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| issue date = 11/13/1987
| issue date = 11/13/1987
| title = Forwards Executive Summary of Investigation,Results of Investigation & Conclusions Re Integrity of Containment Operating Deck Hatches.Hatches Would Not Fail During Accident or Earthquake.Noted Improvements to Covers Made
| title = Forwards Executive Summary of Investigation,Results of Investigation & Conclusions Re Integrity of Containment Operating Deck Hatches.Hatches Would Not Fail During Accident or Earthquake.Noted Improvements to Covers Made
| author name = ALEXICH M P
| author name = Alexich M
| author affiliation = INDIANA MICHIGAN POWER CO. (FORMERLY INDIANA & MICHIG
| author affiliation = INDIANA MICHIGAN POWER CO. (FORMERLY INDIANA & MICHIG
| addressee name = MURLEY T E
| addressee name = Murley T
| addressee affiliation = NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
| addressee affiliation = NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
| docket = 05000315, 05000316
| docket = 05000315, 05000316

Revision as of 16:06, 18 June 2019

Forwards Executive Summary of Investigation,Results of Investigation & Conclusions Re Integrity of Containment Operating Deck Hatches.Hatches Would Not Fail During Accident or Earthquake.Noted Improvements to Covers Made
ML17325A450
Person / Time
Site: Cook  American Electric Power icon.png
Issue date: 11/13/1987
From: Alexich M
INDIANA MICHIGAN POWER CO. (FORMERLY INDIANA & MICHIG
To: Murley T
NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
AEP:NRC:1043, NUDOCS 8711190120
Download: ML17325A450 (53)


Text

ACCESSION NBR FAC IL: 50-315 50-'316 AUTH.NAME ALEX ICH>M.P.RECIP.NAME MURLEYJ T.E.REGULATO INFORMATION DISTRIBUTION STEM (RIDS)8711190120 DOC.DATE: 87/11/13 NOTARIZED'O DOCKET Donald C.Cook Nuclear Power Plant>Unit 1>Indiana 5 05000315 Donald C.Cook Nuclear Power Plant>Unit 2>Indiana 0 05000316 AUTHOR AFFILIATION Indiana Michigan Power Co.(formerly Indiana Sc Michigan Ele RECIPIENT AFFILIATION Document Control Branch (Document Control Desk)

SUBJECT:

Forwards executive summary of.investigation>

results of-investigation Sc conclusions re integrity of containment operating deck hatches.DISTRIBUTION CODE:*002D COPIES RECEIVED: LTR i ENCL J.SIZE: TITLE: OR Submittal:

General Distribution NOTES: RECIPIENT ID CODE/NAME PD3-3 LA WIGGINGTON>

D INTERNAL: ARM/DAF/LFMB NRR/DEST/CEB NRR/DEBT/RSB S/ILRB 01 EXTERNAL: EG8cG BRUSKE>S NRC PDR COPIES LTTR ENCL 0 1 1 0 1 1 1 1 1 1 1 REC IP IENT ID CODE/NAME PD3-3 PD NRR/DEST/ADS NRR/DEST/MTB NRR/DOE*/TSB OGC/HDS1 RES/DE/EIB LPDR NSIC COPIES LTTR ENCL 5 5 1 1 1 1 1 1 1 0 1 1 1 1 1 TOTAL NUMBER OF COPIES REQUIRED: LTTR 21 ENCL 18 tt g h Indiana Michigan Power Company One Summit Square P.O.Box 60 Fort Wayne, IN 46801 219 425 2111 AEP:NRC:1043 Donald C.Cook Nuclear Plant Units 1 and 2 Docket Nos.50-315 and 50-316 License Nos.DPR-58 and DPR-74 INVESTIGATION OF INTEGRITY OF CONTAINMENT OPERATING DECK HATCHES EXECUTIVE

SUMMARY

U.S.Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.CD 20555 Attn: T.E.Murley November 13, 1987

Dear Dr.Murley:

On August 24, 1987, Unit 1 was in the last stages of a refueling outage, when a bolt supporting a reactor pump hatch cover (512)broke while being torqued.Because.of this, other bolts in that hatch were examined.Some of the other bolts appeared to have been cut off and rewelded to the seal channel.The bolts in the other containment operating deck hatches in both Units 1 and 2 were then examined.Iiatches in both units were found t:o have suspect bolts.Unit 2 was immediately shut down and an investigation of the contairunent hatch cover anchorages was initiated.

