ML19309D003
| ML19309D003 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 02/29/1980 |
| From: | Counsil W NORTHEAST UTILITIES |
| To: | Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| References | |
| IEB-79-27, NUDOCS 8004090466 | |
| Download: ML19309D003 (8) | |
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February 29, 1980 Docket No. 50-336 Mr. Boyce H. Grier Director, Region I Office of Inspection and Enforcement U. S. Nuclear Regulatory Ccmmission 631 Park Avenue King of Prussia, Pennsylvania 19406 Gentlemen:
MILLSTONE NUCLEAR POWER STATION UNIT 2 120 VAC INSTRUMENT POWER SYSTEM The attached infomation is provided in response to IE Bulletin 79-27, regarding loss of instrument control power and its effect on the ability to achieve cold shutdown.
ACTION TO BE TAKEN BY LICENSEES 1.
Review the class 1-E and non-class 1-E buses supplying power to safety and non-safety related instrumentation and control systems which could affect the ability to achieve a cold shutdown condition using existing procedures or procedures developed under item 2 below.
For each bus:
a) identify and review the alam and/or indication provided in the control room to alert the operator to the loss of power to the bus.
b) identify the instrument and control system loads connected to the bus and evaluate the effects of loss of power to these loads including the ability to achieve a cold shutdown condition.
c) describe any proposed design modifications resulting from these reviews and evaluations, and your proposed schtdule for implementing those modifications.
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t Mr. Boyce H. Grier U. S. Nuclear Regulatory Commission Page 2 la Response Millstone Unit 2 has two non vital 120VAC instrument A.C. buses (2-IAC-1 and 2-IAC-2) and four vital 120VAC instrument A.C. buses (2-VIAC-1, 2-VIAC-2, 2-VIAC-3 and 2-VIAC-4).
The non-vital buses are powered from a vital 480VAC bus through a regulating transfomer.
The vital buses are powered nomally from an inverter supplied from the station emergency batteries with back-up power available through a static switch from 2-IAC-1 and 2-IAC-2 for 2-VIAC-3 and 2-VIAC-4 respectively. 2-VIAC-1 and 2-VIAC-2 are provided alternate power from inverters supplied from the station turbine battery.
(See attached Figure).
Loss of power to a non-vital instrument bus is annunciated (one annunciator for each bus) in the main control room when its voltage drops to 105VAC or less. This alam coupled with the loss of certain indications / controls would promptly alert the operator to the mal function.
The following alams are associated with the vital instrument buses:
Inverter trouble Vital instrument AC panel on emergency supply These alams would alert the operator to power supply problems with vital instrument buses.
If power were totally lost to one of the vital buses, channel trips would occur in the associated Reactor Protection System and Engineered Safeguards Actuation Systen.
These channel trips and the alams listed above would promptly alert the operator to the malfunction.
lb Response A review of Instrument and Control System loads connected to vital and non-vital buses has been conducted specifically to detemine if losing one such bus would preclude being able to achieve cold s hutdown.
O Mr. Boyce H. Grier U, S. Nuclear Regulatory Commission Page 3 Loss of any one instrument bus will not result in being unable to bring the plant to cold shutdown.
Sufficient alternate indications and controls plus manual capabilities where necessary are provided.
lc Response In light of lb Response, it can be seen that' no design modifications are required.
However, NNEC0 has long recognized the importance of instrument AC and has previously initiated modifications which will strengthen the supply system for non-vital instrument power.
2.
Prepare emergency procedures or review existing ones that will be used by control room operators, including procedures required to achieve a cold shutdown condition, upon loss of power to each class 1-E and non-class 1-E bus supplying power to safety and non-safety related instrument and control systems.
The emergency procedures should include:
a) the diagnostics /alanns/ indicators / symptom resulting from the review and evaluation conducted per item 1 above.
b) the use of alternate indication and/or control circuits which may be powered from other non-class I-E or class 1-E instrumentation and control buses.
c) methods for restoring power to the bus.
Describe any proposed design modification or administrative controls to be implemented resulting from these procedures, and your proposed schedule for implementing the changes.
2a, b,
Response
A review of Millstone procedures has been completed.
