ML20246P153

Provides Addl Info Re 890221 Application for Amend to License NPF-3,increasing Reactor Protection Sys Response Time for High Flux/Number of Reactor Coolant Pumps on Trip Function.Addl Use of Vipre Code Will Be Justified

ML20246P153
Person / Time
Site:	Davis Besse
Issue date:	09/01/1989
From:	Shelton D TOLEDO EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
1709, TAC-72015, NUDOCS 8909110058
Download: ML20246P153 (11)

Text

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4 TOLEDO EDISDN A Centergr Energy Consurg DONALD C. SHELTON vce Present- Nuclese

"* ""#3" Docket Number 50-346 License Number NPF-3 4 Serial Number 1709 September 1, 1989 United States Nuclear Regulatory Commission Document Control Desk Washington, D. C. 20555

Subject:

License Amendment Application to Increase the Reactor Protection System Response Time Requirement for the "High Flux / Number of Reactor Coolant Pumps On" Trip Function, Supplemental Information (TAC Number 72015)

Gentlemen:

Toledo Edison submitted a license amendment request (Serial Number 1634, dated February 21, 1989) which proposed changes to the Davis-Besse Nuclear Power Station (DBNPS), Unit Number 1 Operating License, Appendix A, Technical Specifications Section 3/4.3.1.1, Reactor Protection System Instrumentation, Table 3.3-2, Reactor Protection System Instrumentation Response Times. The request proposed an. increase in the response time requirement for the High Flux / Number of Reactor Coolant Pumps On (power / pumps) trip function of the Reactor Protection System (RPS) provided in Table 3.3-2 from 451 milliseconds to 631 milliseconds. The proposed change is based on an analysis performed ,

with a Nuclear Regulatory Commission (NRC), topically approved, computer code system named VIPRE.

The proposed changes vere discussed in a telephone conference call between Toledo Edison personnel and NRC staff personnel on May 10, 1989. As a result ,

of the telephone conference call (documented in Toledo Edison's letter Serial l Number 1690 dated July 19, 1989, to the NRC), Toledo Edison committed to provide the following information to supplement the original request:

A description of the type of noding studies which vere performed.

An explanation of why VIPRE is reasonable for this application.

Acknowledgment that the Critical Heat Flux (CHF) correlation for VIPRE is different from that of LYNKT and discussion that VIPRE provides the 95/95 confidence level for CHF.

8909110058 890901 J PDR ADOCK 05000346 CDj P PDC

,i 1 THE TOLEDO EDISON COMPANY EDISON PLAZA 300 MAD! SON AVENUE TOLEDO. OHIO 43652 i

_-___________-______-___-___-___Q

r l Dockat Nuxb:r 50-346 Lic:nsa Numb:r NPF-3

. Serial Number 1709 Page 2 Confirmation statement that this is a one time specific use of VIPRE j l and Toledo Edison agreement that additional justification vill be l necessary in order to use VIPRE in other applications.

Toledo Edisan confirmed in Serial Number 1690 that this is a one time specific L use of VIFRE and that further justification vill be needed to use VIPRE in other applications. The remainder of the supplemental information is provided in the Attachment. Based on this supplemental information to support the NRC staff review, Toledo Edison requests that this amendment be issued by December 31, 1989.

If you have any questions concerning this matter, please contact Mr. R. V. Schrauder, Nuclear Licensing Manager, at (419) 249-2366.

Very truly yours, GBK/dlm Attachment cc: P. M. Byron, DB-1 NRC Senior Resident Inspector A. B. Davis, Regional Administrator, NRC Region III T. V. Vambach, DB-1 NRC Senior Project Manager State of Ohio

4 Docket Number 50-346 License Number NPF-3 Serial Number 1709

- Attachment Page 1 of 9 ADDITIONAL INFORMATION TO SUPPORT USE OF THE VIPRE CODE FOR TOTAL ~ LOSS OF FLOW TRANSIENT ANALYSIS l

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Docket Number 50-346 License Number NPF-3 Serial Number 1709 Attachment Page 2 of 9 Backcround The VIPRE-01 11-channel transient model described in Reference 1 i was used to reanalyze a total loss of reactor coolant-flow transient. The reanalysis was performed in order to verify that the pump-power monitor response time could be increased from 451 milliseconds to 631 milliseconds.

The original VIPRE-01 model developed by Toledo Edison consisted of 18 channels as illustrated in Figure 1. This model was developed for steady-state analyses. The 18-channel model was revised for transient analyses as follows; 1) number cf lumped channels were reduced from 12 to 5, 2) hot subchannel rods were modeled as conduction rods, 3) variable gap conductance model was used and 4) forcing functions were incorporated as necessary.

