ML20127G712

Safety Evaluation Accepting Increase in Safety Valve Set Points,Per 730913 Request to Change TS

ML20127G712
Person / Time
Site:	Monticello
Issue date:	10/02/1973
From:	James Shea, Ziemann D US ATOMIC ENERGY COMMISSION (AEC)
To:
Shared Package
ML20127G703	List: ML20127G701 ML20127G712
References
NUDOCS 9211170412
Download: ML20127G712 (6)

Text

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UNITED STATES __ATCMIC ENERGY COMMISSION SAFETT EVALIETION BT TIIE DIRECTORATE OF LICENSING BDRT11ERN STATES POWER COMPANY DOCKET jp. 50-263 Northern States Power Company (NSP) by letter dated September 13. 1973, has proposed to change the Technical Specifications of Provisional Operating License No. DPR-22 to permit operation of the Monticello Nuclear Power Plant with the four safety valve set points at 1240 peig instead of two at 1210 and two at 1220 peig and to require four safety vaivas where three of four installed valves were required previously.

We have reviewed the proposed Technical Specifications changes and the safety analysis provided as attachments to the NSP letter.

According to the Finsi Safety Analysis Report (PSAR), three of four relief valves and two of four safety valves (ref.1) provided sufficient capacity to guard against excessive pressure due to turbine trip with-out bypass, conservatively assued.ng resetor scram from a high flux signal instead of from the turbine valve trip signal. NSF in a later assessment (ref. 2) of relief and safety valve performance changed the basis for steam safety valve capacity deterr.inations to simultaneous closure of all MSIVs assuming delayed reactor scram due to high neutron

' flux signal because this transient is more severe. For this transient, the peak steam pressure was calculated to be 1283 psig using the scram -

reactivity curve corresponding to an exposure threshold of 2250 MWD /STU (ref. 3). We accepted the revised basis for calculating safety valve requirements and changed the Technical Specifications (ref. 4) to show the revised pressure peak assuming three relief and two safety valves operated as designed following MSIV closure with delayed reactor scram due to high neutron flux.

Slower relief valve opening times (ref. 5) caused a- reduction in the exposure threshold from 2250 to 2000 MWD /STU and prompted examination of the advantages that could be gained by setting safety valves at 1240 psig to allow an increase in transient peak pressure while main-taining the 25 poi GE design margin to the safety valve set point (ref. 6 and 7).

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t Aseerding to the reanalysis of safety valve performanos attaehad to l the September 13, 1973 NSF letter, the overpressure peak for closure t of all four MSIYs, assuming delayed remeter seram from the high flus

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j signal and a new and-of-eyele (30C) estan reactivity serve, the vessel pressure (at the bottom of the pressure vessel) is 1304 pois or

{ 67 pet below the anzimum ogsrpressure design limit of 1375 pois.

i Newever, the basis for the saleslation was changed to require that all four safety / relief valves and four safety valves open. In the previous analysis, only three safety / relief and two safety valves were reqeired. Therefers, the severity of the transients using the l

4 fuel esposure threshold at 2000 HifD/STU and BOC are not directly comparable. . Inquiry brought the telephone response by NSF that the fo11 swing esubinations of safety / relief and safety valves had been l evaluated at 100% power with 0.8 second relief valve response times i

i and delayed fluz scram af ter simultaneous closure (within 3 seconds) ef all MSIYa:

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1. 4 safety / relief valves and 0 safety valves

2. 3 safety / relief valves and 2 safety valves l 3. 2 safety / relief valves and 4 safety valves and the margin to 1375 peig design limit r - ina greater than 25 psi.

The margin to the pressure design limit has, therefore, been reduced I from 92 psi to apprezimately 25 poi under sintime aircumstanees. We j have concluded that this margin, with allevance for reliability con-

' siderations, is acosptable and the safety valves may, therefore, be .

set at 1240 pois instead of 1210 and 1220 pois. We note that both valve types, i.e. , the pilmt-eparated safety / relief. valve and the spring-loaded safety valve are pressure actuated (self metuated) and are not dependent on any other source of power to prevent overpressure.

We understand that sensitivity caloslations are currently being per-a formed by NSP to determine the peak transient pressure effect of

,taeressing the safety / relief valve set pressure to 1090 peig (from 1080)

- so that allouanse eam be made for s6t point drift er variations.

Pending eagletion of this study and the analysis for the remainder of fuel eyels 2. however, Heaties11e operations should samtinue to be esaservatively restricted by requiring the same este control rod f laventory attained at 1200 MUD /8TU specified by NSF prior to the Septsaber 29, 1973 shutdown. (Shutdown to modify reliaf valve responsa ,'

time and the increase in the safety valve set points.)

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3 Da the basis of our evaluation, we have concluded that the increase in safety valve set point and the requirement for all four safety valves to be is service does not present an unreviewed safety con-sideratism or significant hasards consideration and there is reasonable aseurance that the health and safety of the public will not be endan-sered by operation of the reactor with the safety valve set points increased by 20 poi for two valves and 30 psi for the remaining two safety valves. The Technical Specifications should therefore be changed as proposed.

