EFFECTS OF IRRADIATION TEMPERATURE ON EMBRITTLEMENT OF NUCLEAR PRESSURE VESSEL STEELS

Transcription

1 Fahmy M. Haggag 1 EFFECTS OF IRRADIATION TEMPERATURE ON EMBRITTLEMENT OF NUCLEAR PRESSURE VESSEL STEELS Reference: F.M. Haggag, Effects of Irradiation Temperature on Embrittlement of Nuclear Pressure Vessel Steels, Effects of Radiation on Materials: 16 th International Symposium, ASTM STP 1175, Arvind S. Kumar, David S. Gelles, Randy K. Nanstad, and Edward A. Little, Eds., American Society for Testing and Materials, Philadelphia, ABSTRACT: The effects of neutron irradiation on the steel reactor vessel for the modular hightemperature gas-cooled reactor (MHTGR) are being investigated, primarily because the operating temperatures are low [121 to 288 C ( F)] compared to those for commercial light-water reactors (LWRs) [~288 C (550 F)]. The need for design data on the reference temperature (RT NDT ) shift necessitated the irradiation at different temperatures of A 533 grade B class 1 plates, A 508 class 3 forging, and welds used for the vessel shell, vessel closure head, and vessel flange. This paper presents regular- and mini-tensile, Automated Ball Indentation (ABI) and Charpy V-notch (CVN) impact test results from five irradiation capsules of this program. The first four capsules were irradiated in the University of Buffalo Reactor (UBR) to an effective fast fluence of neutrons/cm 2 [ neutrons/cm 2 (>1 MeV)] at temperatures of 288, 204, 163, and 121 C (550, 400, 325, and 250 F), respectively. The fifth capsule (designated ORNL-7) was irradiated in the Ford Nuclear Reactor (FNR) of the University of Michigan at 60 C (140 F) to an effective fast fluence of neutrons/cm 2 [ ] (>1 MeV) ]. The yield and ultimate strength of both A 553 grade B class 1 plate materials of the MHTGR increased with decreasing irradiation temperature. Similarly, the 41-J CVN transition temperature shift increased with decreasing irradiation temperature (in agreement with the increase in yield strength). The mini-tensile and Automated Ball Indentation (ABI) test results (yield strength and flow properties) were in good agreement with those from standard tensile specimens. The minitensile and ABI test results were also used on a model which utilizes the changes on yield strength to estimate the CVN ductile-to-brittle transition temperature shift due to irradiation. The model predictions were compared with the CVN test results obtained here and in earlier work. KEYWORDS: high-temperature gas-cooled reactor, Charpy impact specimens, transition temperature shift, nuclear pressure vessel, steel, welds, forging, irradiation temperature, tensile, automated ball indentation, fluence, embrittlement, drop-weight 1 Development Staff Member, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge TN

