United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-99, Supplement 1: Degradation of Steel Containments

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

                              February 14, 1991


Information Notice No. 86-99, SUPPLEMENT 1:  DEGRADATION OF STEEL 
                                                 CONTAINMENTS


Addressees: 

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

Purpose: 

This supplement to Information Notice (IN) 86-99 is intended to alert 
addressees to additional information about a potential degradation problem 
regarding corrosion in steel containments.  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 supplement to the information notice do not 
constitute NRC requirements; therefore, no specific action or written 
response is required. 

Discussion: 

IN 86-99 was issued on December 8, 1986, in response to the discovery of 
significant corrosion on the external surface of the carbon steel drywell in 
the sand bed region of the Oyster Creek plant.  This supplement updates the 
status of Oyster Creek containment corrosion and the licensee's mitigation 
program. 

Since drywell corrosion was detected in 1986, the licensee instituted 
periodic wall thickness measurements by the ultrasonic testing (UT) 
technique to determine corrosion rates.  The most severe corrosion was found 
in the sand bed region at a nominal elevation of 11'-3".  The highest 
corrosion rate determined was 35.2 +/- 6.8 mils per year.  To mitigate the 
corrosion in the sand bed region, water was drained from the sand bed and 
cathodic protection (CP) was installed in the bays with the greatest wall 
thinning in early 1989.  Subsequent UT thickness measurements in these bays 
indicated that CP was ineffective.  The licensee's consultants indicated 
that it would be necessary to flood the sand bed and to install CP in all 
the bays to make the CP system effective.  The licensee decided that large 
amounts of water in the sand bed would be counterproductive. 





9102080329 
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                                                  IN 86-99, Supplement 1
                                                  February 14, 1991
                                                  Page 2 of 3


In the spherical portion of the drywell above the sand bed region, the 
highest corrosion rate determined was 4.6 +/- 1.6 mils per year at a nominal 
elevation of 51'.  In the cylindrical portion of the drywell above the 
spherical portion, where minor corrosion was discovered and was thought to 
have originated mostly during construction, no significant wall thinning was 
detected (at a nominal elevation of 87').  However, this is the region in 
which the nominal thickness of the wall has the least margin, thus requiring 
periodic monitoring of actual thickness.

The licensee has instituted a drywell program to arrest corrosion and to 
ensure containment integrity for the full licensed term of the plant.  The 
licensee has taken action to investigate, identify, and correct leak paths 
into the drywell gap and plans to take more action to survey leakage and 
prevent it.  The stainless steel liners in the refueling cavity and the 
equipment pool developed cracks along the perimeter of the liner plates 
where they were welded to embedded channels.  For the refueling cavity, all 
potential leakage pathways have been thoroughly checked and liner cracks are 
sealed with adhesive stainless steel tape before a strippable coating is 
applied.  Since the refueling cavity is flooded only during refueling, no 
leakage concerns exist at other times.  At the end of an outage, the 
refueling cavity is drained, and the tape and strippable coating are 
removed.  The licensee found leaks related to the equipment pool and stopped 
them with liner weld repairs.  The equipment pool also will be protected 
with a strippable coating during flooded periods of operation.  

The licensee believes that a thorough program has been established for 
managing leakage that could affect drywell integrity due to corrosion from 
moisture ingress into the drywell gap.  Recent surveillance of the sand bed 
drains indicates that the sand bed is free of water.  To further mitigate 
drywell corrosion, the licensee is considering removing the sand, 
insulation, gap filler material, and corrosion film and applying a 
protective coating to the exterior drywell surface.  The licensee is 
proceeding with the analysis, engineering and planning to support removing 
the sand from the drywell sand bed region in the near future.  Removal of 
the insulation and gap filler material from the drywell gap is being 
evaluated for future consideration. 

The BWR Owners Group is surveying its members to determine whether other 
plants are experiencing water leakage into the drywell gap and possible 
corrosion of the exterior surfaces in the sand bed region as well as in the 
spherical and cylindrical parts of the drywell. 

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                                                  IN 86-99, Supplement 1
                                                  February 14, 1991
                                                  Page 3 of 3


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




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation


Technical Contacts:  Frank J. Witt, NRR 
                     (301) 492-0767 

                     C.P. Tan, NRR
                     (301) 492-3315


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