Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 87-19
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
April 9, 1987
Information Notice No. 87-19: PERFORATION AND CRACKING OF ROD CLUSTER
CONTROL ASSEMBLIES
Addressees:
All Westinghouse nuclear power pressurized-water reactor (PWR) facilities
holding an operating license or a construction permit.
Purpose:
This notice is provided to inform recipients of a potentially significant
safety problem that could result from the perforation and cracking of the
rod cluster control assemblies (RCCAs) in Westinghouse PWRs. It is expected
that recipients will review the information for applicability and consider
action, as appropriate to preclude a similar problem from 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:
An estimate that was intended to be conservative indicated that the RCCAs
would last for at least 15 years before the absorber cladding, a thin tube,
would show excessive thinning as a result of sliding wear. These components
were inspected at Point Beach Nuclear Plant, Unit 2, in 1983 after 13 years
of operation. As a result of this inspection Point Beach reported on August
18, 1983 that sliding wear was minor, but one control rod had a 2-in. crack
near the tip of the rod and severe fretting wear had occurred on several
tubes. Subsequent inspections at the Kewaunee and Haddam Neck nuclear power
plants, which have been in operation for more than 12 years, confirmed the
fretting wear. In addition, Haddam Neck reported tube cracking in 32 of 47
RCCAs.
In the event of a breach of the tubing resulting from wall thinning,
perforation, or cracking, the immediate effect is the introduction of
activation products from the neutron absorber material into the reactor
coolant. Although there are large margins, another concern is the potential
reduction in shutdown margin and negative reactivity worth.
8704080095
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IN 87-19
April 9, 1987
Page 2 of 3
Discussion:
Each RCCA contains 16 rods. The rods at Point Beach, Kewaunee, and Haddam
Neck were constructed with an outer tube of 0.019-in.-thick 304 stainless
steel that retains the absorber material (80% silver, 15% indium, 5%
cadmium). Some newer plants use hafnium as the absorber material, while
others use boron carbide surrounded by a 0.038-in.-thick tube. The control
RCCAs are inserted or withdrawn to compensate for various reactivity changes
during operation of the reactor and can trip to provide shutdown capability.
The shutdown RCCAs are fully withdrawn from the core when the reactor is
critical.
At Kewaunee, marks of fretting wear about 1 inch in length, were found
adjacent to the guide blocks that position the rods when the RCCAs are in
their withdrawn position. The 1-in.-thick stainless steel blocks are spaced
on 12-in. centers and each rod in the cluster passes through all eight of
the blocks. At Point Beach the tubing wore in two modes: fretting and
sliding of the rods over the guide blocks during rod motion. Five RCCAs at
Haddam Neck had wall thinning resulting from fretting and four of these were
actually wearing into the absorber material. All of the others had fretting
wear, but to a lesser extent.
The fretting resulted from flow-induced vibratory contact between the rods
and the guide blocks during long periods of steady-state power operation.
Vibration is hydraulically induced by flow of the reactor coolant; therefore
it is a continuous process when the reactor coolant pumps are in operation.
According to Westinghouse Electric Corp. fretting wear encompassed one-third
of the circumference of the rod and the depth varied, with the amount of
time the RCCAs were in the withdrawn position.
At Point Beach significant number of short hairline cracks at the lower
extremity of the tubing were observed near the end plug region of the rod.
The cracks extended axially for 4 in. and penetrated the stainless tubing,
exposing the absorber material. No circumferential cracks were found.
Examination of the cracks showed that irradiation-induced swelling of the
absorber was the principal cause of tensile stress in the cladding, which
resulted in cracking after substantial irradiation.
Where excessively worn rods were found, they have been replaced. While the
issue is being studied by NRC and the industry, several licensees have been
given approval to slightly change the position of the fully withdrawn RCCA
in order to distribute the wear among different locations on the tubing.
Westinghouse Electric Corp. reported that an increase in the amount of the
silver isotope, Ag-110m in the reactor water is a reliable indication of
exposure of absorber material due to cracking or fretting wear.
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IN 87-19
April 9, 1987
Page 3 of 3
The NRC is continuing review of the safety significance of this information
to determine whether further NRC action is warranted. 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 regional office or this office.
Edward L. Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Paul Cortland, IE
(301) 492-4175
Attachment:
List of Recently Issued IE Information Notices
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