ML19341B827
| ML19341B827 | |
| Person / Time | |
|---|---|
| Site: | Midland |
| Issue date: | 02/20/1981 |
| From: | Jackie Cook CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III) |
| References | |
| 10CDR-50.55E, 10CFR-050.55E, 10CFR-50.55E, 11201, 81-03-#1, 81-3-#1, NUDOCS 8102270771 | |
| Download: ML19341B827 (9) | |
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4.s Mr J G Keppler, Regicnal Director 1
5 Office of Inspection & Enforcenent i4 US Nuclear Regulatory Co=issica Regica III 799 Roosevelt Road Glen Ellyn, IL 60137 MIDLAND PROJECT -
DOCKET NOS 50-329, 50-330 CRACKS IN BCRATED WATER S*0 RAGE TANK FOUNDATICN FILE:
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73*10*01, 02362(S), 01100(E) SERIAL: 11201 This letter confirms the 50 55(e) item cencerning the existence of cracks in the berated water storage tank foundation. This cendition was reported by telephone ccaversation to R Sutphin and G Gallagher, USNRC Regien III, on January 22, 1981.
Inclesure 1 provides a description of the condition and the planned corrective action.
Another report, either interi= cr final, vill be sent on er before April 3, 1981.
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'4RB/1r : Manage =ent Corrective Action Report MCAR-1, Report No LS, dated January 29, 1981 and Associated MCAR LS, Interi-Repert 1, dated February 17,1961, "The Existence of Cracks in the Berated Water Storage Tank Foundation - Units 1 and 2" CC: Director, Office of Inspection & Enforcement Att Mr Victor Stello, USNRC (15)
Director, Office of Management Info:. nation & Fregram Control, USNRC (1)
S RJCock, USNRC Resident Inspecter Midler.d Nuclear Plant (1)
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2 Serial 11201 81-03 #1 CC: CBechhoefer, ASLB Panel GAL $nenberger, ASLB Panel FPCowan, ASLB Panel AS&L Appeal Panel IWCherry, Esq MSinclair CRStephecs, USNRC WDPaten, Esq. USNRC FJKelly, Esq, Attorney General SHFreemaa, Esq, Asst Attorney General GTTaylor, Esq, Asst Attorney General WHMarshall GJMerritt, Esq, TNK&J Great Lakes QA Managers t
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Enclecure 1 CUALITY ASSURANCE PROGRAM 1-03 #1 0 2 l 2 8fBANAGEMENT CORRECTIVE ACTION REPORT M C AR-1 REPORT NO.:
48 JOS NO.:
7220 O NO.:
DATE:
January 79, 1981 1 DESCRIPTION * (lociv$ng References):
Excavations on the Borated Vater Tank Foundation for Unit I disclosed cracks in the areas identified by the recent analysis as possibly being overstressed.
This condition has been reported to the NRC by the Consumers Power Conpany as a potential 10 CFR 50.55(e) item.
RECOMMENDED ACTION * (OptionW):
1.
Deter =ine the inpact of the condition of the ring foundation on the tank's inte-grity and ability to perform under all required conditions.
2.
Deter =ine what further actions will be required to maintain the design criteria for the Borated Water Tank to perfor= its intended function.
3.
Coordinate and issue the first interim status report by February 9, 1981.
REFERRED TO:
C[ Engineennc C Construction O CA Management C
C Procurement
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s Bechtel Associates Professional Corporation 022437
SUBJECT:
MCAR 48 (issued 1/29/81)
The Existence of Cracks in the Borated k'ater Storage Tank Foundation - Units I and 2 INTERIM REPORT 1 DATE:
Februa ry 17, 1981 PROJECT:
Consumers Power Company Midland Plant Units I and 2 Bechtel Job 7220 Introdu; tion The 52-foot diameter, stainless steel borated water storage tanks are 32 feet high arid rest on compacted, granular backfill material contained within a reinforced-concrete ring wall. On one side i= an integral valve pit that houses connections and valves for the two 18-inch diameter pipelines that service each tank.
Figure 31.1 fron the response to NRC 10 CFR 50.54(f), Question 31 is attached for reference (see Figure 3).
During the load test on the Unit I tank (conducted in compliance with the response to 10 CFR 50.54(f), Question 4), a diccrepancy was noted between measurements of settlements recorded at the jobsite and the computed displacements derived from the structural analysis used at that time.