The attachments to this letter provide a detailed summary of our investigation, the results of the investigation, and our conclusions concerning the integrity of the D.C.Cook Units 1 and 2 containment operating, deck hatches.As discussed in the attached summary, we have concluded that both the Unit 1 and Unit 2 containment operating deck hatches would not have failed during an accident and/or design basis earthquake.

In addition, the as-found conditions met the design basis as described in the D.C.Cook FSAR, Section 5.2.2.4.We have, however, made or plan to make certain enhancement(s) to the anchorage of some of the hatch covers as described in our report.8711190120 871113 PDR ADQCK 050003i 5 P PDR Dr.T~E.Murley-2-AEP:NRC:1043 This document has been prepared following Corporate procedures which incorporate a reasonable set of controls to ensure its accuracy and completeness prior to signature by the undersigned.

Sincerely, M..Ale ch Vice President cm cc: John E.Dolan W.G.Smith, Jr.-Bridgman R.C.Callen G.Bruchmann G.Charnoff NRC Resident Inspector-Bridgman D.H.Danielson, NRC-Region III A.B.Davis-Region III 4 h~~'

D.C.COOK NUCLEAR PLANT-UNIT gl CONTAINMENT INVESTIGATION

-REACTOR COOLANT PUMP&CRDM HATCHES EXECUTIVE

SUMMARY

Hatch Design A.Location The four (4), Unit gl Reactor Coolant Pump (RCP)hatches are located on the Containment operating deck, elevation 652'-7 1/2".They are identified by numbers ll, 12, 13 and 14 which designate the unit and the quadrant in the unit where the hatch is located (i.e., RCP hatch Ill is in quadrant gl in Unit gl).The operating deck has one (1)other similar hatch on the operating deck at elevation 652'-7 1/2".This is the CRDM hatch which allows access to the Control Rod Drive Mechanism (CRDM)exhaust fans.(Figure No.1)B.RCP and CRDM Hatch Design The RCP hatches are approximately rectangular in shape and measure 7 feet 6 inches by about ll feet.(Figures No.2&3)The CRDM hatch is rectangular in shape and measures 3 feet by 6 feet.(Figure No.23)Each of the four (4)RCP hatches and the CRDM hatch has a steel tray which holds lead bricks.This steel tray is situated below but is independent of the hatch cover which is a welded plate frame filled with concrete.The lead tray and the hatch cover rest on ledges formed in the concrete deck by an angle frame embedment.(Figure No.4)Each RCP hatch cover is anchored to the operating deck by either 18 or 19-3/4" or 7/8" diameter ASTM A193, grade B7 high strength anchor bolts which fit into pockets in the cover.The CRDM hatch is anchored to the operating deck by 12-3/4" diameter ASTM A193 high strength anchor bolts which fit into pockets in the hatch cover.(Figures No.6&24)A seal rod welded to the bottom of the hatch cover fits into a seal channel welded atop the angle that forms the upper ledge which supports the hatch cover.The seal channel which runs around the periphery of the hatch has a continuous notch cut into it, into which an elastomeric seal material is fitted.The seal rod on the bottom of the hatch cover seats into the elastomeric material in the notch.in the seal channel to seal the hatch.(Figure No.24)

The operating deck is a reinforced concrete slab, thirty-four and one-half inches (34 1/2")thick.The anchor bolts are embedded twenty-one and three quarter inches (21 3/4")into the concrete.Each bolt is anchored by a 5 inch square by 1 inch thick steel plate embedded in the concrete.II.As Found Condition of Unit gl Hatches A.Background On August 24, 1987, Unit gl was in the last stages of a refueling outage and Unit g2 was operating.

While reinstalling the cover on RCP hatch f12 in Unit gl, bolt 510 broke while being torqued.Because of this, the other bolts in that hatch were examined and several were found with the appearance of having been cut off and rewelded to the seal channel.The remaining bolts in the nine (9)other RCP and CRDM hatches in both units were then examined by plant QC personnel and hatches in both units were found to have suspect bolts.Unit g2 was immediately shut down.This and subsequent NDE and visual inspections identified eight (8)bolts which had discontinuities at the level of the seal channel and three (3)bolts which were missing in Unit 51.A summary of the results of the NDE and visual examinations follows.B.Description of As Found Unit gl Configuration Hatch Ill All bolts were found to be sound.Hatch 412 4 bolts cut off and rewelded-bolt 410, Ill, gl2, and 513.Note that bolt 410 broke in the weld while being torqued.Hatch 413 2 bolts missing-bolts g9 and 415.This had been documented and analyzed previously.

