Junk Cook Plant Engineering Assumption

God help us all if all their engineering assumption are as inaccurate as this. The industry is riddle with not conservative engineering assumptions.

Donald C. Cook Nuclear Plant Unit 2

LICENSEE EVENT REPORT 316/2016-001-01

Manual Reactor Trip Due to Moisture Separator Heater Expansion Joint Failure

ROOT CAUSE

The Root Cause was determined to be an organizational failure to recognize the risk significance of, and to adequately correct or mitigate, previously identified vibration issues with the Unit 2 B Right MSR crossover expansion joint tie rod and bellows in a timely fashion.

CONTRIBUTING CAUSES

Tack welds were inappropriately applied to nuts on the tie rod areas under tensile load. This was inconsistent with the intent of the Engineering Change which installed the threaded tie rods and was the result of poor communication between engineering workgroups. These tack welds on ASTM A 193 B7 material created stress risers which accelerated tie rod high cycle fatigue failure.

The B crossover lines are subject to higher levels of vibration than the A and C crossover lines. The vibration amplitudes produced stresses in the B tie rods that led to the tie rod high cycle fatigue failure.

The B crossover line vibration issue was removed from the plant high priority list following U2C22 before the vibration condition was resolved. Unit 2 returned to service and operated for several months with elevated vibration levels on the MSR B crossover lines before vibration amplitudes were addressed. Since the issue was no longer on the plant high priority list, the issue was no longer a station priority and workgroups lost focus on the issue.