ML19259A734

Forwards Results of Study Re Extending Time to Start Pcrv Depressurization in the Event of Loss of Forced Circulation. Concludes There Are Less Limiting Conditions Which Increase the 2 Hour Time Limit

ML19259A734
Person / Time
Site:	Fort Saint Vrain
Issue date:	01/04/1979
From:	Justin Fuller PUBLIC SERVICE CO. OF COLORADO
To:	Gammill W Office of Nuclear Reactor Regulation
References
P-79006, NUDOCS 7901100231
Download: ML19259A734 (8)

Text

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public service company e conomde P.O. BOX 840 DENVER, COLORADO 80201

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January 4,1979 Fort St. Vrain Unit No. 1 P-79006 Mr. William P. Gammill Assistant Director for Standardization and Advanced Reactors Division of Project Management U.S. Nuclear Regulatory Commission Washington, D.C. 20555

Subject:

Extending Time to Start PCRV Depressurization - LOFC Gent i c,1en :

At the meeting held in Denver on November 3 and 4,1978, Mr. Ireland of the Staff indicated that he was aware that PSC and GAC were evaluating the possibility of delaying the initiation of PCRV depressurization beyond the two hours presently stated in the Fort St. Vrain procedures in 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 V

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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 at 2-hours following an LOFC regardless of plant status. The 2-hour delay time 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 for any additional time which may be available under less than limiting condi tions .

Results of the analyses indicate that there are less limiting plant conditions which significantly increase the 2-hour time limit to initiate PCRV depressurization following the onset of an t.0FC. The greatest increase in the 2-hour time limit is obtained when the rt?ctor has been shutcown and maintained in a shutdown condition for a period of time prior to the occurrence of the LOFC (Figure 1). The next most significant increase is obtained when the average core outlet temperature is less than about ICCO*F at the onset of the LOFC (Figure 2). The low average core outlet temperature can 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 scram from any power level prior to the onset of the LOFC. The least amount of relief in the 2-hour delay time is obtained from the case where the LOFC occurs 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 at 70% and above (Figure 3).

Discussions A. Limits Establishina Allowable Celay Time to Initiate PCRV Cecressurization Follcwina Onset of a LOFC There are three potential limits against which reactor conditions must be measured under LOFC ccnditions:

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

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

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

. . . 2_

B. Assumotions Used in Analysis s

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

For all of the cases analy:ed, an equilibrium decay heat (FSAR Figure D.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 utilized in the analjsis.

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

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

3 Initial temperatures throughcut the core were determined by tne peaking factors, orificing strategy, and the primary system operating parameters defined in Reference 3 for conditions above 40% power level. For pcwer levels below 40% of rated actual FSV start-up data was used. Under shutdown conditions, the core was assu ed to be uniform at 410*F (the average fuel temperature after shutdcwn presented in Reference 3)

C. Results and Conclusions Two of the '.hree limits establishing the maximum allowable time to initiate Pf RV depressurization following an LCFC are asscciated with PCRV upper plerem te peratures. The RECA code which accounts for the affects of naturel convection in the core cavity was used to calculate the upper plenum ',er.peratures.

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

For each case examined, all three cotentially limiting conditiens were reviewed. PCRV depressurization was aut:matically initiated when tne average upcer plenum gas temperature reached the 1350*F limit imcosed to maintain acceptable helium purification train perfor ance. Analysis of the transient proceeded during the subsequent rapid depressurizaticn and 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,

I ,. .

s core fuel temperatures were reviewed at the time when depressurization was just complete. Again in every case the fuel temperatures did not exceed those of the 105% power, 2-hour initiation of depressurization base case. On the basis of this finding, it was concluded that none of these cases would place greater heat lead requirements on the LTA (due to fission product release) than the base case. Hence, the 1350*F average upper plenum gas temperature limit determined the time available to initiate depressurization at all conditions. (It should be noted, however, that the 30-hour liner temperature condition was also limiting for the base case.)

Figure 1 summarizes the results for the time available to initiate depressurization following an LOFC as a function of t9.;e from reactor shutdown. These results show that the time available increases signif-icantly with hours of reactor shutdown. This is especially true beyond 550-hours when the afterheat falls off rapidly (see FSAR Figure 0.1-9).

Figure 2 summarizes the results for the time available to initiate depressurizaitn as a function of average core outlet temperature at the start of the LOFC. The figure shows that reducing average core outlet temperature has a minimal effect until temperatures below about 1000*F are reached. The !cwer average core outlet temperature can be the result of low power operation (less than 25%) at the onset of the LOFC or a main loop rundown after reactor scram and prior to the start of the LOFC.

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

Comparison of these results indicates that the time available prior to the start of depressurization is closely associated with core tem-peratures at the time of LOFC. This is because the average upper plenum helium gas temperature is dependent upon natural convection in the core ca"ity which redistributes existing energy from the core into the gas.

Crnvection, in turn, is asscciated with region-to-regions temperature differences. These differences are functions of regicn peaking factors (RPFs) and core orificing strategy. Reductions in equilibrated pcwer level may result in increased RPFs due to rodding of the core. Con-sequently, there is no increase in the time available prior to start of depressurization until equilibrated powers below 70% are achieved.

Below this level, increases in time are modest since conditions in the core 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 ccoling cannot significantly alter the redistribution of energy within the core cavity in the relatively short time pericd involved.

de 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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