ML19321B164
| ML19321B164 | |
| Person / Time | |
|---|---|
| Site: | Seabrook, Vermont Yankee, Yankee Rowe, Maine Yankee |
| Issue date: | 07/24/1980 |
| From: | Vandenburgh D YANKEE ATOMIC ELECTRIC CO. |
| To: | Ippolito T Office of Nuclear Reactor Regulation |
| References | |
| TASK-08-01.A, TASK-8-1.A, TASK-RR WYR-80-83, NUDOCS 8007280439 | |
| Download: ML19321B164 (7) | |
Text
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A Telephone 617 366-90ll e
Twx 7'O - 3 90 C 73 9 YANKEE ATOMIC ELECTRIC COMPANY 8.3.2.1 em.
WYR 80-83
- mLf 20 Turnpoke Road Westborough, Massachusetts 01581
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July 24, 1980 United States Nuclear Regulatory Commission Washington, D. C.
20555 Attention:
Of fice of Nuclear Reactor Regulation Mr. T. A. Ippolito, Chie f Operating Reactors Branch #3 Division of Operating keactors
References:
1.
License No. DPR-3 (Docket No. 50-29) 2.
License No. DPR-28 (Docket No. 50-271) 3.
License No. DPR-36 (Docket No. 50-309) 4.
Do cke t No. 50-443 and 50-444 5.
USNRC Letter, dated 8/12/7f (typical) 6.
USNRC Letter, dated 6/3/77 (typical) 7.
USNRC Letter, dated LO/16/79 8.
VYNPC Letter No. WVY-76-ll4, dated 9/16/76 (typical) 9.
VYNPC Letter No. WVY-77-65, dated 7/18/77 (typical)
- 10. VYNPC Letter No. WVY-79-139, dated 12/6/79
Subject:
Mitigating the Ef fects of Grid Degradation on Safety Related Electrical Equipment Dear Sir.
This letter is being written by Yankee Atomic Flectric Company on behalf of the Yankee Rowc, Vermont Yankee, Maine Yankee, and Seabrook nuclear stations. These facilities have been identified as References (!, 2, 3 and 4).
BACKCROUND INFORMATION The NRC position on degraded grid voltage (Reference 5, 6 and 7) requires automatic disconnection of the supply f rom the grid (of fsite power supply) to the plant emergency buses any time the voltage drops below a pre-determined limit. The NRC is concerned that a sustained variation outside the safety related equipment's design rated limit could result in a loss of capability if the equipment were simultaneously required to perform its safety function.
Yankee Atomic has steadfastly opposed the NRC's position on degraded grid voltage because we believe that any changes made in equipment or circuitry 8007280
Unitcd Stctos Nuciccr R2gulctory Commission July 24, 1980 Attcnticn:
Mr. T. A. Ipp211to Pcge 2 L
should increase rather than decrease the level of overall nuclear plant safety; it is our opinion that in attempting to resolve one safety concern additional safety concerns should not be introduced in the process.
Yankee Atomic has proposed an alternative position (References 8,-9 & 10) which relies on operator action in lieu of an automatic trip to resolve the NRC's basic concerns on degraded grid voltage. On receipt of our letter dated 12/6/79 (Reference 10), the NRC requested a meeting to discuss Yankee Atomic's generic position on grid undervoltage. Tne meeting was held at the NRC offices on May 5, 1980.
At the above meeting. Yankee Atomic engineers acknowledged that the NRC's concerns for continued operation of safety related equipment under degraded voltage conditions were valid, but, stated that they could not ignore the fact that additional safety concerns were being introduced by the NRC position.
These safety concerns were categorized into three areas:
a.
Violations of GDC-17, b.
Disintegration of the entire grid, c.
Being left with a less reliable source of power or no source of power.
It was pointed out that the Yankee Atomic position (References 8, 9 & 10) relied on the station operator to assess the situation relating to grid degradation and to take appropriate action to ensure that degradation was being corrected. Failing this he would take additional steps to protect l
safety-related equipment from the influence of degraded grid voltage. The operator action would ensure that a further deterioration of safety would not result from any action directed at correcting the degradation.
