ML20052E383

Forwards Results of Analyses Addressing SER Open Item Re Adequacy of Steam Flow to Reheater During Conditions of Turbine Trip W/O Bypass & Requests Removal of Associated Licensing Condition

ML20052E383
Person / Time
Site:	Fermi
Issue date:	04/27/1982
From:	Tauber H DETROIT EDISON CO.
To:	Kintner L Office of Nuclear Reactor Regulation
References
EF2-57-134, NUDOCS 8205110071
Download: ML20052E383 (11)

Text

H:rry Tcuber

[UdN.Nhfnq c< nstr ft-on Detroit Ecison EEdm~

April 27, 1982 EF2 - 57,134 C

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%gMD Mr. L. L. Kintner U.

S. Nuclear Regulatory Commission

$f4y10 Office of Nuclear Reactor Regulation zrrs C

Division of Licensing

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Washington, D. C. 20555

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References:

(1)

Enrico Fermi Atomic Power Plant, Unit 2 NRC Docket No. 50-341 (2)

NUREG-0798, " Safety Evaluation Report, Enrico Fermi Atomic Power Plant, Unit No.

2",

Supplement No.

1, Section 15.1, September, 1981

Subject:

Reheater Bypass Flow Analysis The purpose of this letter is to provide the results of analyses that address NRC concerns with regard to the ade-quacy of steam flow to the reheater during the conditions of turbine trip without bypass and to request the removal of the associated licensing condition (Reference 2).

The attached analyses address the adequacy of such bypass steam flow by providing additional conservatism that responds to NRC concerns.

Therefore, Detroit Edison requests that the condition that measurements for reheater steam flow, as described in Reference 2, be removed from the Operating License.

Sincerely,

?

r/M Al As<.

Attachment

' gl ec:

Mr. B.

Little l

%T 8205110071 820427 PDR ADOCK 05000 E

SUPPLEMENT TO FERMI-2 REHEATER FLOW STUDY INTRODUCTION This report provides the results of a series of RETRAN analyses to supple-ment an earlier studyl performed to demonstrate that the reheater steam flow during a turbine trip will be equal to or greater than that showing in Figure 15B.0-3 of the Fermi-2 FSAR and reproduced here as Figure 1.

That FSAR flow pattern was utilized in pressure transient calculations performed with ODYN for FSAR Chapter 15 analyses.

The current analyses were performed to respond to NRC concerns that the previous work was not sufficiently conservative with regard to certain modeling assumptions.2 Therefore, the most conservative analyses performed in the previous submittall is defined here as the reference model; then modifications are made to this reference model to address the most recent NRC concerns and the results reported in this submittal.

i REFERENCE MODEL Thebasicmodelconfigurationisthatlabeled"RETRANVERSION2"inFigure2 of the earlier report and reprinted here. The results of the most conserva-tive analyses performed with this configuration in the earlier study is that labeled "8 N0DE/5 NODE" and shown in Figure 5 of that report.

The assumptions and analyses leading to this reheater flow constitute the reference model in the current study.

MODEL MODIFICATIONS NRC has expressed the following concerns regarding certain assumptions implicit in the reference model.

1.

The homogeneous equilibrium two phase flow assumption incorporated in all the flow paths of the RETRAN model would tend to allow too much steam flow from the reheater into the seal tank during the overpressurization transient.

This in turn could lead to an overestimate of the steam flow to the reheater.

2.

There are uncertainties in the flow dependent heat transfer from steam in the reheater tubes to the tube metal. If the initial heat transfer coefficient were actually higher than assumed, then the initial metal temperature would also be higher than previously assumed to maintain the same steady-state heat transfer rate from steam to metal. The higher initial metal temperature would then effectively reduce the capacity of the tube metal to receive heat from the steam during the transient, reducing the rate of condensation and causing pressure to increase and impede steam flow.

The following modifications were made to address the foregoing two concerns respectively.

1.

The total flow out of the reheater to the seal tank was ramped to zero within 0.2 seconds of the initiation of the transient. Therefore, effectively no steam is allowed to flow to the seal tank during the transient.

2.

The flow dependent heat transfer coefficient from tube side steam to metal tubing used for initial conditions was arbitrarily doubled to produce a conservative (high) initial metal temperature. However, as soon as the transient was initiated, the coefficient was ramped back to its normal value within 0.2 seconds to maintain a conservative (lower) transfer of heat from steam to metal as the transient progresses.

Both of the conservative assumptions described above were added to the reference model, and runs were performed for full power turbine trips with 0%, 13% and 26% nominal bypass conditions. The resulting steam flows to the reheater for all three cases are shown in Figure 3.

SENSITIVITY TO REHEATER FLOW PATTERNS It is evident from the results of ODYN calculations performed by General Electric as well as from RETRAN calculations that the peak fission power occurs within one second of the start of the turbine trips of interest.

Therefore, the reheater steam flow of primary effect is that during the very early stages of the transient, probably within the first second. Thus, the expected early peaks in reheater flow as shown in Figure 3 for all three cases would be expected to have a greater effect in reducing peak reactor power and heat flux than the lower but sustained reheater flow shown in Figure 1.

This assertion is borne out by the following secies of RETRAN calculations.

A standard NSSS RETRAN turbine trip model employing o

the identical steam line noding as used in the Chapter 15 ODYN analyses (Figure 4) was used to calculate base case responses for three turbine trips at full power (0%,

13%, 26% bypass). As in Chapter 15 of the FSAR, the re-heater flow was portrayed as an additional steam flow to the turbine stop valve with the flow pattern given in Figure 1.

