ML20043F244

LER 90-015-00:on 900507,PORV Variable Setpoints Used for Low Pressure Overpressure Protection Determined to Be Nonconservative for PORV Opening Time.Caused by Design Deficiency.Tech Spec Amend prepared.W/900607 Ltr

ML20043F244
Person / Time
Site:	Fort Calhoun
Issue date:	06/07/1990
From:	Gates W, Holthaus K OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-015, LER-90-15, LIC-90-0471, LIC-90-471, NUDOCS 9006140285
Download: ML20043F244 (5)

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Omaha Public Power District 444 South 16th Stmet Mall ,

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June 7, 1990 LIC-90-0471 U. S. Nuclear Regulatory. Commission Attn: Document Control Desk Mail Station P1-137 Washington, DC 20555

Reference:

Docket No. 50-285 Gentlemen:

Subject:

. Licensee Event Report 90-15 for the Fort Calhoun Station  ;

Please find attached Licensee Event Report 90-15 dated June 7, 1990.  ;

This report'is being submitted on a voluntary basis.

If you should have any questions, please contact me.

ll Sincerely,.

dhf)dhW

/ d W. G. Gates

, Division Manager '

Nuclear Operations

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The Power Operated Relief Valves (PORV) variable setpoints used for Low Temperature Overpressure Protection at Fort Calhoun Station were determined to be non-conservative for a PORV opening time greater than 0.0 seconds, assuming the Reactor Coolant System pressure-temperature limits and operating restrictions in effect. However, an investigation revealed no periods of plant operations since the installation of the variable setpoint PORV system that would have adversely impacted the ability of the PORV's to mitigate a low temperature pressure transient and protect reactor vessel integrity.

The primary cause of this condition was determined to be deficiencies in the design process for the variable setpoint PORV system.

Corrective actions include improvements in the design change process, changes to plant procedures to provide administrative limits to prevent exceeding the Reactor Coolant System pressure-temperature limits, and an amendment to Technical Specifications to incorporate the low temperature overpressure limitations, i

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010 01 2 oF 0 14 ria . = J w= waac e msAw nn Two Power Operated Relief Valves (FORV's) at Fort Calhoun Station protect the' Reactor Coolant System (RCS) from Low Temperature M rpressure transients.

These relief valves utilize a variable setpoint to )rovide pressure relief of the RCS at varying temperatures and pressures. Wit 1 an RCS temperature reduction the ductility of the steel in the reactor vessel is greatly reduc.ed. The reduction in ductility increases the probability for brittle fractum M the reactor vessel and for this reason requires temperature depeno w. variable PORV setpoints for pressure relief during a low temperature overpressure transient, TMs variable setpoint system was installed in 1984.

These variable PORV s apo k s, along with certain operating restrictions at low temperatures and p e v.ures, make up the low temperature overpressure protection (LTOP) system. The variable PORV setpoints are also referred to as the LTOP setpoints. The LTOP setpoints should be set such that during a pressure transient, the reactor vessel pressure-temperature (P-T) limits would not be exceeded. One of the factors that is currently utilized in the calculation of the LTOP setpoints is the stroke time of the PORV's.

The lack of surveillance tests for verification of PORV stroke time has become an industry concern and has been addressed by NRC Information Notice 89-32.

The Omaha Public Power District (0 PPD) response to IEN 89-32 was to write a-surveillance test for verifying the stroke time of the PORV's. The PORV opening times measured during surveillance testing were .32 and .36 seconds (includinguncertainties)forthetwovalves. In an attempt to establish the proper acce)tance criteria for valve stroke time, OPPD personnel reviewed the basis for tie variable PORV setpoints, and found that an instantaneous opening time (i.e.,0.0 seconds)hadbeenassumedforthePORV's. Since this could roduce non-conservative setpoints, OPPD requested Combustion potentially Engineering p(CE) to perform an analysis to determine the maximum PORV opening time that could be allowed based on the existing Fort Calhoun Station P-T limits, PORV setpt,ints, and operating restrictions. CE determined that the existing PORV setpoints were non-conservative; however, with a few restrictions on operating limitations, a PORV opening time of up to 0.8 seconds could be justified.

OPPD conducted an investigation to determine if any of the more restrictive o)erating limits had ever been exceeded in the past. The investigation showed t1at there were no periods of plant operations since the installation of the variable setpoint PORV system that would have adversely impacted the ability of the PORV's to mitigate a pressure transient and protect reactor vessel integrity. Prior to installation of the variable setpoint system, different design bases and assumptions were in effect and the PORV setpoints were conservative.

The condition was verified on May 7, 1990 while the plant was in refueling shutdown. OPPD personnel contacted the NRC by conference call on May 7, 1990 to inform them of the problem. This condition was determined to not meet any of the specific reportability criteria of 10 CFR 50.72 or 50.73; however, this report is submitted voluntarily because of industry and NRC interest.

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Sig((,*k 7,'8,y,h I 0 l 1l5 oF Fort Calhoun Station Unit No.1 o is lo lo lo l 2l8 l 5 9 10 0 10 01 3 6{4 rixr,, M . w. w w r m m on The variable setpoint PORV system was designed in 1984.by personnel in the OPPD Electrical Engineering Design department. The scope of the modification was to design a variable set)oint system to re) lace the dual setpoint PORV system which was in use at tie time. During tie design phase, no consideration was given to how the system would operate during a pressure transientsuchasaninadvertentreactorcoolantpump(RCP)startatlow temperatures. The system was only designed for operation during normal heatup and cooldown operation. Communications between appropriate design groups appear to have been limited to a review of the instrument uncertainties which were being used for the system design.

The primary cause of the event was determined to be a lack of adequate design review and lack of an interface between the design group and the technical support department.-

An investigation of past operations since installation of the variable setpoint system showed that there were no incidents during heatup or cooldown operations which adversely affected the ability of the PORV's to mitigate a pressure transient and protect reactor vessel integrity. Also, there has never been a pressure transient which challenged the PORV's during heatup or cooldown at Fort Calhoun Station. The plant had always been operated within the restrictions for which the PORV opening times were conservative. The impact on safety is therefore considered to be minimal.

The following corrective actions address the incorrect LTOP calculations and setpoints:

1. Improved administrative guidance assures appropriate considerations will be made during the design and review phases of modifications.

Production Engineering Division procedure " Preparation of Design Packages"(GEI-3),approvedinApril1989,providesforamoredetailed review of design modifications.

2. The incorporation of the Technical Support department into Design Engineering during the 1988 OPPD reorganization placed the design and transient analysis functions under one manager, thereby improving the interface between the two organizations.

3. Operational limits have been established to ensure continued protection of reactor vessel integrity by compliance with RCS pressure-temperature limits. The changes include modified reactor coolant pump restart-criteria and HPSI enable / disable temperatures. Also, a range has been established for entrance to shutdown cooling, and the 20 degree F/ hour limiting plant cooldown rate will be initiated at a higher RCS temperature. These limits were incorporated into the Fort Calhoun Station Operating Manual prior to Cycle 13 startup.

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4. - A Technical Specification Amendment will be prepared to reflect the results of an updated analysis performed by CE to determine new LTOP setpoints for 15 EFPY. The proposed Technical Specification Amendment:

will'be submitted by July 31, 1990. Submittal of proposed changes for the present 14 EFPY prior to that time would be of little benefit as the values will soon be changed to reflect the update for 15 EFPY, The incorporation of the administrative limits listed above will provide j guidance'for operation during the interim period..

! Other Licensee Event Reports addressing potentially non-conservative setpoints

, are LER's89-021 and 88-016.

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