2 HAGGAG ON EFFECT OF IRRADIATION TEMPERATURE 173 The need for MHTGR design data on the reference temperature (RT NDT ) shift necessitated the irradiation at different temperatures of A 533 grade B class 1 plates, A 508 class 3 forging, and welds to be used for the vessel shell, vessel closure head, and vessel flange. The current irradiation plan, which includes 14 capsules, addresses the effects of irradiation temperature, neutron flux, fluence, and spectrum, and thermal aging on base plates, forging, and weld materials. The mechanical property evaluation includes drop-weight, Charpy V-notch (CVN) impact, standard (regular size) tensile, and mini-tensile specimen tests. Other material characterization includes metallography, fractography, chemical analysis, and other miniature specimen and/or non-destructive test techniques [e.g. automated ball indentation (ABI) tests conducted on broken halves of previously tested specimens]. Because of the different irradiation conditions, various test materials, and limited irradiation volume in each capsule, miniature tensile specimens were included in each capsule. Furthermore, due to the small volume of these mini-tensile specimens (24 specimens are packaged to the equivalent size of one standard CVN specimen), more materials were included in every capsule in addition to the MHTGR specimens. This paper presents results from five irradiation capsules (ORNL-1 through 4, and ORNL-7) of this program. IRRADIATION MATRIX AND EXPERIMENTAL PROCEDURE The current irradiation plan, which includes 14 capsules described in Table 1, addresses the effects if irradiation temperature, neutron flux spectrum, neutron flux (damage rate), and thermal aging (not shown in Table 1) on base plate, forging, and weld materials. Table 1 shows the two materials irradiated in each capsule, irradiation temperature, effective fast (>1 MeV) fluence, and the objective and relationship of each capsule to others in the matrix. Charpy V- notch, standard (regular round size) tensile, and mini-tensile specimens from two heats of A 533 grade B class 1 plate (containing 0.07 and 0.14% Cu) were included in each capsule. Additional mini-tensile specimens from A 212 grade B, A 350 grade LF3, nozzle weld, seam weld [all from the High Flux Isotope Reactor (HFIR) archive materials], and high-copper weld [weld 73W from the U.S. Nuclear Regulatory Commission (NRC) sponsored Fifth Irradiation Series of the Heavy- Section Steel Irradiation Program, containing 0.31% Cu] were included in each capsule. In Table 1, the first capsule number is the Oak Ridge National Laboratory (ORNL) designation (ORNL-1 through -14), while the second number is the capsule designation by reactor facility, where UBR and FNR represent University of Buffalo and Ford Nuclear Reactor, respectively. The first four capsules (ORNL-1 through ORNL-4) were irradiated by Materials Engineering Associates (MEA) in the UBR to an effective fast fluence of neutrons/cm 2 [ neutrons/cm 2 (>1 MeV)] at temperatures of 288, 204,163, and 121 C (550, 400, 325, and 250 F), respectively. The irradiation temperature was controlled to within ±8 C (±15 F). The MEA designations for these four capsules were UBR-81A, UBR-81B, UBR-82A, and UBR-82B. The effective fast (>1 MeV) neutron fluence was determined using the weighing factors given in Table 2. Capsule ORNL-7 was irradiated by MEA in the FNR to an effective fast fluence of neutrons/cm 2 [ neutrons/cm 2 (>1 MeV)] at 60 C (140 F). The purpose of this capsule (designated MEA as FNR-3A) was to investigate low temperature irradiation similar to the HFIR vessel operating temperature and applicable to vessel structural support components. The MHGTR neutron spectrum is also shown in Table 2. The effect of neutron spectrum on mechanical property degradation will be investigated in later capsules where the neutron spectrum will be tailored, by using specially designed shielded capsules, to match that shown in Table 2. These capsules (ORNL-6, ORNL-9 through -11) will be irradiated in the FNR at the University of Michigan. Capsules ORNL-5, ORNL-8, and ORNL-12 were also irradiated by MEA in the FNR facility under the MEA designation FNR-3A, FNR-6B, and FNR-6A, respectively.

3 174 16TH RADIATION Table 1--Summary of irradiation matrix for MHTGR reactor vessel materials

4 HAGGAG ON EFFECT OF IRRADIATION TEMPERATURE 175 However, irradiated specimen testing is not complete at this time. The test results of specimens from these capsules will be published later. RESULTS AND DISCUSSION Chemical analyses of these two heats of A 533 grade B class 1 plates are shown in Table 3. The results of drop-weight and upper-shelf energy tests, conducted according to ASTM standards E 208 and E 23, respectively, are shown in Table 4. The room temperature tensile test results from regular-sized test specimens are shown in Fig. 1 (a) and (b). The yield and ultimate strengths of both A 533 grade B class 1 steel plate materials increased with decreasing irradiation temperature. The 41-J CVN transition temperature shift increased with decreasing irradiation temperature (in agreement with the increase in yield strength). The CVN impact energy and fracture appearance test results from the two heats are shown in Figs. 2 through 5, respectively. Sample test results of the CVN impact energy curves with the individual data points are shown in Figs. 6 and 7 (one curve in the unirradiated condition and a curve at one irradiation temperature are shown for each heat of the material). The CVN test results are also summarized in Table 5. Furthermore, the measured 41-J transition temperature shifts (ΔT 41 ) were compared to those predicted using the NRC Regulatory Guide 1.99, Rev. 2, [1] for irradiation at 288 C. Table 5 shows that the predicted values of the CVN ΔT 41 were consistently conservative or higher than the actual shifts. For the high-copper plate (0.14% Cu) of capsule ORNL-4 in Table 1, the measured CVN 41-J transition temperature shift at 121 C (250 F) is 1.6 C, which is not consistent with the increase in yield strength. This discrepancy will be investigated by repeating the irradiation capsule (see capsule ORNL-12 in Table 1 of the irradiation test matrix). Capsule ORNL-12 is just completed and the test results from double the number of the CVN specimens will be reported elsewhere. Also, some additional analyses are being conducted on the load-time traces of the CVN test specimens from capsule ORNL-4. Twenty-four miniature tensile specimens were packed to an equivalent size of one CVN specimen as shown in Fig. 8. The room-temperature test results from the miniature tensile specimens (both unirradiated and irradiated in the four capsules ORNL-1 through 4) for the two heats of the A 533 grade B class 1 materials were in excellent agreement with those from the regular-size specimens as shown in Fig. 9. Uniform elongation slightly decreased with decreasing irradiation temperature. The increase in yield strength was found to correlate with the increase in the 41-J CVN transition temperature shift as shown in Fig. 10 (the solid symbols were not included in the regression analysis, since irradiation of CVN specimens at 288 C resulted in very small transition temperature shifts). Also, in this figure, a single point from two ABI test results on A 212 grade B class 1steel [2] is in agreement with the miniature tensile data. Details of the ABI test technique and results on various materials and welds (including materials with different irradiation levels) are given in refs. 2 through 6. Figure 11 shows several specimens where ABI tests were conducted on their surfaces including irradiated ones (in two vials). The slope of the linear in Fig. 10 is 0.45, which is somewhat lower than that reported in ref. 7 (~0.65). However, it should be mentioned here that the correlation in ref. 7 was developed from surveillance data from LWRs where the irradiation was conducted at 288 C (550 F) and to higher fluences than these MHTGR specimens. Also in ref. 7, test reactor data were presented where the correlation coefficient ranged from 0.43 to 0.65.