A second series of three turbine trip cases using the o

same model and for the same bypass conditions were run with the only change being the replacement of the Figure 1 reheater flow with the appropriate reheater flows pre-viously computed as described above and shown in Figure 3. 1 l

n A final calculation is performed for the 0% bypass case o

that uses a reheater flow similar to that assumed in the FSAR but modified so that the 10% flow value is i

maintained for only one second (rather than two) before i

ramping to zero in three additional seconds.

See dashed a

curve of Figure 1.

?

The resulting peak fission power, heat flux, and reactor dome pressure for the three base cases and their counterparts using the computed reheater flows are shown in Table 1.

In all three bypass cases, the peak values for the parameters of interest are lower when using the explicitly calcu-lated reheater flows of Figure 3 than for the assumed FSAR reheater flow of Figure 1.

The effects on peak fission power, heat flux, and dome pressure when the FSAR reheater flow pattern of Figure 1 is replaced by the reheater flow shown by the dashed curve of Figure 1 is also shown in Table 1 for the 0% bypass The significant result is that all of those peak parameters are case.

essentially identical (Case 1 vs. Case 7 of Table 1) and thus insensitive to the nature of the reheater steam flow after one second.

DISCUSSION The model and assumptions used in the current analyses in the calculation of reheater steam flow during a turbine trip employ many conservative assumptions as summarized below:

o Essentially no credit is taken for heat transfer from the reheater tubes to shell side steam.

Essentially no credit is taken for steam flow from the o

reheater to the seal tank.

o The initial condition heat transfer coefficient fram tube side steam to metal tubing is made conservatively high, and yet no credit is taken for the higher value during the transient.

Moreover, nodalization studies were performed for the steam line, reheater line, and reheater itself to assure that sufficient nodes were employed in the modeling. The resulting composite model is felt to produce a very conservative (low) steam flow to the reheater during the first few seconds of a turbine trip transient.

i The only turbine trip situation during the first fuel cycle that depends on bypass steam flow through the reheater to maintain sufficiently low.ACPR is for nominal 0% bypass conditions near the end of the cycle. Despite the conservative model employed, the calculated reheater steam flow for this 0%

bypass case greatly exceeds the constant flow assumed in the FSAR for the first two seconds of the transient. Moreover, it should be noted that since the calculated flows for all of the bypass cases are quite high during 1 i

1 i

i

o-the first second of the transient, they result in lower calculated maximum fuel pin heat fluxes (Table 1) than obtained by assuming the FSAR reheater steam flows shown in Figure 1.

This is simply due to the fact that with fission power and heat flux peaking within one second (as shown by both RETRAN and ODYN), the reheater bypass flow within about the first second produces the dominant effect on peak fuel pin heat flux, and of course re-heater flows beyond about two seconds are irrelevant.

CONCLUSIONS Only the 0% bypass case requires credit for any reheater bypass flow to achieve an acceptable ACPR for end of cycle one conditions. A very conservative modeling scheme for the 0% bypass turbine trip results in greater reheater bypass flow during the first two seconds than portrayed in the FSAR. However, for all bypass condition's (0%, 13%, 26%), the conservatively calculated reheater flows lead to lower peak fuel pin heat fluxes than obtained by using the assumed reheater flow patterns portrayed in the FSAR even though the calculated reheater flows for the 13% and 26%

bypass conditions drop below the curve used in the FSAR as the transient progresses.

The more favorable results obtained for the calculated reheater flow are to be expected since the bypass steam flow in about the first second of the transient is by far the most important with regard to effects on peak reactor power and heat flux. This assertion is further supported by the analyses that show essentially no differences in peak power and heat flux when the reheater steam flow is assumed constant at 10% for two seconds as in the FSAR or for just one second.

(Case 1 vs. Case 7 of Table 1).

Therefore, the siCPR computed by General Electric for all turbine trip conditions remain conservative and there should be no need for reheater steamflowmeasurementstobemadeduringstartupascuyrentlycalledfor in the Fermi-2 Safety Evaluation Report (Supplement 1),

REFERENCES co 1.

-Edison letter, " Reheater Bypass Flow Analysis," EF2-54,540, August 31, 1981 2.

Safety Evaluation Report, Enrico Fermi Atomic Power Plant, Unit No. 2, NUREG-0798, Supplement 1, September 1981 (Section 15.1, p.15-1).

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TABLE 1

'Ef fect of Reheater Flow Patterns on Safety Parameters for Fermi-2 Full Power Turbine Trips CASE INPUT RESULTS Feak Dome Reheater Flow Peak Power Peak Heat Flux Pressure Rise

% Bypass Assumption

% t(sec) t(sec) psi t(sec) 1 0

FSAR 252 0.9 117 1.1 153 2.5 2

13 FSAR 196 0.9 109 1.1 142 2.9 3

26 FSAR 155 0.9 105 1.2 129 3.4 4

0 Fig. 3 182 0.9 108 1.2 145 2.9 5

13 Fig. 3 147 0.9 105 1.2 134 3.4 6

26 Fig. 3 124 0.9 102 1.2 122 4.4 7

0 Fig. 1 252 0.9 117 1.1 153 2.5 (dashed curve)

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FIGURE 1 ENRICO FERMI ATOMIC POWER PLANT UNIT 2 FINAL SAFETY ANALYSIS REPORT FIGURE 15B.0 3 l

MAIN STEAM FLOW AFTER TURBINE TRIP,

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