ML20206J462
| ML20206J462 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 06/23/1986 |
| From: | Whittier G Maine Yankee |
| To: | Thadani A Office of Nuclear Reactor Regulation |
| References | |
| RTR-NUREG-0737, RTR-NUREG-737 7639L-GDW, GDW-86-143, MN-86-79, NUDOCS 8606270175 | |
| Download: ML20206J462 (7) | |
Text
MAME HARHEE ATOMICPOWERComPARUe avaugr,K,Te%'s g
(207) 623-3521 e
June 23, 1986 HN-86-79 GDH-86-143 Director of Nuclear Reactor Regulation United States Nuclear Regulatory Commission Washington, D. C.
20555 Attention:
Mr. Ashok C. Thadani, Director PHR Project Directorate #8 Division of Licensing
References:
(a)
License No. DPR-36 (Docket No. 50-309)
(b) USNRC Letter to HYAPCo dated May 15, 1986 (c) HYAPCo Letter to USNRC dated September 6, 1985-(MN-85-159)
Subject:
Inadequate Core Cooling Instrumentation System for Maine Yankee Atomic Power Station Gentlemen:
Reference (b) requested that Maine Yankee respond to certain staff concerns regarding the design of Haine Yankee's Inadequate Core Cooling Instrumentation (ICCI). These concerns and our responses are provided in the attached.
Maine Yankee completed installation of ICCI in 1984.
During the 1985 refueling outage, Maine Yankee modified the inadequate core cooling instrumentation and related systems in response to NRC questions by replacing the Primary Inventory Trend System (PITS) transmitters and relocating PITS sensing lines. He have proposed further design changes in Reference (c):
1.
Power supply change to one PITS transmitter.
2.
Addition of PITS trend information on SPDS.
3.
Addition of CET information on SPDS.
As indicated in the attached, we are also proposing to recalibrate the PITS transmitters to a nominal reference temperature in response to staff recommendations.
However, we have decided to defer these proposed modifications to ICCI pending your concurrence regarding system acceptability.
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MAtNE VANKEE ATOMIC POWER COMPAD5Y-United States Nuclear Regulatory Commission Page Two Attention: Mr. Ashok C. Thadani, Director MN-86-79 He believe that the Inadequate Core Cooling Instrumentation as installed at Maine Yankee in conjunction with other existing instrumentation, provides an " unambiguous easy-to-interpret indication of inadequate core cooling", as required by NUREG-0737. As indicated above, we have made and have proposed several changes in response to the staff's suggestions which we believe would improve the system. He believe the installed system with the suggested changes are responsive to staff recommendations, meet the intent of NUREG-0737, and should, therefore, be acceptable.
He would be pleased to meet with you if you feel our response does not adequately resolve your concerns.
Very truly yours, MAINE YANKEE ATOMIC POWER COMPANY i
_.0.L9 N G. D. Whittier, Manager Nuclear Engineering and Licensing GDH/bjp Enclosure cc: Dr. Thomas E. Murley Mr. Pat Sears Mr. Cornelius F. Holden a
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MAINE YANKEE ATOMIC POWER COMPlW5Y PRIMARY INVENTORY TREND SYSTEM (PITS) 1.
NRC Concern:
PITS has no provisions for indication of system voiding with pumped flow as required by NUREG-0737, Item II.F.2, Clarification 4(a).
1.
Maine Yankee Resoonse:
NUREG-0737 Item II.F.2, Clarification 4(a) states, in part, "The indication of ICC must be unambiguous in that it should have the following properties:
(a)
It must indicate the inadequate core cooling caused by various phenomena (i.e., high-void fraction-pumped flow as well as stagnant boil-off...."
Maine Yankee's inadequate core cooling instrumentation provides indication of system voiding with pumped flow.
Specifically, the subcooled margin monitor would indicate system voiding with pumped flow.
o The criterion quoted above requires the ICCI taken as a whole to be capable of detecting system voiding with pumped flow. As indicated above, Maine Yankee's ICCI system meets that objective.
The above quoted criterion does not apply to each subset of the ICCI, e.g. PITS.
There is no specific guidance in NUREG-0737, that we are aware of, for PITS to detect system voiding with pumped flow.
The NRC recently approved Maine Yankee's reactor coolant pump trip criterion of 25*F subcooling. Therefore, by procedure, reactor coolant pumps will have been tripped prior to PITS being utilized.
He believe the guidance provided in NUREG-0737 Item II.F.2 Clarification 4(a) has been acceptably implemented.
2.
NRC Concern:
PITS is a single train instrumentation system consisting of three ranges of overlapping differential pressure indication necessary to cover the full range of reactor coolant system inventory which does not meet the requirements of NUREG-0737, Item II.F.2, Appendix B, Criterion (2).
2.
Maine Yankee Resoonse:
NUREG-0737 Item II.F.2, Appendix B, Criterion (2) states in part "No single failure within either the accident-monitoring instrumentation, its auxiliary supporting features or its power sources concurrent with the failures that are a condition or result of a soecific accident should prevent the operator from being presented the information necessary for him to determine the safety status of the plant and to bring the plant to a safe condition and maintain it in a safe condition following that accident."
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A MAINE VONKEE OTOMIC POWER COMPAP5Y
. i This criteri.on requires the ability to withstand a single failure.
It does not require redundancy as the only acceptable way to meet the single failure criterion.
The Maine Yankee PITS ~can withstand any single failure and the operator would still be able to evaluate the condition of the plant and bring it to safe condition.
