ML17341B334

Proposes Mod to Turbine Runback Sys,Deleting Flux Rate Input.Sys Prone to Spurious Runbacks Due to Single Failure of Electrical Component.Encl Safety Evaluation Discusses Consequences of Single Dropped Rod Control Cluster Assembly

ML17341B334
Person / Time
Site:	Turkey Point
Issue date:	08/10/1982
From:	Robert E. Uhrig FLORIDA POWER & LIGHT CO.
To:	Eisenhut D Office of Nuclear Reactor Regulation
References
L-82-343, NUDOCS 8208170123
Download: ML17341B334 (10)

Text

REGULATOR NFORMATION DISTRIBUTION l EM (RIDS)

AGCESSION NBR:8208170123 DOCSDATE: 82/08/10 'NOTARI'ZED NO DOCKET FACIL:50-250 Turkey Point IPlantP 'Unit 3E Florida IPower and. Light C 05000250 50 251 Turkey .Point Pl anti Unit 4r Florida',Power and Light C 050002S1 AUTH'AME AUTHOR AFFILIATION

'UHRIGP R,E ~ Florida Power 8 Light Co.

>RBC IP ~ NAME RECIPIENT AFFILIATION

.EISENHUT<D,G, Division of Licensing

SUBJECT:

Proposes mod to 'turbi.ne runback sys<deleting flux rat input.

Sys prone to spurious r unbacks due to single failure of s.ingle electrical component. Encl safety evaluat.ion discusses consequences of'single dropped rod control cluster assembly'ISTRISUTION CODE: A001'0 COPIES RECEIVED:LTR lE.NCL g SIZE:

TITLE: General Distribution for after Issuance of, Operating License NOTES:

RECIPIENT COPIES RECIPIENT COPIES ID,CODE/NAME LTTR ENCL ID CODE/NAME LT~TR ENCL ORB Oi BC '01 7 7 INTERNAL: ELD/HDS4 0 'NRR/DHFS DEPY08 NRR/DL D IR 1 1 AB NRR/DS I/RAB 1 1 REG F'ILE 04 RGN2 1 EXTERNAL; ACRS 09 10 10 LPDR 03 1 1 NRC 'PDR 02 1 1 NSIC 05 1 1 NTIS 1 1 TOTAL NUMBER OF COPIES REQUIRED: LTTR 28 ENCL 26

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P OX.14000, JUNO BEACH, FL 33408 lllliA FLORIDA POWER & LIGHT COMPANY August 10, 1982 L-82-343 Office of Nuclear Reactor Regulation Attention: Mr. Darrell G. Eisenhut, Director Division of Licensing U.S. Nuclear Regulatory Commission Washington, D.C. 20555

Dear Mr. Eisenhut:

Re: Turkey Point Units 3 5 4 Docket Ho. 50-250 aJId 50-251 Turbine Runback S stem The Turbine Runback System (TRS) at Turkey Point Units 3 8, 4 receives inputs from either the ex-core neutron flux detectors or a rod-on-bottom indication signal. It has been our experience that the TRS is prone to spurious runbacks due to single failure of single electrical components. These spurious runbacks have subjected the plant to unnecessary transients and reduced availability.

Therefore, we are proposing for your review and approval under the guidance of 10 CFR 50.59 a modification to the TRS which deletes the flux rate input.

Tni s modi fication will significantly. reduce the probability of spurious without compromising plant safety. 'unbacks The attached safety evaluation demonstrates that the consequences of a single dropped Rod Cluster Control Assembly (RCCA) assuming manual rod control and no turbine runback i s bounded by the previously analyzed static RCCA misalignment. The static RCCA misalignment has been analyzed in WCAP-9272, "Reload Safety Evaluation Methodology" which is the proprietary topical report submitted to you from Westinghouse on April 15, 1978. The automatic rod control system has. been disconnected at Turkey point Units 3 8 4, thereby assuring operation in the manual mode.'ery truly yours, 8208170123 820810 PDR ADQCK 05000250 P PDR Robert E. Uhrig Vice president Advanced Systems and Technology REU/JEM/mbd Attachments cc: Mr. James P. O'Reilly, Region II Mr. Harold .F. Rei.s, Esquire PEOPLE... SERVING PEOPLE

0' Attac nt to M-PTP-64

,MODIFICATION OF THE TURSIHE RUtiSACK SYSTEM

'The automatic turbine runback feature of Turkey Point Units 3 and 4 is designed to provide, protective action in the event. of a dropped RCCA or dropped bank.

