UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001

January 16, 2003

NRC INFORMATION NOTICE 2003-02: RECENT EXPERIENCE WITH REACTOR

COOLANT SYSTEM LEAKAGE AND BORIC ACID

CORROSION

Addressees

All holders of operating licenses or construction permits for pressurized water reactors (PWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform

addressees of recently observed reactor coolant leakage at two pressurized water reactor

facilities, one of which resulted in the subsequent degradation of the reactor pressure vessel

head. 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 in this

information notice are not NRC requirements; therefore no specific action or written response is

required.

Description of Circumstances

Sequoyah Unit 2 

On December 26, 2002, the unit tripped from full power as a result of low reactor coolant

system (RCS) flow due to a ground fault in a reactor coolant pump motor winding. In the

ensuing shutdown to correct the pump problem, the licensee initiated a search to locate and

correct a suspected RCS leak that, prior to the trip, had resulted in elevated moisture and

activity levels inside containment. During this inspection, the licensee identified an

accumulation of boric acid on the reactor vessel head insulation that resulted from a leaking

reactor vessel level indication system (RVLIS) compression fitting. The leakage had seeped

through a seam in the insulation onto the reactor pressure vessel (RPV) head and resulted in

minor boric acid corrosion of the head. This RVLIS compression fitting had been disconnected

and reconnected during the May 2002 refueling outage. The licensee also identified a small

leak through a canopy seal weld on an empty control rod drive mechanism (CRDM) penetration

that did not result in any boric acid corrosion of the reactor vessel head.

Based on the location of the leaking RVLIS fitting, the temperature of the leakage fluid was

close to the ambient temperature outside the vessel insulation. The licensee estimated the

mass of boric acid crystals on this insulation surface at about 9 kilograms (20 pounds). A seam

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in the insulation was in this area. On removing the insulation and cleaning the area, the

licensee observed boric acid corrosion of the head near the flange. The licensee determined

that the amount of material loss from the head was small, in the shape of a groove less than

one centimeter (cm) [0.3 inch] wide, about twelve cm [4.6 inches] long, and at most about onethird cm [0.125 inch] deep.

The licensee’s evaluation indicated that 98 percent or better of the structural wall remained

intact and that no abrupt corners existed in the degraded area. The licensee justified continued

operation based on the minor extent of the degradation.

Comanche Peak Unit 1

On November 30, 2002, a control rod dropped into the core. The licensee suspected a fault in

the CRDM coils. Failing to identify the cause of the dropped rod while at reduced power, the

licensee decided to shut down. While continuing to troubleshoot the CRDM problem in Mode 3,

the licensee observed a leak around the CRDM housing. The leak was from a CRDM canopy

seal weld. Water from the leaking canopy seal weld apparently entered the CRDM coils,

causing coil failure. Boric acid crystals were found around the leak site, on the vessel head

insulation, and on the reactor pressure vessel head. The licensee repaired the canopy weld

with a weld overlay and cleaned the CRDM housing, the head insulation, and the head to

remove the boric acid deposits. The amount of boric acid crystals recovered from the head was

about 1 kilogram (2 pounds). The licensee did not find any reactor coolant pressure boundary

degradation. 

Other operating experiences of similar character may be found in the generic communications

listed in NRC Bulletin 2002-01, “Reactor Pressure Vessel Head Degradation and Reactor

Coolant Pressure Boundary Integrity.”

Discussion

A number of mechanical and welded connections exist above the reactor pressure vessel head

that, historically, have leaked at a number of plants. This leakage of borated water may lead to

degradation of the low alloy steel reactor vessel head by boric acid corrosion. At Sequoyah

Unit 2, the leakage resulted in relatively minor degradation of the reactor vessel head. At

Comanche Peak Unit 1, the leakage resulted in no apparent degradation of the RCS pressure

boundary. In the Sequoyah Unit 2 and Comanche Peak Unit 1 events, the unidentified reactor

coolant leakage had not shown a discernible increase from the very low levels that typically

occur at a PWR facility. 

Common assumptions that RCS leakage onto a hot surface, such as the reactor pressure

vessel head, will not cause corrosion may not be justified and are the subject of ongoing

research. Usually, small quantities of water coming into contact with a surface as hot as the

reactor vessel head would be expected to flash and leave a noncorrosive dry boric acid residue

on the surface. However, at Sequoyah Unit 2 the resulting condition produced an environment

in which boric acid corrosion could occur. This experience challenges current assumptions with

respect to the potential effects of RCS leakage. The NRC is continuing to consider the safety

and regulatory aspects of this experience.
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This information notice requires no specific action or written response. If you have any

questions about the information in this notice, please contact the technical contact listed below

or the appropriate project manager from the NRC’s Office of Nuclear Reactor Regulation

(NRR). 

/RA/

William D. Beckner, Program Director

Operating Reactor Improvements Program

Division of Regulatory Improvement Programs

Office of Nuclear Reactor Regulation

Technical contacts: V. Hodge, NRR E. Sullivan, NRR

301-415-1861 301-415-2796

E-mail: cvh@nrc.gov E-mail: ejs@nrc.gov

Attachment: List of Recently Issued NRC Information Notices