ML20076N028
| ML20076N028 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 03/20/1991 |
| From: | Pelletier J VERMONT YANKEE NUCLEAR POWER CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| BVY-91-29, NUDOCS 9103270114 | |
| Download: ML20076N028 (7) | |
Text
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VERMONT YANKEE N.UCLEAn POWER CORPORATION N
Ferry Road. Brattleboro, VT 053017002 k
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'bNited States Nuclear Regulatory Commission ATTN Document Control Desk Washington, D.C.
20555 Dear Sir SUBJECT Implementation of Alternate Rod Injection System (ARI) diversity requirements in 10 CFR 50.62 (ATWS Hule) at Vermont Yankee
REFERENCES:
a)
License No. DPR-28 (Docket No. 50-271) b)
- Letter, R.
Wessman, USNRC, to io.
A.
Tremblay, Vermont Yankee Nuclear Power Station, NVY 91-01, dated 11 January 1991 The purpose of this letter is to provide Vermont Yankee's response to the information requested in Reference b) regarding diversity of trip units between the Alternate Rod Injection System (ARI) and the reactor trip system.
CURRENT INSTALLATION In 1980, prior; to formulation of the ATWS Rule, Vermont Yankee installed an analog instrumentation system.
This system replaced Cooling System (ECCS) instrumentation. portions of the Reactor Protection System (hP and Emergency Core In addit ECCS instrumentation, actuation signals were provided for Alternate Rod Injection (ARI)icipated Transients Without SCRAM (ATWS).and Recirculating P associated with Ant This system utilized the state of the art Rosemount Trip and Calibration System, including series 510 trip units.
In order to comply with the Environmental Qualification Rule, 10 CFR 50.49, Vermont Yankee upgraded the trip units to series 710.
Because our installation was completed prior to the publication of the ATWS Rule, we utilized Rosemount trip units in both RPS and ARI actuation systems.
At the present time, this equipment is still installed and has functioned admirably for over ten years.
This configuration (7e; not comply with the staff's position regarding j
diversity.
COMPLIANCE WITH RECULATORY REQUIREMENTS l
To comply with NRC's recent interpretation of the diversity requirements of the ATWS Rule, Vermont - Yankee plans to install additional equipment which will provide diverse actuation et the
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VERMONT VANKEE NUCLE AR POWER CORPORATION e
U.nited States Nuclear Regulatory Commission March 20, 1991 Page 2 Alternate Rod Injection for the reactor low-low level signal.
Vermont Yankee believes that the availability of the high neutron flux SCRAM prov1&s the required diversity for the ARI high reactor pressure initiation signal. A detailed description of the '::omplete ATWS Mitigation System is contained in Enclosure 1.
Because our Rosemount trip units are located in the Reactor Building in areas subject to harsh environmental conditions, the required modifications constitute a major design change to Vermont Yankee Station.
Outage-dependent modificatinns to the plant have been carefully planned and scheduled in order to be implemented during the next refueling outage in 1992.
We are currently preparing our design change packages for this outage.
It is not possible to include this design change in the engineering effort for that outage without jeopardizing other work which we have previously negotiated with NRC.
Therefore, we anticipate installing the ATWS Diversity modification during the subsequent refueling outage in 1993.
We request that you review the enclosed scheme prior to our beginning detailed engineering work.
Desed upon our current schedule, we need your approval of this scheme by January 1,1592, to allow the d1 tailed design to be completed in time for installation during the 1993 outage.
The above schedule F:s been discussed with and agreed to by our Project Manager.
Vermont tankee will continue to monitor industry and NRC activities regarding ATWS diversity and, should we become aware of a method of meeting the diversity requirements of the ATWS rule that may be more suitable, we will inform you as roon as possible.
I Very truly yours,
- PM ames P. Pelletier Vice President, Engineering l
jbb cci USNRC Region T Administrator USNRC Resident Inspector - VYNPS USNRC Project Panager - VYNPS STATE OF VERMONT
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WINDHAM COUNTY
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Then personally appeared before me, James P.