Detecti.on -of a. dropped RCCA'r bank occurs by either a rod-on-bottom signal'evice or by a change in .neutron flux as seen, by the excore detectors . The rod-on-bottom signal provides separate indication for each RCCA in the core and one signal is sufficient to initiate the turbine runback. Also, a change in flux as seen by one of the four excore detectors will cause the turbine, load to be reduced.

The turbine load is reduced. to a pre-set value of 70%. At the same time, automatic withdrawal of.'he control rods is prevented by a rod .withdrawal block. This scenario is.-discussed and analyzed in Section 14.1.4 of the Turkey Point FSAR.

The design .of .the automatic turbine runback's prone to:spurious runbacks (i.e.,

runbacks not caused by-a RCCA drop) because there is no coincidence logic used in the initiation of the runback. Thus, a single failure of an electrical component (burnout of a .rod position indicator signal, failure of one'excore detector, etc.,)

instead of actii7g to prevent, protective action will, in fact, cause a turbine runback when it is not needed. This causes unnecessary plant transients'nd

'esults in a significant 1'oss,in operability and availability. Operating history at the 'Turkey, Point units shows that seven spurious runbacks have occurred in

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the last two years. No runbacks occurred due. to a dropped 'RCCA or bank-.

The majority of the spurious runbacks have resulted'rom failures in the. flux rate input to the runback logic. If this input could be deleted, operability would be greatly. improved, Since the turbine runback system is designed'; to provide pro-tection for a dropped RCCA/bank, this acci'dent must be re-evaluated in'ight of any changes to the system.

In any accident analysis, a limiting single failure for that transient is assumed.

In the event of' dropped bank (assuming the flux rate input has been dele'ted),

between four and eight rod-on-bottom signals will be generated,, one for each rod inMhe bank.. A failure of any one~gmLhas .no impact, since there are .still .

several other signals available, and only o'e is needed to initiate the turbine runback. Therefore, .the, dropped bank analysis is not a'ected by this change Page 1'f 3

0 ii ~) '4 to the turbine runback logic, and he FSAR analysis presented'n, ~ection 14.1.4 remains applicable.

However, for a single dropped RCCA, the failure of one rod-on-bottom signal means that no runback will occur,, since the only signal .generated, failed. (If the

,flux rate, input .is used, this will,initiate runback, if a rod-on-bottom signal fails for a dropped RCCA.)'herefore, this accident must be reanalyzed assuming no turbine, runback occurs.

r The transient for a dropped RCCA is ca'lculated using the same methods as described in. Section 1'5.1..4., The LOFTRAN code is used to model the plant response.. The LOFTRAN.code is a detailed digital computer program, which simulates neutron kinetics,,the pressurizer and its rel'ief and. safety valves, pressurizer spray and heaters, rod control system, and steam generators and. their rel'ief and,safety valves. Pertinent plant variables, including, temperature, pressure, and power level, are computed. Most negative moderator and doppl.er temperature coefficients are used to maximize the core heat flux.

Figure .1 illust'rates,the transient for a typical dropped rod worth of 150 pcm in

,manual rod:.control. The core heat flux initially drops and then returns to,the initial power level due to reactivity feedback.'emperatu're and pressure drop to a lower value. New equilibrium conditions are reached, with the core at full power and reduced temperature and pressure. These conditions are less limiting than

.those, for which the static full length misali'gnment of an RCCA is analyzed in the Rel'oad 'Safety Evaluation for each cycle., In this analysis, the, reactor is at full V

power with nominal temperature and pressure (including uncertainties). At the same power 1'evel, the DNBR benefit. due to the .reduction in temperature which occurs in the .transient case more than compensates for the DNBR penal.ty caused by the transient drop in pressure. Thus, the single dropped RCCA analysis assuming manual rod'ontrol and no turbine runback is bounded by static RCCA misal.ignment:

A dropped RCCA in. automatic rod control is not 'bounded: by this analysis since a

power overshoot above 102K could, occur..

In su+nary, Westinghouse finds. that it is acceptable to delete the flux rate

portion of the turbine runback system, provi'ded the plants, remain in manual rod control. T'e detection and,analysis~of dro ed

banks are not affected by this change. A dropped RCCA,which .does .not result .in a runback is bounded'y static RCCA misalignnsent.

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