The following changes will be made to strengthen them in the areas identified below:
(a) The Vital Instrument A.C. System Operating Procedure will be revised to incorporate additional infonnation to aid operators in identifying a loss of power.
Additionally procedures will be revised to include a listing of instrument loads which are utilized in bringing the plant to cold shutdown and what alternate instrumentation is available.
C1 Mr. Boyce H. Grier U. S. Nuclear Regulatory Commission Page 4 (b) The Non-Vital Instrument A.C. System Operating Procedure will be revised to include a listing of instrument loads which are utilized in bringing the plant to cold shutdown and what alternative instrumentation is available.
With respect to Item 2c, existing procedures were found to be sufficient.
The changes described above will be completed by June 1,1980.
3.
Re-review IE Circular No. 79-02, Failure of 120 Volt Vital AC Power Supplies, dated January 11, 1979, to include both class 1-E and non-class 1-E safety related power supply inverters.
Based on a review of operating experience and your re-review of IE Circular No. 79-02, describe any proposed design modifi-cations or administrative controls to be implemented as a result of the re-review.
Response
NNEC0 has reevaluated Millstone 2's power supply systems with respect to IE Circular 79-02.
Our detemination of March 9,1979 remains the same. A copy of our position at that time is attached for your infomation.
Summary / Conclusions The review of Millstone 2 Vital and Non-Vital Instrument A.C.
Systems in response to IE Bulletin 79-27 has not revealed the need to implement design modifications in order to bring the plant to cold shutdown with any one instrument bus de-energized.
- However, certain procedural changes, when implemented, will provide operators with necessary additional infomation when power is lost to an emergency bus.
It should be pointed out that our detemination of instrument bus loads was done with specific emphasis of the ability to achieve cold shutdown. As such it was not necessary to identify the source of power for every instrument throughout the plant.
Although the source of power for every device at Millstone 2 can be detemined, no composite list has been generated.
To compile this infomation represents a significant undertaking. However, in recognizing the value of such a listing Millstone 2 has initiated action to compile this infomation with a tentative completion date of January 1,1981.
Mr. Boyce H. Grier U. S. Nuclear Regulatory Comission Page 5 Once this task is canpleted, further evaluation will be done to detennine if design modifications and/or procedure revisions are wa rranted.
I trust this infonnation is responsive to your request.
Very truly yours, Northeast Nuclear Ene y Company T(/]V W. G. Counsil Vice President L
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J. F. Opeka March 9, 1979 MP 3003 Ed C.
Unit 2 Superintendent Inverter Reliability Reference (a)
NRC-IE Circular 79-02 Failure of 120 Volt Vital AC Power Supplies (CR2819)
Reference (b)
NRC-IE Information Notice 79-04 Degradation of Engineered Safety Features (CR2851)
Review of the references indicates that Millstone either does not have the equipment or has proper administrative controls to reduce the proba-bility of such an occurrence at Millstone.
In summary:
1.
Unit 2 does not utilize time delay circuitry in its inverter units.
2.
Unit 2 does not use a 480/120 volt transformer as described in the ci rcular.
3.
Unit 2 does utilize the 125% overcurrent and the 80% undervoltage for the protection transfer circuitry.
Unit 2 has not had a history of inexplicable transfer.
A.
The retest controls of job orders on Category I equipment (inverters) are sufficient administratively to ensure operability after being subjected to maintenance.
Based on the above no further response is felt to be required or changes necessary.
Reference (b) also indicated two other problems at Arkansas.
One problem was that degraded voltage of the off site supply could result in the unavailability of ESF equipment.
This situation has already been exten-sively reviewed and corrective action taken.
The other problem is that loss of one of two off site circuits would also result in the loss of the other circuit.
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1-i This is highly unlikely at Millstone since the feeds.to the. Unit 1 RSST (Unit 2's alternate off site source) and the Unit 2 RSST are electrically separated by the breaker and a half scheme in the switchyard.
ECF/ RAP:llm cc:
S.' E. Scace R. A. Place Unit 2 Files 1.3.6.5B (79-02) 1.3.6.5C (79-04) 6.13 CR 2819 Foider/
CR 2851 Folder f
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