ML070860281

Review and Analysis of the Davis-Besse March 2002 Reactor Pressure Vessel Head Wastage Event, Appendix a, Finite Element Stress Analysis of Davis-Besse CRDM Nozzle 3 Penetration

ML070860281
Person / Time
Site:	Davis Besse
Issue date:	03/15/2007
From:	Ballinger R, Bullen D, Latanision R Altran Solutions Corp, Exponent Failure Analysis Associates, Exponent
To:	FirstEnergy Nuclear Operating Co, Office of Nuclear Reactor Regulation
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References
FOIA/PA-2007-0299
Download: ML070860281 (31)

Text

Appendix A Finite Element Stress Analysis of Davis-Besse CRDM Nozzle 3 Penetration

Appendix A Finite Element Stress Analysis of Davis-Besse CRDM Nozzle 3 Penetration Exponent developed a three-dimensional finite element model of the control rod drive mechanism (CRDM) Nozzle-3 penetration in the Davis Besse reactor pressure vessel (RPV) head for evaluation of weld residual and operational stresses in and around the J-groove weld. The modeled loading history included the buttering deposit, nozzle expansion, thirteen weld passes, the hydrostatic test and normal operating conditions.

The stress results can be used to predict crack growth in the nozzle wall.

Finite Element Model The finite element model of the Nozzle-3 penetration, shown in Figure A.1, represents a portion of the hemispherical head extending half the distance from Nozzle 3 to neighboring Nozzles 1, 6 and 11. Advantage was taken of a plane of geometrical symmetry passing through the nozzle centerline along a meridian of the hemispherical RPV head. This plane of symmetry allowed for simulation of the full nozzle using a model representing only half of the nozzle. Appropriate boundary conditions applied on the symmetry (or cut) plane cause the half-symmetry model to behave just like the full nozzle. This common modeling technique optimizes solution efficiency by avoiding redundant calculations.

The finite element model was developed from dimensional information obtained from the following Babcock & Wilcox (B&W) fabrication drawings and quality control records:

• Drawing No. 154632 E21

• Drawing No. 154631 E42

• Drawing No. 154628 E53

• Quality Control Inspection Drawing No. 156631 E-R24 BN63097.001 B0T0 1106 DB05 A-1

• Quality Control Inspection Drawing No. 154628 E45 Unlike most other similar B&W reactors, the Davis-Besse head was fabricated without counter bores in the CRDM nozzle penetrations6. This means that the nozzle shrink-fit interference was not uniform around the circumference of the nozzle due to the curvature of the head. The interference contact zone extended higher on the nozzle on the uphill side and lower on the nozzle on the downhill side.

Material Properties Four different materials models were used in the analysis. In each case, material properties such as the conductivity, specific heat, thermal expansion coefficient, elastic modulus and strength data were modeled as a function of temperature.

The hemispherical head was modeled as ASME SA-533, Grade B, Class 1 alloy steel.

The properties were taken from the ASME Boiler and Pressure Vessel Code7. The elastic modulus is given as a function of temperature in Figure A.3, the specific heat as a function of temperature in Figure A.4, the thermal conductivity as a function of temperature in Figure A.5, the thermal expansion as a function of temperature in Figure A.6 and strength as a function of the temperature in Figure A.7. The head material was modeled as elastic-perfectly plastic with temperature-dependent yield strength based on the yield curve in Figure A.7. The input stress-strain relations for the head material are shown in Figure A.8.

The cladding on the inside of the head is modeled as Type 308 stainless steel. The elastic modulus as a function of temperature is given in Figure A.9. Yield, tensile, and flow stress as a function of temperature for Type 308 stainless steel cladding is given in Figure A.10. Poisons ratio was taken as 0.29 over the entire temperature range. The density of Type 308 stainless steel is 499.4 lb/ft3. The thermal expansion coefficient as a function of temperature is shown in Figure A.11, the specific heat as a function of temperature in Figure A.12 and the thermal conductivity as a function of temperature in Figure A.13.

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The nozzle is modeled as the nickel-chrome-iron alloy Inconel 600. The stress-strain data for Inconel 600 was adapted from the ASME Code and Special Metals product brochures.

