ML20011E227

LER 89-021-00:on 891010,util Informed by C-E of Potential Nonconservative Setpoint in Reactor Protection Sys Thermal Margin/Low Pressure Trip Unit.Caused by Error in Incorporating Transient Setpoint analyses.W/900201 Ltr

ML20011E227
Person / Time
Site:	Fort Calhoun
Issue date:	02/01/1990
From:	Gates W, Voss K OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-89-021, LER-89-21, LIC-89-1155, NUDOCS 9002120036
Download: ML20011E227 (7)

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4- . February 1, 1990 3 .LIC-89-1155-y 1

~U.S,LNuclear'RegulatoryCommission '

x Attn: Document Control Desk- i Maii Station.P1-137  !

'-Washington,_DC,20555-

Reference:

-- Docket'No. 50-285

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y Gentlemen:-

Subjects' Licensee. Event Report 89-021 for the Fort Calhoun Station L -Please-find attached Licensee Event Report 89-021 dated February 1,  !

H :1990~. This report is being submitted as a voluntary LER.

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L If you-should have any questions, please contact me.

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.W. G. Gates  ;

Division Manager y; Nuclear' Operations-WGG/ tem Attachment c: 'R. D.' Martin, NRC Regional Administrator-A.;Bournia, NRC Project Manager ,

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Fort Calhoun Station Unit No. 1 olsiolotol2i 8l5 i lor l 0,6 717 4: ..i Potential Nonconservative RPS Thermal Margin / Low Pressure Setpoints .

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CombustionEngineering(CE)wasrequestedbyOmahaPubilePowerDistrict(0 PPD) sis for Fort Calhoun to review The Station. the DNB Cycle 12 Excess Limiting Safety Load System Transient Setting Analy(LSSS) Setpoint Analysis was also reviewed. On October 10 1989 OPPD was formally informed of the i

possibility)ofhavinganon-conservativesetpointintheReactorProtective-Thermal System (RPS MargI theincorporationoftheTransientPowerDecalibration(TPD)inthesetpoint analyses. The potential safety impact on the plant was assessed by reviewing the Cycle 12 Excess Load Transient analysis and Departure from Nucleate Boiling Limiting Safety System Setting analysis to verify that the plant has been operating conservatively. The results of the review concluded that the plant had been operating within the design basis and would continue to do so for the remainder of Cycle 12 based on a comparison between predicted and observed core parameters. Corrective actions include development of im) roved administrative controls and training for the reload analysis process. T11s report is submitted as a voluntary LER.

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0l2 l 1 __ 0;O 0l2 or 0j 6 The Thermal Margin / Low Pressure (TM/LP) trip protects the core from violating theallowableDeparturefromNucleateBoilingRatio(DNBR) limit. Technical Specifications recuire that the minimum DNBR be maintained above 1.18. This ensures fuel cladting integrity by maintaining adequate heat transfer from the cladding to the coolant. The TM/LP Trip function provides this protection through the calculation of a variable setpoint using an equation (Pvar) defined in Figure 1-3 of the Technical Specifications. The Pvar equation yields a reactor trip setpoint that is a function of the core power, the core inlet temperature and the Axial Shape Index. The core power used is the greater of thethermal,(delta-T)powerorthenuclear(excoredetector) power. The calculated trip setpoint is compared with the measured pressurizer pressure.

If the measured pressurizer pressure value is less than or equal to the setpoint,thentheTM/LPtripunitintheReactorProtectionSystem(RPS)will initiate a reactor trip.

TheIntegratedRadialPeakingFactor(Fr)istheratioofthepeakpinpowerto theaverageintegratedpinpowerinthecore,excludingthetilt. The Total Integrated Radia, Peaking Factor (Frt) is the radial >eaking factor multiplied by a correction factor that accounts for the tilt. T1e limits on Frt assure that the assum)tions made in the setpoint analysis remain valid during o>eration at t1e various Control Element Assembly (CEA) group insertion limits.

