05000275/LER-2009-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 03-22-2009 |
|---|---|
| Report date: | 05-19-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 44927 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 2752009001R00 - NRC Website | |
I. Plant Conditions
Unit 1 was in Mode 3 (Hot Standby).at normal operating reactor coolant temperature and pressure in preparation for restart following the fifteenth refueling outage (1R15).
II. Description of Problem
A. Background
The Diablo Canyon Power Plants (DCPP) Units 1 and 2 are Pressurized Water Reactors (PWR) with four Reactor Coolant Loops (RCL)[AB] to circulate reactor coolant to each of the four steam generators (SG)[SG].
Each SG is a vertical U-tube design provided by the Nuclear Steam Supply System (NSSS) vendor, Westinghouse. Due to reaching the end of the useful life of the SG tubes, DCPP has installed Replacement Steam Generators (RSG) with an improved design andmaterial.
The SGs are installed on a support system designed to resist deadweight, thermal expansion, pipe break and seismic loadings while allowing free thermal motion from cold to normal operating conditions. The design and details of the SG support structures are further discussed in Final Safety Analysis Report Update (FSAR), Section 5.5.13.
Four vertical support columns* on each RSG transfer downward and uplift loads from the RSGs to the containment structure. Each support column is connected to the RSG support foot through a column adapter and eight hold-down bolts. A total of sixteen washer plates (some single and some with multiple bolt holes) are installed under the heads of the hold-down bolts to spread the bearing load evenly on the column adapter bearing surface. A critical characteristic of these washer plates is to sit flat against the column adapter. At two locations on RSG 1-3, gaps existed between two of the sixteen washer plates and column adapters such that some hold-down bolts were not effective in resisting design loadings.
B. Event Description
On March 20, 2009, at 13:20 PDT, Unit 1 entered Mode 4 (Hot Shutdown) following 1R15.
On March 21, 2009, at 9:22 PDT, Unit '1 entered Mode 3.
On March 22, 2009, at 13:34, plant operators entered Technical LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Specification (TS) 3.0.3 due to verification that two of the sixteen washer plates on RSG 1-3 were improperly installed.
On March 22, 2009, at 15:36 PDT, plant operators exited TS 3.0.3 following installation of a shim between the RSG 1-3 anchor column adapter and the washer plate of sufficient thickness to clear the weld metal interference at each of the two nonconforming locations.
On March 22, 2009, at 16:00 PDT it was determined that an eight-hour nonemergency report in accordance with 10 CFR 50.72(b)(3)(ii)(B) was required.
On March 22, 2009, at approximately 22:44 PDT plant operators made a nonemergency event notification (EN#44927) in accordance with 10 CFR 50.72(b)(3)(ii)(B).
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event None.
D. Other Systems or Secondary Functions Affected No additional safety systems were adversely affected by this event.
E. Method of Discovery During an exit interview, a contract pipe fitter performing work for the RSG project raised a concern regarding an unacceptable gap that may exist between the RSG column adapter and the washer plates. A plant walk down verified that two out of sixteen washer plates on RSG 1-3 were not seated in the column adapter due to interfering excess weld metal on the interior recesses of the column adapter.
F. Operator Actions Utility licensed plant operators stabilized Unit 1 in Mode 3 at normal operating temperature and pressure.
Plant operators made a nonemergency phone notification (EN#44927) in accordance with 10 CFR 50.72(b)(3)(ii)(B).
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) G.�Safety System Responses No safety systems were required to respond.
III.�Cause of the Problem
A. Immediate Cause
The gaps between the washer plates and column adapters were caused by interfering excess weld metal on the interior recesses of the column adapters and inadequate verification of the washer installation.
B. Root Cause
Limitations associated with the use of a fit-up template to identify excess weld material resulted in insufficient excess weld material removal.
The field installation instructions over-relied upon the preinstallation template fit-up, resulting in inadequate verification requirements.
A human performance error by RSG contract personnel resulted in one washer plate being installed upside down.
IV.�Assessment of Safety Consequences There were no safety consequences as a result of this event.
The Unit 1 reactor was maintained in Mode 3 at stable pressure and temperature during the condition with appropriate equipment available and TS-required equipment operable; thus, any at-power accidents postulated in the FSAR Updated were precluded.
In the unlikely event of a postulated high seismic event an analysis of the as-found RSG 1-3 support system verified that there may have been localized yielding of the connection but failure would not occur. The nonconforming condition could have resulted in limited damage to the hold-down bolts and washer plates at the nonconforming locations, but the RSG supports would have performed their safety function to restrain the RSG and attached piping systems.
Therefore, the event is not to be considered risk significant and it did not adversely affect the health and safety of the public.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) ■TEXT V.�Corrective Actions A. Immediate Corrective Actions Plant operators entered TS 3.0.3 and maintained Unit 1 in Mode 3 at normal operating temperature and pressure. This event was entered into the DCPP corrective action program for resolution (50231240).
RSG contract personnel installed shims between the RSG 1-3 anchor column adapter and the washer plate of sufficient thickness to clear the weld metal interference at each of the two nonconforming locations.
B. Corrective Actions to Prevent Recurrence (CAPR) The RSG installation contractor will incorporate this event into the Lessons Learned so as to include the critical characteristics for inspection on work packages for future RSG projects at other stations.
RSG personnel and DCPP personnel verified that all Unit 1 RSG support anchors are properly installed and that the extent of the condition was isolated to RSG 1-3. More rigorous verification requirements employed during the Unit 2 RSG project ensured that each of the configurations were adequate.
VI.�Additional Information
A. Failed Components
None.
B. Previous Similar Events
None.
C. Industry Reports INPO Operating Experience Report OE 28508 issued for this condition.