United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-31: Swelling and Cracking of Hafnium Control Rods

                                  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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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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                                                            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 
..
Page Last Reviewed/Updated Tuesday, November 12, 2013