05000483/LER-2015-001

LER-2015-001, Completion of a Shutdown Required by the Technical Specifications - TS 3.4.13

Callaway Plant Unit 1
Event date:	07-23-2015
Report date:	12-02-2016
Reporting criterion:	10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown
Initial Reporting
ENS 51253	10 CFR 50.72(b)(2)(xi), Notification to Government Agency or News Release, 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown
LER closed by
IR 05000483/2016004 (25 January 2017)
4832015001R01 - NRC Website

LER 15-001-01 for Callaway Plant, Unit 1, Regarding Completion of a Shutdown Required by the Technical Specifications - TS 3.4.13

ML16340A730
Person / Time
Site:	Callaway
Issue date:	12/02/2016
From:	Cox B L Union Electric Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
ULNRC-06338 LER 15-001-01
Download: ML16340A730 (8)

1. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):

BBV0400 [EIIS code V] is a drain valve in the Auxiliary Spray line from the Chemical Volume and Control System (CVCS) [EIIS system code CB] to the pressurizer in the Reactor Coolant System [EIIS system code AB]. The valve is an American Society of Mechanical Engineers (ASME) Code Section III Class 2, 3/4-inch welded globe valve. The BBV0400 drain line connection to the Auxiliary Spray header contains a 3/8-inch orifice [EIIS code OR] (upstream of the BBV0400 valve) that serves as the designed Safety Class break between the ASME Class 1 and Class 2 portions of this piping. A non-ASME Class 1, 2, or 3 blind flange is located downstream of valve BBV0400.

The 3/8-inch orifice is a design feature for mitigation of a line break that may be postulated to occur in the Class 2 piping upstream of the drain valve. Specifically, the orifice enables the normal makeup system to the Reactor Coolant System (i.e., the CVCS) to be capable of providing makeup flow while maintaining pressurizer level by limiting the maximum break flow to less than the available charging flow.

2. INITIAL PLANT CONDITIONS:

The plant was in MODE 1 at approximately 100 percent power on July 22, 2015.

3. EVENT DESCRIPTION:

On July 22, 2015, Callaway Plant was operating normally in Mode 1, at approximately 100% reactor power. At 2109, the particulate channel on a containment radiation monitor alarmed in Alert status. This was followed shortly with an alarm on another containment radiation monitor.

A ten-minute water inventory balance of the Reactor Coolant System (RCS) was performed and it identified a 1.2 gallon per minute (gpm) leak. Technical Specification (TS) 3.4.13 Condition A was entered due to unidentified RCS leakage greater than 1.0 gpm.

Shortly after midnight on July 23, 2015, a containment entry was made. A steam cloud was identified to be coming from the Pressurizer Spray Valve cubicle. Technical Specification (TS) 3.4.13 Condition B was entered at 0053 based on the inability to rule out pressure boundary leakage. Per TS 3.4.13 Condition B, shut down of the plant due to the inability to rule out pressure boundary leakage was commenced.

A plant load reduction was started at 0115. This was the initiation of a shutdown required by the TS. The NRC was notified of the shutdown initiation via Event Notification 51253. MODE 2 was entered at 0513. MODE 3 was entered at 0600. This was completion of a shutdown required by the TS.

Cooldown of the RCS was initiated at 1210. During the cooldown, the B Main Feedwater Pump was tripped. This led to a manual Auxiliary Feedwater initiation which is discussed in LER 2015- 002-00.

At an RCS pressure of approximately 950 psig, a containment entry was made. The leak location was determined to be at the flange downstream of valve BBV0400, CVCS Aux Spray Drain Valve. At this time it was determined that the leakage was unidentified leakage, not RCS pressure boundary leakage. BBV0400 was closed one quarter turn and leakage reduced to approximately 60 drops per minute which is less than the 1 gpm TS limit for unidentified leakage. TS 3.4.13 Conditions B and A were exited at 1725.

The flange downstream of valve BBV0400 was removed, and its Gore-Tex gasket was removed.

