05000247/LER-2002-005
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2472002005R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Westinghouse 4-Loop Pressurized Water Reactor
EVENT IDENTIFICATION
Central Control Room Wall Identified As Being In Non-Conformance With Design Drawings
EVENT DATE
October 8, 2002
REFERENCES
Condition Reporting System Number: CR-IP2-2002-09027, CR-IP2-2002-09060
PAST SIMILAR EVENTS
NRC Inspection Report Item(s) 50-247/02-02-02 and 50-247/02-010-001
EVENT DESCRIPTION
On October 8, 2002, an operability assessment of the Indian Point Unit 2 Central Control Room (CCR) south wall determined that in the event of a postulated design basis earthquake, the wall would be subjected to excessive differential building motion. The subject masonry constructed wall is located in the Indian Point Unit 1 Superheater Building and forms part of the Unit 2 CCR envelope along the south side of the Control Building structure between column lines G and F4/F5 in the east-west plane and column line 10.4 in the north-south plane. The wall is approximately 15.25 feet wide by 18.4 feet high, spanning between the control room floor at El. 53' and the Superheater Building roof at El. 72'. The original design specified the wall to be an 8-inch thick concrete masonry unit (CMU) and glazed brick composite construction. The as-found condition of the wall was identified as being in non-conformance with existing design drawings. The discrepancies include a steel angle (L3x3x1/4) mortared to a cut-out in the wall at the location of an existing steel girt channel, approximately 12 feet above the floor. Three tie angles (L5x3x1/4) span between the embedded angle and building steel of the CCR, and are welded to both.
Additionally, the masonry block wall was cut back to approximately 3.5 inches thick to clear the upper girt.
The embedded angle and the tie angles were identified on design drawings for removal, but were not removed during construction of the wall. Consequently, during a seismic event, this condition poses a seismic interaction concern, and could result in cracking of the wall and a potential breach of the CCR envelope, preventing the Control Room Air Filtration System [EllS:VI] from performing its design function of limiting dose to the operators. This condition was discovered during an ongoing extent of condition review of masonry wall design deficiencies. Indian Point Unit 2 Technical Specification 3.3.H.1 states that, "The control room air filtration system shall be operable at all times when containment integrity is required." Furthermore, Technical Specification 3.3.H.2 states, "From the date that the control room air filtration system becomes and remains inoperable for any reason, operations requiring containment integrity are permissible only during the succeeding 3.5 days.
EVENT DESCRIPTION (Con't) In accordance with Technical Specification 3.3.H.2, on October 8, at approximately 1330 hours0.0154 days <br />0.369 hours <br />0.0022 weeks <br />5.06065e-4 months <br />, the control room air filtration system was declared inoperable, and the limiting condition of operation was entered in order to implement corrective actions. The corrective actions included cutting and removing the three tie angles, and filling in the cut back area of the wall with new masonry block. On October 9, at approximately 1320 hours0.0153 days <br />0.367 hours <br />0.00218 weeks <br />5.0226e-4 months <br />, upon completion of the necessary actions, the limiting condition of operation was exited. At the time of discovery, the plant was steady state at 100 percent power.