All other VIPRE input parameters are identical to the 18-channel model. The resulting 11-channel transient model is described in Reference 1. The benchmark analysis discussed in Reference 1 l Verified that the 11-channel transient model predicts j conservative minimum DNBRs during a loss of flow transient.

l As part of Toledo Edison's ongoing core thermal-hydraulic model development efforts, the 18-channel steady-state and 11-channel transient models were independently reviewed by the University of Cincinnati (Reference 2). The review concluded that the models are acceptable and no significant concerns were identified.

Sensitivity studies have been performed (Reference 2) to provide additional verification that the VIPRE 11-channel model can be used to analyze a total loss of flow transient. Results of these sensitivity studies are discussed in this attachment. Additional information concerning the total loss of flow transient analysis as described in Reference 1 is also provided.

Steady-state sensitivity studies have been performed using the 18-channel model at nominal and low flow conditions. The studies were performed by changing various VIPRE input parameters and correlations and then determining the change in minimum DNBR relative to a base case. Sensitivity studies for transient conditions have also been performed using the 11-channel transient model. In so doing, it can be verified that the appropriate VIPRE input parameters and correlations have been chosen and that the proper geometries have been modeled.

Docket Number 50-346 License Number NPF-3 Serial Number 1709 Attachment Page 3 of 9 L VIPRE Sensitivity Studies

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Steady-state sensitivity studies have been performed for the following'VIPRE input parameters at nominal and low flow conditions.

Axial Friction Factor

-Lateral Resistance Coefficient Grid Form Loss Coefficient Magnitude Grid Form Loss Coefficient Location Centroid Distance Results indicate that for expected variances in the above parameters, the minimum DNBR changes by less than 1 point (1 point = 0.01 DNBR).

The following. input options were also analyzed, Heat Transfer Correlations Solution Scheme Convergence Criteria Results show that the 18-channel model produces conservative results relative to the input options that were studied The axial noding, radial noding, hot subchannel location and pin power distribution sensitivity studies are of particular interest and will be briefly discussed on an individual basis.

Axial Noding Sensitivity:

The analysis presented in Reference 1 used 48 axial nodes. Axial noding sensitivity studies were performed with the 18-channel model by both decreasing and increasing the number of axial nodes from the base case of 48 axial nodes. The results are summarized below.

Delta DNBR Relative Number of Nodes to Base Case (noints)

L 48 Base Case I

77 2.0 25 -2.0 l

I

l 4

p Docket Number 50-346 i License Number NPF-3 l- '

Serial Number 1709 L . Attachment L Page 4 of 9 l

Results demonstrate that the minimum DNBR decreases when the number of axial nodes are decreased which is conservative. ,

Since the accuracy of VIPRE increases when more axial nodes are  !

used, selection of the number of axial nodes is a tradeoff between accuracy and computer time. Using 48 nodes is conservative with respect to 77 nodes and produces reasonable computer times.

Radial Noding Sensitivity:

Radial noding was analyzed by both increasing and decreasing the number of channels with respect to the 18-channel steady-state model. The effect of decreasing the number of radial nodes was evaluated using an 11-channel steady-state model. The 11-channel steady-state model (Figure 2) was constructed by decreasing the number of lumped channels from 12 to 5. As in the 18-channel model, six individual subchannels were modeled in the hot assembly. The noding schemes for the steady-state and transient 11-channel models are identical. Sensitivity study results show that.the minimum DNBR computed with the 11-channel model is greater than the 18-channel minimum DNBR by 0.7 points at low

' flow conditions. This indicates that lumping of the channels for the 11-channel model is slightly nonconservative with respect to the 18-channel model. However, this nonconservatism is not considered significant and the 11-channel model noding scheme for the lumped channels is acceptable.

The effects of increasing the number of subchannels were evaluated with a steady-state 31-channel model (Figure 2). This model utilizes a hot assembly containing 21 individual subchannels. The noding used for assemblies other than the hot assembly is identical to the 18-channel model. Sensitivity study results show that, at low flow conditions, the minimum DNBR.

computed with the 31-channel model is 0.2 points greater than the 18-channel model's minimum DNBR. This demonstrates that the use of 6 subchannels in the hot assembly for the 18-channel and 11-channel models is slightly conservative.

