ML20030B043
| ML20030B043 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 07/24/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20030B042 | List: |
| References | |
| NUDOCS 8108050104 | |
| Download: ML20030B043 (3) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION J
o t-l WASMNGTON. D. C. 20665
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Enclosure SAFETY EVALUATION REPORT BY THE OFFICE OF NUCLEAR REACTOR REGULATION CONTROL R0D GUIDE TUBE WEAR IN FACILITIES DESIGNED BY BABC0CK AND WILCOX (B&W)
A degradation of control rod guide tube walls was observed during post irradia-tion examinations of fuel assemblies removed from several pressurized water reactors.
In the investigation of this problem the NRC requested infonnation t
from B&W designed facilities related to the susceptibility of their facilities to significant guide tube wear. B&W prepared report BAW-1623, June 1980,
" Control Rod Guide Tube Wear Measurement Program" for the B&W Mk-B User's Group l
to provide this requested information. The findings of this report are as follows.
i Several flow-related mechanisms may cause Control Rod Assembly (CRA) vibration and guide tube wear; two of these appear to be the most probable. The first is turbulent flow along the rod or crossflow across the control rods in the upper internals, resulting in wear at the lower tip of the rod when in the park posi-l tion. The second hypothesized mechanism is axial flow inside the guide tube and l-a turbulent vibration response.
In both cases, an increase in primary coolant flow could result in higher rod vibration and possible higher wear; however it is felt that turbulent flow with some possible crossflow in the internals area is the most probable cause of rod vibration.
B&W's 15x15 array fuel assembly design includes 16 Zircaloy guide tubes for the control rods. Each CRA has 16 stainless steel clad contr31 rods connected to a stainlass steel spider which is in turn connected to the drive ' mechanism. Full length guidance for each CRA is provided by the guides in the upper plenum assembly and in the fuel assembly. The control rod tip always remains inside the fuel assembly guide tube during normal operation. When the control rod is in the full out (parked) position, the tip of the control rod is approximately nine inches inside the fuel assembly guide tube. The CRAs and guide tubes are l
designed with flexibility and clearances to pennit-freedom of motion within the j
fuel assembly guide tubes throughout the stroke of the CRA.
The B&W upper reactor internals design, in addition to the full-length guidance for the CRAs, has two features that minimize crossflow excitation and turbulence on the CRAs. The first is an upper plenum, which directs approximately 82% of the flow in an axial direction while allowing only 18% of the flow to be taken 8108050104 810754 PDR ADOCK 05000346
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_2-o This strong tendency towards out directly into the outlet nozzles.
axial flow will minimize any dependence of CRA vibration on outlet nozzle location. The second feature is the use of the full-length upper plenum tubes enclosing each CRA in the upper reactor internals.
These full-length tubes provided additional protection.for the CRAs against crossflow and turbulence.
Fuel assemblies were selected from across the core and for various f
effective full-power days (EFPD) of control rod operation in the safety position to determine -the effect of core-position, time, fluence exposure, To obtain the effect of flow rates on guide tube wear, fuel as-semblies were examined at Oconee 1, Oconee 3, and Rancho Seco, the nominal etc.
Two flows of which run from approximately 109 to 114% of design flow.
The first concern is (nat potential areas of concern were investigated.
any one guide tube could wear through at the control rod park position The (approximately 9 inches from the top of the control rod guide tube).
l second concern is that smaller amounty of wear in all 16 guide tubes in a l
fuel assembly could cause high stresses in that fuel assembly.
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The results from the Rancho Seco examination (high flow rate) indicates In general, that the largest mearared wear was 57% (through-wall wear).
l the wear was located approximately 9 inches down from the top of the guide l
tube, which corresponds to the locatio,n of the lower tip of the control l
This location also cor-rods of the safety banks in their park position.
responds to the control rod park position for control banks in some reactors.
Four Rancho Seco fuel assemblies indicated wear ranging from 6.9 to 14.9%
The other five assemblies for one and one half cycles of operation.
examined indicated no wear.
The results from the Oconee examination (low flow rates) indicate less wear than Rancho Seco, with a maximum measured indication of 27% (in Oconee 3).
The average wear for eac., Oconee fuel assembly was low, with a maximum of G.7%.
The second concern was that each guide tube would have some wear and that I
A stress analysis was performed that unacceptable stresses could occur.
This showed that through-wall wear for a single tube was acceptable.
j Two analysis also considered the case with some wear in each guide tube.
types of wear were considered - localized wear on only. one side of the The results of this tube and uniform wear around its circumference.
analysis show an allowable of 100% wear for localized (one-sided) defects Therefore, if the average-and 55% wear for uniform circumference wear.
wear in any fuel assembly (average of 16 tubes) does not exceed 55% uni-form wear, the #uel assembly will maintain a positive design margin.
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1 The results of both the Oconee and Rancho Seco measurements show small average wear with-the maximum average of 14.9% for one Rape.ho Seco assembly with 1 1/2 cycles of operation. The 14.9% was ~ one-sided wear
'which is compared to an allowable wear of 100%.
The results f*om the Rancho Seco statistical analysis indicate the pro-l bability of one guide tube wearing a hole is very low and is not expected.
The probaoility that a hole will occur in 150 weeks of operation is 0.000011. The results from this analysis also indicate that the proba-bility of the average of 16 tubes in one assembly-reaching 55% wear is j;
also very low 'and is not expected. The probability of the average of 16 l
tubes reaching 55% wear is 0.000001 for 150 weeks of operation. This case assumes uniform circumferential wear, which is not expected. ' The more likely case is wear occurring on only one side of the guide tube.
This case has a higher allowable wear (100% versus 55%) and hence a signi-ficantly smaller probability-of reaching this wear.
(The assumed maximum core residence time for a fuel assembly with an installed CRA is 150
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l weeks.)
The NRC staff has found that the B&W Report accounts for all of the major variables that affect the control rod guide tube wear process. Based on our review, we conclude that the control rod guide tube wear has an acceptably _ low likelihood of keeping within allowable wear limits.
Dated:
JULY 2 4 E81 l
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