ML20024E475
| ML20024E475 | |
| Person / Time | |
|---|---|
| Site: | Big Rock Point File:Consumers Energy icon.png |
| Issue date: | 08/09/1983 |
| From: | Bordine T CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | Crutchfield D Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8308150115 | |
| Download: ML20024E475 (8) | |
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Consumers Power Company Generet offices: 1945 West Parnall Road, Jackson, MI 49201 * (517) 788-0550 August 9, 1983 Dennis M Crutchfield, Chief Operating Reactors Branch No 5 Nuclear Reactor Regulation US Nuclear Regulatory Commission Washington, DC 20555 DOCKET 50-155 - LICENSE DPR BIG ROCK POINT PLANT - RESPONSE TO STAFF QUESTIONS ON MCPR TECHNICAL SPECIFICATION CHANGE REQUEST A conference call was held between Consumers Power Company and the NRC on June 3, 1983. The purpose of the call was to discuss the Minimum Critical Power Ratio (MCPR) Technical Specification Change Request supplemental information submitted on April 26, 1983. As a result of that conversation, RETRAN analysis parameters and COBRA analysis assumptions were developed for further clarification.
Our answers to the questions presented by the Staff during the above referenced telecon are attached. As stated in our December 20, 1982 submittal, this request is necessary for Big Rock Point to resume power at the licensed thermal rating of 240 MW.
Your prompt approval is requested.
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(, Thomas C-BotTine Staff Licensing Engineer CC Administrator, Region III, USNRC NRC Resident Inspector-Big Rock Point Attachmant AcoI oc0783-0237a142' 5%Ng l
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4 RETRAN SENSITIVITY ANALYSIS Question Show that the RETRAN analysis is conservative by evaluating the sensitivity of the model (in terms of AMCPR/ initial MCPR) and provide a basis why the analysis assumed a conservative value for the following parameters:
a) Void Coefficient, Doppler Coefficient, Scram Reactivity, and Core Pressure Drop (or Core 4
l Bypass Flow) b) Steam Line Pressure Loss (Loss Coefficient) c) Separator Inertia, Node Length / Node Area (L/A) d) Fuel Heat Transfer Model e) Prompt Neutron Heating f) System Inertia (L/A) and Separator AP g) Void Fraction Model (Homogeneous Assumption)
Response
a) The results of sensitivity studies that had been performed in the latter part of 1981 were submitted to the NRC April 26, 1983. These studies reported on the sensitivity of Turbine Valve Closure Time, Core Bypass Flow, Scram Rod Worth, Scram Time, $/A, Doppler Coefficient, and Void.
Coefficient. The results of further sensitivity studies follow. One-difference in the following analysis from the previous sensitivity study was that the new analysis uses the RETRAN/ COBRA model developed for the Technical Specification Change Request of December 20, 1982. Therefore, this study has an initial Critical Power Ratio (CPR) of 1.591 compared to 1.681 of the 1981' study.
b). Steam Line Pressure Loss - The steam line loss coefficients in junctions i
140, 150, 160, 170 and 180 (see Attachment 1) were increased 50% and decreased 50% from the base case. The results below show that MCPR is insensitive to changes in steam line pressure losses.
0.5 x Base Case
- 1.5 x Base Case Base Case Coefficients Coefficients Initial CPR 1.591 1.591
.1.591 MCPR 1.320 1.321 1.320 MCPR change from' base case
-N/A.
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1 x 100%
N/A
+0.08%
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Big Rock Point Plant August 9, 1983 c) Steam Separator Inertia - The steam separator inertia (L/A) base case of 0.63 was increased by 100% to 1.26.
A neglible decrease in MCPR was observed as a result of doubling the separator inertia. The results are shown below.
Base Case 2.0 x Base Case Inertia Initial CPR 1.591 1.591 MCPR 1.320 1.318 MCPR change from base case N/A
-0.002 MCPR-MCPR( ase) x 100%
N/A
-0.15%
M R(Base) d) Fuel Heat Transfer Model - The gap conductivity was varied to show sensitivity of MCPR to changes in the heat transfer parameters for the fuel.
In the RETRAN model, the base case gap conductivities are listed in Table I (" Gap Conductivities"). These conductivities were selected so the fuel temperatures and stored energies at different power levels matched those calculated in XN-76-21, " Design Report for Big Rock Point Reactor Reload G-3 Fuel, Addendum 4", August 1976. The fuel temperature in XN 21 was calculated by the GAPEX computer code. The results of the sensitivity runs are shown below. As can be seen, for even large changes in gap conductivity, MCPR changes very little.
Base Case
+25%
+300%
Initial CPR 1.591 1.591 1.591 MCPR 1.320 1.308 1.298 MCPR change from base case N/A
-0.012
-0.022 MCPR-MCPR(Base) x 100%
N/A
-0.9%
-1.7%
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RETRAN Analysis 3
Big Rock Point Plant August 9, 1983 TABLE I GAP CONDUCTIVITIES Base Case Temp ( F)
(Btu /ft-hr *F)
+25%
+300%
0.0 0.044 0.055 0.132 333.0 0.052 0.065 0.156 900.0 0.12
.0150 0.36 1200.0 0.1745 0.218125 0.5235 2500.0 0.1745 0.218125 0.5235 e) Prompt Neutron Heating - In the RETRAN/ COBRA analysis for the HCPR Technical Specification Change Request, 97% of the heat was assumed to be generated in the fuel and 3% in the moderator, which is consistent with XN-76-21.
