05000306/LER-2014-001
| Prairie Island Nuclear Generating Plant Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(vii)(B), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 49685 | 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat, 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 3062014001R01 - NRC Website | |
identify precautions or limitations regarding the required ULS shim adjustment during Mode 3.
On June 19, 2013, Work Package (WP) for hot gap measurements was issued, which includes lower lateral support (LLS) one-bumper at a time restriction during Mode 3. There was no ULS Mode 3 restriction or an acceptable method of checking hot gaps as found in the Unit 1 Steam Generator Replacement WPs. Work package was approved by vendor and Xcel Energy.
On December 29, 2013, at 1327 CST, Unit 2 reached Normal Operating Pressure (NOP) / Normal Operating Temperature (NOT) per 2C1.2-M3, Mode 3.
On December 29, 2013, at 1552 CST, SGR project reported at 1545 that 21SG & 22SG have the LS- 6 shim on the lower lateral supports stuck in position. Shims were loose at 535 degrees and stuck at 547 degrees.
On December 30, 2013, at 0333 CST, Unit 2 reached Normal Operating Pressure (NOP) / Normal Operating Temperature (NOT) per 2C1.2-M3, Mode 3.
On December 30, 2013, the field team had difficulty achieving the repeatable measurements of the ULS shim gaps when the shims were in place. Craft, Field Engineers (FEs), and Quality Control (QC) measured the shims gaps separately using feeler gages and arrived at different gap clearances.
On December 30, 2013, a meeting was held by vendor engineering, Foremen, FEs, QC, Xcel Energy, and others. The resolution to the gap clearance differences was to remove the shim packs and create shims to fill the gaps, using C-clamps to hold the shim sections together for consistent measurements. Once the shim packs were installed there would be no further measurements. There was no discussion on restriction to remove one ULS shim pack at a time.
On December 30, 2013, Craft workers were directed by vendor mechanical superintendent to remove all shim packs to plant elevation 755' for detailed measurement and adjustment.
1 EIIS System Code - SG APPROVED BY OMB: NO. 3150-0104 EXPIRES: 0/31/2017 hours. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the Records and FOIA/Privacy Service Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by Internet e-mail to Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to Impose an Information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
On December 30, 2013, at approximately 2100 CST, all ULS shims were removed to plant elevation 755' for gap measurement and adjustment.
On December 31, 2013, at 0630 CST, the dayshift Xcel Project Oversight noted on arrival in containment that all the shim packs on SG 21 were removed and only positions 6 and 12 were installed on SG 22.
On December 31, 2013, at 0933 CST, both RCS Loops were declared inoperable and entry into Tech Specs 3.4.5 Condition D commenced, which placed the Unit 2 Reactor Coolant System (RCS) in an unanalyzed condition for seismic and pipe rupture analyses and was reportable under 10 Code of Federal Regulations (CFR) 50.72(b)(3)(ii)(B).
EVENT ANALYSIS
The Steam Generator is supported on a structural system consisting of four vertical columns fitted at the top and bottom with a double clevis and pin assembly. The vertical column clevis base plates are bolted to the SG support feet and permit movement in the horizontal plane to accommodate reactor coolant pipe thermal expansion. Horizontal restraint is accomplished at two locations. The lower lateral support is located at the support feet and the upper lateral support is located near the center of gravity below the transition cone. This combination of upper and lower supports and included stops and hydraulic shock suppressors limit and control horizontal movement for pipe rupture and seismic effects.
On December 31, 2013, at 0933 CST, both RCS Loops were declared inoperable and entry made into Tech Specs 3.4.5 Condition D, due to the Unit 2 RCS in an unanalyzed condition for Seismic and pipe rupture analyses. The unanalyzed condition was reportable under 10CFR50.73(a)(2)(ii)(B), any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety.
Based on further analysis, it was determined that with both RCS loops being inoperable, this condition was a loss of safety function, reportable under 10CFR50.73(a)(2)(v)(B), any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to remove residual heat.
Additionally, this condition was reportable under 10CFR50.73(a)(2)(vii)(B), any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system designed to remove residual heat, and 10CFR50.73(a)(2)(ix)(A)(2), any event or condition that as a result of a single cause could have prevented the fulfillment of a safety function for two or more trains or channels in different systems that are needed to remove residual heat.
APPROVED BY OMB: NO. 3150-0104 EXPIRES: 0/31/2017 hours. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the Records and FOIA/Privacy Service Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
There were no actual nuclear, radiological, industrial, and/or environmental events. There is a regulatory impact.
A potential consequence of excessive gap may be increased stresses in the piping and support system beyond allowable design basis and subsequent reduction in design margin. Shim gap requirements are established to restrain movement of the Steam Generator in a seismic event, primary line break, or secondary line break and ensure analyzed stresses in the system remain within design basis allowable levels.
An additional supplement to this LER will be provided following the completion of the evaluations being performed to determine the potential consequences. These evaluations are considering the impacts of seismic events, primary line breaks and secondary line breaks on the Reactor Coolant system structural integrity.
CAUSE
Root Cause, RC-01: The Steam Generator Replacement Project (SGRP) failed to document on the design drawings their assumption that the Unit 2 ULS shim packs would be measured and adjusted one shim pack at a time while making adjustments in Mode 3.
CORRECTIVE ACTION
Immediate and interim actions taken:
- Restored the shims to an interim condition to support operability and performed QC verification and notified operations. Completed on 12/31/2013.
- Revised WPs for the ULS to contain the required configuration control requirement. Field Change Requests were issued to include configuration requirements in the WP. These requirements were based on engineering input, including evaluations performed in the Field Change Notice which evaluated the minimum bearing area required at the shims to meet the maximum design load (seismic and pipe rupture). Completed on 12/31/2013.
- Completed Issue Resolution items identified on the Safety Stop Work Form dated 12/31/2013 as follows: (1) Determined barrier breakdown was failure to detail requirements on design documents and work packages along with less than adequate communication, (2) Determined and implemented corrective actions to revise work packages to prevent reoccurrence, (3) Communicated lessons learned through site wide communications and briefing of work crews, and (4) Provided for additional Xcel oversight of remaining shim work. Completed 12/31/2013.
APPROVED BY OMB: NO. 3160.0104 EXPIRES: 0/31/2017 hours. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the Records and FOIA/Privacy Service Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by Internet e-mall to Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to Impose an Information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the Information collection.
- Revise Unit 1 ULS and LLS design drawings to document the configuration control requirements for the adjustable shims in all operating modes. Due 7/30/2014.
- Revise Unit 2 ULS and LLS design drawings to document the configuration control requirements for the adjustable shims in all operating modes. Due 7/30/2014.
PREVIOUS SIMILAR EVENTS
None.