United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-108, Supplement 1: Degradation of Reactor Coolant System Pressure Boundary Resulting from Boric Acid Corrosion

                                                  SSINS No.:  6835 
                                                  IN 86-108, Supplement 1 

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

                               April 20, 1987

Information Notice No. 86-108, SUPPLEMENT 1:  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 another severe instance of boric acid 
induced corrosion of ferritic steel components on the pressure boundary of a 
PWR. 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: 

On March 13, 1987, personnel at Turkey Point Unit 4 discovered more than 500 
lbs. of boric acid crystals on the RV head. There also was a large amount of 
boric acid crystals in the exhaust cooling ducts for the control rod drive 
mechanisms (CRDMs). After removal of this boric acid and steam cleaning of 
the RV head, severe corrosion of various components on the RV head was 
noted. Boric acid corrosion of a reactor coolant system (an HPI nozzle at 
Arkansas Nuclear One, Unit 1) was discussed previously in Information Notice 
86-108 of December 1986. 

This event at Turkey Point Unit 4 has once again demonstrated that boric 
acid will rapidly corrode ferritic (carbon) steel components and it also 
again demonstrated that if a small leakage occurs near hot surfaces and/or 
surroundings, then the boric acid solution will boil and concentrate, 
becoming more acidic and thus more corrosive. In addition, the evaporation 
of the water will cause the boric acid crystals to accumulate at that point.

The source of the boric acid crystals was found to be a leaking lower 
instru-ment tube seal, (conoseal), on one of the incore instrument tubes. 
This seal is a flanged joint with an oval metal gasket that is held together 
by clamps bolted in place. This seal, which is inside the CRDM cooling 
shroud, was observed to 

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                                                  IN 86-108, Supplement 1 
                                                  April 20, 1987 
                                                  Page 2 of 3 

have a very small leak as evidenced by some boric acid crystals during a 
plant outage in August 1986. An evaluation by the licensee at that time 
concluded that plant startup was acceptable provided the seal was inspected 
again within 6 months. The seal was again inspected while the plant was shut
down for an unrelated problem in October 1986. The leak rate was still 
judged to be acceptable for plant operation. About 1  of boric acid 
crystals were found and removed from the RV head at that time. However, the 
CRDM cooling ducts were apparently not closely inspected at that time. The 
licensee's evaluation was updated to require another inspection within 6 
months and the plant was started up. 

On March 13, 1987, Westinghouse, the NSSS vendor, completed a review of 
boric acid corrosion rates, as earlier requested by the licensee, and 
reported that the corrosion rate might be much faster than assumed when the 
licensee's evaluation was performed. This reassessment was based in part on 
the experience of a PWR in Europe that had experienced an accumulation of 
boric acid crystals on the RV head in 1970. Turkey Point Unit 4 was in hot 
shutdown at this time and the licensee promptly cooled the unit to cold 
shutdown to inspect the RV head and discovered the conditions discussed. 

The leakage from the conoseal apparently ran down one side of the RV head 
insulation and much of it leaked under the insulation to the bare RV head. 
In addition, a large amount of vapor was apparently carried up into the CRDM
cooling coils and ducts where it condensed and deposited boric acid 
crystals. Of 58 RV head bolts, 3 are severely corroded above the associated 
nuts and will be replaced. The CRDM cooling shroud support is severely 
corroded in the affected sector and the entire shroud will be replaced. The 
conoseal clamps also were corroded. The RV head will be removed to inspect 
for additional damage. The RV head, bolts, and other components will be 
non-destructively tested (NDT) to check for additional damage. 

The licensee has made a preliminary determination that the leak rate from 
the conoseal was less than O.25 gpm between startup in October 1986 and the 
recent shutdown. The total leakage from the reactor coolant system during 
this period was equal to or less than 0.45 gpm. The average reactor coolant 
system boron concentration during this period was about 500 ppm. 

The NRC dispatched an augmented inspection team (AIT) to the plant site on 
March 18, 1987. The NRC's and licensee's investigations into this event are 
still in progress. 

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                                                  IN 86-108, Supplement 1 
                                                  April 20, 1987 
                                                  Page 3 of 3 

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




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

Technical Contacts:  Henry A. Bailey, AEOD 
                     301-492-9006

                     John B. MacKinnon, AEOD 
                     301-492-9087 

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