At the meeting, the NRC stated that they had an ongoing concern with operator action; they did nor have confidence in operator action and for that reason were opposed to pl#< ing any reliance on it.
l As one alternative (henceforth known as Alternative 1), the NRC suggested that we consider interlocking the automatic trip with an accident signal. An automatic trip of the offsite power supply would then result only if a simultaneous grid degradation and an accident occurred.
Another alternative (henceforth known as Alternative 2), suggested by the NRC for our consideration subsequent to the meeting, was that we interlock the automatic trip with a signal indicating that the main generator was off-line.
An automatic trip of the of fsite power supply would then result only if grid degradation occurred when the generator was not synchronized to the grid.
Both the above alternatives assumed that manual operator action would be utilized in modes when the automatic trip was not applicable.
DISCUSSION:
We have considered the two NRC suggestions and have analyzed the merits of each scheme. These alternatives have then Deen compared with both the original NRC position (Reference 5, 6 & 7) and the Yankee Atomic position (References 8, 9 & 10) on degraded grid voltage.
i j
Unit;d Stct20 Nuclect R2gulctory Commiccion July 24, 1980 Att:nticn:
Mr. T. A. Ippolito Page 3 Alternative _1 This alternative requires that the offsite power circuit breaker be automatically tripped if a simultaneous grid degradation and accident occurred.
The circuit breaker connecting of fsite power to the emergency bus -is shown in Figure 1.
The first level undervoltage relay is shown as device 27A. The second level undervoltage relay is shown as device 27B.
Both relays sense vdttage on the emergency bus.
When voltage is degraded below that required to ensure the continued operation of safety-related equipment, the second level voltage relay 27B will be activated.
Contacts of relay 27B will close in the breaker trip circuit as well as in the alarm circuit. The breaker will trip automatically if an accident signal is also received.
If bus voltage is severely degraded or lost altogether, the first level voltage relay 27A will also be activated. Contacts of relay 27A in the breaker trip circuit will cause an instantaneous trip of the circuit breaker.
With Alternative 1, a grid degradation experienced without an accident signal will only cause an alarm. Established plant procedures require the operatcr to take specific steps to assess the magnitude and expected duration of the grid degradation. If he is not assured that the discurbance is transitory, and that recovery is imminent, he may choose to manually trip the of fsite power circuit breakers af ter ensuring that a further deterioration of safety will not result from his proposed action.
Advantages 1.
Violations of CDC 17 are precluded.
2.
An accident signal would itself cause a trip of the main generator; therefore, any resulting collapse or disintegration of the grid could not be attributed to this circuit modification.
3.
The reliance on operator action in the event of a simultaneous accident and degraded grid condition is avoided.
4.
Operator action would be maintained for all non-accident conditions, thus precluding our concerns expressed in References (8, 9 and 10).
Disadvantages 1.
If the reactor is at power and all onsite ac power is determined to be unavailable, (i.e. all diesel generators lost) the reactor will be brought to a cold shutdown condition in accordance with technical specification requirements.
If, during this mode, an accident signal and a grid degradation were to occur, the offsite power supply breaker would trip leaving the plant with a total loss of all onsite and offsite ac power.
i
Unit;d Stat;o Nuciccr R;guletory Commiecion July 24, 1980 Att ntion:
Mr. T. A. Ippolito Paga 4 L
This scenario is being identified in spite of its extremely low probability because firstly, the basis for the NRC position on grid degradation is to design for a simultaneous accident and grid degradation, and secondly, loss of all onsite ac power has occurred at a number of facilities. The combined probability, however, remaining extremely low.
Al'ternative 2 Th'is alternative requires that the offsite power circuit breaker be tripped automatically if a grid degradation occurs when the generator is not connected to the grid.
The circuit breaker connecting offsite power to the emergency bus is shown in Figure 2.
The first level undervoltage relay is shown as device 27A.
The second level undervoltage relay is shown as device 27B.
Both relays sense voltage on the emergency bus.
When voltage is degraded below that required to ensure the continued operation of safety-related equipment, the second level voltage relay 278 will be activated.
