a. Sodium fire. During maintenance a pipe had been plugged at two places and cut. The plugs were being kept frozen by the use of two fans, but a welder switched off one fan that was obstructing his welding operation. The plug then melted and released sodium from the open end of the pipe.

A contributor to the incident was the fact that the rest of the system was pressurized and therefore the sodium emerged under pressure. A fire ensued and a considerable clean-up job resulted.

b. Fuel handling drop. One step in the fuel handling procedure from the core to an in-vessel storage location was designed to give the operator a better feel for what was going on. Thus the only nonautomated part of the handling process was one designed to give the operator some psychological control.

The hold-down device spreads six adjacent assemblies, while a grapple engages the top knob of the subassembly. The operator then, remotely, engages a lifting arm (see Fig. 4.40) below the knob and closes a pin to lock-in the knob. He then performs a “wiggle” test to ensure the connection before the assembly is lifted out and transferred. In this particular case the operator did not fully engage the knob and the pin was closed with the knob out. When lifted, the knob rested on the two ends of the grapple and was lifted out of the core, over a well outside the core but inside the vessel and

Knob on assembly head

Correct operation Incorrect operation

Fig. 4.40. Diagram of a fuel grapple failure in the EBR-II fuel handling incident (43).

then it dropped off. No damage occurred, and the assembly was easily retrieved from a previously installed catch basket in this position.

In this case, total automation might have been safer. The operator is not the most reliable of machines, and the probability of failure is unity in most cases.

c. Lost oscillator bearings. Bearings from an oscillator installed to do some response tests were lost from the oscillator and they jammed the control rod helical drives. However, the safety rods were not affected.

There were a number of contributory factors to this incident also. Bad design specified using ball bearings in the temporary piece of equipment (the oscillator) despite the fact that they were purposely not used in any permanent equipment. There was also a loss of quality control. The bearings were not stainless steel even though they were marked as such.