The 0.2%-offset yield points were taken from Special Metals data and Heat M3935 Cert.

The stress-strain data of Alloy 600 at 2300°F was taken from EPRI TR-103696, Fig. 7-3.

The final input stress-strain curves are shown in Figure A.14. Elastic modulus as a function of temperature was taken from the ASME Code and is shown in Figure A.15 and Poissons ratio as a function of temperature is shown in Figure A.16. Temperature-dependent thermal properties for Inconel 600 were obtained from the ASME Code. The coefficient of thermal expansion is given in Figure A.17, the thermal conductivity in Figure A.18, the specific heat in Figure A.19 and the emissivity Figure A.20. Density varies little with temperature, so a constant value of 528.8 lb/ft3 was used.

The J-groove weld and buttering was modeled as Inconel 182 (Alloy 182) weld filler metal. The thermal and elastic properties of the weld filler metal are the same as the INCONEL Alloy 600 properties. The work hardening behavior for Inconel 182 is shown in Figure A.21.

Loading History The Davis-Besse RPV head assembly was fabricated in a number of steps. Several key steps influenced the stress states in and around the CRDM nozzle J-groove welds. The hemispherical alloy-steel head was clad on the inside with a layer of Type 308 stainless steel weld metal. Following this, the CRDM nozzle penetrations were bored through the head and cladding. Then the J-grooves were cut on the inside end of each CRDM penetration to accept the multi-pass welds which join the CRDM nozzles to the head and create a watertight seal. The surface of the J-grooves was then buttered (or clad) with a layer of Alloy 182 weld metal. At this point, the entire head was given a thermal stress relief treatment at 1125°F. The CRDM nozzles were matched to the head penetrations and each was ground to final dimensions that provided a targeted interference fit of 0.0013 to 0.0015 inch. To install each slightly oversized nozzle into its head penetration, the nozzle was cooled to around -320°F in liquid nitrogen to shrink it and then inserted into the penetration. As the nozzle warmed to ambient temperature, it expanded into BN63097.001 B0T0 1106 DB05 A-3

contact with the head, locking it in position. Lastly, the multi-pass J-groove welds were applied to join the nozzles with the head and provide a watertight seal.

There was no post-welding thermal stress relief treatment given to the head after the J-groove welds were applied. Therefore, tensile residual stresses from weld cooling remained in and around the J-groove welds after head assembly fabrication. Prior to service, the entire reactor pressure vessel must pass an ASME-Code required hydrostatic pressure test to 3,125 psi (1.25 times its design pressure of 2,500 psi). This pressure test leads to some localized yielding in and around the J-groove welds due to their already high tensile residual stresses from welding. This yielding during the pressure test results in a slight reduction in the residual stresses.

In service, the reactor pressure vessel was subjected to the full-load operational pressure and temperature of the primary cooling water. During Cycle 13 at Davis-Besse, the RCS pressure averaged about 2155 psi and the temperature averaged just over 605°F.

The finite element model of Nozzle 3 was analyzed in the following steps to simulate the actual sequence of fabrication, testing, and operational loading:

• solidification and cooling of the J-groove weld buttering from 2240°F to 70°F,

• stress relief of the head, cladding, and J-groove buttering at 1125°F,

• cooling of the nozzle to -320°F, installation of the nozzle into the head, and warming of the nozzle back to 70°F to achieve the interference fit,

• sequential application of thirteen weld passes, each deposited at 2240°F and cooled below a maximum inter-pass temperature of 350°F,

• pressurization of the wetted internal surfaces up to the hydrostatic test pressure of 3,125 psi and back to zero pressure, and BN63097.001 B0T0 1106 DB05 A-4

• pressurization to the operating pressure of 2,155 psi at a temperature of 605°F.

Throughout the simulation, the boundary planes of the modeled head segment were constrained to allow only radial displacement. Such boundary conditions cause the model to behave as if it were part of the complete hemispherical head.

