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January 4,1979Fort St. VrainUnit No. 1P-79006

Mr. William P. GammillAssistant Director for Standardization

and Advanced ReactorsDivision of Project ManagementU.S. Nuclear Regulatory CommissionWashington, D.C. 20555

Subject: Extending Time to Start PCRVDepressurization - LOFC

Gent i c,1en :

At the meeting held in Denver on November 3 and 4,1978, Mr. Irelandof the Staff indicated that he was aware that PSC and GAC were evaluatingthe possibility of delaying the initiation of PCRV depressurizationbeyond the two hours presently stated in the Fort St. Vrain proceduresin the event of a LOFC event at other than full-power conditions. Mr.Ireland verbally requested the results of this study.

As requested, please find the attached for your information.

If there are any questions, please feel free to let us know.

Very truly yours,

.K. Fuller, Vice President

Engineering and Planning

JKF/FES:ler

Attachment

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TIME AVAILABLE TO INITIATE CEPRESSURIZATION-

. FOLLOWING AN LOFC AT FORT ST. VRAIN

Summary

Currently, F5V procedures state that depressurization must be initiated at2-hours following an LOFC regardless of plant status. The 2-hour delaytime is based on the assumption that the LOFC occurs from 105% power con-ditions for both core average outlet temperature and power. In order to

permit greater operational flexibility, it is desirable to take credit forany additional time which may be available under less than limitingcondi tions .

Results of the analyses indicate that there are less limiting plant conditionswhich significantly increase the 2-hour time limit to initiate PCRVdepressurization following the onset of an t.0FC. The greatest increase inthe 2-hour time limit is obtained when the rt?ctor has been shutcown andmaintained in a shutdown condition for a period of time prior to theoccurrence of the LOFC (Figure 1). The next most significant increase isobtained when the average core outlet temperature is less than about ICCO*Fat the onset of the LOFC (Figure 2). The low average core outlet temperaturecan be the result of low power operation (less than 25% of rated) at the on-set of the LOFC or as a result of a main loop rundown after reactor scramfrom any power level prior to the onset of the LOFC. The least amount ofrelief in the 2-hour delay time is obtained from the case where the LOFCoccurs during power operation above 25% of rated. At 255 power operation,the delay time is 2-1/2 hcurs decreasing to 2-hours for power operation at70% and above (Figure 3).

Discussions

A. Limits Establishina Allowable Celay Time to Initiate PCRV CecressurizationFollcwina Onset of a LOFC

There are three potential limits against which reactor conditions mustbe measured under LOFC ccnditions:

1) Average gas temperature in the upper plenum must remain below 1350 Fat the start of the PCRV depressurization so that heat loads onhelium purification train front end ccmponents will not exceed coolingcapabilities during the early (high ficw) portions of the de-pressurization transient (Reference 1).

2) Top head liner temperature must be maintained belcw 1500*F duringthe first 30-hours folicwing a simultanecus LOFC and loss of linercooling to permit reflooding (Reference 2).

3) Heat loads on the icw temcerature adsorber (LTA) due to radioactiveXenon and Krypton cepcsited during depressurizaticn must remainwithin LTA cooling cacabilities until depressurizaticn is complete(Reference 1).

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B. Assumotions Used in Analysiss

The following are the assumptions used in the study to assure theanalysis results are conservative.

For all of the cases analy:ed, an equilibrium decay heat (FSAR FigureD.1-9) based upon an initial core power of 105% was assumed. Thus,the actual d%y heat will always be less than the decay heat utilizedin the analjsis.

Core radial peaking factor configuration have an important influenceupon the amount of natural convection experienced during an LOFCcondition. Accordingly, a conservative equilibrium core radial peakingfactor configuration was selected and utilized throughout the analysis.

Core orificing has also been shown to have an imoortant influence uponnatural convection during LOFC conditions. Where core average outlettemperatures were greater than 950*F, orificing resulting in conserva-tively high outlet temperatures for high radial peaking factor regionswas utilized. The maximum value of core outlet temperature dispersionwas chosen at 250*F; the maximum allowed by the Technical Specification(LCO 4.1-7) including instruant error. For conditions where coreaverage outlet temperature was below 950*F, all orifices were set to thesame effective position (as is the practice at low power levels durings ta rt-up) . Investigation of an alternate orificing scheme under theseconditions demonstrated the conservatism of this approach.