5 176 16TH RADIATION TABLE 2--MHTGR neutron spectrum and weighing factors for calculating the effective fast fluence (>1 MeV) TABLE 3--Chemical analysis of MHTGR pressure vessel steel plates (A 553 grade B class 1) TABLE 4--Unirradiated properties of A 533 grade B class 1 pressure vessel steel plates

6 HAGGAG ON EFFECT OF IRRADIATION TEMPERATURE 177 Fig.1--Effect of irradiation temperature on tensile properties of A 533 grade B class 1 plates [fluence of neutrons/cm 2 (>1 MeV)]. (a) Plate G, 0.07% Cu, (b) Plate H, 0.14% Cu.

7 178 16TH RADIATION Fig.2--Effect of irradiation temperature on the Charpy impact energy and the 41-J transition temperature shift (ΔT 41 ) for A 533 grade B class 1 steel, plate G, 0.07% Cu. Fig. 3--Effect of irradiation temperature on the Charpy fracture appearance and the 50% shear transition temperature shift for A 533 grade B class 1 steel, plate G, 0.07% Cu.

8 HAGGAG ON EFFECT OF IRRADIATION TEMPERATURE 179 Fig. 4--Effect of irradiation temperature on the Charpy impact energy and the 41-J transition temperature shift (ΔT 41 ) for A 533 grade B class 1 steel, plate H, 0.14% Cu. Fig.5--Effect of irradiation temperature on the Charpy fracture appearance and the 50% shear transition temperature shift for A 533 grade B class 1 steel, plate H, 0.14% Cu.

9 180 16TH RADIATION Fig. 6--Charpy V-notch impact energy of A 533 grade B class 1 pressure vessel steel, plate G, 0.07% Cu, unirradiated, and 163 C irradiation data. Fig. 7--Charpy V-notch impact energy of A 533 grade B class 1 pressure vessel steel, plate H, 0.14% Cu, unirradiated, and 163 C irradiation data.

10 HAGGAG ON EFFECT OF IRRADIATION TEMPERATURE 181 Table 5--Measured and predicted transition temperature shifts from Charpy impact tests on A 533 grade B class 1 steel after irradiation to neutrons/cm 2 (>1 MeV) Fig. 8--Miniature tensile specimens [24 specimens packaged equivalent to one Charpy V-notch specimen ( cm)].

11 182 16TH RADIATION Fig. 9--Comparison between regular size and miniature tensile specimens of A533 grade B class 1 pressure vessel steel. Fig. 10--Correlation between transition temperature shift (ΔT 41 ) and the change in yield strength due to neutron irradiation.