Pelletier, who, being duly sworh, did state that he is Vice President, Engineering, of Vermont Yankee Nuclea r Power Cgt,pp% } on, tation that he is duly authorized to execute and file the foregoing document i n ),#f 6 a b %
the behalf of versont Yankee Nuclear Power Corporation, and that the sta*
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ATWS MITIGATION ACTL% TION SYSTEM Safety Objective As described in the Vermont Yankee Final Safety Analyds Report, the objective of the ATWS mitigation rystem is to provide a means to reauce the probability of a failure to scram or to mitigate the consequences it. the unlikely event that an a.7ticipated transient occun and control rods fail to insert.
Safety Design Basis 1.
_ The ATWS mitigation system shallinitiate functions that prevent damage to the nuclear system pmcess barrier as a result of an abnormal transient that does not result in a scram.
2.
Malfunctions in the ATWS mitigation systems will not effect the reactor protection systems.
Description The nTWS mitigation system pmvides actuation signals to reduce reactivity and shutdown the reactor. Reactivity is reduc'ed and the reactor shutdown by two actions.
These two actions are:
- Motor-generator recirculation pump trip (RPT), and Alternate rod injection (ARI)
Actuation signals to initiate these two actions are derived from instrumentation systems which monitor reactor pressum and reactor level. A block diagram for a typical proposed channel of the ARl/RPT initiation instrumentation is shown in Figure 1.
The remainder of this discussion will address only the ARI and r ammon portions of the system.
The outputs of the electronic transmitters are connected to electronic trip units. The trip unit actuates when a preset input value is reached. The output of the trip unit actuates a-relay whose contacts are used to produce a.two-out-of-two taken once logic.
The parameters which initiate the-ARI system are reactor low low level, after a time delay of approximately ten seconds, or high reactor pressure. A ten second delay on low low level is used to avoid making the consequences of a postulated LOCA more
-sCvere.
In addition to the equipment sh'own in Figure 1, reset and manual actuation switches are mounted on the recirculation pump portion of the main control board. Upon receipt of a manual actuation signal, reactor. water level low-low trip signal, or reactor high pressure trip signal, the logic will seal in and hold the trip until the reset switch is depressed. A 1
Logic Channel trip sigaal will result in energizing one of two solenoid valves that cause reactor rod injection.
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Rgure 1 Proposed configuration J ARI Initiation Power for the ARI actuation system is provided from AC and DC sources which are completely independent from the power sources which supply the mactor protection system.
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Annunciation is provided to indicate the status of the four sub channels in the two-out-of two logic.-
SAFETY FUNCTION The Al'ernate RodJnjection system is provided to reduce the probability of a failure of control rods to insert on demand. Upon receipt of the initiating signal described above, o
.this symm will'act to bleed air pressure off the scram pilot valve air header, thus L-producing a scram separately and independently from other reactor scram functions.
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DIVERSITY ANALYSIS One channel of the level and pressure signals which initiate the reactor protection scrams.
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Sensors Although Rosemount transmitters are used in both the reacto: protection system and the alternate rod injection actuation system this is allowed by the ATWS Rule,10 CFR 50.62 (c)(3) which states:
Each bolling water reactor must have an alternate rod Ltdection (ARI) system that is diverse (from the reactor trip system)from sensor output to thefinal actuation device.
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Figure 2 Reactor Protection System Analog to Digital Converters The reactor level signal which initiates the reactor protection system is generated by a Rosemount trip unit. The reactor level signal which initiates alternate rod injection will be generated by trip unit which is made by a manufacturer other than Rosemoun' Both of these devices convert the analog output of the transmitter to a switched voltap signal capable of operating an auxiliary relay. These devices are manufactured by different companies, thereby achieving equipment diversity. The auxiliary relays which are used in the logic systems for these two initiation signals are made by different manufacturers, also achieving equipment diversity. In addition, they are powered by diverse power 3
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sources, i.e. direct current and alternating current, and are diverse in their me6cd of operation,i.e. energize to trip and de energize to trip.