As a result, the analysis was modified to include a finite element model of the soil subgrade. A number of analyses were completed using various values for the modulus of elasticity (E) of the soil.
The results of the analyses predicted that greater than allowable moments existed at several locations in the foundation structure (see Figure 1).
The foundation at these locations was examined to rerify whether visible signs of high reinforcement strain existed. Cracks were found in the structure at those locations indicated by the analysis as having greater than allowable moments. The largest crack measured 0.063 inch.
Sub-sequently, tSe Unit 2 tank foundation was also examined; similar cracks were found, and the largest crack measured 0.035 inch.
The smaller f3 bearing area of the Unit 2 valve pit partially explains why the cracks in the Unit 2 tank foundation are smaller than the cracks in the Unit I tank foundation.
Significance of Cracks The cracks in the foundation structure of the Unit I tank indicate tnat high reinforcement strain exists in the ring waii foundation. As shown in Figure 2, the tank's outside periphery is atta:t.ed to the ring wall foundation by equally spaced anchor bolts that transfer induced forces through an anchor chair. A check with jobsite construction personnel indicated that all anchor bolts were installed in a snug-tight condition
Bechtel Associates ProfessionalCorporation "C^^ '8 022437 Interim Report 1 February 17, 1981 Page 2 consistent with installation requirements and were in contact with the top of the anchor chair when the tank was erected. However, recent field observations indicate that gaps exist between the anchor bolt nut and anchor bolt chair in several locations around the periphery of the tank; whereas, in other locations the horizontal plates of the anchor bolt chairs are deflected, indicating high tensile stress in the bolts.
This anchor bolt behavior results from the distortion of the foundation ring which is caused by differential settlement; this agrees with the conclusions of the modified structural analysis. The tension induced in the bolts is similar to the tension induced by a preload phenomenon and does not affect the ability of the bolts to resist design loads.
Al-though the ring wall is cracked, it is capable of providing the neces-sary dead load anchorage for the anchor bolts and confinement of the tank foundation support material. Therefore, resistance will be paavided for externally applied forces which induce tension in the anchor bolts.
Because the tank is made of ductile material (stainless steel), it is capable, through plastic deformation, of redistributing resulting load:. Although failure of the tank to hold water is not considered probable at the present time, without remedial action future behavior is difficult to establish.
Corrosion of the reinforcing steel in the ring bean. conbined with increased displacewents, may cause problems in the t ure.
The concrete ring wall confines the foundation material which is loaded by the tank; this confinement causes hoop tension in the ring wall. Cracks equal in size to those observed in the ring wall expose the reinforcing steel to the groundwater and atmosphere.
If the reinforcing steel carrying the hoop tension were to corrode significantly, the ability of the ring wall to resist the hoop tension would be reduced. Reduction of overall strength of the ring wall, conbined with increased displacenents (both vertically and laterally) of the ring wall nay occur, causing stres; concentrations in the tank near anchor chairs.
Based on the foregoing discussion, we believe that the cracking of the ring beau does not create a present safety problem for the tank. However, without additional extensive analysis, it cannot be conclusively shown t ha t the safety of future plant operations would not have been affected if the deficiency had gone uncorrected. Therefore, the condition is being treated as reportable under the provisions of 10 CFR 50.55(e).
Probable Cause Review of field observations and analytical investigations leads to a preliminary understanding of the probable cause. When the tank was loaded, the bearing pressure on the tank area was increased to appoximately 2 ksf greater than the valve pit area; however, this 2 ksf differential was not accounted for in the calculation for ring beam reinforcing. As a result, the valve pit restrained the tank foundation from settling uni-l formly, causing bending at the ring wall / valve pit junction.
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Bechtel Associates ProfessionalCorporation "C^^ '8 022437 Interin Report 1 February 17, 1981 Page 3 Future Actions A number of corrective action alternatives are currently under consid-eration that ensure the borated water storage tanks ability to perform its safety function:
1.
Surcharge the valve pit to reduce crack widths and reduce the bending moments and, if required, reinforce the ring foundation Partially excavate the soil beneath the valve pit to increase a
the bearing pressure under the valve pit and, therefore, reduce the bending monents in the ring 3.
Use as is, show that the intended safety function can be fulfilled for the expected service period 4.
Surcharge and then disconnect valve pit 5.
Partially or totally reconstruct the foundation structure Analysis of these and other alternatives is ongoing. A discussion of alternatives will be incorporated in future interim reports.
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