United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-10: Liner Plate Corrosion in Concrete Containments

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                          WASHINGTON, D.C. 20555-0001

                                March 13, 1997


NRC INFORMATION NOTICE 97-10:  LINER PLATE CORROSION IN CONCRETE CONTAINMENTS


Addressees

All holders of operating licenses or construction permits for power reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to occurrences of corrosion in the liner plates of
reinforced and pre-stressed concrete containments, and to detrimental effects
such corrosion could have on containment reliability and availability under
design-basis and beyond-design-basis events.  It is expected that recipients
will review this information for applicability to their facilities and
consider actions, as appropriate.  However, suggestions contained in this
information notice are not NRC requirements; therefore, no specific action or
written response is required.   

Background

Criterion 16 of the General Design Criteria of 10 CFR Part 50 requires that
the reactor containment and associated systems shall be provided to establish
an essentially leaktight barrier against the uncontrolled release of
radioactivity to the environment.  To meet this leak-tightness requirement,
the inside surfaces of concrete containments are lined with thin metallic
plates, generally between 6.2 mm (1/4 in.) and 9.5 mm (3/8 in.) thick.  The
liner plates are attached to the concrete by means of stud anchors or
structural rolled shapes or both.  

The design process assumes that the liner plates do not carry loads.  However,
the normal loads, such as from concrete shrinkage, creep and thermal changes,
imposed on the concrete containment structure are transferred to the liner
plates through the anchorage system.  Internal pressure and temperature loads
are directly applied to the liner plate.  Thus, under design-basis conditions,
the liner plate could experience significant strains.  Section III, Division 2
of the American Society of Mechanical Engineers (ASME) Boiler and Pressure
Vessel Code (also called American Concrete Institute Standard 359), "Code for
Concrete Reactor Vessels and Containments," allows liner tensile strains up to
0.004 cm/cm (inch/inch) of material for normal operating stresses, and up to
0.01 cm/cm (inch/inch) for stresses resulting from the postulated
environmental and accident conditions.  American Society for Testing and
Materials (ASTM) Standard A516 (or equivalent) steel is typically used in the 




9703140144.                                                            IN 97-10
                                                            March 13, 1997
                                                            Page 2 of 3


construction of containments.  The minimum ultimate tensile strain of the
liner material is  0.2 cm/cm (inch/inch).  Thus, there is a minimum safety
factor of 20 above the theoretically calculated liner strains.

Any corrosion (metal thinning) of the liner plate could change the failure
threshold of the liner plate under a challenging environmental or accident
condition.  Thinning changes the geometry of the liner plate, creating
different transitions and strain concentration conditions.   This may reduce
the design margin of safety against postulated accident and environmental
loads.

Description of Circumstances

Inspections of containment liners have shown various degrees of corrosion.

�     In January 1993, an NRC inspector pointed out corrosion of the drywell
      liner at Unit 2 of the Brunswick plant.  The liner was corroded at
      various spots at the junction of the base floor and the liner.  A
      subsequent examination of Unit 1 showed similar corrosion.

�     During the NRC staff's structural assessment review in June 1992, the
      staff noted peeled coating and spots of liner corrosion at Trojan (not
      operating) and at Beaver Valley Unit 1.

�     Before integrated leak rate testing of the containment at Salem Unit 2
      in 1993, the licensee's staff noted minor corrosion of the containment
      liner.  

�     During the NRC staff's structural assessment review in April 1992,
      discoloration of the vertical portion of the containment liner was
      observed at an insulation joint at Robinson Unit 2.

Discussion

Of the five occurrences cited above, four (Trojan and Beaver Valley Unit 1,
Salem Unit 2, and Robinson Unit 2) were found to be benign from the standpoint
of safety.  However, the licensees were sensitized to the conditions for
future monitoring and repairs, as appropriate.  Corrosion of the liner plate
at Brunswick Units 1 and 2 was significant from the standpoint of safety.  The
sealing material along the circumference at the junction of the drywell wall
and the bottom floor had significantly degraded from the water accumulation at
the junction.  The liner plate was found to have pitted significantly (as much
as 50 percent of the original thickness) at various locations along the
circumference.  Before the restart of the two Brunswick units, the licensee
cleaned the joint areas, repaired the pitted liner plate areas, and resealed
the entire gap at the junction with dense silicone elastomer.  The repair of
the pitted areas consisted of (1) welding the pitted areas, (2) examining the
repaired areas in accordance with the ASME Section III, Division 2, and (3)
recoating the repaired areas.  
.                                                            IN 97-10
                                                            March 13, 1997
                                                            Page 3 of 3


Corrosion of a liner plate can occur at a number of places where the metal can
be exposed to moisture, or where moisture can condense (behind insulation) or
accumulate.  Potential locations for liner plate corrosion are (1) the
junction of the containment cylinder and intermediate floors and basemat
concrete (PWRs and Mark III BWRs), (2) the junction of the drywell and the
base or intermediate concrete floors (Mark I, Mark II concrete containments),
(3) adjacent to crane girder rails and supports attached to the liner plate
(concrete containments), (4) water-soaked areas where carbon steel liner plate
is used (Mark II and Mark III containments), and (5) behind insulation and
ice-condenser baskets. 

An amendment to 10 CFR 50.55a became effective September 9, 1996.  This
amendment, by endorsing the use of Subsections IWE and IWL of Section XI of
the ASME Boiler and Pressure Vessel Code, provides detailed requirements for
inservice inspection of containment structures.  Inspection (which includes
examination, evaluation, repair, and replacement) of concrete containment
liner plate in accordance with the 10 CFR 50.55a requirements involves
consideration of the potential corrosion areas.  

This information notice requires no specific action or written response.  If
you have any questions about information in this notice, please contact one of
the technical contacts listed below or the appropriate NRR project manager.


                                          signed by M.M. Slosson

                                    Thomas T. Martin, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical Contacts:  Hans Ashar, NRR                     
                     (301) 415-2851                   
                     E-mail:  hga@nrc.gov             

                     R. A. Benedict, NRR
                     (301) 415-1157
                     E-mail:  rab1@nrc.gov
Page Last Reviewed/Updated Tuesday, December 03, 2013