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