The Enrico Fermi reactor suffered a complete blockage of two sub­assemblies as a result of a piece of structure being forced free by the coolant flow. The consequences were a melt-down of those subassemblies with some damage to surrounding subassemblies which were only partially blocked. There was no evidence that the failure had spread, although the accident occurred at low power and in an area of low power rating (15a, b).

A good lesson has been learned from this accident and now all LMFBR subassemblies are specifically designed against such blockage. The EBR-II nozzle design is shown in Fig. 5.7 to include side entry paths for the coolant. Most designs now include both vertical and horizontal entry paths. The only possibility for blockage could be a slow build-up of crud due to coolant contamination. Such a slow build-up could be detected either as a local flow reduction in the subassembly or by resulting higher outlet temperatures long before the blockage reached dangerous proportions. However the usual quality assurance programs also ensure that crud is of negligible quantity in the circuit. Thus a total subassembly flow blockage is not a credible CDA initiator.