ML20148Q204
| ML20148Q204 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 04/08/1988 |
| From: | Andrews R OMAHA PUBLIC POWER DISTRICT |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| Shared Package | |
| ML20148Q210 | List: |
| References | |
| LIC-88-240, NUDOCS 8804120358 | |
| Download: ML20148Q204 (8) | |
Text
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g Omaha Public Power District 1623 Harnetj Omaha. Nebraska 68102-2247 402/536 4000 April 8, 1988 LIC-88-240 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555
Reference:
Docket No. 50-285 Gentlemen:
SUBJECT:
Teleconference between NRC and OPPD on Compliance with ATWS Rule i
On February 10, 1988, representatives from Omaha Public Power District (0 PPD) held a teleconference with the individuals from the NRC responsible for reviewing OPPD's submittal on 10 CFR 50.62, the ATWS Rule.
The main topic of discussion was the Diverse Scram System which has been designed and installed by OPPD. To document the discussion and to provide a complete review of OPPD's system, the NRC requested that a letter be prepared detailing the areas discussed.
The points of discussion with OPPD's responses are as follows:
1.
The NRC requested a drawing which shows the circuit breakers used by the Diverse Scram System and the H-contactors used by the Reactor Prctection System to actuate a reactor trip.
A drawing which details the intercon-nection between these actuation devices and the turbine trip syrtem was also requested.
Responit: Drawing E-23866-411-013 (File #1587) shows the circuit breakers, and a-contactors which actuate a reactor trip by interrupting power to the c'utch power supplies. This drawing also depicts the interconnection between the clutch power supplies and the turbine trip circuitry.
When the clutch power supplies are deenergized, clutch power relays K1, K2, K3 and K4 actuate a turbine trip.
This drawing is attached with this letter and has been marked to show t'1e areas of interest.
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U. S. Nuclear R:gulatory Commission LIC-88-240 Page 2 2.
The NRC expressed a concern regarding replacement of Diverse Scram System components which might occur during maintenance operations. Assurance is needed that components used in the Diverse Scram System are indeed diverse from those used in the Reactor Protection System and that those parts are not chtnged at a later date to components which are not diverse.
Resoonse: The equipment critical to the operation and function of the Diverse Scram System has been classified as CQE equipment.
Since each component critical to the Diverse Scram System has been classified as CQE, its tag and part number are identified on the drawings for Fort Calhonn.
A change in part number due to a maintenance activity would not be per-mitted without adequate engineering review and documentation. Since a change such as this would be a planned change in a component owned by 0 PPD, l
the activity would be classified as a modification per Standing Order G-21 Section 1.4.7.
The modification process would then provide the required l
review and documentation before the component change would be allowed to occur.
3.
The NRC requested that OPPD provide the planned maintenance and surveil-lance requirements for the Diverse Scram System.
Resoonse: OPPD is prepared to submit a facility license change to incorporate the Diverse Scram System functional requirements into the Technical Specifications upon receipt of an SER. This change will incorporate the following requirements:
Check:
A comparison of independent pressure indications will be l
performed each shift.
Calibrate:
A full system calibration will be performed each refueling.
l Testing:
Individual channels of the Diverse Scram System (including channel bistable units) will be tested each month.
These maintenance and surveillance requirements will parallel those for the high pressurizer pressure trip of the Reactor Protection System.
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4.
The NRC requested a discussion of the common mode failure possibilities for l
l the power supplies of the Diverse Scram and Reactor Protection Systems l
including a discussion of the affects of frequency degradation on the power I
supplies.
Resoonse: To provide the reviewer with background information on the electrical distribution system at Fort Calhoun, a brief overview of the 120 VAC system is provided. The power supplies used in the Reactor Protection l
Syster. (RPS) and Diverse Scram System (DSS) are listed and a discussion of common mode failure possibilities for the power supplies of the two systems is then presented.
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U. S. Nuclear Regulatory Commission LIC-88-240 l
Page 3 120 VAC System j
The 120 VAC instrument system is comprised of six separate buses, four of which supply power to safety related instrumentation.
