Information Notice No. 82-27: Fuel Rod Degradation Resulting from Baffle Water-Jet Impingement

 

SSINS No.: 6835 IN 82-27 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 August 5, 1982 Information Notice No. 82-27: FUEL ROD DEGRADATION RESULTING FROM BAFFLE WATER-JET IMPINGEMENT Addressees: All nuclear power reactor facilities holding an operating license (OL) or construction permit (CP). Purpose: This information notice is provided as a notification of an incident that may have safety significance. It is expected that recipients will review the information for applicability to their facilities. No specific action or response is required at this time. Description of Circumstances: On May 6, 1982, Portland General Electric submitted a Licensee Event Report (LER) 344/82-06, describing abnormal fuel clad degradation identified during a pre-planned fuel inspection to locate suspected leaking fuel assemblies. Fuel rod damage involved 17 fuel assemblies examined at the end of Cycle 4 operation. Portions of fuel rods were found missing and loose fuel pellets were discovered and retrieved from reactor vessel internals and the refueling cavity. Visual inspections revealed severe perimeter fuel rod failures in 8 fuel assemblies. Failures in the remaining 9 assemblies were detected by sipping operations, but did not exhibit visual damage. This general type of fuel rod damage has been encountered previously. In April 1980, fuel rod failures were found in two fuel assemblies at the end of Cycle 2 for the Trojan reactor (LER 344/80-06. Similar events were also reported for the Point Beach 1 reactor during the years 1975 and 1977. These occurrences are delineated in LER 266/75-18 and 76-12, respectively. IE Circular No. 80-17 likewise has cited fuel rod failures for foreign plants. The aforementioned fuel rod damage, involving 10 of the 17 assemblies, can be attributed to the water-jetting-induced motion of fuel rods in fuel assemblies that are adjacent to baffle plate joint locations with enlarged gaps. Two mechanisms for baffle gap-related rod failures have been determined. The first of these is the outside corner or center-injection jetting failure that is similar to clad degradation as described in LER 344/80-06. (See Figure 1.) In this case, the water jet impacts on the third rod from the corner and causes its failure in the lower axial regions from direct water impingement, combined with induced rod whirling and vibration. The second type of baffle gap-related failure is the inside corner or corner-injection jetting failure, whereby a jet . IN 82-27 August 5, 1982 Page 2 of 4 of water flows parallel to the fuel bundle perimeter face between the fuel and the adjacent baffle plate. This flow also causes fuel rod whirling and vibration to occur at the first few rod locations. Such water-jetting parallel to these fuel rods ultimately results in failure of the rods and in the likelihood of failure propagation to adjacent rods. In general, the water-jetting-induced rod motion causes fuel rod fretting because of abnormal clad wear against the Inconel grid assemblies, which consist of slotted straps interlocked in an "egg-crate" arrangement. For the Trojan design, holes in the core barrel provide pathways for bypass flow which is diverted from the annulus between the vessel and the core barrel walls, to a downward direction through holes in the horizontal "former" plate segments. (See Figure 2.) This small portion of the coolant flow between the baffle plates and the core barrel, provides additional cooling for the barrel. However, most of the coolant flow from the vessel cold-leg inlet nozzles is downward through the annulus between the core barrel and the vessel walls, then into a plenum at the bottom of the vessel. It then reverses direction and flows upward through the core. A pressure differential is thereby established between the downward coolant flow in the vessel-barrel annulus and through the "former" plates, and the reversed upward flow through the core inside the . IN 82-27 August 5, 1982 Page 3 of 4 barrel. Coolant cross-flow through the enlarged baffle gaps results in high velocity jetting because of this pressure differential. The baffle water-jet then impinges on fuel rods and induces excessive rod motion, producing severe clad degradation. One fix of the baffle joint is to peen the entire joint to reduce the gaps between baffle plates. However, as noted in the most recent Trojan Event Report (82-06) relating to the problem, the gaps in the corner-injection baffle joints were apparently enlarged during the peening of the center- injection baffle joints at the end of Cycle 3 operation (1981). In addition, peening did not entirely dispose of the problem with the center-injection assemblies, since damage was found in three fuel assemblies of this type. A more successful interim solution includes the use of modified fuel assemblies to replace those damaged assemblies adjacent to baffle plate joints. Such modified fuel assemblies which utilize stainless steel fuel rods have been licensed and used previously. Two fuel assemblies were modified for Cycle 3 operation at . IN 82-27 August 5, 1982 Page 4 of 4 Trojan, as a result of the baffle jet problem described in LER 344/80-06. Three Zircaloy-clad rods in each of two assemblies were replaced with solid stainless steel rods of the same diameter and length as the fuel rods. Damage was not observed in the modified assemblies at the end of Cycle 3 or at the end of Cycle 4. These assemblies were located adjacent to baffle corners where water-jetting-induced fuel rod degradation had occurred. LER 344/82-06 cites the continued use of modified fuel assemblies with stainless steel rods, in addition to inserting partial grids that will provide midspan support to fuel rods adjacent to corner-injection baffle joints. The partial grids serve to increase the frequencies and decrease the amplitudes of the fundamental modes of vibration for fuel rods, and thus raise the threshold for rod vibration, which causes clad damage. With regard to the center-injection assemblies, the most vulnerable fuel rod (the one nearly aligned with the baffle plate gap) and the two adjacent fuel rods in the first row are to be replaced with stainless steel rods to minimize the possibility of additional baffle-jet-induce failures. Westinghouse is presently evaluating proposed permanent fixes for the baffle water-jetting problem. Although the baffle water-jetting problem has been experienced in a limited number of Westinghouse PWRs, this information notice is being distributed to all licensees and construction permit holders, including PWRs whose core baffle designs may have features which contribute to fuel rod failures as previously described. Such fuel degradation may result in relatively high primary coolant activity and thereby impede periodic maintenance-related functions and/or pose radiological hazards to personnel. Edward L. Jordan, Director Division of Engineering and Quality Assurance Office of Inspection and Enforcement Technical Contact: R.M. Young 49-29672 Attachment: List of Recently Issued IE Information Notices

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