ML20154A680

Safety Evaluation Supporting B&W Owners Group Generic Rept, Design Requirements of Diverse Scram Sys & ATWS Mitigation Sys Actuation Circuity

ML20154A680
Person / Time
Issue date:	09/15/1987
From:	Office of Nuclear Reactor Regulation
To:
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ML20154A683	List: ML20154A680 ML20195G473
References
TAC-57540, NUDOCS 8809130053
Download: ML20154A680 (7)

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SAFETY EVALUATION FOR "DESIGN PEQUIREMENTS FOR 055 (DIN RSE SCRAM SYSTEM) AND AMSAC ETWSMITlGATIONSYStEMACTUATIONCIRCUf1RY)"

.(TAC NO. 57540) 1.0 INTk000CT10N 10 CFR 30.6't specifies "Requirements for reduction of ria from anticipated tran;ients uthout scram (ATWS) events for liyr.t..ater-cooled nuclear power plants." This rule requires two specific systems for PWRs of B&W desion:

(1)

& diverse scram system (055) d: signed to reduce,',he probability of an ATWS (preventive systes.) and (2) a diverse system to ::utomatically initiate a turbine trip and auxiliary feedwater initiation (AMSAC) under conditions indicative of an AnS (mitigatter, system).

By letter J. T. Enos (B&W Owners Group) to H. L. Thompson (NRC), "B&W Owners Group ATWS Design Basis.* October 9, IVBb, the subject document was forwarded to NRC for review and approval.,

this document provides the generic design basis for the ATWS modifications required by 10 r.FR 50.62 for B&W 177 and 20SFA plants.

Plant specif!?

modifications will be developed from this design basis.

The scope of this safety evaluation is limited to Section 3B, ' Functional Requirements *; Iters 8.1 (Function), B.2 (In;ut), B.8 (Stsrtup B-

• ). and B.O (Trip Setpoint). Section 2, ' Aeview of ATWS Analyses", was also..yiewed f roe tae standpoint of how tho functional requirements depend on the results of these. '1yses.

It should be noted that 5U.62 contains no perfonnance criter

..e 055 C e AM!AC other than that the systems initiate "under condit'-

micatis c r ATWS". Nevertheless, it follows that the systems 5, function in response to ATWS events as icer.tified sihul.

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by the ai m-rerfonneo in st.pprt of the ATWS rulemaking which o

sittal. These analyses have been reviewed by the are sun" staff in th ;...

of 1,he ATWS ruleut* inq as indicated in NUREG-0460, IWS Task For u port, and SECY-83-293.

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2.0 STAFF EVALVATION The transients analyzed by the B&W Owners Group are those recormended by the y

staft in NUREG-0460. Vol. 11.Section IV. These transients are anticipated operatior,a1 occurrences as required by 10 CFR 50.62 and defined by Appendix A to Part 50 as transients expected one or more times during the. life of the plant. As such, LOCA is not considered except for sr.411 leuks and a stuck open relief valvi.

Ef,W selected the following perfortnance indie,es to be used in determining if an event is limiting and thus, to be considered in the design of the DSS /AMSAC.

(1) System pressure within limitt, of service level "C*,Section III. Division I of ASME code.

(2) Fuel Integrity - If DNB occurs, cladding temperatures should be "less than 2200 'F, and the extent o'f fuel failure shall be small, so as to not significantly distort the core, impede core cooling, or prevent safe shutdown".

(3) Radiological Consequences within 10 CFR 200 timits.

(4) Containment Pressure within design pressure.

(5) Reacter

• capable of being shut down and the core maintained amenable to long tarn eccling."

These eriteria are consistent with M,Ai-03SO recommendations and are, therefore, a:ceptable.

Based on BAW-1610 and earlier calculations, B&W has concluded that loss of feedwater and loss of offsite power A'TWS events cause high pressure and were considered by B&W as limiting transients in the design of the 055 and AMSAC.

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Based upon review of the B&W calculations, the staf f finds use of these transients for evaluating the design of the DSS and AMSAC acceptable.

B&W has adopted an "arbitrary design goal" of 3250 psia for the preventive DSS and 4000 psia for the AMSAC mitigation system.

B&W states that a pressure of 4000 psia will satisfy service level "C" stresses for rnost components, but somecomponents(e.g., pumps)mayrequirelowerpressures(3700tc3800 psia).

The 055 goal of 3250 psia is acceptab'e because it provides a snargin to service level "C" stress and is consistent with previous staff recomendations. The AMSAC goal of 4000 psia does not seem consistent witn the perfortnance indices establishec by B&W, Additional discussion of the AMSAC pressure goal is included below.

B&W has perfortned additional calculations of the loss of feedwater transient with DSS and AMSAC it.itiation to aid in the design of the 055 setpoints required to meet the 3250 psia goal. These subsequent calculations made use.

ot the NUREG-0460 recemendations aild are, therefere, acceptable.

B&W has al,*o perfortned additional plant specific calculations of the loss of feedmater transient with AKSAC alone. These calculations resulted in peak pressures of 3621 psia to 4190 psia.

.he AMSAC calculations ass med a rnoderator terpera ture coef ficient (MTC) of 901, l. wever, whien is slightly less conservative than the 951 Mit. recomended by the staff.

It appears, therefore, that scre B&W plants wounc not meet the service level *C" perfcrmance indices estab'ished by B&W curing sorne period in the life of the plant.

It was known when the ATWS rule was written that the AMSAC would not be as effective for B&W and CE plants and, therefore, the DSS was required on B&W and CE plants, t>ut not on Westinghouse plants where the AMSAC is enore j

effective.