Hatch 514 1 bolt missing-bolt 47.This had been documented and analyzed previously.

CRDM Hatch 4 bolts cut off and rewelded-bolts gl, g6, g7 and 412.

The bolt layout and the locations of the suspect bolts are shown in Figure No.25.III.Analysis-Unit gl RCP and CRDM Hatches All of the hatches were analyzed for: o The operation of Unit gl in the as-found condition.

o The operation of Unit gl after modification of the anchorage.

o The original anchorage design.In the analyses, an equivalent pressure load of 18 psi was applied to each hatch cover and its anchorages.

The equivalent pressure load included the effects of dead load, earthquake and accident in accordance with the load factor equations of the D.C.Cook Plant updated FSAR.The Structural Design Section of AEPSC analyzed all of the hatches in both units.The hatch covers and their anchorage were evaluated using the computer program GTSTRUDL and manual computations.

RCP Hatches gl2, f13 and$14 and the CRDM hatch were analyzed assuming that the cut and rewelded bolts and the missing bolts did not exist, although as noted in the Unit g2 Hatch Investigatxon Executive Summary, the in-situ, plant pull tests made on the RCP hatch 423 bolts in Unit g2 established that the cut off and rewelded bolts were capable of carrying a considerable amount of load.The analysis of RCP hatch 512 was made assuming that bolts gl0, gll, 512 and gl3 did not exist.Without these bolts, the hatch was found to be structurally adequate under the design loads and would not have unseated in an accident and/or earthquake.

The analysis of RCP hatch 513 was made assuming that the two missing bolts g9 and 515 did not exist.This was a known condition that had been analyzed previously.

The hatch was found to be structurally adequate under the design loads and would not have unseated in an accident and/or earthquake.

The analysis of RCP hatch gl4 was made assuming that the missing bolt, 57 did not exist.This case was enveloped by the RCP hatch 513 analysis.This was a known condition that had been analyzed previously.

The hatch was found to be structurally adequate and would not'have unseated in an accident and/or earthquake.

o The analysis of the CRDM hatch was made assuming that the four (4)cut and rewelded bolts did not exist.The hatch was found to be structurally adequate and would not have unseated in an accident and/or earthquake.

The analysis of the Unit gl RCP and CRDM hatches is contained in Calc./Anal.

No.DC-D-3195-165-SC.

A copy of the analysis of RCP hatch 412 was given to the NRC Region III personnel who attended the September 10, 1987 presentation on the Unit gl hatches at D'.C.Cook Plant.IV.Restoration of Unit gl RCP and CRDM Hatches V.Based on the results of the analyses of the as-found condition of the Unit gl operating deck hatches and because the bolts which were cut off and rewelded to the seal channel are required to carry small tensile loads, it was decided to enhance the anchorage of only RCP Hatch 512 at present.k To enhance the anchorage along the crane wall side of RCP hatch 512, three hold down plates were designed and bolted to existing embedded plates in the crane wall above the hatch opening.The remaining cut and rewelded bolts can be used to tighten down on the hatch cover.The hold down plates sit atop the hatch cover and hold the hatch cover seal rod in place along the crane wall during an accident and/or earthquake.

These hold down plates aid in holding down the hatch cover, but they are not required to maintain the integrity of the hatch because the remaining effective anchor bolts are capable of doing that without help.(Figures No.26&27).Based on the experience gained in restoring the bolts cut off and rewelded in Unit 52, AEP has decided to restore all the Unit gl bolts that were cut off and rewelded or missing in a way similar to that which was done on Unit g2.This work is planned for the next scheduled Unit gl outage in 1989.(Figures No.12 through 15, 17, 19 and 21 show the type of restoration planned for the Unit gl bolts.)Documentation of As'-found Hatch Bolt Configurations As part of the investigation, we made extensive searches and conducted pertinent personnel interviews in attempting to locate records that would address the as-found bolt configurations.

Both construction and operation records were reviewed and pursued.Personnel involved with the construction history and personnel involved with the operations history of the D.C.Cook Plant were interviewed.

I' Based on our searches and interviews, we believe that the applicable bolts were probably cut and rewelded prior to the unit going operational.