The analysis of the Nozzle-3 model involved both transient thermal and thermal-mechanical solutions. A coupled analysis technique was employed wherein a transient thermal solution and a thermal-mechanical solution is obtained for each small increment of load application. This type of incremental coupled analysis was necessary to appropriately simulate the nonlinear response (in both stress and displacement) of the nozzle to temperature and pressure loads.

J-groove Weld Buttering To simulate the buttering weld deposit and cool down, the elements representing the buttering were initially set to 2240°F. The remote cut boundaries of the truncated head portion of the model were held at 70°F during the weld deposition and cool down stages.

Radiation boundary conditions were applied to the exposed faces of the buttering.

Convective boundary conditions were applied to the exposed faces of the buttering and the top and bottom faces of the head. For both the head and nozzle, turbulent free convective film coefficients were used with an ambient-air sink temperature of 70°F.

Heat was also allowed to conduct away from the weld through the head. The buttering weld was allowed to cool to near ambient temperature. During the buttering analysis, the nozzle portion of the model was deactivated to simulate the fact that the nozzle was not yet installed.

Head Stress Relief After the buttering step, the temperature of the entire model was raised uniformly to the stress-relief temperature of 1125°F and then reduced back to 70°F. Since the yield strength of the materials is lower at higher temperatures, this step reduced the residual stresses around the buttering weld. However, the stress relief does not remove all BN63097.001 B0T0 1106 DB05 A-5

residual stress from the head, and the buttering and head cladding layers remained in tension after this step.

Nozzle Installation with Interference Fit Following the stress relief step, the nozzle portion of the model was activated and shrunk by cooling to -320°F. Frictional contact interfaces on the outside of the nozzle and the inside of the head penetration bore were activated, and the nozzle was warmed back to 70°F, engaging contact between the nozzle and head to accurately simulate the interference fit. The coefficient of friction between the head and nozzle was taken as 0.2.

At the end of the nozzle heating, there was complete contact between the nozzle and head.

J-groove Weld Deposition After the nozzle heat up, the thirteen weld passes were deposited sequentially starting at the root of the J-groove. The elements representing each weld pass were activated at 2240°F and allowed to cool to below 350°F before the next pass was deposited. The boundaries of the truncated head portion of the model remote from the nozzle were again held at 70°F for these thermal loading steps. Radiation boundary conditions were applied to the exposed faces of the weld and buttering as well as the adjacent, exposed exterior faces of the nozzle. Convective boundary conditions were applied as before on the exposed faces of the weld and buttering as well as the top and bottom of the head.

Hydrostatic Testing and Full-load Operation Following completion of the J-groove weld deposition steps, the hydrostatic test was simulated by ramping up the pressure on the inner surfaces of the nozzle and head to 3,125 psi and back down to zero. In the final step of the analysis, to simulate the full-load operating condition, pressure on the inner surfaces of the nozzle and head was ramped up the operating pressure of 2155 psi and the temperature of the entire model was simultaneously raised to 605°F.

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Stress Results Residual stresses are present in the region near the weld from the welding process. Under full-load operating conditions, the hoop stress in the vicinity of the downhill side of the weld on the symmetry plane is shown in Figure A.22. This stress tends to cause axial cracking. The stresses in this region are used to determine the driving force for axial crack growth. As can be seen from Figure A.22, both the weld and the buttering have yielded. Some small regions of the nozzle and the head that are in the immediate vicinity of the weld have also yielded but the stress decays quickly with distance from the weld.

The axial stress in the same region is shown in Figure A.23. The axial stress is about half of the hoop stress in this region.

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Figure A.1 Half-symmetry finite element model of Davis-Besse CRDM Nozzle-3 head penetration.

Figure A.2 Close-up view of 13-pass J-groove weld in Nozzle-3 model.

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SA-533 Gr.B Cl.1 Alloy Steel Elastic Modulus 35000 30000 Elastic Modulus (ksi) 25000 20000 15000 10000 5000 0

0 500 1000 1500 2000 2500 Temperature (°F)

Figure A.3 Elastic modulus of ASME SA-533 Grade B Steel as a function of temperature7,8.

