The inner and outer pyrolytic carbon layers both shrink and creep during irradiation. A portion of the gas pressure (30 MPa for a power of 150 mW/particle) is transmitted through the IPyC layer into the SiC. This pressure continually increases as irradiation of the particle continue, thereby contributing to a tensile stress in the SiC layer. However, countering the effect of the pressure load is the shrinkage of the IPyC and OPyC during irradiation, which pulls the SiC inward. Failure of the particle is normally expected to occur if the stress in the SiC layer reaches the fracture strength of SiC. The pressure induced failure can be reduce by providing an ample margin in the thickness of the buffer layer and by changing the fuel kernel from UO2 to UCO, which would produce a decrease in the amount of CO produced (the main gas responsible for the internal pressure). Pyrolytic carbon cracking can be eliminated by reducing the anisotropy of the PyC layer.
Chemical interaction of the SiC coating layer with fission products is another possible performance limitation of the TRISO-coated fuel particle. Previous irradiation experiments indicate that fission products of palladium and lanthanides react with the SiC layer. Corrosion of the SiC layer could lead to fracture of the coating layers or provide a localised fast diffusion path, which degrades the capability of retaining fission products within the particle. The corrosion of SiC by the fission product palladium has been observed in almost all kinds of fuel compositions, and is considered as one of the key factors influencing the fuel performance. To avoid the degradation of the coating layers caused by extensive corrosion of the SiC layer, it is necessary to limit the fuel temperature, irradiation time or increase the thickness of the SiC layer. These limitations narrow the range of operation conditions of the HTRs.
Although not a failure mechanism, the migration and release of silver (produced by the decay of Uranium) is considered an important issue since it determines the plan maintainability and service requirements. Silver can migrate through intact particles and be released into the reactor coolant system, where it will deposit on cold surfaces. Even though the release mechanism is not very well understood, it has been demonstrated that silver diffuses through the grain boundaries in SiC (Fig. 1)