ML19290F424
| ML19290F424 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 07/16/1980 |
| From: | Moore V Office of Nuclear Reactor Regulation |
| To: | Tedesco R Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML18025B195 | List:
|
| References | |
| FOIA-80-587, REF-GTECI-A-09, REF-GTECI-SY, TASK-A-09, TASK-A-9, TASK-OR NUDOCS 8008110650 | |
| Download: ML19290F424 (5) | |
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THis DOCUMENT CONTAINS POOR QUALITY PAGES
-.7 Occket No. 50-339 JUL 15 g a MEMORNIDUM FOR: Robert L. Tedesco, Assistant Director for Licensing, Div'ision of Licensing FRC:1:
Voss A. I' core, Acting Deputy Director, Division of Human Factors Safety SUSJECT:
SAFETY EVALUATION REFORT SUPPLE"ENT I.*:PUT:
NORTH ANNA UNIT NO. 2 - ANTICIPATED TRNiSIENTS
'AITHOUT SCRA'4 ( AT' 5) rne Procedures and Test Review 3 ranch has cc:npleted its review of the North: Anna Unit No. 2 E,nergency Procedures to detemine if interim requirements to mitigate Anticipated Transients Without Scram (AT45) events have been included.
Our-SER provides input for Section 7.2.4 and Generic Issue A-3.
The review was perfor:ned at requested by Frank Schroeder's memo of June 9,'1330, to you and was consistent with the guidance provided in his June 23, 1330 nemor.indum to you. Generic Issues Branch has reviewed the SER input and concu s with our conclusions.
~ 9EW Y Yoss A. Moore, Acting Deputy Director Division of Human Factors Safety
Enclosure:
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A-9 Anticioated Transients Without Scram (ATUS)
Nuclear plants have safety and control systems to limit the consequences of termporary abnormal operating conditions or " anticipated transients."
Some deviations from normal operating conditions may be minor; others, occurring less frequently, may impose significant demands on plant equipment.
In some anticipated transients, rapidly shu' ting down the nuclear reactor (initiating a " scram"), and thus rapidly reducing the generation of heat in the reactor core, is an important safety measure.
If there were a potentially severe " anticipated transient" and the reactor shutdown system did not " scram" as desired, then an " anticipated transient without scram," or ATWS, would have occurred.
The anticipated transient without scram issue and the requirements that must be met by the applicant prior to operation of North Anna Unit 2 are discussed in Section 7.2.4 of this supplement, The requirements set forth are for the interim period pending completion of Task A-9 and implementation of additional requirements if found to be necessary.
VEPCo has submitted procedures for anticipated transients without scram.
These procedures were reviewed as part of the staff's review of the VEPCo emergency procedures disc::ssed in Section I.C.1 of this supplement.
We have concluded that the emergency procedures adequately 9.ddress ATWS mitigating actions for operation at full power.
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7.0 INSTRUttENTATION AND CONTROL 7.2 Reactor Trio System 7.2.4 Anticipated Transients Without Scram
Background
In a pressurized water reactor, the anticipated transients which require prempt action to shut down the reactor in order to avoid plant damage and possible offsite effects can be classified in two groups:
those that isolate the reactor from the heat sink, and those that do not.
(A list of these transients is included in Appendix IV of Volume II of NUREG-0460, April 1978.)
In general, the consequences of both of these types of events are an increase in reactor power or system pressure, or both.
In Section 6.3 of NUREG-0460. Volume I, potentially unacceptable consequences of anticipated transients without scram events for pressurized water reactors of designs like North Anna are indicated to include (1) pressure rises that could threaten the integrity of the reactor coolant pressure boundary, (2) loss of core cooling, and (3) leakage of radioactive material from the facility.
In NUREG-0460, we concluded that for plants which fall within the envelope of the Westinghouse generic anticipated transient without scram analyses, the anticipated transient without scram acceptance criteria will not be violated if the actuation circuitry of turbine trip and auxiliary feedwater systems which are relied upon to mitigate anticipated transient without scram consequences are sufficiently reliable and are separate and diverse from the reactor protection system. Additionally, the functionability of valves required for long-term cooling following the postulated anticipated transient without scram events has to be demonstrated.
The NRC's Regulatory Requirements Review Committee has completed its review and concurred with our approach described in Volume 3 of NUREG-0460 insofar as it applies to North Anna Unit 2 We issued requests for the industry to supply generic analyses to confirm the anticipated transient without scram mitigation capability described in Volume 3 of NUREG-0460.
The staff evaluation of these reports was published as NUREG-0460, Volume 4, in.' larch 1980.
We plan to present our recommendations on anticipated traasients without scram to the Commission in July 1980.
The Commission would determine required modifications to resolve anticipated transient without scram concerns as sell as the required schedule for implementation of such modifications.
ilorth Anna Unit 2 would, of course, be subject to the Commission decision in this matter.
The following discusses the bases for operation of North Anna Unit 2 at power levels up to full power while final resolution of anticipated transients without scram is before the Commission.
In NUREG-0460, Volume 3, we state:
"The staff has maintained since 1973 (for example, see pages 69 and 70 of WASH-1270) and reaffirms today that the present likelihood of severe consequences arisiag from an ATWS event is acceptably small and presently there is no undue risk to the public from ATWS.
This conclusion is based on engineering judgement in view of :
(a) the estimated arrival rate of anticipated transients with potentially severe consequences in the event of scram failure; (b) the favorable operating experience with current scram systems; and (c) the limited number of operating reactors."
In view of these considerations and our expectation that the necessary plant modifications will be implemented in one to four years following Commission decision on anticipated transients without scram, we have generally concluded that pressurized water plants can continue to operate because the risk from anticipated transient without scram events in this time period is acceptably small. As a prudent course, in order to further reduce the risk from anticipated transient without scram events during the interim period before completing the plant modifications determined by the Commission to be necessary, we have required that the following steps be taken:
(1 )
Emergency procedures be developed to train operators to recognize an anticipated transient without scram event, including consideration of scram indicators, rod position indicators, flux monitors, pressurizer level and pressure indicators, pressurizer relief valve and safety valve indicators, and any other alarms annunicated in the control room with emphasis on alarms not processed through the electrical portion of the reactor : cram system.
(2)
Operators be trained to take actions in the event of an anticipated transient without scram, including consideration of manually scramming the reactor by using the manual scram button, prompt actuation of the auxiliary feedwater system to assure delivery of the full capacity of this system, and initiation of turbine trip.
The operator should also be trained to initiate boration by actuation of the high pressure safety injecticn system to bring the plant to a safe shutdown condition.
We consider these procedural requirements an acceptable basis for interim operation of the North Anna Unit 2 plant based on our understanding of the plant responsa to postulated anticipated transient without scram events, In response to our requirements on operator training and emergency procedures, Virginia Electric and Power Company submitted on January 10, 1980, emergency operating procedures for the postulated anticipated transient without scrum (ATWS) events.
Following review of the procedures by the staff, VEPCo revised the procedures to accommodate staff comments.
The revised procedures were reviewed as part of the staff review of emergency procedures addressed in Section I.C.1 of this supplement.
The instructions provided in the procedure for Reactor Trip permit the operator to diagnose an ATWS event and take appropriate actions required for minimizing its effects and bringing the plant to a safe shutdown condition, The instructions include the descriptions of the automatic responses of the plant as well as the operator's actions taken immediately after he diagnoses ATWS and later when he attempts to bring the plant to a cold shutdown condition.
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