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SA-533 Grade B Class 1 Alloy Steel Specific Heat 0.45 0.40 Specific Heat (BTU/lb/°F) 0.35 0.30 0.25 0.20 0.15 ASME Code Data from Reference 8 used 0.10 d to extrapolate ASME Code 0.05 curve above 1500 F.

0.00 0 500 1000 1500 2000 2500 Temperature (°F)

Figure A.4 Specific heat as a function of temperature for SA-533 Grade B Class 1 alloy steel 7,8.

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SA-533 Grade B Class 1 Alloy Steel Thermal Conductivity 40.0 Data for low and medium carbon steels from Thermal Conductivity (BTU/hr/ft/°F)

Reference 8 used to extrapolate ASME Code curve 35.0 above 1500 F.

30.0 25.0 ASME Code data 20.0 15.0 10.0 5.0 0.0 0 500 1000 1500 2000 2500 Temperature (°F)

Figure A.5 Thermal conductivity as a function of temperature for SA-533 Grade B Class 1 Alloy Steel 7,8.

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SA-533 Grade B Class 1 Alloy Steel Coefficient of Thermal Expansion 10 Thermal Expansion Coeff. (10 /°F)

-6 9

(Extrapolation >1500F) 8 7

6 5

0 500 1000 1500 2000 2500 Temperature (°F)

Figure A.6 Coefficient of thermal expansion for SA-533 Grade B Class 1 Alloy Steel 7 BN63097.001 B0T0 1106 DB05 A-12

SA-533 Grade B Class1 Alloy Steel Yield and Tensile Strength 90 80 Yield Strength Tensile Strength 70 60 Stress (ksi) 50 40 30 20 10 0

0 500 1000 1500 2000 2500 Temperature (°F)

Figure A.7 Strength as a function of temperature for SA-533 Grade B Class 1 Alloy Steel BN63097.001 B0T0 1106 DB05 A-13

SA-533 Gr.B Cl.1 Alloy Steel Stress Strain Behavior 50 0°F, 70°F 200°F 400°F 600°F 40 800°F Stress (ksi) 30 1000°F 20 1200°F 10 1500°F 0

0 0.001 0.002 0.003 0.004 0.005 0.006 Strain Figure A.8 Stress Strain behavior of SA-533 Grade B Class 1 Alloy Steel BN63097.001 B0T0 1106 DB05 A-14

Elastic Modulus Type 308 Stainless Steel Cladding 30000 25000 Elastic Modulus (ksi) 20000 15000 10000 5000 0

0 500 1000 1500 2000 2500 Temperature (°F)

Figure A.9 Elastic modulus as a function of temperature for 308 stainless steel cladding BN63097.001 B0T0 1106 DB05 A-15

Strength of Type 308 Stainless Steel Cladding 60 Tensile 50 40 Strength (ksi)

Flow 30 Yield 20 10 0

0 500 1000 1500 2000 2500 Temperature (°F)

Figure A.10 Strength as a function of temperature for Type 308 Stainless Steel BN63097.001 B0T0 1106 DB05 A-16

Thermal Expansion of Type 308 Stainless Steel Cladding 15 Thermal Expansion Coefficient (x10-6 /°F) 12 9

6 3

0 0 500 1000 1500 2000 2500 3000 Temperature (°F)

Figure A.11 Coefficient of thermal expansion as a function of temperature for series 308 stainless steel BN63097.001 B0T0 1106 DB05 A-17

Specific Heat of Type 308 Stainless Steel Cladding 0.15 0.13 Specific Heat (Btu/lb/°F) 0.10 0.08 0.05 0 500 1000 1500 2000 2500 3000 Temperature (°F)

Figure A.12 Specific Heat as a function of temperature for series 308 stainless steel BN63097.001 B0T0 1106 DB05 A-18

Thermal Conductivity of Type 308 Stainless Steel Cladding 20.00 Thermal Conductivity (Btu/hr/ft/°F) 15.00 10.00 5.00 0.00 0 500 1000 1500 2000 2500 3000 Temperature (°F)

Figure A.13 Thermal Conductivity as a function of temperature for series 308 stainless steel BN63097.001 B0T0 1106 DB05 A-19