As noted above, the leakage past BBV0400 valve seat was observed to be 60 drops per minute. A Flexitallic gasket was placed in the flange, and the flange was bolted and tightened to approximately 150 ft-lbs. Later, a RCS heat-up was commenced. At normal operating temperature and pressure, no leakage was observed at BBV0400 valve and flange. The reactor and secondary plant startup was continued.

4. ASSESSMENT OF SAFETY CONSEQUENCES:

As mentioned above, the BBV0400 drain line connection to the Auxiliary Spray header contains a 3/8" orifice that serves as the designed Safety Class break between the ASME Class 1 and Class 2 portions of the piping. This 3/8" orifice is a design feature that enables the normal makeup system to be capable of providing makeup flow while maintaining pressurizer level by limiting the maximum break flow to less than the available charging flow. Although the leak was determined to exceed the TS limit for unidentified leakage, the leak was well within the capacity of a charging pump, as it was much less than the flow that would result from a postulated break in the Class 2 drain line. Thus, there was no significant safety impact on the plant.

In terms of a Probabilistic Risk Assessment (PRA), the leak was classified as a Very Small LOCA. The associated success criteria included charging and secondary or alternate core cooling with RHR in the injection mode, but it did not require RHR recirculation.

The incremental change in core damage frequency was determined to be less than 1E-6; therefore, this event was of very low risk significance.

Use of the PRA model to evaluate the event provides for a comprehensive, quantitative assessment of the potential safety consequences and implications of the event, including consideration of alternative conditions beyond those analyzed in the FSAR.

5. REPORTING REQUIREMENTS:

This LER is submitted pursuant to 10 CFR 50.73(a)(2)(i)(A) to report a completion of a shutdown required by the Technical Specifications. The event, i.e., initiation of a shutdown required by the Technical Specifications due to the leak from the piping around valve BBV0400, was reported to the NRC via Event Notification51253 on July 23, 2015.

The manual actuation of the auxiliary feedwater system was reported in Callaway Plant LER 2015-002-00.

6. CAUSE OF THE EVENT:

The cause of the plant shutdown was an RCS leak in excess of the limit specified in the Technical Specifications. It was determined that the leak was due to seat leakage through the RCS Pressurizer CVCS Auxiliary Spray Supply Drain valve BBV0400 and then through the non-safety related pipe flange immediately downstream of the valve. This leakage ultimately placed the plant in Condition B of TS 3.4.13, requiring the plant to be in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> per Required Action B.1.

The valve at location BBV0400 was removed during Refuel Outage 21 (RF21) and replaced with a new valve. The removed valve was subjected to forensic analyses. The root cause of the leak was determined to be that valve BBV0400 was not fully closed at normal closing force in RF20. Contributing to this cause was a manufacturing defect that prevented the valve from being fully closed at hand-tight torque when it was closed in RF20. The defect was in the bevel machining on the valve disc and seat.

Additionally, the gasket in the flange downstream of valve BBV0400 failed.

7. CORRECTIVE ACTIONS:

After identification of the leakage, the leakage past the seat of valve BBV0400 was reduced to approximately 60 drops/minute which is below the TS limit for unidentified leakage of 1 gallon per minute. The valve was subsequently replaced in April 2016 during Refueling Outage RF21.

Additionally, a plant procedure was revised to add valve BBV00400 to the list of selected that valves are closed in MODE 3 using normal force or additional force if leakage is identified.

The Gore-Tex gasket in the flange downstream of valve BBV0400 was replaced with a flexitallic gasket.

8. PREVIOUS SIMILAR EVENTS:

A review of Callaway LERs for the past three years did not find any in which Callaway reported completion of a shutdown required by the Technical Specifications.

In July 2013, Callaway submitted LER 2013-006-00 for RCS leakage from a crack in a socket weld located where a 3/4-inch vent valve is connected to B train injection piping which in turn is connected to a cold leg of the RCS. The estimated leakage rate through the crack was 6 drops per minute.

The plant's corrective action system also documents an event from November 2014 where a sample from a containment radiation monitor showed indication of a possible RCS leak. A socket weld downstream of the CVCS Letdown Orifice B Outlet Throttle Valve developed a leak which required isolation of normal letdown and placing excess letdown in service.