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
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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

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