05000318/LER-2013-005

LER-2013-005, Manufacturing Defect Results in Shutdown Due to Dropped Control Element Assembly

Calvert Cliffs Nuclear Power Plant, Unit 2
Event date:	09-05-2013
Report date:	10-25-2013
Reporting criterion:	10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown
3182013005R00 - NRC Website

A. PRE-EVENT PLANT CONDITIONS:

On September 5, 2013 Unit 2 was operating at 99.5 percent power. There were no inoperable systems, structures, or components that would have impacted this event.

B. EVENT:

At 1016 on September 5, 2013, Unit 2 Control Element Assembly (CEA) #27 dropped to the bottom of the reactor core due to its 240 Volt circuit breaker tripping open while the CEA was being operated during quarterly CEA surveillance testing. Following the dropped CEA, reactor power stabilized at 91 percent. Operators entered Technical Specification 3.1.4, Action B for CEA #27 being misaligned from its group by greater than 15 inches. When CEA #27 could not be realigned within the required Completion Time, operators entered Technical Specification 3.1.4, Action C, and reduced power to 70 percent. When CEA #27 was not able to be realigned within the required Completion Time of Action C operators entered Technical Specification 3.1.4, Action F and commenced a required unit shutdown to Mode 3. Operators commenced the shutdown of Unit 2 at 1404 on September 5, 2013. Unit 2 entered Mode 3, Hot Standby, at 1735 on September 5, 2013.

Initial investigation into the cause of CEA #27 dropped rod identified its Control Element Drive Mechanism (CEDM) lift coil wire was grounded internally to the coil housing due to a chafed wire. During the resulting forced outage, CEDM #27 coil stack was replaced. Unit 2's forced outage ended at 0808 on September 10, 2013 and the unit returned to full power at 2130 on September 10, 2013.

TO THE EVENT:

The ground on CEDM #27 lift coil resulted in CEA #27 dropping to the bottom of the reactor core. When this issue could not be corrected within the Required Action Completion Times for as required by Technical Specification Required Action 3.1.4.F.1.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

September 5, 2013 1016 — CEA #27 dropped to its fully inserted position while CEA #27 was undergoing rod motion during a quarterly surveillance testing. Operators implemented Abnormal Operating Procedure-1B, CEA Malfunction. Reactor power stabilized at 91 percent.

1207 — Commenced reducing reactor power to 70 percent in accordance with Technical Specification Required Action 3.1.4.C.1.

, .,.

Completion Times required by the Technical Specifications, operators placed the unit in Mode 3.

Operators subsequently placed the unit in Mode 5 to facilitate troubleshooting and repairs.

Following replacement of CEDM #27 coil stack, operators began unit startup and returned the unit to full power on September 10, 2013.

H. SAFETY SYSTEM RESPONSES:

There were no demands for safety system actuations during this event.

circumferential displacement of the coil within the coil housing and the misalignment of the lift coil lead wire within the coil housing nipple. A contributing cause was a design flaw that left the coil stack lead wires susceptible to grounding within the coil housing due to; a) rubbing/chafing of the wire insulation due to service stress, and b) hardening of the coil lead wire insulation over time due to thermal and radiation effects. The root cause in combination with the contributing cause allowed the lift coil lead wire insulation to degrade which resulted over time in the lead wire contacting the coil housing nipple thus causing an arc to ground at the contact point.

III. ANALYSIS OF EVENT:

This event is reportable in accordance with:

10 CFR 50.73(a)(2)(i)(A) — "The completion of any nuclear plant shutdown required by the plant's Technical Specifications;" Operators completed the Technical Specification required shutdown on September 5, 2013 when efforts to realign the CEA to its proper position were not able to be completed within the required Technical Specification Completion Time. This means the event is reportable under 10 CFR 50.73(a)(2)(i)(A).

This event did not result in any actual nuclear safety consequences. While the failure of CEDM #27 lift coil prevented the CEA from being positioned, it did not prevent the safety function of the CEDM (when a reactor trip signal is initiated the CEDM grippers disengage, allowing the CEA to fall to the bottom of the reactor core due to gravitational forces) from occurring as designed. Additionally the dropped CEA event is analyzed in Section 14.11 of the Calvert Cliffs' Updated Final Safety Analysis Report. The most limiting drop event is an uncontrolled CEA insertion at hot full power. This current event is therefore bounded by the Section 14.11 safety analysis.

The risk associated with the CEA drop event is considered to have a relatively low impact.

While the probabilistic risk assessment does not model the failure likelihood of a single dropped CEA, the failure could be a precursor to a plant trip. A plant trip has a Conditional Core Damage Probability of less than 1E-06 and a Conditional Large Release Probability of less than 1E-07. A dropped CEA is considered to be within the bounds of a plant trip. As a result, the risk associated with this event is considered small and is not significant from a probabilistic risk assessment perspective.

The event has limited impact on the Nuclear Regulatory Commission's Reactor Oversight Process Performance Indicators for Unit 2. The performance indicator for Unplanned Power Change per 7000 Critical hours rose to 0.9 which remains within the green band.

due to an internal short circuit caused by overheating due to degradation in the CEDM cooling system. This event is not similar to the Unit 2 CEA #27 failure because it involved a short circuit CEDM coil stacks were replaced in 2006 with the design change that installed protective heat shrink wrap along the length of where the coil stack lead wires penetrate the coil housing nipple.

Condition Report IRE-021-712 — During the Unit 2 2007 refueling outage, technicians, while reconnecting CEDMs, identified that CEDM #63 lift coil indicated a grounded condition. The ground occurred where the leads from the lift coil protruded from the coil itself. The most likely cause of the ground was the physical repositioning of the coil to the new reactor vessel head which caused the insulation to be cut and resulted in the grounded condition.

C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EllS) COMPONENT

COMPONENT

IEEE 803 IEEE 805

SYSTEM ID

CEDM Lift Coil

D. SPECIAL COMMENTS:

None.