Within the reactor protection system two diverse means are provided to trip the reactte when the pressure exceeds an allowable limit. These diverse signals are a dirret measurement of reactor pressure and the measurement of neutron flux. In a boiling water reactor an increase in reactor pressure causes a decrease in void fraction. This results in an increase in neutron Cux which is detected by the local power range monitors. Whea the neutron Dux exceeds the allowable limit, a trip of the reactor trip system is initiated.
To provide assurance that an anticipated transient will not prevent shutdown of the reactor the ATWS initiation system includes a high reactor pressure signal. This signal is produced by a Rosemount series 710 trip card. This signal is diverse from the high neutron Dux signal which provides a trip input to the tractor trip system. Equipment diversity is established since the equipment for these two signals iade by different manufacturers. Measurement diversity is established since one sigt
.easures pressure directly and the other measures a related parameter, neutron Dux. Power supply diversity is es%11shed since one signal is powered by the reactor protection system power supply and the other is powered by the completely separate ECCS batteiy system. Based upon these factors Vennont Yankee has determined that the degree of diversity provided between the existing ATWS high pirssure trip and the existing neutron flux trip of the rractor protection system meets the intent of 10 CFR 50.62.
In addition for those events where the high reactor pressure is caused by an isolation of the main steam lines or turbine generator, signals are provided from the main steam isohtlon valve and turbine stop valve limit switches to the reactor protection system.
These signals result in a scram if either the main steam isolation valves or the turbine r, top valves close. These signals provide an additional diverse scram for these events.
RELAYS l
The auxiliary relays used in the tractor protection system are manufactured by Agastat L
and General Electric. These relays are nonnally energized by a 120 volt AC source and de energize to initiate a scram. The relays used in the alternate rod injection system are manufactured by Agastat and a vendor other than General Electric. The ARI relays are L
nonnally de-energized and are energized by 125. volts DC to initiate the system.
f Although there is some commonality between the Agastat relays in the RPS and ARI l
systems, they are diverse in the type of power, i.e. AC versus DC, and their manner of application, i.e. nonnally energized versus normally de-energized. These differences are L
sufficient to preclude the types of common mode failures which diversity seeks to avold.
l On this basis additional diversity for these irlays are deemed unnecessary.
l The scram solenoid valves used in the reactor protection system are mar ifactured by ASCO for General Electric Company.
These valves are manufactured to GE 4
l specifications. %ey are normally energized by 120 volts AC and de-energize to scram.
De solenoid valves which actuate to cause an alternate rod injection are also manufactuted by ASCO. They are nomially de energized and are energized by 125 volts DC to vent air from the scram air header. Although both valves are manufactured by the same company, they are diverse in the type of power, i.e. AC versus DC, and their manner of application, i.e. normally energized versus nonnally de energized. These differences are sufficient to preclude the types of common mode failures which diversity seeks to avold. On this basis additional diversity of these solenoid valves is deemed unnecessary.
SAFETY EVALUATION
'1k; circuitry described above for ARI initiation is completely independent of the reactor protection system. his equipment is powered from sources which are completely independent from the sources which supply the reactor protection system.
This
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configuration ensures that no single failure can prevent an ARI initiation.
The setpoints at which the ARI is actuated have been selected so that the ARI will not interfere with the reactor protection systems. The effectivenns of the RPS is not impaired by any malfunction of the alternate rod injection sy: tem.
The ARI is designed as Class I seismic equipment. All of the equipment is suitable for all environmental conditions to which it may be subjected.
C.ONCLUSION 9
The alternate rod injection is capable of meeting its safety objective by performing the fanctions detailed in the safety design basis when required. The equipment utilized has been designed and installed to meet the applicable design criteria. This results in a system which increases the safety of the plant and effectively eliminates or mitigates Anticipated Transients Without Scram.
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