ML20107G842
| ML20107G842 | |
| Person / Time | |
|---|---|
| Site: | Robinson  |
| Issue date: | 10/12/1984 |
| From: | Beatty G CAROLINA POWER & LIGHT CO. |
| To: | James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| References | |
| EIB-84-3, IEB-84-03, IEB-84-3, RSEP-84-731, NUDOCS 8411080279 | |
| Download: ML20107G842 (9) | |
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Form 244 A T' 4 i,.
.4 Carolina Power & L;ght dompany 84 BCT23 AII: ga Company Correspondence POST OFFICE BOX 790 HARTSVILLE, SOUTH CAROLINA 29550 OCT 121984 Robinson File No:
13510A Serial: RSEP/84-731 Mr. James P..O'Reilly Regional Administrator U. S. Nuclear Regulatory Commission Suite 2900-101 Marietta Street, N. W.
Atlanta, Georgia 30303 H. B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO. 2.
DOCKET NO. 50-261 LICENSE NO. DP-23 IEB-84-03
Dear Mr. O'Reilly:
In a letter dated August 31, 1984, Carolina Power and Light Company (CP&L) provided a preliminary response to the subject bulletin informing the NRC that the seal arrangement at H. B. Robinson Unit 2 (HBR2) was significantly different from that at Haddam Neck. This response discusses these differences and concludes that the pneuma-seal at HBR2 will not " push through" the annulus as is understood to be failure the mechanism at Haddam Neck.
I.
Annulus Seal Description The opening between the reactor vessel and the reactor cavity liner is a nominal 2" annulus. Originally, a one foot wide metal ring, along with a set of square 0-rings and the liberal use of a sealing compound performed the annulus sealing function during refuelings. Since 1978, a single pneuma-seal has.been used as the primary seal for the 2" annulus. The same metal ring has been cut into 3 sections for ease of handling and is installed by resting it on the pneuma-seal and bolting it to the flanges on either side of the 2" annulus. See Figure 1.
The intent of resting r
this metal ring on the pneuma-seal was to protect the pneuma-seal from overhead objects and to help hold the pneuma-seal down. The pneuma-seal is designed so that, in the unlikely event of damage to the inflatable section, the wedge configuration at the top holds itself into the annulus and prevents excessive leakage.
8411080279 841012 PDR ADOCK 05000261 PDR G
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Lett r to Jamaa P. O'R illy
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Pcg2 2 II. Potential for Seal Failure The failure mechanism at Haddam Neck is understood to have been a " push through" of a properly inflated seal due to the hydraulic force of the head of water.
Figure 2 compares the Haddam Neck and HBR2 pneuma-seals. One
-significant differer.ce is that HBR2 has more rubber in the wedge and top portions of the pneuma-seal. Another difference is that at HBR2, the cavity gap or annulus opening is smaller than that at Haddam Neck. These significant differencec would not allow the pneuma-seal to " push through" the annulus at HBR2.
Calculations were performed which cupport the position that the HBR2 pneuma-seal is appropriately sized and will not " push through" the cavity annulus due to the forces of the maximum cavity water level.
The maximum downward deflection of the head of the pressure seal was computed using a beam model. This model accounts for the properties of rubber as well as the geometry of the seal. The i-idel does not include frictional forces due to the edges supporting the seal and the internal pressure of 30 psi. The additional stiffness due to the plate is also ignored. These result in a conservative value for the deflection of the seal due to the water pressure. Including a safety factor of four (4) for the normal water level of twenty-five feet (25') deep, the maximum deflection of the top of the head of the seal is.120 inches. Results of this calculation demonstrates, therefore, that four times the level of water could not result in the movement of a properly installed seal through the gap between the reactor pressure vessel and the cavity liner.
A test was performed on a one foot section of a deflated spare pneuma-seal. The test configuration utilized two steel plates to create a non-uniform gap (2 3/16" to 2 1/4").
The pneuma-seal surfaces were wetted to simulate operating conditions. A constant pulling force equivalent to approximately 65 ft. of water was applied for 30 minutes to the deflated seal with no measurable movement of the seal. The calculation and test results will be available at the H. B. Robinson Plant In addition, the metal ring resting on the pneuma-seal results in the water pressure being uniformly distributed across the top of the pneuma-seal. See Figure 3 Also, the use of a single pneuma-seal between two immovable objects at HBR2 insures the size of the annulus is fixed and not subject to potential shifting of a seal ring.
[L Littcr to Mr., Jim:s P. O'Reilly Page'3 III. Evaluation of Consequences' Based on the above tests and calculations of a deflated pneuma-seal which demonstrates that the pneuma-seal will not fall through the annulus, CP&L considers the pneuma-seal r. passive seal system. Therefore, failure of the pneuma-seal is not a' creditable event.
However, existing procedures and operator actions in the event of a decreasing cavity level during fuel movement were reviewed. This review considered the various cavity and spent fuel building elevation shown in Figure 4.
Based on this review, procedures will be reviaed to provide additional guidance in the unlikely event that some unforeseen problems
. occur. These revisions are addressed in Section V below.
'IV.-
Action Taken The metal ring that rests on the pneuma-seal at HBR2 has a rib on the center line of the bottom of the metal ring which made the actual contact with the pneuza-seal. This metal ring has been turned over so that the rib is on top, and the. metal plate now makes full contact with the
. pneuma-seal. See Figure 5.
V.
Action to be Taken Prior.to moving fuel back to the containment from the Spent Fuel Pit, the Refueling Emergency Procedure will be revised to address actions to take in case of a decreasing level in the Refueling Cavity exceeding the make-up capacity.
This procedure will address isolating the Spent Fuel Pit from the reactor cavity and will provide instructions for relocating fuel in transient to locations which will ensure the fuel remains covered with watcr. As an additional conservative action, operations in containment will be restricted such that'the operator will have to be concerned with the relocation of only one spent fuel assembly.
VI.. Conclusion Calculations, along with a physical test of a pneuma-seal at HBR2, have clearly demonstrated that the pneuma-seal will not experience a " push through" gross failure as occurred at Haddam Neck and is. considered as a passive sealing system. However, procedures which deal with handling fuel
T LLetter. to James'P. O'Railly Page li during refueling operations have been reviewed and will be upgraded prior
' to moving-fuel'back to the reactor cavity which will address appropriate
- actions,to take in the case of'a decreasing level in the refueling cavity.
If you have an'y questions concerning this response, please. contact my staff or me.
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[uyP.Beatty/[r.
Manager Robinson' Nuclear Project Department
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FIGURE I H.B. ROBINSON SEAL. CONFIG'URATION WITH BACKING RING. IN' ' ORIGINAL' POSITION w
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