Each instrument bus is supplied by a separate inverter from the 125 VDC system. The arrange-ment of the inverter supplies is such that two of the safety related inverters are fed from one 125 VDC (battery-backed) bus while the other two safety related inverters are fed from the other 125 VLC (battery backed) bus.
The inverters receive an external synchronization signal from independent 480 volt (Class IE) sources and revert to internal clock signals upon loss of the external signal.
These provisions keep the inverters in synchronous operation with the system in the frequency range of 59 to 61 Hz with an automatic reversion to internal reference if system frequency departs from these limits.
The inverters have an output voltage regulation of 2 percent (steady state), and total harmonic content not exceeding 5 percent. All instrument buses provide annunciation in the main control room upon detection of low bus voltage.
The 120 VAC power sources for both the DSS and the RPS are the four safety related 120 VAC instrument buses.
The DSS and the RPS are fed from separate breakers on the instrument buses.
Reactor Protection System Power Sucolies The RPS performs its function by monitoring vital plant parameters, interpreting the parameters, and, based upon a 2-cut-of-4 logic, actuating a reactor trip when the parameter limits are exceeded.
Power supplies essential to the RPS functions are as follows (all power supplies are fed from the 120 VAC system):
Instrument Loop Power Supplies m
Instrument loop power supplies provide power to process transmitters, indicators, and RPS inputs. The RPS loop power supplies for the high pressure trip input are msnufactured by GE/MAC.
m Computation Modules Analog computations performed to support RPS functions are made by Bell &
Howell (now known as DeVar) computation modules which receive power from Bell & Howell power supplies.
I a Bistable Trip Units The RPS uses bistable trip units to compare the process signal inputs to predetermined setpoint values. When values exceeding the setpoint limits are detected, the bistable trip unit actuates a channel trip for the applicable parameter. The bistable trip units are supplied from Power / Mate power supplies.
I U. S. Nuclear Regulatory Commission LIC-88-240 Page 4 e Logic Matrix Relays Logic matrix relays are actuated by the bistable trip units. The contacts of the logic matrix relays are configured to actuate a reactor trip in a two-out-of-four logic scheme.
The logic matrix relays are energized by Power / Mate power supplies, a Logic Matrix Contacts The logic matrix relay contacts for the RPS are energized by 120 VAC power from the 120 VAC safety related instrument buses.
The logic matrix relay contacts open to actuate a reactor trip.
Diverse Scram System Power Sucolies l
The DSS performs its function by monitoring the Reactor Coolant System pressure, processing the pressure input signal, and, based upon a 2-out-of-4 logic, actuating a rehetor trip when the established pressure limit is exceeded. The 120 VAC dependent power supplies for the DSS are as follows:
a Instrument Panel Power Supplies The 120 VAC feed from the safety related instrument bus is routed directly from the distribution panel breaker to the Foxboro instrument rack power supply. This power supply is the source of power for the entire instrument rack.
Each DSS channel is configured in an independent instrument rack.
m Nest Power Supplies The Foxboro instrument rack power supplies feed DC power to up to 7 nest power supplies contained within the instrument rack. The nest power supply of each DSS channel services the loop power supply, the alarm trip unit, and the relay contact output modules (matrix and alarm relays) with DC power.
e Loop Power Supplies The Foxboro loop power supplies are used to supply the pressure transmitters with power.
i Power Supoly Common Mode Failure Possibili,tiqi In analyzing possible common cause failures for the AC power sources, five causes are of primary concern.
The possible scenarios are:
transient overvoltage conditions, loss of power, grounding of the power source, short circuit events, and frequency degradation. A discussion of each scenario and expected system responses is as follows:
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U. S. Nuclear Regulatory Commission LIC-88-240 Page 5 1.
Transient Overvoltage Conditions A transient overvoltage condition in the 120 VAC system would be iso-lated at the DSS instrument rack power supply or at the various RPS power supplies.
Since the power supplies of the DSS are diverse from those used in the RPS, diverse responses to the transient overvoltage are expected.
The only credible failure due to a transient over-voltage condition is the loss of the affected power supply which, in the case of the DSS instrument rack power supply, results in a channel trip because the transient overvoltage would be isolated at the rack power supply.
