NRC NEWS

 U.S. NUCLEAR REGULATORY COMMISSION 

 Office of Public Affairs Telephone: 301/415-8200

 Washington, DC 20555-001 E-mail: opa@nrc.gov 

 Web Site: http://www.nrc.gov/OPA

No. 01-096 August 3, 2001

NRC ASKS FOR INFORMATION FROM CERTAIN NUCLEAR PLANT LICENSEES 

ON REACTOR VESSELS

The Nuclear Regulatory Commission has issued a bulletin seeking information from several nuclear

power licensees regarding the structural integrity of reactor pressure vessel head penetrations.

The bulletin is being sent to the licensees of 69 pressurized water reactors (PWRs) as the result of recent

discoveries of cracked and leaking penetration nozzles in the top of reactor pressure vessels. These nozzles are

fabricated from Alloy 600 material. 

These discoveries have raised concerns about the structural integrity of reactor penetration nozzles in the

top of reactor pressure vessels at PWRs throughout the industry. Control rod drive shafts pass through

penetration nozzles which sit at the top of a reactor vessel head. Control rod drive mechanisms (CRDMs) are

used to guide the movement of control rods in and out of a reactor core.

Axial cracking (along the length) of these nozzles has previously been observed but has not been

considered a safety concern requiring immediate attention. This issue was addressed by the NRC in Generic

Letter 97-01, issued in 1997. Inspections and repairs have generally been made during plant maintenance

outages. However, earlier this year, circumferential cracking (part way around the width of the nozzle, above

the structural retaining weld) was discovered at two PWRs.

One function of the nozzles is to maintain the reactor coolant system pressure boundary. Cracking of

the control rod drive mechanism nozzles represent a degradation of the primary reactor coolant system

boundary, and hence, is potentially safety significant. 

 NRC is requesting information from PWR licensees to determine whether current inspection practices

are adequate and whether any additional regulatory response by the agency is necessary. 

The NRC bulletin requests PWR licensees to provide information related to the structural integrity of

these reactor pressure vessel head penetration nozzles, ìas needed to ensure the integrity of their reactor coolant

system pressure boundaries and demonstrate compliance with applicable regulatory requirements.î The bulletin

also requires that PWR licensees submit a written response indicating whether the requested information will be

submitted within the required 30-day time period. NRC licensees unable to satisfy this requirement will have to

submit a written description of any alternative course of action they propose to take, including the basis for its

acceptability. 

NRC has established a web site at: http://www.nrc.gov/NRC/REACTOR/ALLOY-600/index.html
to keep the public informed of generic activities on cracking of Alloy 600 material in PWRs. This page provides

links to information regarding the cracking identified to date, along with documentation of NRC interactions

with its licensees (industry submittals, meeting notices, presentation materials, meeting summaries and the full

text of this bulletin). The NRC will continue to update this web page as new information becomes available. 

 NRC will meet with representatives of the Nuclear Energy Institute and PWR licensees on August 15 to

discuss the agencyís expectations regarding licensee responses to the bulletin. The meeting will be held from

9:00 to 11:00 a.m. in room T-2B3 of the agencyís Two White Flint North building, 11545 Rockville Pike,

Rockville, Maryland. The meeting is open to public observation.

EDITORS: A drawing illustrating a typical pressurized water reactor vessel upper head is attached.

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TYPICAL PWR VESSEL UPPER HEAD

CIRCLE SHOWS TYPICAL LOCATION OF OBSERVED PRIMARY

WATER STRESS CORROSION CRACKING (PWSCC)

.