IR 05000282/1992027

Insp Repts 50-282/92-27 & 50-306/92-27 on 921209-10.No Violations Noted.Major Areas Inspected:Cooling Water Sys for Potential Water Hammer Occurrence During Loss of Offsite Power Event

ML20126F883
Person / Time
Site:	Prairie Island
Issue date:	12/24/1992
From:	Burgess B, Yin I NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
To:
Shared Package
ML20126F881	List: IR 05000282/1992027 ML20126F878
References
50-282-92-27, 50-306-92-27, NUDOCS 9212310185
Download: ML20126F883 (5)

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U.S. NUCLEAR REGULATORY COMMISSION i

REGION III

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I Reporte No. 50-282/92027(DRS); 50-306/92027(DRS)

Docket Nos. 50-282; 50-306 Licensos No. DPR-42; DPR-60 Licensoot Northern States Power Company ,

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414 Nicollet Mall Minneapolis, MN 55401 Pacility Name Prairie Island Nuclear Generating Plant, f Units 1 and 2

Inspection At: Prairio Island Sito '

Wolch, Minnesota Inspection conducted: December 9 - 10, 1992 Inspector: / 2h7[ b

' Isa Yin / Date-Approved By: bk

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Hurgebs, Chief N ~

Date 2 fL operational Programs section

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Inspection Summary Ingpection on December 9 - 10, 1992 (Report No. 50-2.82/92027(DRS)f N o _. 50-306/92027(DESH Areas Insnocted: Special, announced inspection of cooling water syntom for potential water-hammor-occurrenco during a loss of offsite power even ,

~Results: Based on the review, the inspector concluded that under -

the worst design basis condition, water-hammer could-occur in the -

cooling water system. The inspector discussed the possibility-of i additional check valvo and system operationaltocts'to confirm the possibility of a water' hammer occurrence and to minimize the offect when it nappen .

- 9212310185 921224- - - -

PDR ADOCK 05000282

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DETAILS i Persons Contag1pA l

Rorthern States __Eqwcr Comnany '

K. Albrecht, General Superintendent, Engineering B. Stephens, Superintendent, System Engineering, Mechanical M. Carlson, Technical Support Engineer Introduct12D The inspection was conducted to assess cooling water (CL)

system configuration and operation for the occurrence of water hamme Similar inspections were conducted at other NRC RIII plants. The scope of this inspection included discussion with licensoo staff, review of P& ids, piping "

drawings, correspondence, and the following documents:

• DBD-SYS-35, Design Bases Document for Cooling Water System, Revision 0, dated January 11, 199 * Operation Precedure-B35, Cooling Water System, Revision 1, dated March 23, 198 . General System Confiquration The CL system consists of five pumps feeding a ring header shared by the two reactor unit At present, two of the pumps are diosol driven and three are motor drive Two of these five pumps must be in service during normal plant operation, and the third pump may be needed during the hotter summer days. c;.e of the two diesel driven pumps has sufficient capacity to supply cooling water to both emergency diesel generators (EDGs) during a Loss Of Offsite Power (LOOP) ovent and subsequent reactor trip-of both unit After installation of two additional EDGs, one of the three motor driven pumps.will be powered from a Class 1E power suppl . Water Hammer Event During a LOOP event, the two dicsc1 driven pumps would be ready for operation within 20 seconds, and the Class 1E motor-driven pump would be ready for operation within 35 seconds. Any water column separation during_the time between all pumps stopped and a selected pump start is prevented by two sets of check valves (CVs). The first set consisted of five 20" CVs at the pump discharge nozzle areas, located at EL. 684.5' to-695.9', above the pump suction bays (EL. 654.5').

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The second set consisted of eight 6" CVs, located at EL. 729.5'-to 742',. below the highest pc,ints of the system (EL. 765').