Tie peaking factors assumed in the safety analysis are limiting assumptions chosen to provide the most conservative setpoint analysis, i

The setpoint analysis utilizes inputs from a number of different analyses, including the Thermal Hydraulics, the Transient Safety and the Axial Shape Indexanalyses,andcalculatestheRPStripunitsetpoints. One of the variables that must be included in the reload analysis is the Transient Power Decalibration(TPD) term. The TPD term accounts for the temperature shadowing and rod shadowing effects on the excore detector indicated power level and RTD response time for the delta-T power calculation. Temperature shadowing occurs when changes in the core inlet temperature take place. The result is a change in the reactor vessel downcomer coolant density which produces an increase or decrease in the moderation of the neutrons normally detected by the excore detectors. The result is a mismatch between the actual core power and the excore detectors' indicated power. The rod shadowing effect is caused by CEA movement resulting in shielding of the excore detectors, which also results in a mismatch between actual core power and the indicated power. The setpoint methodology allows the personnel performing the analysis to account for the TPD in either the Excess Load transient analysis or the Limiting Safety System Setting (LSSS)setpointanalyses.

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0l 0 0l 3 or 0[6 rm - . ,,- - =c % = = nn Durin the preparation of the Cycle 10 setpoints, Omaha Public Power District (OPPD used an updated version of the CESEC computer code that contained expli it modeling of the excore detector and RCS temperature responses, which is used in the determination of the core nuclear power and delta-T power levels, respectively. The new modeling allowed a better approximation of the magnitude of the TPD term that must be used in the setpoint analysis. On March 1,1984OPPDcontactedCombustionEngineering(CE)proposingtheremovalofthe nonzero TPD term to take advantage of the new com> uter modeling. On April 17, 1984 CE informed OPPD that the TPD term could eitler be reduced in the Excess Load Transient analysis or removed from the setpoint analysis. However, there was apparently some misunderstanding by 0 PPD personnel as to the intent of the guidance in this correspondence. During the Cycle 10 reload analysis the TPD term was reduced by OPPD in both the transient analysis and the setpoint I analysis.  :

Prior to the start of the Cycle 11 reload analysis there was a large turnover of personnel within the OPPD department that performs the analyses. This  :

resulted in increased reliance on the assumptions used for the methodology in Cycle 10. The Cycle 11 analysis was the first at OPPD to employ a computer- l aided method to perform the setpoint analysis. The analysis used the Cycle 10 analysis as a guideline. preparation Since the analyst of the Cycle 11 performing the Cycle 11 setpoint analysis was not as experienced as the previous analyst, the TPD term error of Cycle 10 was undetected and propagated  ;

into the Cycle 11 analysi.% )

In April of 1988, CE was contracted by OPPD to perform the Cycle 12 setpoint analysis because at that time there were no qualified OPPD personnel available i to complete the analysis. The Cycle 12 setpoint analysis was also to be used '

as a training exercise that would aid OPPD personnel in becoming qualified to perform the analysis. OPPD personnel performed the transient analyses for Cycle 12 that were used as inputs for the setpoint analysis. The TPD error again was undetected and propagated into the Cycle 12 analysis.

During the performance of the Cycle 12 setpoint analysis, errors leading to non-conservative Cycle 11 TM/LP trip setpoints were discovered and reported to NRC as LER 88-16. CE determined that there were no additional errors in the setpoint analysis for Cycle 11. On August 11, 1988, as a result of NRC Violation 88-22-01 and an enforcement conference on the events detailed in LER 88-16, OPPD further committed to have all the Cycle 12 reload analyses reviewed by CE. Prior to the start of Cycle 12, CE was contracted by OPPD to review the Cycle 12 reload application and the supporting analyses. No errors, including the TPD term error, were found by CE during this review. CE also performed a review of the Cycle 10 setpoint analysis which revealed no problems.

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0l2 l 1 0l0 0l4 or 0j 6 The TPD term error was subsequently found during another review performed by CE at the request of OPPD. This review was conducted as part of the follow-up for a delta-T power flow stratification event that occurred at Fort Calhoun Station on September 13, 1989 during Cycle 12. The purpose of this review was to verify that the Excess Load Transient analysis was conservative and to assess the margin in the Departure from Nucleate Boiling Limiting Safety SystemSetting(DNBLSSS)portionofthesetpointanalysis.