The normal values for heat generated in the fuel range from 90 to 95%. The 97% figure used in the RETRAN/ COBRA is conservative because reducing the heat generated in the fuel correspondingly reduces the heat flux and increases the MCPR.
In addition, a prompt moderator heating fraction (ie, greater than 3%) results in an earlier void regeneration and thus the power excursion due to the pressure surge is terminated sooner, f) System Inertia and Separator AP - Sensitivity studies were not done on these parameters because they would have no effect on the MCPR for the turbine trip without bypass transient. The MCPR in the transient occurs very early (<3 seconds) before any variations in flow can manifest themselves. The results of varying the steam line pressure loss and core bypass sesnitivity studies verified this. The A CPR did not change intense cases even though the parameters were varied very significantly.
g) Void Fraction Model (Homogeneous Assumption) - In a homogeneous model the vapor velocity equals the liquid velocity as opposed to a slip model where the vapor velocity is greater than the liquid velocity.
In the homogeneous model, the void fraction is over predicted as compared to a slip model.
In the turbine trip without bypass transient where the void collapse produces a rapid reactivity addition, it is conservative to use the homogeneous model because it produces the largest reactivity addition.
In addition, as noted in our April 26, 1983 submittal, the analysis assumed the most negative beginning-of-cycle-void coefficient and the coefficient was increased by 25% (multiplied by 1.25) for design conservatism.
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DESCRIPTION OF COBRA ANALYSIS MAJOR ASSUMPTIONS Question: Describe the method used to calculate MCPR (ie, COBRA analysis),
discussing major analysis assumptions including:
j a) Friction-Factor and Form-Loss Coefficients - Describe the specific inputs to friction-factor and form-lors coefficients, b) Void Fraction Model c) Subcooled Void Model d) Two-Phase Friction Multiplier e) Link between RETRAN and COBRA f) Time-Step Size g) Nodalization h) Heat Transfer Model in different flow regions node length /
node area (L/A) input and Gap Conductance Describe why each assumption is conservative.
Response
1 a) A one quarter core geometry was used to perform the thermal margin analysis. The single phase friction factor and form-loss coefficients shown in Table II, " Big Rock Point Friction and Form-Loss Coefficients," are based upon experimental data obtained by Exxon (Reference 1) during the hydraulic and i
fretting corrosion tests performed using the Reload G fuel design. The current fuel design has the same hydraulic characteristics as the Reload G fuel. Any minor differences would be insignificant compared to the high form-loss coefficients of the flow distribution orifice in each fuel assembly channel.
1 b) The Modified Armond Void Correlation (as defined in the COBRA users guide) was used for this analysis b: sed upon our analysis of the data contained in Reference 2.
Several correlations were compared with the General Electric data and the Modified Armond Void Correlation was judged to provide the best results 4
for the data obtained near the operating pressure of the Big Rock Point Plant core.
c) A subcooled void fraction model was not utilized for this analysis. The XN-2 Critical Power Correlation (Reference 3) is based upon channel average fluid properties and subcooled voids were not considered in the development of the correlation.
d) The Armond Model (also from the COBRA users guide) was selected t
l for the two phase friction multiplier. A sensitivity study was performed using the homogeneous two phase friction multiplier correlation. The ACPR calculated using the Armond Model is the same as the ACPR for the homogeneous correlations.
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COBRA Antlysis Major Assumptions (Contd) 2 Big Rock Point Plant August 9, 1983 e) The RETRAN analysis provided the boundary condition of normalized heat flux, core flow rate, core inlet enthalpy and core exit pressure for the COBRA thermal margin analysis. The data is read directly from the RETRAN restart file. The location of each of the four above variables plus the location of the transient time parameter are input parameters for COBRA.
f) A time step size of 0.2 seconds was selected for most of the COBRA analyses. The line printer plot produced at the end of the COBRA output is reviewed to ensure that the minimum CPR was found.
If the CPR curve was flat near the minimum, the time step was reduced to ensure that the MCPR was calculated.
g) The axial node size selected for this series of thermal margin calculations is four inches. Since Exxon XN-2 " Critical Power Correlation", (Reference 3) is based upon channel average properties, a small axial node size is not required. The node size was reduced to approximately three inches for the Turbine Trip without Bypass Transient and the calculated thermal margins did not change.
h) The COBRA code analysis did not contain a thermal model of the fuel rods, therefore, the pellet-to-clad gap conductance was not a required input variable. The normalized heat flux was obtained from the RETRAN analysis as described above.
A conservative thermal margin analysis has been obtained by using a "best estimate" model of the Big Rock Point geometry with conserva-tive reactor physics parameters, an engineering heat flux factor of 4% and an' engineering enthalpy rise factor of 10%.
3 REFERENCES
- 1) " Evaluation of hydraulic and Fretting Corrosion Behavior of Big Rock Point Reload G Fuel", XN-73-20, (June 1973).
(Exxon proprietary report.)
- 2) " Steady State and Transient Void Fraction in Two-Phase Flow Systems.
Final Report for the Program of Two-Phase Flow Investigation.", GEAP-5417, (January 1967).
- 3) "The XN-2 Critical Power Correlation," XN-75-34 (August 1, 1975).
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4 TABLE II l
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Inlet K*
Standard channel 13.5 Peripherical channel 69.4 Lower End Fitting 0.80 4
Spacer Grid 1.02 Upper End Fitting 1.06 1
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- Based on nominal flow area of a fuel assembly in'the flow channel.
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