Hot Subchannel Location Sensitivity:

The hot subchannel location for the VIPRE models previously discussed is subchannel number 2. Using the 31-channel model, when the' hot subchannel is moved to subchannel number 8, the minimum DNBR increases 4 points. This is expected since when subchannel number 2 is the hot subchannel, it is " communicating"

Docket Number 50-346 License Number NPF-3 Serial Number 1709 Attachment Page 5 of 9 with only 3 adjacent subchannels. When subchannel number 8 is the hot subchannel, it communicates with all 4 adjacent subchannels which is more realistic. Therefore, it can be concluded that modeling subchannel number 2 as the hot subchannel is conservative.

i Pin Power Distribution Sensitivity:

The hot assembly pin power distribution used for VIPRE modeling was generated by normalizing a typical pin power distribution to the design pin peak. The resulting power distribution has one pin equal to the design pin peak which defines the location of the hot subchannel. The other three rods which contribute power to the hot subchannel have peaks that are slightly less than the design peak. A sensitivity study was performed that set all rods contributing power to the hot subchannel equal to the design peak. This " flat" distribution reduced the minimum DNBR 2.5 points at low flow conditions. This value is equivalent to the maximum nonconservative difference in minimum DNBR expected between the power distribution used in VIPRE and all other possible distributions. Considering that an actual flat distribution is unlikely, and the associated 2.5 point nonconservatism is small which is offset by model characteristics that are conservative, it is concluded that the power distribution used in the analysis presented in Reference 1 is acceptable.

Two transient sensitivity studies were performed with the 11-channel transient model described in Reference 1. Using the total loss of flow transient described in Reference 1 as a basis, the number of timesteps and the number of conduction rods were analyzed. Results show that the 11-channel transient model described in Reference 1 is appropriate for the analysis of a total loss of flow transient.

In conclusion, the sensitivity studies performed with 1) the steady-state models at low flow conditions and 2) the 11-channel transient model, verify that the 11-channel transient model described in Reference 1 is appropriate for the analysis of a total loss of flow transient with a 631 millisecond delay time.

Use of the VIPRE models for other transients or design conditions will require additional sensitivity studies.

Docket. Number 50-346 License Number NPF-3 Serial Number 1709 Attachment' Page.6 of 9 Additional Informatio.p_Concernina Total Loss of Flow Transient.

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The total' loss of flow transient analysis described in Reference 1 utilized conservative transient forcing functions for neutron power versus time (prior to reactor trip), core exit pressure versus time andxcore inlet temperature versus time. The transient was initiated from 102 %RTP with minimum design flow.

Data pertaining to 1) the shape of the flow coastdown curve and

2) neutron power versus time after start of rod insertion are based on fuel vendor data.

The VIPRE analysis presented in Reference 1 used the B&W-2 critical heat flux correlation. The minimum DNBR computed by VIPRE during the total loss of flow transient was reported in Reference 1 as 1.89. This value was then compared to the design limit of 1.30 which is.specified in the Davis-Besse-USAR.

The critical heat flux limit that is applicable to a particular

. design code is normally determined by comparing critical heat

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flux predictions to measured values. The critical heat flux design limit is set such that the critical heat. flux is predicted

with a 95/95 confidence level. The applicability of the 1.30 limit to'VIPRE has not been verified with measured critical heat flux data. However, based on VIPRE benchmark analyses (Reference 1).to the LYNXT code (B&W-2 limit = 1.30), the "true" B&W-2 limit for VIPRE is expected to be within +/- 10 points of 1.30. Thus, it can.be concluded that the 1.89 minimum DNBR during the transient is acceptable. This method of analysis is justifiable due to the large amount of DNBR margin between the minimum DNBR value'of 1.89 during the transient and the expected limit of approximately 1.30.

The total loss of flow transient discussed in Reference 1 and the sensitivity studies described in this-attachment were performed with VIPRE models that are applicable to Mark B4 and Mark B5 fuel designs fabricated by Babcock & Wilcox. These fuel designs are being used for the current cycle (number 6). The next reload batch of fuel (for cycle 7) will utilize the Mark B8 fuel design.

The BWC' critical heat flux correlation has been used in DNBR design analyses of Mark B8 fuel. Verification that the total loss of flow transient is acceptable (pump-power response time =

631 milliseconds) for cores containing Mark B8 fuel will be performed as part of the fuel vendor's reload analyses. Based on DNBR margins available between the calculated minimum DNBR during

-the transient and the safety limit, it is expected that use of Mark B8 fuel for future cycles will not impact the 631 millisecond delay time determined by reference 1.

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I Docket Number 50-346

, License Number NPF-3 L' :- Serial Number 1709

' Attachment Page 7 of 9 References

1. Toledo Edison License Amendment Request, Serial Number 1634, February 21, 1989.

2. Toledo Edison Calculational Package Identification Number; dbl *S*VIPRE*890815

n Docket Number 50-346 License Number NPF-3 Serial Number 1709 Attachment

- Page 8 of 9 FIGURE 1 VIPRE 18-CHANNEL MODEL RADIAL NODING DIAGRAM l

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. l Docket Number 50-346 License Number NPF-3 Serial Number 1709 Attachment Page 9 of 9 FIGURE 2 VIPRE 11-CHANNEL AND 31-CHANNEL MODELS RADIAL NODING DIAGRAMS 11 Channel Layout 1 - 8  ;;

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