Although these two sets of CVs are installed to prevent water column separation, during a CL pre-operational test in-1973, a water hammer occurre Due to-lack of documentation, the extent of the problem and associated-damage, if any, is unknown. The cause of the event was determined to be water column separation due to a 35 second delay between start of the safety-related diesel driven pumps subsequent to stopping the motor driven pump The corrective action taken was te reduce the time delay to 20 seconds. The corrective action appeared to be adequate in that no damage was identified on any of the heat exchangers or area coolers. However, some damage on-pipe hangers was reported during In-service Inspection (ISI) of the CL syste The inspector concidered the planned system modification involving one-motor driven pump connected to a Class 1E power supply. . Since pump start will be delayed 35 seconds during a LOOP, the potential for a water hammer event will be re-established. Consideration of'the worst case '

condition (both diesel driven pumps unavailable during a LOOP due to equipment tag-out or failure to start), resulted in che potential for a water hammer event caused by the start of a single motor driven pump. This water hammer-will generate about 60% of the force calculated during the 1973 water hammer. This condition-was evaluated to be acceptable by the l'9ensee due to a lack of damage-identified subsequent to the 1973 water hammer even While concerning with the licensee's assessment, the inspector discussed the following with the licensee:

• System post modification test to simulate the potential water hammer : condition, or impose operational constraints for the worst case design basis' conditio * Incorporate the worst case design basis water hammer loading condition during a LOOP into the DBD-SYS-3 Licensee management agreed-that they would consider.the inspector's recommendation S. Hanaer Damace Based on the inspector's request,.the licensee performed a record search for all past piping and hanger damag There was no damage recorded for the CL system during the last 10 years of-ISI program review except for the following:

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• Hanger CWRH-41 at EL. 746' was found with a bent-rod in September 198 * Hanger CWRH-365 at EL. 713' on-a CL return header was found with a minor concrete crack near one base plate, and one slightly bent I-beam in August 1988. The deficiencies were attributed to welding during constructio * Hanger CWRH-1 at EL. 750.b' was found with a bent rod in September 198 With the exception of CWRH-365, no engineering evaluation for severity and root cause was completed for CWRH-41 and In response, the licensee stated that the ISI program was-modified to require review of ISI discrepancies and generation of Nonconforming-Item Reports when necessar . CV Maintenance The two sets of CVs were important in the prevention of water column-separation during a LOOP. The inspector reviewed the licensee's CV maintenance progra Among the eight 6" CVs, only two were opened for maintenance in June 1991. Both valves were found having deteriorated internals, and were repaired. The licensee stated that acoustic and magnetic tests will be conducted for the remaining six CVs in the near futur Among the five 20" CVs, four were replaced between 1985 to 199 The first replacement in 1985 was due to deteriorated internals, but all subsequent CV replacements were performed-in accordance with the preventive maintenance progra The licensee stated that the present.CV program complies with ASME Section XI, and has addressed the NRC correrns stated in IE Bulletin 83-03, and Information Notics 68-7 . Solenoid Operated Valves (SOVs)

Kewaunee Nuclear Power Plant, the sister plant of Prairie Island Nuclear Generating Plant, reported water hammer occurrences; and attributed the cause to fast opening and closing Solenoid Operated Valve (NRC Region III Inspection Report 50-305/92023). SOVs are.also employed.at Prairie Island to control CL flow to area and equipment coolers, but are different from the type installed at Kewaunee. SOVs in Kewaunee are the direct electric coil operating type, and open or close in 0.04 to 0.06 second * *

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SOVs-in Prairie Island are pilot operated, and/ require to two seconds to open or close, depending on the amount of differential pressure across the valv The licensee stated that there have not been any problems reported regarding water hammers caused by the opening or closing of fast acting SOV . Exit Interview I

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The Region III inspector met with the licensee representatives (denoted in Paragraph 1) at the conclusion of the inspection on December 10, 1992. The inspector summarized the purpose and findings of the inspection. The licensee representatives acknowledged this information. The inspector also discussed the likely informational content of the inspection report with regard to documents or processes reviewed during the inspection. The licensee representatives did not identify any such documents and processes as proprietar !

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