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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 8903160427 . IN 89-31 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. .. IN 89-31 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 ..
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