Initial temperatures throughcut the core were determined by tne peaking3

factors, orificing strategy, and the primary system operating parametersdefined in Reference 3 for conditions above 40% power level. For pcwerlevels below 40% of rated actual FSV start-up data was used. Undershutdown conditions, the core was assu ed to be uniform at 410*F (theaverage fuel temperature after shutdcwn presented in Reference 3)

C. Results and Conclusions

Two of the '.hree limits establishing the maximum allowable time toinitiate Pf RV depressurization following an LCFC are asscciated with PCRVupper plerem te peratures. The RECA code which accounts for the affectsof naturel convection in the core cavity was used to calculate the upperplenum ',er.peratures.

The third limit was that fuel temperatures and consequent fission productrelease do not excead the low temperature adsorber (LTA) capacity. Thecode CORCON was used to calculate fuel te:meratures as was done for theLOFC accident in FSV FSAR Appendix D.

For each case examined, all three cotentially limiting conditiens werereviewed. PCRV depressurization was aut:matically initiated when tneaverage upcer plenum gas temperature reached the 1350*F limit imcosedto maintain acceptable helium purification train perfor ance. Analysisof the transient proceeded during the subsequent rapid depressurizaticnand out to a ocint beyond 30-hours frem the initiation of :ne LOFC. In'

every case the liner temperature had not yet reached 1500*F. In additicn,

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core fuel temperatures were reviewed at the time when depressurizations

was just complete. Again in every case the fuel temperatures did notexceed those of the 105% power, 2-hour initiation of depressurizationbase case. On the basis of this finding, it was concluded that noneof these cases would place greater heat lead requirements on the LTA(due to fission product release) than the base case. Hence, the 1350*Faverage upper plenum gas temperature limit determined the time availableto initiate depressurization at all conditions. (It should be noted,however, that the 30-hour liner temperature condition was also limitingfor the base case.)

Figure 1 summarizes the results for the time available to initiatedepressurization following an LOFC as a function of t9.;e from reactorshutdown. These results show that the time available increases signif-icantly with hours of reactor shutdown. This is especially true beyond550-hours when the afterheat falls off rapidly (see FSAR Figure 0.1-9).

Figure 2 summarizes the results for the time available to initiatedepressurizaitn as a function of average core outlet temperature atthe start of the LOFC. The figure shows that reducing average coreoutlet temperature has a minimal effect until temperatures below about1000*F are reached. The !cwer average core outlet temperature can bethe result of low power operation (less than 25%) at the onset of theLOFC or a main loop rundown after reactor scram and prior to the startof the LOFC.

- Figure 3 summarizes the analysis results assuming the LOFC occurs duringpower operation above 25% of rated and there is no forced circulationfollowing the reactor scram. An additional one-half hour is gained onthe two-hour time limit at 25% pcwer operation. There is no relief inthe two-hour time limit for power operation at or above 70% of rated.

Comparison of these results indicates that the time available prior tothe start of depressurization is closely associated with core tem-peratures at the time of LOFC. This is because the average upper plenumhelium gas temperature is dependent upon natural convection in the coreca"ity which redistributes existing energy from the core into the gas.Crnvection, in turn, is asscciated with region-to-regions temperaturedifferences. These differences are functions of regicn peaking factors(RPFs) and core orificing strategy. Reductions in equilibrated pcwerlevel may result in increased RPFs due to rodding of the core. Con-sequently, there is no increase in the time available prior to startof depressurization until equilibrated powers below 70% are achieved.Below this level, increases in time are modest since conditions in thecore which enhance natural convection are not greatly relieved. It is

only as the absolute core temperatures are reduced that the time avail-able pricr to initiation of depressurization is greatly increased.

Finally, it should be noted that the presence or absence of liner ccol-ing has very little effect upcn these results because liner ccolingcannot significantly alter the redistribution of energy within the corecavity in the relatively short time pericd involved.

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References

1. P-77250, dated December 22, 1977.

2. P-77221, dated November 1,1977.

3. DC-1-4, " Plant Operating Parameters," Issue B, dated January 30, 1970.

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