Local failure might include local subchannel blockages, defected fuel, or the failure of a pin that inadvertently included more highly enriched fuel than the design called for. Section 4.4 has already discussed such accidents and their possible consequences, including the possibility of a propagation of that failure to other pins and other assemblies.

The previous discussion showed that a propagation of failure from a case that resulted only in the release of some fission gases was very unlikely and even when molten fuel was involved, current analysis seemed to show that failure propagation did not occur. The most probable local defect was a subchannel blockage due to a failure of the wire wrap or grid supports or possibly from crud deposition. Such a case is the subject of much analysis and experiment aimed at determining the possible course of events and at showing that such a failure would be localized within a single assembly.

Propagation of a failure in which only gas release occurs is not considered a credible CDA initiator. A failure following a very low probability local blockage is also not expected to give rise to more than localized core damage, but this has not yet been conclusively demonstrated. No such failure propagation has ever been observed; nevertheless the accident should be retained as a low ranking candidate CDA initiator for further consideration.