05000250/LER-2012-001

LER-2012-001, Containment Concrete Thickness At Spalled Patch Does Not Meet Technical Specification Design Value

Docket Number
Event date:	3-21-2012
Report date:	5-21-2012
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2502012001R00 - NRC Website

DESCRIPTION OF THE EVENT

On March 21, 2012, during the 40 year tendon surveillance inspection being conducted in Refueling Outage 25, a patch on the exterior surface of the Unit 3 Containment was observed to be spalling, resulting in falling pieces of concrete grout. The location of the spall was in the northwest quadrant, above elevation 42 feet.

Investigation found that the spall occurred in a concrete patch that was located along the top edge of the original Containment construction opening. It is presumed that this patch was placed when the opening was filled in at the end of construction, though this cannot be verified. The area was approximately 2 inches deep, and shallow re-bar was exposed at the top edge of the patched area. The rebar showed evidence of minor corrosion.

Technical Specification 5.2.1.c specifies that the minimum Containment concrete wall thickness is 3.75 feet.

Plant drawing 5610-C-132, Revision 4, "Containment Structure Wall Elevation & Det. — Unit 3," shows the thickness of the Containment concrete exterior wall as 3.75 feet with no tolerance on thickness. Therefore, literal compliance with the Technical Specifications leads a conservatively conclusion that the spalled area was reduced to less than 3.75 feet.

Civil engineering review of this spalling indicated that it was an isolated case where the concrete patch became loose due to aging and the effects of environmental conditions that may have been exacerbated by the perceived quality of the original repair. No other visible degraded concrete areas of concern were identified during the engineering walkdowns.

Condition Report 1746929 was initiated to determine causes and corrective actions. This report is submitted in accordance with 10 CFR 50.73(a)(2)(i)(B).

CAUSE OF THE EVENT

The cause of the event is an isolated case wherein a concrete patch became loose. Aging and environmental conditions, as well as the possible original quality of the repair, caused loosening of the patch and the resultant spalling.

ANALYSIS

Primary Containment provides the boundary that ensures that the release of radioactive materials from the containment atmosphere will be restricted to those leakage paths and associated leak rates assumed in the safety analyses. This restriction, in conjunction with the leakage rate limitation, will limit the site boundary radiation doses to within the dose guideline values of 10 CFR Part 100 during accident conditions.

The Containment structure is of post-tensioned concrete design, whose principal design function is to withstand the internal pressure caused by a Maximum Hypothetical Accident as defined in Updated Final Safety Analysis (UFSAR) Section 5.1.1. The internal pressure is resisted by application of an opposing external pressure type load on the structure via the post-tensioning tendons (UFSAR Section 5.1.2).

Since the plant drawings specify the same thickness as the Technical Specifications, literal compliance with the Technical Specifications leads to a conservative conclusion that the spall reduced the thickness of the wall to below 3.75 feet in this localized area.

Engineering evaluation of the spalled area was performed and yielded the following:

ASME Section XI, Subsection IWL-3211 states, "The condition of the concrete surface is acceptable if the responsible engineer determines that there is no evidence of damage or degradation...sufficient to warrant further evaluation or performance or repair/replacement activities." The identified condition meets the category as specified in ASME Section XI. The spalled grout is non-structural, and the rust on the exposed rebar is minor.

The identified localized area of shallow spalling will not adversely affect the integrity of the Containment building, and the building is structurally acceptable in its present condition.

CORRECTIVE ACTIONS

Repairs to the spalled location have been completed in the field.

PREVIOUS SIMILAR EVENTSN None