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90

Information Notice No. 93­90 Unisolatable Reactor Coolant System

Leak Following Repeated Applications of Leak Sealant

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

December 1, 1993

NRC INFORMATION NOTICE 93‐90: UNISOLATABLE REACTOR COOLANT SYSTEM LEAK

FOLLOWING REPEATED APPLICATIONS OF LEAK SEALANT

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

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

notice to alert addressees to an application of an on‐line leak sealing

process which substantially degraded the integrity of the reactor coolant

pressure boundary. 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 information

notice are not NRC requirements; therefore, no specific action or written

response is required.

Description of Circumstances

On May 24, 1993, at the Millstone Nuclear Power Station, Unit 2, Northeast

Utilities (the licensee) identified a leak in a body‐to‐bonnet gasket on

valve 2‐CH‐442. This valve is a Velan 2‐inch gate valve which is used to

manually isolate the letdown portion of the chemical and volume control system

from the reactor coolant system for maintenance and local leak rate testing of

containment isolation valves. The valve is upstream of the automatic letdown

isolation valves, and itself cannot be isolated from the reactor coolant

system. Between June 4, and August 5, 1993, leak sealant was injected

approximately 30 times in an attempt to stop the leakage from the body‐to‐

bonnet joint. Five sealant injections were performed by Leak Repairs, Inc.,

and the remainder were performed by Furmanite, Inc.; these activities were

performed under the direction of the licensee. The injections had marginal

results; after sealing, the valve would remain leak free for various periods

ranging up to 18 days in length and would then start to leak again.

On June 12, 1993, following the 7th leak‐seal injection, technicians attempted

to install a body‐to‐bonnet peripheral clamp to provide a boundary for the

leak‐sealant compound. The clamp could not be installed because of fit‐up

problems with the irregularly shaped valve bonnet. The bonnet had been

installed 90 degrees away from its normal orientation, further complicating
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IN 93‐90

December 1, 1993

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the attempted clamp installation. Subsequent on‐line attempts to seal the

leak included mechanical peening to prevent injection material from extruding

out of the gasket area. This peening closed the body‐to‐bonnet joint.

On August 5, 1993, leak sealant was injected to obtain the dry valve surfaces

required for welding a second (hybrid) clamp across the body‐to‐bonnet joint.

During the injection, technicians observed that the leakage increased

dramatically, from slight seepage to a 3.1‐meter [10‐foot] steam jet. The

technicians also reported that the valve bonnet appeared to lift and that one

body‐to‐bonnet stud moved. In response to the increased leakage, the Director

of Millstone, Unit 2, ordered a controlled normal shutdown. During the

shutdown, the maximum reactor coolant system leakage was 16.3 liters

[4.3 gallons] per minute.

Discussion

When valve 2‐CH‐442 was disassembled, it was discovered that one stud was

broken. Because the body of valve 2‐CH‐442 is an unisolatable reactor coolant

system pressure boundary, the broken stud changed the character of the

incident from a routine gasket‐related failure to a significant structural

failure. An ABB‐CE metallurgical study indicates that the stud broke in

response to loads applied as part of the on‐line leak sealing process. The

most likely potential sources of loads were evaluated. These likely load

sources are: (1) drilling, tapping, and injection port installation,

(2) peening, and (3) injecting. Subsequent testing performed for the licensee

at the ABB‐CE facility indicated that stud loads produced by drilling,

tapping, and injection port installation were moderate. However, when body‐

to‐bonnet joints are peened to the point that the edges contact, the adjacent

bolt can be loaded to failure due to the wedging action of the deformed metal.

Inspection revealed that two of the four studs had drill holes in them from

the injection port and clamp installation drilling processes. The broken stud

and one other stud each had 0.49 centimeter [0.19 inch] diameter holes that

penetrated 0.25 centimeter [0.10 inch] and 0.28 centimeter [0.11 inch] deep,

respectively. The holes did not appear to contribute to the stud failure.

The licensee had intended to limit the injection port locations to low stress

zones. However, the injection ports had actually been drilled in restricted

high‐stress zones located near the studs. Later analysis determined that the

drilling caused very little stress on the studs.

The leak‐sealant injection procedure at Millstone permitted some peening

between the body and bonnet to prevent sealant extrusion (it did not allow

peening along the entire perimeter of the valve). Inspection of valve

2‐CH‐442 revealed a significant amount of metal moved by the peening process.

Peening essentially obscured the body‐to‐bonnet interface, leaving a groove‐

like indentation along the split line, and metal was peened so that it was in

contact with all four bonnet studs. In addition, chisel‐like marks were .

IN 93‐90

December 1, 1993

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evident around the leak‐sealant injection ports where hand peening was

performed. The licensee determined that the extensive peening was responsible
for the stud failure.

The licensee had made no provision to limit the amount of leak sealant

injected into the valve. As a result, a total of approximately 2.16 liters

[0.57 gallon] of leak sealant was injected into the body‐to‐bonnet joint.

The repeated attempts to seal the valve at Millstone Unit 2 indicated that an

adequate engineering evaluation was not performed. The licensee evaluation

did not adequately consider the effects of the sealing process and the borated

water on the fasteners. Also, the evaluation did not adequately consider the

amount or effect of sealant entering the system after repeated injections.

Further, the evaluation did not adequately consider the operational and safety

consequences of structural failure of the component or the fasteners during

and after the leak‐seal attempts. In addition, management and quality

assurance oversight did not identify the failures to follow procedures, the

failures to adhere to engineering documents and the lack of weight given to

personnel safety considerations.

Events such as the one discussed above have the potential to cause a loss‐of‐

coolant accident and to result in personnel injury or death. This event

illustrates the importance of properly performed engineering and safety

evaluations and the importance of considering occupational safety hazards in

support of on‐line leak sealant use. When ASME Code Class 1 pressure boundary

components are involved, these considerations are especially important to

public safety.

This information notice 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 Office of

Nuclear Reactor Regulation (NRR) project manager.

/s/'d by BKGrimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contacts: Eric J. Benner, NRR

(301) 504‐1171

Geoffrey P. Hornseth, NRR

(301) 504‐2756

Charles G. Hammer, NRR

(301) 504‐2791

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