Certain construction meeting minutes indicate that work activities, related to the alignment of the hatches, continued for an extensive period of time.Though not conclusive, the possibility exists that the applicable bolts were cut and rewelded during the extensive hatch alignment period.No other records were found which address the bolts which were cut and rewelded.VI.Conclusions We believe, based on the analyses that have been performed on the Unit: Nl RCP and CRDM hatches, that the hatch covers would not have failed during an accident and/or earthquake and that in the as found condition, these hatches met the design basis in section 5.2.2.4 of the FSAR.The plant was operated with a secure divider barrier between the upper and lower containments.

The modifications made to the anchorage of RCP hatch 412 enhance the structural integrity of the hatch.

D.C.COOK NUCLEAR PLANT-UNIT 52 CONTAINMENT INVESTIGATION REACTOR COOLANT PUMP HATCHES EXECUTIVE

SUMMARY

Hatch Design A.Location B.The four (4), Unit 52 Reactor Coolant Pump (RCP)hatches are located on the Containment operating deck, elevation 652'-7 1/2".They are identified by numbers 21, 22, 23 and 24 which designate the unit and the quadrant in the unit where the hatch is located (i.e., RCP hatch 521'is in quadrant gl in Unit 42).The operating deck has one (1)other similar hatch on the operating deck at elevation 652'-7 1/2".This is the CRDM hatch which allows access to the Control Rod Drive Mechanism (CRDM)exhaust fans.(Figure No.1)RCP Hatch Design The RCP hatches are approximately rectangular in shape and measure 7 feet 6 inches by about ll feet.(Figures No.2&3)Each of the four (4)RCP hatches has a steel tray which holds lead bricks.This steel tray is situated below but is independent of the hatch cover which is a welded plate frame filled with concrete.The lead tray and the hatch cover rest on ledges formed in the concrete deck by an angle frame embedment.(Figure No.4)Each RCP hatch cover is anchored to the operating deck by either 18 or 19-3/4" or 7/8" diameter ASTM A193g grade B7 high strength anchor bolts which fit into pockets in the cover.(Figures No.5&6)A seal bar welded to the bottom of the hatch cover fits into a seal channel welded atop the angle that forms the upper ledge which supports the hatch cover.The seal channel which runs around the periphery of the hatch has a continuous notch cut into it, into which an elastomeric seal material is fitted.The seal bar on the bottom of the hatch cover seats into the elastomeric material in the notch in the seal channel to seal the hatch.(Figure No.5)The operating deck is a reinforced concrete slab, thirty-four and one-half inches (34 1/2")thick.The anchor bolts are embedded twenty-one and three quarter inches (21 3/4")into the concrete.Each bolt is anchored by a 5 inch square by 1 inch thick steel plate embedded in the concrete.

II.As Found Condition of Unit g2 Hatches A.Background On August 24, 1987, Unit gl was in the last stages of a refueling outage and Unit 52 was operating.

While reinstalling the cover on RCP hatch 412 in Unit gl, bolt 410 broke while being torqued.Because of this, the other bolts in that hatch were examined and several were found with the appearance of having been cut off and rewelded to the seal channel.The remaining bolts in the nine (9)other RCP and CRDM hatches in both units were then examined by plant QC personnel and hatches in both units were found to have suspect bolts.Unit 52 was immediately shut down.This and subsequent NDE inspections identified twenty (20)bolts in Unit 52 which had discontinuities at the level of the seal channel.A summary of the results of the NDE examinations follows.B.Description of As Found Unit g2 Configuration Hatch$21 3 bolts cut off and rewelded-bolts g3, g4&g5.1 stud with no embedded anchor bolt in concrete below-bolt 417.Hatch 522 1 bolt cut off and rewelded-bolt gl.1 stud with no embedded anchor bolt in concrete below-bolt g3.Hatch 423 12 bolts cut off and rewelded-bolts gl, 52, g8 thru gl6, and gl9.1 stud with no embedded anchor bolt in concrete below 418.Hatch 524 1 bolt cut off and rewelded-bolt gl.'CRDM Hatch All bolts were found to be sound.The bolt layout and the locations of the suspect bolts are shown in Figure No.7.