EVENT ANALYSIS
In November 2001, a number of CCR masonry walls at Indian Point 2 were inspected relative to the issues identified in IE Bulletin 80-11, "Masonry Wall Design." Based upon observations at floor level, the condition of the CCR south wall was deemed to be within acceptable design requirements. In October 2002, while performing repairs to the control room side of the south wall, several design discrepancies were noted, which had not been identified previously. This observation was made possible due to the scaffolding erected to perform the repairs and removal of the egg crate ceiling and transite boards. The discrepancies include a steel angle (L3x3x1/4) mortared to a cut-out in the wall at the location of an existing steel girt channel, approximately 12 feet above the floor. Three tie angles (L5x3x1/4) span between the embedded angle and building steel of the CCR, and are welded to both. Additionally, the masonry block wall was cut back to approximately 3.5 inches thick to clear the upper girt. The affected wall is located within the Superheater Building, and provides a building envelope for the CCR. The tie angles were attached to the mortared angle within the wall at one end (Superheater Building) and to the CCR building roof steel at the other. The tie angles effectively bridged across the two buildings. Based on a review of the building drawing, expansion gaps are provided at the El. 53' floor of 1.0 inch and at El. 72' roof of 1.5 inches. These expansion gaps permit normal building growth due to thermal expansion (from ambient temperature variations) and limit building interaction if a seismic event should occur. Based upon an evaluation of the wall in its as-found condition, the resulting calculated loads due to building motion would result in a local yielding of the CCR roof truss steel, and cracking of the concrete masonry wall, a gross breach of the wall is unlikely since the wall and CCR steel will deflect to accommodate building differential motion. Potential exists for severe flexural cracking and local shear failure of the block at the wall ends near the location of decreased wall thickness. As such, it is concluded that the wall, in the as- found configuration can be considered structurally operable, in that catastrophic collapse is unlikely to occur to the CCR side of the wall in the event a postulated earthquake had occurred. Breach of the wall, in terms of CCR envelop is likely in local sections at the edges of the wall at the reduced wall section, due to the decreased flexibility at this section to accommodate the building displacements. Consequently, during a seismic event, this condition poses a seismic interaction concern, and could result in cracking of the wall and a potential breach of the CCR envelope, preventing the Control Room Air Filtration System from performing its design function of limiting dose to the operators. This report is being made pursuant to 10 CFR 50.73(a)(2)(v)(D), which requires that, "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident." Pursuant to 10 CFR 50.72(b)(3)(v)(D), on October 8, 2002 the NRC was notified (Event Number 39259) of this condition upon its discovery.
CAUSE OF OCCURRENCE
The cause for this condition is human performance-related, and is attributed to a failure to adhere to the specified structural detail requirements of the CCR south wall during initial construction. Although the embedded angle and the tie angles were identified on design drawings for removal, these were not removed during the construction of the wall.
EVENT SAFETY SIGNIFICANCE
The Control Room Air Filtration System is designed to ensure that control room operators are adequately protected against the effects of accidental releases of toxic chemicals or radioactive gases, and that the plant can be safely operated or shut down under design-basis accident conditions. An assessment of the safety consequences and implications of the degraded wall condition was performed. Although the control room habitability envelope is affected, a seismic event does not cause an immediate radiological hazard.
Therefore, a full complement of accident mitigation equipment would remain available, and the expected response by the operators to the event would not be substantially impacted. Based upon an evaluation of the wall in its as-found condition, catastrophic collapse is unlikely to occur to the CCR side of the wall during a postulated seismic event. As such, engineered safeguards components within the CCR remain available. The location and localized nature of the postulated wall degradation, and the presence of intervening structures that act as a barrier to the entry of toxic gas releases into the area of the degradation, are sufficient to maintain the validity of the existing toxic chemical study conclusion that the control room operators are adequately protected. Because localized failure of the affected wall has been determined to not impact any control room equipment, the ability to safely shutdown the plant in the event of a design basis accident has not been reduced. Thus, the failure of the CCR envelope would not be expected to impact the likelihood of successful accident mitigation. Based upon the above, this event has been determined to be of minimal safety significance.
CORRECTIVE ACTIONS
In accordance with Technical Specification 3.3.H.2, on October 8, at approximately 1330 hours0.0154 days <br />0.369 hours <br />0.0022 weeks <br />5.06065e-4 months <br />, the control room air filtration system was declared inoperable, and the limiting condition of operation was entered in order to implement corrective actions. The corrective actions included cutting a two-inch minimum section out of each tie angle, and filling in the cut back area of the wall with new masonry material and mortar. On October 9, at approximately 1320 hours0.0153 days <br />0.367 hours <br />0.00218 weeks <br />5.0226e-4 months <br />, upon completion of the necessary actions, the limiting condition of operation was exited. An extent of condition review to determine the presence of any other walls with similar seismic interaction concerns will be completed no later than January 31, 2003.
PREVIOUS OCCURRENCES
This condition was discovered during an ongoing extent of condition review of construction deficiencies associated with the CCR west fire barrier wall. Those deficiencies were originally identified in February 2002 and are documented in NRC Inspection Reports 50-247/02-02 and 50-247/02-010.