pyrocarbon shield is ten thousandths of a millimeter thick and serves to divert the crack along the fiber rather than allowing it through. This means that cracking may deform the material, but it will not break it. Although this type of composite material can provide thermomechanical protection, in no way can it guarantee the leaktightness required for nuclear fuel cladding. The only way to overcome this problem is to develop a liner to be used in conjunction with the material.

Radiation resistance studies

Composites have passed mechanical qualification tests under all kinds of temperature and chemical environment conditions. Researchers at the Nuclear Energy Division are conducting a series of studies on these materials, consisting of irradiation experiments, modeling, and technological research. A great deal of modeling work remains to be done. The physicists have adopted a pragmatic approach. They begin by characterizing a strand under experimental conditions that are as similar as possible to those found in the reactor. Then they input the data obtained into a model describing the interaction between the woven strands and the coupling between the woven strands and the matrix. The experiment, the first phase of which began in October 2008 and ends in December 2009, involves irradiating a sample of composite material in the Osiris experimental reactor and taking an in situ measurement of strand deformation under stress. Another CEA’s laboratory has just acquired equipment for manufacturing SiC/SiCf composites. The feedback from the irradiation tests, along with numerical simulation results, will be used to optimize the material qualification process. Companies working in the sector will be able to manufacture these materials once the technology is stabilized.

S. Astorg – Le Journal de Saclay N°43 – April 2009