ML19309A417
| ML19309A417 | |
| Person / Time | |
|---|---|
| Site: | Oconee, Rancho Seco  |
| Issue date: | 08/23/1974 |
| From: | Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| To: | |
| References | |
| NUDOCS 8003270862 | |
| Download: ML19309A417 (5) | |
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SACRAMENTO MUNICIPAL UTILITY DISTRICT C 6201 S Street, Box 15830 Sacramento, California 95813: (916) 452-3211 2
August 23, 1974 1
Dr. Donald F. Knuth, Director Directorate of R6tulatory Operations U. S. Atomic Energi Jommission Washington, D. C.
20545 Rancho Seco Nuclear Generating Station No. 1 Raactor Coolant Flow Miscalibration AEC Docket No.56-312
Dear Dr. Knuth:
We recently were informed by our nuclear steam system vendor, Babcock & Wilcox, of a generic investigation made as a result of the identi-fication of a miscalibration of the RC flow in RPS power / imbalance / flow at Oconee I.
In accordance with our procedures, this nonconformance has been f~~s
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reviewed and has been determined to be a reportable "significant deficiency" as defined by 10CFR50.55(e).
Description of Deficiency - The calibration procedures for the reactor coolant flow instrumentation and reactor protective system (RPS) power / imbalance / flow channel did not specifically state:
(1) the reactor coolant flow rate basis (design flow or actual measuralflow) or, (2) the reactor coolant density conditions that should be used for calibration.
The RPS power / imbalance / flow trip is designed to trip the reactor if the measured neutron flux is greater than or equal to the allowed flux as determined from the measured reactor coolant flow multiplied by the flux ratio less a correction for the measured core axial power imbalance. The power / imbalance / flow trip settings were based upon the flux flow ratio and imbalance limits as defined in the Technica) Specifications and the reactor coolant flow upon 100% design flow.
The power / imbalance / flow trip utilizes the measured reactor cool-ant flow as an index to establish the imbalance trip envelope. Two sources of calibration errors have been identified that resulted in a non-conserva-
'tive RPS trip setting and could have resulted in a violation of the Techni-cal Specification imbalance limits.
The first calibration error was the use of the design reactor coolant flow rather than the actual measured flow value in the calibration rh I
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U Dr. Donald F. Knuth August 23, 1974 Procedure. The actur.1 is 109% of the design value and with the power /
imbalance / flow trip calibrated on the design flow basis, the actual flow rate input to the RPS will allow the axial imbalance trip setting to be 109%
of the Technical Specification limit.
The second calibration error was the use of an incorrect reactor coolant system temperature distribution for the density correction to the reactor coolant flow in the calibration procedures. The RPS power / imbalance /
trip setting was based at the zero power reactor coolant temperature and was conservative between 0 and 58% rated power; however, above 58% rated power the imbalance trip envelope could exceed the Technical Specification limits.
The core axial imbalance vs flow trip protects the core against central fuel melting.
Protection from excessive core imbalance occurs from three soarces, first the limits precautions in the plant operating procedures, secondly the limits on rod positions nd imbalance in the Technical Specifications and thirdly the reactor prot 4ction system.
With the RPS flow channels miscalibrated as described above, the O-ability of the RPS to protect against central fuel melting caused by core axial imbalance could have been compromised. However, for the excessive imbalance condition to exist previous misopc..ation of the control rods would have to occur.
For Oconee I, r.he RPS trip setpoint, as allowed by the Technical Specifications, would allow a core power / core axial imbalance condition which could produce 20.1 kw/ft. With the miscalibration of the RPS flow channel for design flow equals 100% flow, the maximum linear heat rate at RPS trip could be n-21.9 kw/f t.
Calibration of the flow channels at zero rather than 100% power would allow an additional 0.5 kw/f t or a maximum heat rate of 22.4 kw/f t prior to initiation of a reactor trip signal.
Ex-ceeding the 20.1 kw/f t limit would allow a higher probability of central fuel melting in a small percentage of the fuel rods in the core.
Oconee I was however, operated within the Technical Specifications operational power / imbalance envelope and no Technical Specification or safety. limit was exceeded prior to making procedural corrections. Plants after Oconee I have been calibrated using the corrected procedure.
Similar comparisons of tech spec and actual kw/ft limits with the miscalibration are shown in Table I.
Figure 1, Simplified Diagram of RPS Flow Channel, and Figure 2, Maximum Setting Voltage V3 vs Reactor Power are included for clarification.
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O 3-August 23, 1974 Dr. Donald F. Knuth TABLE I CENTRAL FUEL MELT CRITERIA VIOLATION TECH SPEG ACT. FLOW /
KW/FT NSS KW/FT DESIGN, %
KW/FT(1)
MAX. KW/Pr(2)
VIOLATION 3
20.1 109 21.9 22.4 11.4%
4 19.8 110 21.8 22.3 12.6%
5 19.6 108.3 21.2 21.7 10.7%
8 20.1 108 21.7
'22.2 10.4%
11(3) 20.4 106.7 21.8 22.3 9.3%
(1) Based on miscalibration of actual flow equal 100%
(2) Based on miscalibration of actual flow equal 100% at zero power (3) NSS 11 is data on Rancho Seco i
Corrective Action - The field calibration procedure at Rancho Seco j
for the summing amplifier was changed such that the gain factor (K ) is set 1
by utilizing the actual system flow that would exist at 100% power. This is the proper base from which the imbalance correction should be made and l
yields setpoints which are clearly conservative and verified by analysis.
Other documents which provide instructions on claibration of the RPS for reactor coolant flow have also taen revised to clearly identify that actual flow at 100% power is to be used.
4 Should additional information on this problem be needed, or if we can be of further assistance, do not hesicate to call.
Sincerely yours, J.J. Mattimoe Assistant General Manager and Chief Engineer
' Attachments cc:
R. H. Engelken Region V i
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FIGURE 2 MAXIMUM SETTING VOLTAGE V3 VS REACTOR POWER l
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