Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 22, 1989
Information Notice No. 89-31: SWELLING AND CRACKING OF HAFNIUM
CONTROL RODS
Addressees:
All holders of operating licenses or construction permits for pressurized
water reactors (PWRs) with Hafnium control rods.
Purpose:
This information notice is being provided to alert addressees of swelling and
cracking of Hafnium control rods at several PWRs. It is expected that reci-
pients will review the information for applicability to their facilities and
consider actions, as appropriate, to avoid similar problems. However, sug-
gestions contained in this information notice do not constitute NRC require-
ments; therefore, no specific action or written response is required.
Description of Circumstances:
During eddy current and profilometry measurements of the control rods at the
Wolf Creek Generating Station, abnormal wear and swelling were identified in
53 Hafnium rod cluster control assemblies (RCCAs). As a result, the Wolf
Creek licensee informed the RCCA vendor, Westinghouse Electric Corporation, of
their inspection results. Subsequently, Westinghouse examined previous
inspection data from Union Electric Co.'s Callaway Plant and confirmed that
Hafnium control rod swelling was also occurring at Callaway. In addition,
there has been some foreign experience with these Hafnium control rods
breaking due to either swelling or hydriding and fretting.
Discussion:
Westinghouse introduced Hafnium material in RCCAs as a replacement for silver-
indium-cadmium (Ag-In-Cd) in the 1970s. Some control rod cladding wear was
expected; however, the extent of swelling and cracking discovered at the Wolf
Creek and Callaway reactors was not.
Westinghouse believes that the Hafnium control rod swelling and cracking are
caused by hydriding of the Hafnium. Hydriding of the control rod occurs when
hydrogen from the reactor coolant diffuses through the stainless steel
cladding and reacts with Hafnium to form Hafnium hydride. The hydriding
phenomenon was addressed by Westinghouse during the development of the Hafnium
control rods
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March 22, 1989
Page 2 of 3
and was not considered to be significant. During autoclave testing of the
control rods in the 1970s, hydriding occurred uniformly over the entire
length of the rod and resulted in only a very minor volumetric increase.
At the Wolf Creek and Callaway reactors, however, the abnormal wear and local-
ized swelling of the control rods occurred at several locations, with the
largest number of swollen sections located in the upper regions of the control
rods. According to Westinghouse, Hafnium reacts with air during the
fabrication process and quickly forms a protective oxide layer on the Hafnium
rod surface that inhibits the hydriding process. It is postulated that
initial misalignment of the Hafnium rods with the cladding during the assembly
of the RCCAs can result in loose particles or spalling of the stainless steel
cladding near the top of the inner diameter of the cladding. During thermal
cycling, the differential thermal expansion between the Hafnium and the
stainless steel cladding causes the particles to come in contact with the
Hafnium and to penetrate or remove the protective oxide layer. After the
oxide layer is penetrated, the diffusion of hydrogen into the Hafnium occurs.
Because the Hafnium and the cladding have a constant interface at the bottom
of the control rod, the greatest relative movement between the Hafnium and the
stainless steel occurs at the top of the control rod and results in increased
hydriding.
The principal safety concern associated with swelling and cracking caused by
hydriding is the inability of a control rod to fully insert into the core
because of interference between control rods and the guide cards. The safety
analysis prepared by Westinghouse to evaluate the potential consequences of
control rod swelling and end plug cracking indicates that there is no possi-
bility of interference or locking of the control rods. The maximum
theoretical diametrical increase of a control rod due to the hydriding
mechanism has been calculated by Westinghouse to be 0.026 inches. In the
safety analysis, the maximum increase was conservatively assumed to be 0.029
inches and resulted in a maximum control rod diameter of 0.410 inches. Since
the passage way in the guide cards through which the control rods travel has a
nominal inside diameter of 0.420 inches, Westinghouse has determined that a
nominal clearance of at least 0.010 inches exists between the control rods and
the guide cards when the maximum diametrical increase is assumed.
Westinghouse has also calculated that operating plants with Hafnium control
rods will experience only a small increase in control rod drop times (less
than 0.1 seconds) assuming that all 24 RCCA individual absorber rods are bent.
In addition, no plant is expected to exceed its technical specification limits
on rod drop time through the end of either a third 18-month cycle or a fourth
12-month cycle.
Although the current information does not suggest the need for immediate
action, the NRC is concerned about the long-term continued operation of
nuclear reactors with swollen or cracked Hafnium control rods. The NRC staff
and Westinghouse personnel are continuing their evaluations on this issue. In
the interim, it is important that addressees take whatever actions are
necessary to ensure that similar problems, if they exist at their facilities,
are detected early and cor-rective actions are taken to preclude any
deterioration of the safety function of the control rods.
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March 22, 1989
Page 3 of 3
No specific action or written response is required by this information notice.
If you have any questions regarding this matter, please contact one of the
technical contacts listed below or the Regional Administrator of the appropri-
ate regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: S.L. Wu, NRR
(301) 492-1065
L. Phillips, NRR
(301) 492-3235
Attachment: List of Recently Issued NRC Information Notices
.. Attachment
IN 89-31
March 22, 1989
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
89-30 High Temperature 3/15/89 All holders of OLs
Environments at or CPs for nuclear
Nuclear Power Plants power reactors.
89-29 Potential Failure of 3/15/89 All holders of OLs
ASEA Brown Boveri or CPs for nuclear
Circuit Breakers power reactors.
During Seismic Event
89-28 Weight and Center of 3/14/89 All holders of OLs
Gravity Discrepancies or CPs for nuclear
for Copes-Vulcan power reactors.
Air-Operated Valves
89-27 Limitations on the Use 3/8/89 All holders of OLs
of Waste Forms and High or CPs for nuclear
Integrity Containers for power reactors,
the Disposal of Low-Level fuel cycle
Radioactive Waste licenses and
certain by-product
materials licenses.
89-26 Instrument Air Supply to 3/7/89 All holders of OLs
Safety-Related Equipment or CPs for nuclear
power reactors.
89-25 Unauthorized Transfer of 3/7/89 All U.S. NRC
Ownership or Control of source, byproduct,
Licensed Activities and special
nuclear material
licensees.
89-24 Nuclear Criticality Safety 3/6/89 All fuel cycle
licensees and
other licensees
possessing more
than critical
mass quantities of
special nuclear
material.
89-23 Environmental Qualification 3/3/89 All holders of OLs
of Litton-Veam CIR Series or CPs for nuclear
Electrical Connectors power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
..
