ML18227D301

Letter Reactor Vessel Overpressurization Events

ML18227D301
Person / Time
Site:	Turkey Point
Issue date:	12/10/1976
From:	Robert E. Uhrig Florida Power & Light Co
To:	Lear G Office of Nuclear Reactor Regulation
References
Download: ML18227D301 (13)

Text

U.S. NUCLL'ABBEGULATOBYCOMMISSION I>>l>>C>>o>>>>>>>> 195 I2 TGI I>>IRC DISTRIBUTION ron RT BO DOCI<ET MATERIAL DOCKET NUMOEB 50-250/.5l.

FILE NUMBER TO:- lfr; George Lear FROM:

Florida Power 6 Light Company Miami, Florida lfr. Robert E. Uhrig DATE OF DOCUMENT 12/10/76 DATE BECEIVED 1?/13/76 jyfLETTEB

~ OBIGINAL Q COPY DESCBIPTION C3 N0 To B I7 E D j3VNCLASSIFI ED INPUT I'OBM FNCLOSUBE NUMOEB OF COPIFS RECEIVED One signed Ltr. v/attached....re their 10/15/76 1tr.

and our 8/11/76 1tr....concerning Reactor Vessel Overpressuri7ation.

(1-I )

REACTOR VESSEL OVERPRESSURXZATXON DISTRIBUTION PER G, KECH 10-21-76 ZCK~O Q'QQ g0 (5-P)

PLANT NAlfE:

.Turlcey Point Units 3 6 4 SAFETY BllANCII CHIEF:

5 LXC ~ ASST:

FOR ACTIOIN/INFORMATION 12 15 76 Lear Parrish RJL PROJECT HANAGER'lliott INTERNALDISTR I BUTION NRC PDR I'

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AC.I>

The present power supply alignment for the solenoid valves controlling air flow to the pressurizer power operated relief valves will be retained.

Installation of the "Reference Mitigating System" will not compromise the existing separation between DC power sources.

d.

A keylock switch or an equivalent administratively controlled switch will be used to enable and disable the low setpoint of each relief valve.

The enable/

disable switches will conform.to the separation criteria requirements for the DC buses for the Turkey Point Plant.

e.

Seismic design of the electronic equipment presently installed in the Turkey Point Plant will be retained.

Additional electronic equipment will be installed so as not to compromise the present seismic qualifica-tions of existing safety systems.

The control air supply from the air accumulators will be seismically designed.

Typical pressurizer power operated relief valves are designed to withstand seismic loading and retain their function during such loading.

The valves will not be degraded by the system modification.

1 g ~

Testability will be provided.

Verification of opera-

'bility is possible prior to solid-system, low-temperature operation by use of the remotely operated isolation valve,, enable/disable

switch, and normal electronics surveillance.

Testing requirements could be incorporated in the operating procedures to assure performance prior to existence of plant conditions requiring operability of the mitigating system.

h.

Figure 3 presents a typical electrical schematic diagram which would be used for each pressurizer power

I

ATTACHMENT (Continued)

I h.

(Continued) operated relief'alve.

The additional pressure channel's bi-stable contact or auxiliary relay contact and the enable/disable switch addressed in "d" above are included.

i.

The loss of an instrument power bus will not result in an isolation of letdown flow and disabling of the "Reference Mitigating System."

By the time the transient analysis is completed, agreement should be reached on what the design criteria for the "Reference Mitigating System" should be in order to minimize the time needed to install an acceptable system.

Ne have inquired as to the availability of electrical and mechanical equipment required for the "Reference Mitigating System."

According to vendors'stimates, delivery of additional equipment needed for the "Reference Mitigating System" could be expected within six months of order placement.

Our desire to resolve this matter by the end of 1977, coupled with the facts that (1) analysis completion is scheduled for the end of March, 1977 and (2) equipment delivery may require an additional six months, make it imperative that the design criteria include sufficient flexibilityto allow us to accomplish the objective of preventing overpressurization transients.

Both.

pressurizer relief valves may be necessary to mitigate the worst-case overpressurization event to be analyzed in oQr bounding analysis.

Contingencies of this nature should be and were con-sidered in selection of design criteria.

The "Reference Mitigating System" design includes conformance to the guidelines of your August 11, 1976 letter, provides for the maximum pressure relief possible with available mechanical equipment, and could be installed by the end of 1977.

Overpressurization events are cause for concern, and reasonable modification of operating plant equipment is sound engineering practice in view of the events which have occurred.

EIowever, Appendix G limits are based on conservative assumptions, and it, should be noted that exceeding Appendix G limits does not necessarily mean that vessel

damage, much less fracture, Sill

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. ATTACHMENT (Continued) s occur.. In fact, additional mar'gin to the Turkey Point pressure-temperature limits could be achieved with the existing Appendix G methodology by performing a plant-specific analysis'hich included actual plant data and more realistic assumptions of flow, fluence, vessel material properties, etc.

Thus, we con-clude that, because there is significant margin between the realistic plant analysis and the conservative Appendix G

analysis, an Appendix G limit does not represent a Safety Limit.I In our October 15, 1976 letter, we stated that. the remote possibility of exceeding Appendix G limits would still exist following installation of any mitigating system.

It was also stated that the proper course of action following such an event would be to analyze the event to verify the acceptability of continued

,operation.

We will follow that course of action if Appendix "G" limits are exceeded at. the Turkey Point Plant.

In our October 15, 1976 letter, we also stated that administrative controls would be used to prevent inadvertent overpressurization of the reactor coolant system by the safety injection accumulators.

Those administrative controls, would include de-energizing the accumulator isolation valve motors in the same manner utilized for power operation.

Specific procedural verification of valve status and motor breaker status as now used to verify that the valves are open and de-energized for power operation will be incorporated in the plant procedures to verify that the valves are closed and de-energized.

The steady state flow capacities of typical pressurizer power operated relief valves and the mass :injection rates for a typical 4 loop Westinghouse plant are presented in Figures 4 and 5

respectively.

Note that the steady state relief Capacity of a single pressurizer power operated relief valve is of the approximate capacity necessary to compensate for a steady state safety injection mass input.

Although the steady state flow rates appear consistent, transient analysis will be necessary to assure the relief capability of the system.

Figure 6 presents the typical flow vs. valve plug position relationships which will be incorporated in the analysis.

The "Reference Nitigating System" design, which incorporates the guidance of your letter, uses installed plant equipment.

as much as possible to avoid equipment procurement delays and provides the maximum pressure relief available without a major plant modification.

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ATTACHi~)ENT (Continued)

A testable relief valve system, as described

above, coupled with increased administrative control of the accumulator isolation valves will provide a,means to mitigate the consequences of an overpressurization event.

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