\d James J. She a Operating Reactors Branch #2 Directorate of Licensing

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Denni s L. Zia==nn , (2tief Operating Reactors Branch #2 Directorate of Licensing Date: OCI 2 I9I) omcz > . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .... ,

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J N1 REFERENCES

1. FSAR - page 4-4.4 "The required safety valve steam flow espaelty is determined by analysing tbs pressure rise aseospanying the main steam flov stoppage resulting from a turbine trip initiated with the rosator i at 1670 MWt. The analysis assumes ne steem bypass system fisw.

no turbine valve trip scram but a reacter seram from indirect means (high flux). The relief and safety valve capacity is assured to total 50% (35% relief and 15% safety of the full power steam generator rate). This capacity correspeeds to assuming that i three of the four relief / safety valves (35.4%) and two of the four safety valves (18.5%) operated."

2. ItsF letter to AEC dated February 13, 1973, transmitting "Results of Transiaat Reanalysis for Monticello Nuclear Generating Plant with End-of-Cycle Core Dynamic Characteristics". A significant change in the shape of the scram reactivity curve cculd occur by the end of fuel cycle 2 (see Figure 1 - the new analysis curve is sometimes referred to as curve B).

Page 4 "It should be noted that the original FSAR analysis used for the safety valve sising transient was the turbine trip without bypass (identical to instantaneous loss of condenser vacuum transient) with flux serse. However, it was determined with later plants that the main steam lina isolation with flux scram could be more severe." Hence this analysis is used in @==H== safety valve adequesy.

Page 5 - Relief Valve Adequacy Transient "A scram signal it initiated at the same time a turbine trip occurs by position switches on the turbine stop valves. This transient causes a rapid pressure increase in the reactor pressure vessel. Primary system relief valves are provided to remove sufficient energy from the reactor to prevent safety valves from -

lifting." Using improved control rod scram times (Figure 2) a:ad four relief valves (three required previously) the peak pressure i

in the steam line at the safety valve location was calculated to l be 1183 pois and since the lowest safety _ valve set point is 1210 pois, the CE design margin between peak pressure and the safety' valve set point of 25 pai is maintained.

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Page 5 and 6 - Safety Valve Adequacy i

Figure 4 shows the transient resulting from closure of all 4 MSIVs .

i within 3 seesade wherein 3 of the 4 relief / safety valves open l

(32% of main steam generattom rate) and only 2 of the 4 eafety valves (18% of main steam generation rate). Neutron flux reaches l the sorse level at about 1.8 seconds, initiating reacter shutdown.

1 The assumed safety valve capacity (Target Rock plus spring safety l

capacities) keeps the peak vessel pressure 92 psi below the peak alloweble ASNE overpressure of 1375 peig. "Therefore, the relief 4

valves plus spring safety valves provide adequate protection

! against assassive overpressuriaation of the nuclear system process barrier with a large margin because of the reduced capet.ities assumed for this analysis."

3. NSP istter to AEC dated June 1,1973 - Request to change the Technical Specifications to require four operable relief valves

instead of three, and slightly shorter control rod scram times

< in accordance with the analysis presented in the attachment to NSF letter dated February 13,1973 (reference 2 above). "P re-

, liminary calculations show that the new analyses present the nost limiting conditions expected during the first 2250 MWD /STU exposure increment of cycle two."

4. AEC approval letter (Change No. 8) dated July 2,1973, to require  ;

four relief valves instead of three as previously required'and slightly faster serse times than previously specified in accordance with NSF change request dated June 1,1973 (reference 3 above) for reactor operation at rated power out to 2250 MWD /STU.

"We are continuing our evaluatica of the shape changes in the scram reactivity curve and the necessity for more restrictive technical specifications but agree that the technical specification changes you have proposed should be made now."

5. NSP 1stter to AEC dated August 1,1973 " Observed Relief Valve Opening Times Different than those Assumd in the Transient Analysis".

General. Electric reports that results of Target Rock relief valve performance tests show a delay in initial opening time of about 0.8 second rather-than 0.2 second as reported in the Monticello FSAR.

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6. NSF 1stter to AEC dated August 21,1973 " Planned Reactor Operation from 2000 MWDIT to the End of cycle 2".

Page 2 "1alief valve modifications will reduce peak vessel pressure felltving transients for the end of cycle 2 as well as subsequent cycles. Safety valve setting increases will maintain or improve the margin between vessel pressure and valve set points" (following turbine trip without steam bypass).

7. ABC Memo to Fils dated September 13, 1973.

We will consider a change to the Technical Specifications to increase safety valve set point from 1210-1220 psig to 1240 psig.

Our final coselusions, in this regard, are dependent on additional analysis for the period beyond 2000 WD/T to be prc.vided by NSP.

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