05000318/LER-2010-002

LER-2010-002, Reactor Coolant System Pressure Boundary Leakage in Valve Leakoff Line Weld

Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. 05000
Event date:	02-23-2010
Report date:	04-22-2010
Reporting criterion:	10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded
Initial Reporting
ENS 45719	10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded
3182010002R00 - NRC Website

A. PRE-EVENT PLANT CONDITIONS

Unit 2 was in Mode 5 (Cold Shutdown) when the condition was discovered.

B. EVENT

On February 23, 2010, while Unit 2 was in Mode 5 during a forced outage, it was verified that the pinhole previously identified at the socket weld attaching the packing leakoff line (auxiliary connection) to the bonnet (stem retaining area) of valve 2HVRC-220 (pressurizer spray bypass valve) constituted a Reactor Coolant System (RCS) pressure boundary leak. The valve is a three-quarter inch manual globe valve that is used to throttle the amount of bypass flow around the pressurizer spray valve.

The Technical Specifications limit RCS pressure boundary leakage to zero. The leak was originally identified (presence of boric acid) during the Mode 3 boric acid walkdown on February 18, 2010. The packing flange bolts and the body-to-bonnet bolts were tightened to stop the leakage from those areas. However, on February 21, 2010 further inspection identified a pinhole in the valve packing leakoff line. On February 23, 2010 it was verified that due to a previous modification, the leakoff line had been relocated from downstream of the valve packing to upstream of the valve packing. Therefore, per Technical Specification definition, the pinhole leak constituted an RCS pressure boundary leak.

Based on visual inspection performed during the post-trip boric acid walkdown, the leak most likely existed during plant operation. A progressive non-destructive examination (NDE) of the pinhole leak site was performed to further characterize the flaw in the socket weld as it was ground out for repair. That NDE concluded that the flaw was a, single pore through the socket weld that caused the leak. Based on a review of the maintenance history and discussions with experienced Calvert Cliffs' personnel, the apparent cause of the pinhole was a latent weld defect created during the original valve manufacturing process. The weld was repaired and inspected satisfactorily on February 24, 2010, prior to startup from the Unit 2 forced outage.

TO THE EVENT

There were no inoperable structures, components, or systems that contributed to the event.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES

� 02/18/2010 During Mode 3 boric acid walkdown per the boric acid corrosion control program, an active boric acid leak was identified at valve 2HVRC-220.

02/19/2010 Valve cleaned off and leak determined to be packing leak and body-to-bonnet leak. Both sets of studs were tightened.

Visual inspection during the Mode 3 boric acid walkdown identified an active boric acid leak at the valve area. The pressure boundary leak was verified after the area was cleaned revealing the pinhole. The pinhole was located in the socket weld attaching a capped leakoff line (one­ half inch pipe) to the bonnet of the valve (three-quarter inch globe valve).

G. MAJOR OPERATOR ACTION

On February 21, 2010 when the actual source of the leak was determined to be a pinhole, Unit 2 was taken from Mode 3 to Mode 5 and the RCS was depressurized to allow repairs.

After the repairs were completed, Unit 2 was taken back to Mode 3 and a normal operating pressure ISI walkdown verified that the weld was leak tight.

H. SAFETY SYSTEM RESPONSES

No safety system responses were expected. None occurred.

II.CCAUSE OF EVENT The event is documented in station condition report number CR-2010-001591. Based on boric acid walkdown visual inspection data, the leak most likely existed during plant operation. A progressive NDE of the pinhole leak site was performed to further characterize the flaw in the socket weld as it was ground out for repair. That NDE concluded that the flaw was a single pore through the socket weld that caused the leak. Based on a review of the maintenance history and discussions with experienced Calvert Cliffs' personnel, the apparent cause of the pinhole was a latent weld defect created during the original valve manufacturing process. The weld was forced outage.

NRC FORM 366A� U.S. NUCLEAR REGULATORY COMMISSION (9-2007) I REV

III. ANALYSIS OF EVENT

The Technical Specifications for RCS operational leakage (Technical Specification 3.4.13.a) limits pressure boundary leakage to zero. If any RCS operational pressure boundary leakage exists, the Technical Specifications require the operating Unit to be in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and to be in Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. Based on review of boric acid walkdown data the leak most likely existed during operation for a time longer than allowed by the Technical Specification. Therefore, this condition is reportable pursuant to 10 CFR 50.73(a) (2) (i) (B).

As stated earlier, the failure was most likely due to a latent weld defect created during the original valve manufacturing process. Since the failure may have occurred due to a material problem that resulted in abnormal degradation of a principal safety barrier (i.e., it was necessary to take corrective actions to restore the barrier's capability), this event is also reportable pursuant to 10 CFR 50.73(a)(2)(ii)(A).

This event did not result in any actual nuclear safety consequences. The RCS pressure boundary leakage was contained within the Containment Building and did not interface with any other system. Unidentified RCS pressure boundary leakage remained within Technical Specification limits during the time the pressure boundary leak may have existed (i.e., during the period of applicability after startup from the spring 2009 Refueling Outage, until the leak was repaired in February 2010, approximately one year maximum). No abnormal containment radiation levels were observed during the period.

From a qualitative perspective, there was no significant increase in core damage frequency or large early release frequency due to the RCS pressure boundary leak. The leak was extremely small for the duration that the condition existed. As such, there is no indication that the rupture likelihood of the leak should be increased. If the leakage rate had increased, then it is likely that a plant shutdown would ensue prior to the leakage reaching the level where the event would be considered a very small loss of cooling accident exceeding charging pump capacity. The conditional core damage probability of a forced shutdown under these conditions would be less than 1E-06 and conditional large early release probability less than 1E-07. There was a small likelihood that this leak would have significantly increased and caused this forced shutdown.

Thus, the overall core damage frequency and large early release frequency impact was well below 1E-06 and 1E-07, respectively.

IV. CORRECTIVE ACTIONS

A.� ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL

STATUS

The weld was repaired. Visual inspection and surface examination (dye penetrant test) were performed satisfactorily. Post-maintenance test during Mode 3 at normal operating pressure was performed satisfactorily.

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (9-2007)

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE

The apparent cause of the pinhole leak was a latent weld defect created during the original valve manufacturing process. Three similar valves purchased under the same purchase order as 2HVRC-220 are currently installed in the plant (two in Unit 1 and one in Unit 2). Active boric acid leaks do not exist at these valves. However, nondestructive examination of the leakoff line­ to-bonnet socket weld on those valves is planned to be performed in future outages.

A. FAILED COMPONENTS

2HVRC-220 pressurizer spray bypass valve.

B. PREVIOUS LERs ON SIMILAR EVENTS

A review of Calvert Cliffs' events over the past several years was performed. No previous Calvert Cliffs occurrences were identified involving an RCS pressure boundary leak due to failure of a valve leakoff line weld.

C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EMS) COMPONENT

REFERRED TO IN THIS LER:

IEEE 803 IEEE 805 Component Function Identifier System Identification 2HVRC-220 Pressurizer spray bypass valve V AB