2.
Loss of Power The loss of power to the power supplies within the DSS and RPS presents various results. A discussion of loss of power to the various power supplies is as follows:
Component Failure RPS Resoonta DSS Resoonse 120 VAC System Channel Trip Channel Trip Rack Power Supply N/A Channel Trip Nest Power Supply N/A Channel Trip Loop Power Supply No Trip No Trip Bistable Trip Unit Channel Trip No Trip Computation Module No Trip N/A Matrix Relay Channel Trip Channel Trip i
The worst case failure as observed from this comparison is loss of power to the loop power supplies. A failure of both an RPS and a DSS power supply is highly unlikely, but is mitigated by the fact that both the RPS and DSS utilize 2-out-of-4 logic systems.
(A failure of this nature is beyond the bounds of the single failure criteria).
3.
Grounding of the Power Source a.
A single ground on either leg of the 120 VAC system would have no affect on either the DSS or the RPS since the 120 VAC system is an ungrounded system.
l b.
A double ground at the power supply connection to the 120 VAC l
system would result in loss of power to the affected power supply.
The loss of power to the power supplies and a comparison of system responses has been made previously.
4.
Short Circuit The only credible failure from a short circuit from a single failure viewpoint would result in the loss of power at the 120 VAC system level. As previously discussed, this would result in an RPS and DSS channel trip.
U. S. Nuclear Regulatory Commission LIC-88-240 Page 6 5.
Frequency Degradation The only credible source of frequency degradation is at the 120 VAC safety related inverters. As previously discussed, the inverters are in synchronous operation with the station's electrical distribution system and maintain a frequency range of 59 to 61 Hz. An automatic reversion to internal reference occurs if the system frequency devi-ates from the 59 to 61 Hz range. Assuming that the system frequency deviates from the prescribed range and that the inverter's internal reference malfunctions, frequency degradation is possible at the 120 VAC system level.
Upon deviating from the 59 to 61 Hz range, a "Sync Loss" alarm is actuated at the inverter with a corresponding "Inverter Trouble" alarm actuated in the main control room.
Response of the RPS and DSS power supplies to frequency degradation would be difficult to predict, but as a worst case could be assumed to cause a failure of the affected channel to trip.
The frequency degra-dation scenario could only be expected to occur in a single channel at any given time. The two-out-of-four logic schemes of the DSS and RPS would mitigate the consequences of frequency degradation.
125 VDC System The two-out-of-four matrices and the trip actuation devices of the DSS use 125 VDC from the station's 125 VDC system.
Possible common cause failures for this system are the same as for the 120 VAC system:
transient overvoltage, loss of power, grounding, and short circuit conditions.
Transient overvoltage conditions on the 125 VDC system would not produce common cause failures between the DSS and the Reactor Protec-tion System because none of the Reactor Protection System components obtain power directly from the DC system.
Loss of power to one of the DC buses would cause a reactor trip from the DSS since two channals would assume a TRIPPED status and the redundant logic matrix would initiate a reactor trip.
If both DC buses lost power, the reactor would trip because the clutch power supplies would have no power. A ground on either leg of the DC supply would not affect the DSS since the system is isolated from the station's grounding system.
Grounding of both legs of the DC system would result in the loss of DC power. A short circuit in the DC system would ultimately result in the loss of DC power to the affected matrix in the DSS.
The only credible common mode failure between the RPS and DSS stemming from a 125 VDC system transient is through the inverters (i.e., a loss of 120 VAC voltage to the systems).
The inverters serve to isolate all transients which do not cause a loss of power.
U. S. Nuclear Regulatory Commissicn LIC-88-240 Page 7 If you should need further information on this subject, please do not hesitate to contact us.
Sincerely, Q.E.
h%
R. L. An rews Division Manager Nuclear Production RLA/me Enclosure c.
LeBoeuf, Lamb, Leiby & MacRae 1333 New Hampshire Ave., N.W.
Washington, DC 20036 R. D. Martin, NRC Regional Administrator Anthony Bournia, NRC Project Manager P. H. Harrell, NRC Senior Resident Inspector
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