An indication of a small change in reactor period during start-up was ignored by the operator because it occurred at 3.00 p. m. and he had previous­ly noticed that considerable line noise occurred at this time of the day from external power supplies, and he associated the signal on the recorder with this.

There was difficulty in determining the reactivity state of the system, or what 6k had been changed during the start-up, because a staggered rod movement system was used in order to stay on a steep reactivity slope. There was no automatic computing equipment to calculate the ensuing reactivity changes.

No high temperatures were observed to indicate that sodium boiling might be occurring. This may have been because flow through those failed assemblies was very small and insufficient to transport high temperature indications from the assembly. There were not sufficient thermocouples and they were not reliable enough to give confident indications of over heating. In addition, previously “hot” assemblies had been moved and were still apparently running a little hot.

The subassemblies were eminently blockable, having simple flat orifice inlets. The inlet design has now been changed to include cruciform pro­jections. The material to block the subassemblies was available. The last — minute design changes had not been properly documented, and diagnosis was difficult because the zirconium liners were not included on the master drawings.

There had been no acoustic “fingerprints” taken prior to the failure, and therefore acoustic detection methods could not be used for diagnosis. They could not be compared to normal steady-state and unfailed signals. There was inadequate operator monitoring analysis. However, this has now been remedied by including a computer in the diagnostic system although it is not yet integrated into the protective system.

These were the contributors to the accident which, in themselves, were all relatively innocuous but, in combination, could have a damaging po­tential. The value of such an incident is that the postmortem provides considerable input of data into safety engineering methods.