III.Test Program-Hatch 523 Bolt g9 in hatch 423 and a section of the seal, channel it was attached to were cut out and a lengthwise slice was made through the specimen to examine the weld.This bolt was selected because from previous analyses we knew that bolt 59 was one of the lesser loaded bolts in the hatch.Cutting out this bolt posed less risk in the restoration of the hatch.The slice was sent to Westinghouse Electric Corporation's General Technology Systems Division (metallurgical laboratory) for testing to establish an analytical baseline.The results of the metallurgical analysis showed that: The Stud was a 0.254 carbon steel.~(Specimen Tensile Strength=85 ksi)The embed bolt was AISI 4140 steel*(Specimen Tensile Strength=150 ksi).The seal channel was a carbon steel similar to ASTM A36 steel*(Specimen Tensile Strength=70 ksi).The weld was a low carbon steel probably made by an E7018 electrode.

  • (Specimen Tensile Strength=95 ksi).*Approximate specimen tensile strengths were obtained from hardness test conversions.

The welds were mapped.Weld Sizes, location and descriptions were indicated on as-found sketches.Bolt stamps identifying bolt material were noted where found.This information was documented by plant QC and site design personnel.

The As-found bolt locations were documented.

All bolts were photographed and videotaped.

A series of samples/mockups were fabricated and tested to approximate the as-found conditions and to establish an analytical baseline.These tests were proposed and implemented by Stevenson and Associates of Cleveland.

AEPSC reviewed and approved the test program.Most of the mockups were fabricated at D.C.Cook Nuclear Plant.The single exception was the embedded angle/seal channel mockups.The tests were made at Heron Labs and Case Western Reserve University in Cleveland under the direction of Stevenson and Associates personnel.

I The cut off and rewelded bolts were pull tested in place.Alp cut and rewelded bolts were pull tested to at least 20 except bolt N9 which had been cut out with a portion of the seal channel and bolt 418 where no embedded bolt was found in the concrete below the seal channel.Bolts g8 and N10 were pulled against the seal channel.All other bolts were pulled against beams which rested on the opegating deck level.All bolts reached a tension of 20 without bregking except bolt gl0 which broke at a tension of 16.5.This bolt broke in the weld.Bolts NS, N13, 414, gl5 and gl6 were retested.Bolt 48 was pulled ygainst the seal channel.The bolt was tensioned to 30 without apparent distress and did not break.Bolt N13 broke at 41.25 , 1/2" above the seal channel in the heat effected zone of the shank of the jolt.Bolt gl4 broke in threads at akload of 24.2.Bolt 415 broke at a load of 24.45 in the threads and at the weld simu(taneously.

Bolt N16 was pulled to a tension of 35.43 , when the test was stopped.Many of these pull tests were videotaped'nd photographed.

The pull tests were conducted by Wiss, Janney and Elsner Associates, Chicago, who used their own equipment.

The pull tests were witnessed by personnel from the Plant QC department, AEPSC Civil Engineering Division, Stevenson and Associates, and Wiss Janney and Elsner.IV.Analysis-RCP Hatches N21, f22, f23 and 424 All of the hatches were analyzed for: o The operation of Unit N2 in the as-found condition.

o The operation of Unit g2 after modification of the anchorage.

o The original anchorage design.In the analyses, an equivalent pressure load of 18 psi was applied to each hatch cover and its anchorages.

The equivalent pressure load included the effects of dead load, earthquake and accident in accordance with the load factor equations of the D.C.Cook Plant updated FSAR.The Structural Design Section of AEPSC analyzed all of the hatches in both units.The hatch covers and their anchorage were evaluated using the computer program GTSTRUDL.

~~

Hatches f21, 522 and 424 were analyzed assuming that the cut and'rewelded bolts did not exist, although as noted above, the in-situ, plant pull tests made on the hatch 523 bolts established that the cut off and rewelded bolts were capable of carrying a considerable amount of load.o The analysis of hatch 521 was made assuming that.bolts g3, g4, 55 and f17 did not exist.Without these bolts, the hatch was found to be structurally adequate under the design loads and would not have unseated in an accident and/or earthquake.

o The analysis of hatch 422 was made assuming that two bolts gl and 53 did not exist.This case was enveloped by hatch 521.The hatch was found to be structurally adequate under the design loads and would not have unseated in an accident and/or earthquake.

o The analysis of hatch f24 was made assuming that one bolt, gl did not exist.This case was enveloped by hatches 521 and 522.The hatch was found to be structurally adequate and would not have unseated in an accident and/or earthquake.

The analysis of the Unit g2 RCP hatches is contained in Calc./Anal.

No.DC-D-3195-167-SC.

