The previous sections have discussed failure criteria from the point of view of anticipated failure mechanisms arising from faults. We also need to know how fuel pins may survive under normal conditions and so cladding survival criteria can be defined in terms of the following.

(a) Once-off conditions due to operational transients which might result in: overstress due to internal loadings; overtemperatures due to high power- to-flow conditions; and defect failure which is exhibited as an aggravation of other modes of failure. These conditions all arise from minor accidents or transient overshoots such as those discussed in Chapter 2.

(b) Cyclic conditions due to repeated minor loadings: stress cycles; temperature cycles due to load changes; external loadings, possibly from flow disturbances; and vibrational fatigue from flow induced vibrations.

(c) Continuous adverse conditions due to normal operation: cladding erosion; cladding corrosion; and internal loadings due to fuel swelling and fission gas pressure.

In these cases the cladding would have a failure because of a reduction in allowable cladding strain before yield. This might, in combination, reduce the failure limits discussed in Section 3.1.1.