In response to the staff's concerns, Maine Yankee performad an assessment of Maine Yankee ICCI. He have concluded that the limiting single failure would be the postulated loss of vital bus 1.
In this event, information to allow the operator to assess the adequacy of core cooling would be provided by one of the three PITS transmitters and the core exit-thermocouples indicated on SPDS. Additional information on RCS system temperature and pressure would be available on the main control board.
This information is sufficient to allow the operator to assess the adequacy of core cooling and to place the plant in a safe condition. This a
evaluation assumes that the power supply upgrade of the PITS transmitters, l
which was proposed in Reference (c) at the recommendation of the staff, has been completed.
The failure of the trend recorder paper drive was also considered.
i
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Although indication from each PITS transmitter would still be provided, trend information would not be as readily available.
The operator could I
still develop trend information manually.
In order to improve the trend information available in the event of the loss of the paper drive, we have proposed to modify the SPDS during the 1987 refueling to provide trend information, Reference (c).
The NRC SER dated November 14, 1984 implied the functional redundancy provided by the three overlapping transmitters and the chart recorder design was acceptable. He believe that the recommendations of NUfiEG-0737 Item II.F.2, Appendix B, Criterion (2) have been acceptably implemented at Maine Yankee and that the installed system will be further enhanced wittJ tha incorporation of the NRC recommendations as we have proposed.
3.
NRC Concern:
PITS is not compensated for pressure, temperature, and density changes and exhibits large errors and uncertainties which can render it unusable for its intended purpose.
3.
Maine Yankee Resoonse:
The purpose of PITS in conjunction with other existing instrumentation, is to provide the operator with an " unambiguous easy-to-interpret indication of inadequate core cooling", as required by NUREG-0737.
It does this by giving him a reliable indicator of coolant mass inventory (not level) within the system. The coolant inventory following a transient may be an unstable and changing mixture of steam, saturated liquid, steam voids and i
subcooled liquid. Therefore, any reference to coolant inventory as a i
" level" woul6 be erroneous. He knew of no instrumentation that can j
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. accurately detect and assess the coolant mixture and determine a specific level if one exists. He believe that any attempt to embellish the system with additional inputs such as temperature or pressure compensation would not improve its accuracy or usefulness and may make the system less reliable.
He believe that PITS is capable of providing a reliable indication of reactor vessel mass inventory and mass inventory trends. He also believe that the system is sufficiently accurate and reliable so as to enable the operator to determine the adequacy of core cooling during postulated accident conditions and, therefore, meets the guidance provided in NUREG 0737.
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MAINE YONKEE ATOMIC POWER COMPANY CORE EXIT THERM 0 COUPLES (CET) 1.
NRC Concern:
The CETs do not meet the requirements of NUREG-0737, Item II.F.2, (3) regarding the backup display.
1.
Maine Yankee Resoonse:
NUREG-0737 Item II.F.2, Attachment 1 (3) states, in part, "A backup display (or displays) should be provided with the capability for selective reading of a minimum of 16 operable thermocouples, 4 from each core quadrant, all within a time frame interval no greater than 6 minutes."
NRC SER to Maine Yankee dated November 14, 1984 asked Maine Yankee to
" Provide the schedule and plan for upgrading the backup core exit thermocouple system... or justify the deviation from the design requirements."
Maine Yankee has elected to justify the deviation with NUREG-0737 Item II.F.2 Attachment 1 (3). As stated in previous correspondence, Combustion Engineering in their report " Generic Thermal-Hydraulic Functional Requirements for Inadequate Core Cooling Instrumentation" (CE-NPSD-199) concluded that one thermocouple per reactor quadrant was sufficient to assess the adequacy of core cooling under accident conditions. Maine Yankee has two CETs per quadrant which can be read on the subcooled margin monitor.
In addition, Maine Yankee has three other CETs per quadrant (for a total of 5 per quadrant) which can be displayed on the plant computer and the SPDS. He believe the installed CETs at Maine Yankee meet the intent of NUREG-0737 and provide an acceptable alternative to the guidance provided therein.
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MQlNE YANKEE ATOMIC POWER COMPANY l
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SUBC00 LING MARGIN MONITOR (SMM)
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1.
NRC Concern:
The SMM system does not meet the requirements of NUREG-0737, Item II.F.2, Appendix B, Criterion (2) with respect to qualification and redundancy.
1.
Maine Yankee Response:
NUREG-0737, Item II.F.2, Appendix B, Criterion (2) states in part "No single failure within either the accident-monitoring instrumentation, its auxiliary supporting features or its power sources concurrent with the failure that are a condition or result of a specific accident should prevent the operator from being presented the information necessary for him to determine the safety status of the plant and to bring the plant to a safe condition and maintain it in a safe condition following that accident."
This criterion does not address qualification requirements. Nevertheless, we believe the SMM as installed for the core region and the reactor vessel head region to be qualified.
The criterion states that the operator should have sufficient information following a postulated single failure to assess the adequacy of core cooling and to place the plant in a safe condition. He believe a limiting single failure would be a loss of vital bus I which powers the subcooled margin monitor displays.
The operator would still have information on subcooled margin from either the plant computer, by using inputs of core exit temperature and pressurizer pressure, or by manual calculation using these same indications along with steam tables. He believe this information is sufficient to allow the operator to assess the adequacy of subcooled margin and place the plant in a safe condition, thereby meeting the guidance provided in this criterion.
In response to recommendations of your staff, we have upgraded the power supply to the pressure transmitters providing an input to the SMM, as described in Reference (c). He believe that with this upgrade, the Maine Yankee SMM design should be acceptable to the staff.
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