Contacts of relay 27B will close in the breaker trip circuit as well as in the alarm circuit. The circuit breaker will trip automatically if it is determined by a logic circuit that the generator is not connected to the grid.
If bus voltage is severely degraded or lost altogether, relay 27A will also be activated. Contacts of relay 27A in the breaker trip circuit will cause an instantaneous trip of the circuit breaker.
With Alternative 2, a grid degradation experienced with the generator 1
connected to the grid will only cause an alarm. Established plant procedures require the operator to take specific stens to assess the magnitude and expected duration of the grid degradation.
If he is not assured that the disturbance is transitory, and that recovery is imminent, he may choose to manually trip the offsite power circuit breakers after ensuring that a further deterioration of safety will not result from his proposed action.
Advantages 1.
Violations of GDC-17 are precluded.
2.
Disintegration of the entire grid is precluded.
3.
This alternative prevents the second level undervo1*. age relay from automatically tripping the of fsite power circuit breaker during plant operation. Our concerns expressed in References (8, 9 and 10) relating to a plant trip are thereby removed.
l Disadvantages If the reactor is at power and all onsite ac power is determined to be unavailable (i.e. all diesel generators lost), the reactor will be brought to a cold shutdown condition in accordance with technical specification requirements. Once the generator is disconnected from the grid, this circuit
Unitsd Stct:0 Nuctsar R2guletery C:mmission July 24, 1980 Attsntion:
Mr. T. A. Ippolito Page 5 i
will cause the circuit breaker to trip if a grid degradation also occurs; the plant will now face a total loss of all onsite and offsite ac power. This situation is extremely undesirable because of the unpredictable consequences of this transient.
CONCLUSION
~
We' have carefully analyzed four possible methods of mitigating the ef fects of gr'id degradation on safety related equipment. These methods were:
(a) NRC Position (b) Yankee Atomic Position (c) Alternative 1 (d) Alternative 2 Of these four methods, we believe Alternative 1 is the most desirable scheme for our facilities. Additionally, the low probability disadvantages of Alternative 1 are outweighed by the advantages.
We, therefore, propose to adopt Alternative l for mitigating the ef fects of a grid degradation. The sensors, and circuit to be utilized will be as detailed in Figure 1, and as described in the text above.
PROPOSED ACTION AND SCHEDULE We are assuming your continued endorsement of Alternative 1, and will therefore immediately commence engineering changes to incorporate this modification for our Yankee Rowe, Vermont Yankce, and Maine Yankee facilities.
It is anticipated that engineering for these changes will be completed by November 1980.
Installation will follow at the first opportune shutdown following completion of engineering and receipt of materials.
Similar changes will be made on our Seabrook facility and will be documented in the FSAR and installed prior to commencement of fuel loading.
Should you have any comments on this proposed course of action and schedule, please notify us by August 15, 1980.
)
Very truly yours, YANKEE ATOMIC ELECTRIC COMPANY
\\_',
D. E. Vandenburgh Senior Vice President
i FIGURE i
TO OFFSITE EE
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UNDERVOLTAGE SENSOES 88 OFFSITE FONER ORCUlT
)
xx xx PT'S BREAKER EMERGENCY BUS ONE UNE. REPRESENTATION
+
=
27A-l ; d
- d 27B-l TO 278-2;d gggg
_ _ ACCIDENT SIGNAL r
CTC BREAKER TRIP Coll SCHEMATIC REPRESENTATidN OF OFFS ITE.
POWER. CIRCUlT~ BREAKER TRIP CIRCUlT l
t FIGURE 2
~
TO OFFSITE POWEE UNDERVOLTAGE SENSORS O27A27B OFFSITE NR CIEIT
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xx xx PT's BRENER EMERGENCY BUS ONE LINE REPRESENTATION
+
2 /A-1 ;d pf 27B-1 TO 27B-2;d 4tggg
- d LOGIC INDICATING 3
GENERATOE IS DISCONNECTED FRON\\ GRID CTC BREAK.ERTRIP COIL 5.t HEMATIC.
R EPRd.SEN TATION OF OFFSITE POWER. CIRCUtT BREAKER. TRIP Cl ECU iT
__. - _ _ _