On September 29, 1989 OPPD was verbally informed by CE of a potential discrepancy in the Cycle 12 Transient and Setpoint analyses. There was a potential non-conservatism in the RPS TM/LP trip unit setpoint due to an error in the incorporation of the TPD term in the setpoint analysis. OPPD Nuclear Engineering department performed a preliminary assessment which indicated the plant was operating within design basis. It was concluded that further discussions with CE were required.

On October 10, 1989, while the plant was operating in mode 1 at 100 percent power, OPPD was informed by a letter from CE that the Pvar equation used in the TM/LP trip units of the RPS could, under certain conditions, be non-conservative. The Pvar equation would be non-conservative when Frt was greater than 1.77 and core inlet temperature was 543 degrees Fahrenheit. The Technical Specification limit for Frt is 1.80 and for core inlet temperature <

is 543 degrees Fahrenheit. As noted previously, the TPD term is required to be accounted for in either the Excess Load Transient analysis or the setpoint analysis. OPPD set the term to zero in the Excess load Transient analysis and CE also set it to zero in the DNB LSSS setpoint analysis.

On October 10, 1989 it was determined that the existing RPS TM/LP setpoints would be conservative for Frt values below 1.77. A review of the actual full power peaking factors for Cycle 12 showed that Frt had not exceeded the value of 1.65. Also, the predicted values for Frt were not expected to exceed 1,73 for the remainder of Cycle 12. Therefore, the TM/LP trip function had not been and would not be outside of the design basis for the plant during Cycle 12 operation. Furthermore, normal operating practice at Fort Calhoun is to maintain the core inlet temperature a) proximately 2 degrees Fahrenheit below the Technical Specification limit. T11s practice provides additional conservatism. It was concluded that the plant was currently operating in a safe configuration and had been in a safe configuration since Cycle 12 startup. The TM/LP reactor trip )rotects the core from exceeding the DNBR limit. Only if the value of Frt lad been higher than 1.77 during Cycle 12 with the core inlet temperature at the Technical Specification limit of 543 degrees Fahrenheit would it have been possible for the DNBR limit to be violated for an event that required TM/LP protection, such as the Excess Load Event.

This event is reported as a voluntary LER for information.

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_ 0 ; 2l1 _ 0;0 0, 5 or 0l 6 rut n - . , e, ,,,,3. v ii7, The root cause for tM vent was the failure to maintain adequate administrative contron a w rsight to ensure that the reload analysis '

remained conservative overa or the Cycle 10 and subsequent reload analyses. There were several contributing causes: lack of overall design process guidelines for the reload analyses which would have allowed the analysts to verify that all the appropriate variables were included, lack of experience for some personnel involved in the Cycle 11 reload analysis, marginal quality of documentation for previous reload analyses, and inadequate communications between CE and OPPD and within the OPPD Nuclear Engineering Department.

The following corrective actions have been completed:

1. The RPS was verified to be operating within the design basis by the Supervisor of Reactor Performance Analysis by reviewing the DNB LSSS and the LHR LSSS setpoint calculations.

2. An Operations Memorandum was issued on October 13, 1989 notifying operations personnel of an administrative change in the maximum limit for Frt to a value of 1.75. This Operations Memorandum provided required corrective actions in the event that Frt exceeds 1.75. The Shift Technical Advisors periodically verify the peaking factor limits are not exceeded in accordance with the existing Technical Specifications requirements.

3. The procedure governing how the calculations are prepared was changed to require a higher level of documentation and review.

4. A formal root cause analysis was completed by the Nuclear Safety Review Group.

5. Additional training for appropriate personnel has been completed on setpoint generation.

6. The Cycle 10 and 11 peaking factors were reviewed to assure the plant was operating within the design basis for those cycles, j

7. To enhance the quality verification process, an overview of the entire reload process, including inputs to the setpoint analyses, has been conducted. This review included the applicable procedures to be followed and the transfer of information between the affected groups.

The following longer term corrective actions are planned:

1. A design )rocess document will be im)1emented, by September 30, 1990, to provide t1e necessary instructions tlat will be followed during a reload l

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2. OPPD will evaluate the feasibility of an oversight comittee to )rovide guidance for the entire reload process for subsequent cycles. T 11s evaluation will be completed prior to initial reactor criticality for Cycle 13.

TherehasbeenonepreviousLER(LER88-016)writtenonerrorsin, generation of RPS setpoints.

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