Information Notice No. 96-64: Modifications to Containment Blowout Panels Without Appropriate Design Controls
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
December 10, 1996
NRC INFORMATION NOTICE 96-64: MODIFICATIONS TO CONTAINMENT BLOWOUT
PANELS WITHOUT APPROPRIATE DESIGN
All holders of operating licenses or construction permits for nuclear power
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert
addressees to the potential for operating outside of the design basis as a result of modifying
structures without appropriate design controls. It is expected that recipients will review the
information for applicability to their facilities and consider actions, as appropriate, to avoid
similar problems. However, suggestions contained in this information notice are not NRC
requirements; therefore, no specific action or written response is required.
Description of Circumstances
North Anna, Units 1 & 2
On February 13, 1996, it was identified that six blowout panels for both units' incore
instrument tunnels had been modified by installing hasps and locks without appropriate
design controls. Additionally, several steam generator and reactor coolant loop containment
cubicle door panels had been modified by tack welding their sheet metal covers to the door
frames and adding stiffener plates around the door handles. These modifications were not
reviewed prior to installation to assure their compliance with design bases assumptions
contained in the Updated Final Safety Analysis Report (UFSAR), and existed for an extended
time period during past operating cycles.
On November 21, 1995, it was recognized that the accident analysis for High Energy Line
Break outside Primary Containment contained a key assumption that had been rendered
invalid. For a postulated High Energy Line Break inside the Reactor Building Main Steam
Tunnel (Steam Tunnel), the Steam Tunnel blowout panels had been credited with rupturing to
allow a vent path to the Turbine Building, thereby preventing compartment overpressurization.
In an effort to reduce Secondary Containment leakage, maintenance was performed on the
blowout panels in 1985 to cover them with fiberglass. As a result, the blowout panels would
rupture at a higher pressure than credited in the High Energy Line Break analysis. This was
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December 10, 1996
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apparently attributable to an inadequate investigation of the blowout panels' design basis
functions prior to the initiation of the maintenance.
On May 10, 1996, a tornado caused structural damage to the metal siding that forms the
secondary containment barrier for the refueling floors on both Units. Subsequent
documented inspections revealed approximately 270 (out of 1496) damaged explosion bolts.
It was observed that not all of the damaged bolts were a result of this tornado. On
August 23, 1996, it was concluded, through calculations, that not all of the design
requirements for the siding would have been met and this would have placed secondary
containment in an inoperable condition. The previously damaged explosion bolts were
damaged by either a pressure load or by ineffective work practices.
Nine Mile Point, Unit 1
On October 25, 1993, it was determined that the relief (blowout) panels in the Nine Mile Point
Unit 1 turbine and reactor buildings would not blow out at the design pressure of 2.15 kph
(45 lb/ft2) because the bolt fasteners for the panels were larger than designed and had a
higher ultimate strength than designed. The initial engineering evaluation of this condition
erroneously determined that the turbine and reactor building panels would blow out at
2.87 kph (60 lb/ft2) and 2.54 kph (53 lb/ft2), respectively, to relieve internal building pressure
prior to structural failure of the buildings, and the panels were declared operable. On
March 27, 1995, it was determined that there was an error in the design assumptions for load
distribution. Revised calculations confirmed that the relief panels would not blow out until the
internal building pressure exceeded the minimum documented building structural design of
3.83 kph (80 lb/ft2). The cause of this event was inadequate quality control measures during
initial construction which resulted in oversize bolts being installed in the relief panels.
The purpose of blowout panels is to prevent overpressure or overtemperature conditions in a
compartment or other enclosure that could be structurally damaged due to a high energy line
break inside the compartment, or that contains equipment that would be damaged by the
overpressure or overtemperature condition. This function may or may not be a "safety-
related" function depending on whether the high energy line break could result in loss of (1)
the capability to reach and maintain safe shutdown, or (2) the capability to mitigate an
accident in which a high energy line break is postulated to occur as a passive or
consequential additional failure. In addition to the overpressure "active" function, blowout
panels may also have a passive function such as tornado missile protection.
Blowout panels are designed to open at specific design differential pressures. Licensees
discovered that as-found configurations either would not allow the panels to open until after
exceeding the design pressure, or in the Quad Cities example, did not meet all of secondary
containment design bases described in the UFSAR. Modifications made to satisfy apparently
valid concerns did not adequately evaluate the effects on the panels' design bases or.
December 10, 1996
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adequately consider the design assumptions used for analyzing containment structures and
supports. The modifications were not recognized as having the potential to place the
structures outside of their design bases.
Previous Similar Generic Communication
NRC Information Notice 96-17, "Reactor Operations Inconsistent with the Updated Final
Safety Analysis Report," dated March 18, 1996, (Accession No. 9603150213).
This information notice requires no written response. If you have any questions about the
information in this notice, please contact the technical contacts listed below or the appropriate
Office of Nuclear Reactor Regulation Project Manager.
1. NRC Inspection Report 50-338/96-01 & 50/339/96-01, dated March 21, 1996,
(Accession Number 9604090115)
2. Cooper Nuclear Station Licensee Event Report 50-298/95-018-01, dated
May 31, 1996, (Accession Number 9606110378)
3. Quad Cities Nuclear Power Station Licensee Event Report 50-254 &
50-265/96-016, dated September 20, 1996, (Accession Number 9609300009)
4. Nine Mile Point Nuclear Station Unit 1 Licensee Event Report 50-220/95-005-01,
dated June 26, 1996, (Accession Number 9607010370)
signed by D.B. Matthews
Thomas T. Martin, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Donald R. Taylor, RII
Neal K. Hunemuller, NRR
Attachment: List of Recently Issued Information Notices
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