It is also clear from the Statervnt of Considerations for the ATVS Rule, that the Comission intended that the DSS and AMSAC should be installed pror ptly without further debate regarding perfomance criteria and analysis. The 1

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Probatihstic Analysis in SECY-83-293 makes clear that the DSS is th'e principal means of ATWS mitigation for B&W plants and that AMSAC provides a small increment of safety improvement. However, consistent with the defense in depth concert it was decided that actuation of AFW and turoine trip would be appropriate.

For this evaluation, then, the principal purpose of the transient analyses is to provide confidence that the functional requirerents are reasonable, in particular, tht input initiation signals for both DSS and AMSAC. Thus, the desigt, goal of 4000 psia for the AMSAC is acceptable.

Hased upon review of the ATWS analyses, B&W concluded that loss of r.rin feecmater or very high reactor primary pressure, or a cocination of both.

could be used as input to the DSS.

Each of these inputs has advantages and disadvantages. The loss of feedwater input responds directly to the cause of one of the liriting transients and would also respond to the loss et offsite power transient which quickly results in loss of feedwater. This input would, therefore, rapidly and directly respond to the limiting ATES events identified by B&W. Tne disadvantage cf the less of feedweter input is that it would not respond to other ATWS events.

The high pressure input is a more general swptom of an ATWS which would result in DSS activation for both the identified limiting transients and a wide range of other ATW', events such as control rod withdrawal and purp i

l coastdown ATWS. The high pressure input is also expected to be a more f

reliable input because it would require less signal processing (e.g., pressure l

to flow conversicn and/or cosparison to flux signal) and would not be sensitive to rinor feedwater flow perturiations. The high pressure input (%s the disaevantage thet it responds to a :.jrptom of ATWS and thus would respond t

later than the loss of feedwater inp t following the liciting transients.

The two recceended inputs may also have advantages and disadvant tes in terrs of implementation that are plant specific. Certain inputs may be scre difficult to implement in one plant than another because of lack of existing instrumentation, penetrations, etc.

5 Review of the ATWS analysis indicate that the DSS is only required for the loss of feedwater and loss of offsite power ATWS events to assure that systen, pressure is limited to less than 3250 psia.

Peak pressure for the other ATWS events, without DSS or AMSAC functions, were all less than 2750 psie. Thus, the staff fit.d5 that use of either the loss of main feedwater or high pressure input, or a combination of the two, as an input signal to the DSS is acceptable. Each licensee rest identify which input signal it selects for the DSS in the plant specific submittal.

B&W reconrends that only loss of nain feedwater De used as input to the AMSAC, The high pressure input is appropriate for the fest acting DSS, but the high pressure signal would occur too late for the AMSAC to be effective.

Therefore, an additional high pressure input sign 61 to the AMSAC would provide little additional mitigation. The staff finds the loss of rain feedwater input to the AMSAC acceptable due to the minimal additional benefit of a high pressure input.

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The supplemental inferration publi,shed in the Federal Register with the final rule indicated that the DSS and AMSAC should net result in increased reactor trips. Thus the design and specific setpoints of the DSS and AMSAC trips should be such that initiation occurs only after normal RPS trips occur or should have occurred.

B&W perform @d sensitivity studies to determine how soon 055 must function s

in order to meet the standard of 3250 psis. Based on this study, a DSS pressure setpoint of 2450 t 50 psia was reconrenced. This setpoint is above the current RPS setpoint and below the lowest relief valve setpoint. These setpoints may be plant specific an may be subject to change in the future, however. The plant specific DSS pressure setpoint should be set to ensure that it is higher than the RPS setpoint and either lower than the lowest relief valve setpoint or designed so that the input pressure is not affected by relief valve flow.

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B&W recorrnends a DSS /AMSAC loss of feedwater "setpoint representative of a complete (100'.) less of main feedwatet flow". The definition of setpoint is acceptable,in prine'iple, but lacks specific detail as to implementation. This lack of detail is due to the fact that systems vary from plant to plant and, as a result the specific irplementation may vary f rom plant to plant.

It ray also be difficult to design a system that is a fast and reliable indication of complete loss of feedwater, without providing the potential for spurious trips due to feedwater perturbations. This system may also interface with and compete with other safety systems (such as systems designed to teminate feedwater to a broken stear. generator). The licensee should ensure that the design of the system does not result in increased trips or degradation of other safety systers, yet provides a reliable trip on complete loss of feedwater.

B&W recorriends bypass of the DSS and APSAC for power levels less than 45%.

Considering the dependence of the inputs on riain feedwater and the #eedwater perturbations typical curing startup, these systems would likely be the cause of a number of trips if not bypassed at startup. The need for the systeps is also less at low power since the iimiting ATVS events would be less severe at low power. The staff, therefore, finds these requirements acceptable.

3.0 C0NCLUSION The staff finds the design basis for c e 055 and AMSAC acceptable under the following conditions:

(1) Each licensee should identify the selection of input to the DSS in their plant specific submittal.

(2) The setpoin's for the pressure input to the DSf ouid be higher than the RPS high pressure trip setpoint and below the i,...ot pressure that relief valves open, or the design of the pressure input should be such that it is not affected t'y relief valve flow.

1 (3) The des.ign of the DSS and AMSAC less of feedwater trip input and setpoints sheuld demonstrate that a reliable and rapid initiation occors under tonditions of complete loss of main feedwater, without an increase in the nurrber of spurious trips or degradation of other safety systems.

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