A copy of the analysis was given to the NRC Region III personnel who attended the September 30, 1987 presentation on the Unit 52 hatches at D.C.Cook Plant.Analysis of Hatch 523-Past Operations in the As-Found Condition.

Hatch 423 was analyzed independently by the Structural Design Section of AEPSC and by Stevenson and Associates, who acted as consultants to AEPSC in the matter of the as-found condition of the RCP Hatches.The test results (discussed above)were used to establish the ultimate tensile strengths of the cut off and rewelded bolts.Hatch 423 was analyzed for the equivalent pressure load described in part IV on page 4.The analysis made by the Structural Design Section of AEPSC found that Hatch 523 was structurally adequate and would not have become unseated during an accident or earthquake.

The draft report on the analyses made by Stevenson and Associates dated October 20, 1987 on Hatch 423 concludes as follows: "Hatch 23 in its"as-found" condition is capable of sustaining a differential pressure of'at least 20.6 psi without failure of any bolts and with deformations well within the capacity of the sealing mechanism".

VI.Restoration of Unit¹2 RCP Hatches AEP modified the as-found configuration of every suspect Unit¹2 anchor bolt to a configuration as close to the original design as was possible.With the exception of bolt¹17 on Hatch¹21, bolt¹3 on Hatch¹22 and bolt¹18 on Hatch¹23, all of the Unit¹2 RCP hatch anchor bolts that were cut and rewelded have been modified in accordance with Figures No.8 thru 22.The rewelded studs h'ave been removed.The embedded anchor bolts have been freed of any connection to the seal channel.The top of the embedded anchor bolts have been threaded and extension nuts have been installed on those threads.ASTM A193 studs have been installed in the other end of the extension nuts to fit into the hatch cover bolt pockets.These bolt pockets were modified to accept the location of the modified anchor bolts.At bolt¹17 on hatch¹21, bolt¹3 on hatch¹22 and bolt¹18 on hatch¹23, no embedded bolt was found to exist.All of these bolts are situated in a corner of the hatch adjacent to the crane wall.The analyses made by both AEPSC and Stevenson and Associates show that these corner bolts carry very little tensile load during an accident and/or earthquake because of the way the hatch cover framing was designed.Therefore, holes were drilled and tapped in the seal channel and ASTM A36 studs were installed in those corners to ensure a tight seal.VII.Documentation of As-found Hatch Bolt Configurations As part of the investigation, we made extensive searches and conducted pertinent personnel interviews in attempting to locate records that would address the as-found bolt configurations.

Both construction and operation records were reviewed and pursued.Personnel involved with the construction history and personnel involved with the operations history of the D.C.Cook Plant were interviewed.

Based on our searches and interviews, we believe that the applicable bolts were probably cut and rewelded prior to the unit going operational.

Certain construction meeting minutes indicate that work activities, related to the alignment of the hatches, continued for an extensive period of time.Though not conclusive, the possibility exists that the applicable bolts were cut and rewelded during the extensive hatch alignment period.No other records were found which address the bolts which were cut and rewelded.

VIII.Conclusions We believe, based on the testing program that was done and on the analyses that have been performed by both AEPSC and by Stevenson and Associates on the Unit N2 RCP Hatches, that the hatch covers would not have failed during an accident and/or earthquake and that in the as found condition, these hatches met the design basis in section 5.2.2.4 of the FSAR.The plant was operated with a secure divider barrier between the upper and lower containments.

The modifications made to the anchorage of the hatches and to the hatch cover bolt pockets enhance the structural integrity of the hatches.

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6.With the hatch cover in the fully seated position, use the hold down plates, positioned as shown on oRAWHQ HC4A, as templates to drill and tap the holes in the existing embed plates.The actual drilling and tapping may be performed with the hatch cover off or in place;7.At the back of the drilled and tapped holes in the embedded plates pockets shall be drilled in the concrete to a depth of 1/2" to accommodate bolt projection.

The pockets shall be cleaned of loose material.No reinforcing steel shall be cut or exposed during this operation.

Contact the Columbus Structural Design Office if reinforcing is encountered.

8.With the hatch covers fully seated, install the hold down plates and tighten the 5/8 inch diameter A 307 bolts to a torque of 35 FT-LBS.Each time the hatch cover is reinstalled the bolts must be torqued to 35 FT-LBS.9.Shims may be used between the hold down plate and the top of the hatch cover to insure.that the hold down plates bear uniformly on the hatch cover.DESIAA'4'lSlg&".

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