United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-108: Degradation of Reactor Coolant System Pressure Boundary Resulting from Boric Acid Corrosion

                                                            SSINS No.:  6835
                                                            IN 86-108 

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555

                              December 29, 1986

Information Notice No. 86-108:  DEGRADATION OF REACTOR COOLANT SYSTEM 
                                   PRESSURE BOUNDARY RESULTING FROM BORIC  
                                   ACID CORROSION 

Addressees: 

All pressurized water reactor (PWR) facilities holding an operating license 
or a construction permit. 

Purpose: 

This notice is to alert recipients of a severe instance of boric acid 
induced corrosion of ferritic steel components in the reactor coolant system 
(RCS). Recipients are expected to review the information for applicability 
to their facilities and consider actions, if appropriate, to preclude 
similar problems occurring at their facilities. However, suggestions 
contained in this information notice do not constitute NRC requirements; 
therefore, no specific action or written response is required. 

Description of Circumstances: 

In October 1986, the Arkansas Nuclear One, Unit 1 (ANO-1) Plant was in cold 
shutdown and was performing nondestructive testing of the high pressure 
injection (HPI) nozzle thermal sleeves. An HPI nozzle is attached directly 
on the side of each of the four RCS cold legs. The metallic insulation was 
removed from the "A" HPI nozzle to allow radiographic examination. Removal 
of this insulation revealed severe corrosion wastage on the exterior of the 
HPI nozzle and some wastage on the RCS cold leg pipe. The corrosion 
apparently was caused by reactor coolant leakage from an HPI isolation valve 
located about 8 feet above the nozzle as shown in the attached Figure 1. 

The wastage began adjacent to where the 3-1/2 inch OD stainless steel safe-
end is welded to the carbon steel HPI nozzle. The safe-end is located 
between the stainless steel HPI line and the carbon steel HPI nozzle. The 
wastage was approximately 1/2 inch at its deepest location (adjacent to the 
stainless-to-carbon steel weld). The HPI nozzle (including cladding) is 
approximately 3/4 inch thick at this point. At the transition weld between 
the safe-end and the carbon steel nozzle, the wastage extended approximately 
20 percent around the circumference of the nozzle in the form of several 
trenches. From this point, the wastage narrowed to two separate trenches 
that became shallower as they progressed more than 10 inches along the 
bottom of the HPI nozzle towards the RCS cold leg. The two trenches then 
continued down the cold leg for approximately 6 inches. The depth of the 
trenches on the cold leg were less than 1/4 inch. 

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                                                         IN 86-108 
                                                         December 29, 1986 
                                                         Page 2 of 3 

The HPI nozzle is constructed of ferritic (ASTM, A-105, grade 2) steel. The 
cold leg also is constructed of ferritic (ASTM, A-106, grade C) steel. The 
HPI nozzle and the cold leg are clad on the inside with stainless steel of 
3/16 inch nominal thickness. 

Leakage from the HPI isolation valve was first noted in August 1985, through
RCS leak detection methods. The measured leakage was approximately 0.08 
gallons per minute (gpm). The leakage was attributed to a valve 
body-to-bonnet leak. The valve's seal ring and yoke clamp were replaced in 
September 1985. A leakage of 0.09 gpm was again detected from this valve 8 
days later during plant startup. This leak rate continued until subsequent 
repair of the valve in February 1986. The insulation was not removed at the 
time of these repairs. After the damaged HPI nozzle was discovered, a 
reddish stain, resulting from the leaching out of iron oxide corrosion 
products, was found on the exterior of the insulation near the damaged area. 

Discussion: 

There have been a number of reported incidents of boric acid corrosion wast-
age of ferritic steels. In 1981 Calvert Cliffs, Unit 2, experienced boric 
acid corrosion wastage on an RCS cold leg near the suction to a reactor 
coolant pump (RCP). This corrosion wastage was from 1/8 to 1/4 inch in depth
and extended about 20 percent around the circumference of the RCS pipe. This
RCS piping is fabricated from ferritic (ASME, SA 516, grade 70) steel. 

Most incidents, however, have been wastage of threaded fasteners. In June 
1982, IE Bulletin 82-02, "Degradation of Threaded Fasteners In The Reactor 
Coolant Pressure Boundary OF PWR Plants," was issued. The closeout of this 
bulletin was addressed in NUREG-1095, May 1985. The affected threaded fast-
eners were of a low alloy, high strength, ferritic steel. A generic issue, 
"Bolting Degradation or Failures in Nuclear Power Plant " is currently under
review by the NRC staff to determine if additional actions are necessary. 
One of the main concerns in this issue is boric acid corrosion. The 1983 
edition of the ASME Code, Section XI, was revised to provide for more 
restrictive requirements for visual examinations of systems containing 
borated water. Part of these requirements is an inspection of insulation at 
the joints for evidence of leaks. This revision is contained in Section 
IWA-5242(a), Insulated Components. 

Boric acid corrosion has been found to be most active where the metal 
surface is cool enough so that it is wetted. If the metal is sufficiently 
hot, then the surface will stay dry and this loss of electrolyte will slow 
the corrosion rate. At ANO-1, borated water leaked from the HPI isolation 
valve in the form of a liquid and then ran down the HPI piping on the inside 
of the insulation to the HPI nozzle. As the leakage approached the cold leg, 
the increased piping temperatures caused evaporation of the water, thus 
increasing the boric acid concentration and lowering the PH of the solution. 
It is believed that the close tolerance between the HPI nozzle and the 
insulation, aided by boric acid crystallization, caused pooling of the 
solution at the nozzle. This pool of highly acidic solution wetted the 
nozzle and resulted in accelerated corrosive 

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                                                         IN 86-108 
                                                         December 29, 1986 
                                                         Page 3 of 3 

attack. Experience has shown that even relatively hot metal can be suffic-
iently cooled on the surface by the flow of the leakage so that the surface 
stays wetted and boric acid corrosion is promoted. In addition, periods dur-
ing which a metal surface is below normal operating temperature may allow 
corrosion in areas that would not otherwise be expected. Boric acid 
corrosion rates in excess of 1 inch depth per year in ferritic steels have 
been experienced in plants and duplicated in laboratory tests where low 
quality steam from borated reactor coolant impinged upon a surface and kept 
it wetted. 

Additional information is contained in EPRI-NP-3784, "A Survey of the Liter-
ature on Low Alloy Steel Fastener Corrosion in PWR Power Plants," December 
1984, and NUREG/CR-2827, "Boric Acid Corrosion of Ferritic Reactor Compon-
ents," July 1982. 

Followup: 

The damaged HPI nozzle has been repaired by grinding out all indications of 
corrosion and rebuilding by welding in those areas with less than the 
minimum required wall thickness. Repair to the cold leg required only 
grinding out the corrosion. All repairs were in accordance with ASME codes. 
The other HPI nozzles were inspected and no evidence of corrosion wastage 
was found. The licensee continues to evaluate methods and procedures to 
minimize recurrence of this type of event. The primary defense is to 
minimize leaks, detect and stop leaks soon after they start, and promptly 
clean up any boric acid residue. Detection of leaks will be enhanced by an 
evaluation of any iron oxide stains on insulation. 

No specific action or written response is required by this information 
notice. If you have questions about this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office. 


                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response  
                                   Office of Inspection and Enforcement 

Technical Contact:  Henry A. Bailey, IE 
                    (301) 492-9006 

Attachments: 
1.   Figure 1:  ANO-1 HPI Line/Nozzle Configuration 
2.   List of Recently Issued IE Information Notices 
Page Last Reviewed/Updated Tuesday, November 12, 2013