ML20028E148
| ML20028E148 | |
| Person / Time | |
|---|---|
| Site: | Bellefonte, 05000000, Washington Public Power Supply System |
| Issue date: | 01/14/1983 |
| From: | Taylor J BABCOCK & WILCOX CO. |
| To: | Deyoung R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE) |
| References | |
| REF-PT21-83-755-000 PT21-83-755, PT21-83-755-000, NUDOCS 8301210029 | |
| Download: ML20028E148 (7) | |
Text
{{#Wiki_filter:' p ie e Babcock & Wilcox soci.ar Power Generation Division a McDermott company 3315 Old Forest Road P.O. Box 1260 Lynchburg, Virginia 24505 (804) 384 5111 January 14, 1983 Mr. R. C. DeYoung, Director Office of Inspection and Enforcement U. S. Nuclear Regulatory Commission Washington, DC 20555
Dear Mr. DeYoung:
The purpose of this letter is to inform the NRC regarding a preliminary safety concern which was recently processed within B&W. It relates to the possible adverse effects of cross flow on control rod assemblies for B&W 205 FA plants. It applies only to the TVA, Bellefonte 1 & 2 and Washington Public Power, WNP-1 plants. The 177 FA plants have a different configuration of the . CRAs and related guide structures in the upper plenum and are therefore not affected by this potential concern. In processing this potential safety concern by our internal procedures it was decided that we would advise the NRC of the concern for information purposes, even though we judged it non-reportable under the requirements of 10 CFR 21. We delayed transmitting this information so that the submittal could include the results of some confirmatory test work still under way for the proposed fix (i.e., installation of sleeves to close the 3" column weldment holes). However, following a recent audit by the Region IV I&E Office, it was decided that we should advise you of this matter at this time. Control Rod Assembly Upper Plenum Guide Tube Supports Control rod assemblies when withdrawn from the core are contained within guide tubes as shown in Figure 2. Each guide tube assembly is made up of 16 peripheral "C" tubes and 8 internal " split" tubes which surround the 24 pins of a control rod assembly (Figu es 2 & 4). The guide tubes and their spacers form an assembly, the control rod brazement (Figure 2), which is contained within the column weldment (Figure 1 & 2). There is space for 81 control assemblies each with their brazement and weldments in a 205 FA reactor vessel. Flow from the core passes upward through and around the column weldments. The core flow from the fuel assemblies containing control assemblies enters the bottom of the column weldments. Most of this flow exits the column weldments at the three rows of holes in the lower part of the weldment. Most of the remaining flow within the weldment exits from the 2 rows of 3" holes in the upper portion of the weldment with only a small part going out of the top of the weldmentsinto the vessel dome (see Figure 1). The flow from both sets of holes in the weldments joins with the majority of the core flow from the fuel assemblies that do not contain control rods, exiting through holes in the upper plenum cylinder and then discharging to the reactor coolant system hot legs through the reactor vessel discharge nozzles (Figures 1, 2 & 3).
-l 8301210029 930114 PDR ADOCK 03000438 S PDR
Babcock &Wilcox Mr. R. C. DeYoung January 14, 1983 Each of the 16 "C" guide tubes contain forty 1/4" holes as shown in Figure
- 2. These holes provide hydraulic relief when the control rod assemblies are in motion and allow cross flow when they are stationary.
Description of Concern The 3" holes drilled in the upper portion of the column weldment surrounding control rod assemblies and smaller 1/4" holes drilled in corresponding locations in the "C" type guide tubes surrounding the individual CRA pins were the areas of concern. Two concerns were identified. The first concern postulated that cross flow velocities through these holes could produce drag forces that may increase rod drop times beyond those assumed in FSAR accident analysis. A second potential
! concern dealt with the possibility that vibration induced wear resulting from high cross flow velocities during normal operation might impair rod insertion, cause rods to separate from the spider holding them or result in perforations in pin cladding causing loss of B4C neutron poison by leaching.
Initial investigations indicated that in certain pin locations excessive cross flow velocities through the 3" dia. holes might be possible. Since cross flow velocities increase from a very low level at the center of the core to a maximum near the periphery, investigations were focused on those CRAs located toward the periphery of the core. Calculations indicated that at the elevation of the upper 3" diameter holes, the estimated cross flow velocities impinging on the guide tubes containing control rod pins could be high at a few locations but would vary considerably within a peripheral column weldment. This variation indicates that only a few of the total number of pins of a peripheral control rod assembly would be subjected to potentially excessive cross flow velocity. In addition, since all the 3" holes in the column weldments are aligned to two perpendicular axes and not to radii of the plenum cylinder, the cross flow patterns within the peripheral column weldments, due to a radially oriented external cross flow, would also vary from location to location (see Figure 3). These facts led to the following conclusions:
- 1. The control rod assemblies likely to see high velocities were those toward the outer periphery of the core.
- 2. The pins of postulated concern, for any given CRA, were limited in number and represented only a small percentage of the total control rod pins in the core.
- 3. The velocities and therefore the wear, if real, would vary widely from pin to pin.
- 4. Postulated wear, since it is produced from small vibratory motion, would propagate slowly.
These conclusions taken with the number of ways that developing pin wear, individual pin failure or B C4 leaching could be discovered, i.e., monthly
I Babcock &Wilcox Mr. R. C. De;ourg January 14, 1983 surveillance testing, incore and out-of-core neutron monitoring, refueling outage visual inspection or startup t-od worth tests led to the conclusion that normal operation cross flow velocities were unlikely to produce either a gross failure mechanism or significant impainrent to rod function that could go undetected. It was therefore judged that a significant increased risk to public health and safety did not exist from this concern. Further investigations were then focused on the possibility of impeded insertion of CRAs due to drag forces produced by cross flow. This potential for a common mode effect on rod insertion appeared to be the more serious concern, since estimates indicated that the drag forces could be high enough to delay rod insertion. It was recognized, however, that longer drop times would have been discovered and fixed as a result of established, required pre-operational rod drop tests. It was, therefore, concluded that no increased risk to the public health and safety could have resulted, even if this issue hadinot been raised. - In summary, it was judged that none of the concerns raised would lead to a substantial safety hazard. Nevertheless it was decided to be a matter requiring a design change. Consequently a change has been developed which eliminates cross flow through the weldments at the upper 3" hole location. This fix provides a sleeve which is expanded in the column weldment to seal off the upper rows of 3" holes eliminating cross flow through the weldment in that upper area. The adequacy of the revised design is being verified by an extensive test program. These changes are expected to be implemented before the first 205 plant goes into operation. Should there be any questions about this matter, please contact Mr. T. L. Baldwin (804) 385-3142 of my staff or me. Very truly yours, J
@ k J. H. Taylor
- Manager, Licensing JHT/fw cc: Mr. R. B Borsum - B&W Bethesda Office Mr. R. 'i. Brickley - NRC, Region IV, Office of I&E Mr. 4. R. Denton - NRC, Washington, DC
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