12 HAGGAG ON EFFECT OF IRRADIATION TEMPERATURE 183 Fig. 11 Specimen geometries used in automated ball indentation (ABI) tests. At the completion of this program and with tensile and ABI tests results from specimens irradiated in all 14 capsules, a better correlation between changes in yield strength and/or other flow properties and transition shifts can be developed. The ABI test technique is particularly advantageous for localized testing (e.g. welds and heat-affected zones), for efficient use of test material and providing more test results for statistical analyses. Furthermore, the large amount of test results from several nuclear pressure vessel materials (plates, welds, and forging) can help to improve our understanding of radiation damaging mechanisms. Also, thermal aging of Charpy V-notch impact and tensile specimens to the same length of irradiation times will aid in separating the effects of thermal aging from the neutron irradiation at the corresponding temperatures. SUMMARY AND CONCLUSIONS 1. Two pressure vessel steel plates were irradiated to a low fluence of neutrons/cm 2 (>1 MeV) in two test reactors. For both the low-copper (0.07% Cu) and medium-copper (0.14%) A 533 grade B class 1 plates, the 41-J, the 68-J, and the 50% shear transition temperature shifts increased by reducing the irradiation temperature from 288 to 204 C, but did not change with further decrease of temperature to 163, 121, or 60 C. The maximum temperature shift was 25 C. 2. Regulatory Guide 1.99, Rev. 2, predictions of the 41-J transition temperature shifts of 7 and 12 C for the 288 C irradiation temperature were conservative for both A 533 grade B class 1 materials. 3. The yield and ultimate tensile strength of two heats of A 533 grade B class 1 pressure vessel steel increased with decreasing irradiation temperature. Uniform elongation slightly decreased with decreasing irradiation temperature.

13 184 16TH RADIATION 4. Miniature tensile and ABI test results were in agreement with standard (regular round size) tensile data for various pressure vessel steels and welds in the as-received and irradiated conditions. 5. The 41-J transition temperature shifts were correlated with changes in the yield strength for all irradiation temperatures. The correlation coefficient of 0.45 is within the range of 0.43 to 0.80 reported by other investigators for specimens irradiated in power and test reactors for relatively higher fluences. 6. Miniature tensile and ABI test techniques could be useful in measuring yield strength and flow properties and understanding radiation damage mechanisms. Effects of neutron spectrum and thermal aging on several nuclear pressure vessel materials are being investigated and the results will be published as they become available. ACKNOWLEDGEMENT This work was sponsored by the Office of New Production Reactors, U.S. Department of Energy, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. Mechanical testing was conducted by E.T. Manneschmidt, R.L. Swain, and J.J. Henry, Jr. The technical peer review by R.K. Nanstad and D.E. McCabe is greatly appreciated. Sincere thanks to Julia L. Bishop for manuscript preparation. REFERENCES [1] Radiation Embrittlement of Reactor Vessel Materials, Regulatory Guide 1.99 (Rev. 2), U.S. Nuclear Regulatory Commission, Washington, D.C., May [2] Haggag, F.M., Nanstad, R.K. and Braski, D.N., Structural Integrity Evaluation Based on an Innovative Field Indentation Microprobe, Innovative Approaches to Irradiation Damage and Fracture Analysis, PVP Vol. 170, D.L. Marriot, T.R. Mager, and W.H. Bamford, Eds., American Society of Mechanical Engineers, New York, 1989, pp [3] Haggag, F.M., Field Indentation Microprobe for Structural Integrity Evaluation, U.S. Patent 4,852,397, August [4] Haggag, F.M., Nanstad, R.K., Hutton, J.T. Thomas, D.L., and Swain, R.L., Use of Automated Ball Indentation Testing to Measure Flow Properties and Estimate Fracture Toughness in Metallic Materials, Applications of Automation Technology to Fatigue and Fracture Testing, ASTM STP 1092, A.A. Braun, N.E. Ausbaugh, and F.M. Smith, Eds., American Society for Testing and Materials, Philadelphia, 1990, pp [5] Haggag, F.M., Application of Flow Properties Microprobe to Evaluate Gradients in Weldment Properties, to be published in the proceedings of the ASM Third International Conference on Trends on Welding Research June 1-5, 1992, Gatlinburg, Tennessee. [6] Haggag, F.M., In-Situ Measurements of Mechanical Properties Using Novel Automated Ball Indentation System, presented at the ASTM Symposium on Small Specimen Test Techniques and Their Applications to Nuclear Reactor Vessel Thermal Annealing and

14 HAGGAG ON EFFECT OF IRRADIATION TEMPERATURE 185 Plant Life Extension, January 29-31, 1992, New Orleans, Louisiana (to be published in ASTM STP 1204). [7] Odette, G.R., Lomborozo, P.M., and Wullaert, R. A., Relationship Between Irradiation Hardening and Embrittlement of Pressure Vessel Steels, Effects of Radiation on Materials: Twelfth International Symposium, ASTM STP 870, F.A. Garner and J.S. Perrin, Eds., American Society for Testing and Materials, Philadelphia, 1985, pp