M A In c o n e l 6 0 0 S tre s s -S tra in D a ta 90 E la stic slo p e s (7 0 °F , 8 0 0 °F , 1 2 0 0 °F , 1 6 0 0 °F )

80 7 0 °F 70 8 00 °F 60 Stress (ksi) 50 12 0 0 °F 40 0 .2 % -o ffse t yie ld p o ints fro m S p e cia l M e ta ls d a ta a nd H e a t M 3 9 3 5 30 C e rt. (R T )

20 16 0 0 °F 10 0

0 .0 0 0 .0 1 0 .0 2 0 .0 3 0 .0 4 0 .05 0 .06 S tra in Figure A.14 Stress strain data for Alloy 600 BN63097.001 B0T0 1106 DB05 A-20

Inconel Alloy 600 Elastic Modulus 35 30 Elastic Modulus (ksi x 1000) 25 20 15 10 5

0 0 500 1000 1500 2000 2500 3000 Temperature (°F)

Figure A.15 Elastic modulus as a function of temperature for Inconel Alloy 600 BN63097.001 B0T0 1106 DB05 A-21

Inconel Alloy 600 Poisson's Ratio 0.40 0.35 Poisson's Ration 0.30 0.25 0.20 0 500 1000 1500 2000 2500 3000 Temperature (°F)

Figure A.16 Poisson's ratio as a function of temperature for Inconel Alloy 600 BN63097.001 B0T0 1106 DB05 A-22

Thermal Expansion of INCONEL Alloy 600 12 Thermal Expansion Coefficient (x10-6 /°F) 9 6

3 0

0 500 1000 1500 2000 2500 3000 Temperature (°F)

Figure A.17 Coefficient of thermal expansion as a function of temperature for INCONEL Alloy 600 BN63097.001 B0T0 1106 DB05 A-23

Thermal Conductivity of INCONEL Alloy 600 18 Thermal Conductivity (Btu/hr/ft/°F) 15 12 9

6 3

0 0 500 1000 1500 2000 2500 3000 Temperature (°F)

Figure A.18 Thermal Conductivity as a function of temperature for INCONEL Alloy 600 BN63097.001 B0T0 1106 DB05 A-24

Specific Heat of INCONEL Alloy 600 0.20 Specific Heat (Btu/lb/°F) 0.15 0.10 0.05 0.00 0 500 1000 1500 2000 2500 3000 Temperature (°F)

Figure A.19 Specific Heat as a function of temperature for INCONEL Alloy 600 BN63097.001 B0T0 1106 DB05 A-25

Emmissivity of INCONEL Alloy 600 1.00 Emmissivity 0.80 0.60 0 500 1000 1500 2000 2500 3000 Temperature (°F)

Figure A.20 Emmissivity as a function of temperature for INCONEL Alloy 600 BN63097.001 B0T0 1106 DB05 A-26

Figure A.21 Elastic-perfectly plastic stress-strain model for Alloy-182 weld filler BN63097.001 B0T0 1106 DB05 A-27

Figure A.22 Hoop stress under full-load operation BN63097.001 B0T0 1106 DB05 A-28

Figure A.23 Axial stress under full-load operation BN63097.001 B0T0 1106 DB05 A-29

References

1. Control Rod Mechanism Housing, Drawing No. 154632 E-R2, Babcock &

Wilcox Company, May 14, 1971.

2. Closure Head Assembly, Drawing No. 154631 E-R4, Babcock & Wilcox Company, September 15, 1971.

3. Closure Head Sub-Assembly, Drawing No. 154628 E-R5, Babcock & Wilcox Company, April 12, 1978.

4. Quality Control Inspection Drawing No. 156631 E-R2, April 24, 1972.

5. Quality Control Inspection Drawing No. 154628 E-R2, March 9, 1972.

6. Closure Head Assembly, Drawing No. 154631 E-R4, Babcock & Wilcox Company, September 15, 1971.

7. ASME Boiler and Pressure Vessel Code, 2001 Edition,Section II, Materials, Part D, Properties, American Society of Mechanical Engineers, 2001.

8. Structural Alloys Handbook, 1996 Ed., CINDAS/Purdue University.

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