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: 16th 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, 1993.
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 (250-550°F)] compared to those for commercial light-water
reactors (LWRs) [~288°C (550°F)]. The need for design data on the reference temperature
(RTNDT) 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 1.1×1018 neutrons/cm2 [0.7×1018 neutrons/cm2 (>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 1.3×1018 neutrons/cm2 [0.8 × 1018] (>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 37831-6151.
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The need for MHTGR design data on the reference temperature (RTNDT) 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 Vnotch, 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 HeavySection 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 1.1 × 1018 neutrons/cm2 [0.68 × 1018
neutrons/cm2 (>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 1.3
× 1018 neutrons/cm2 [0.80×1018 neutrons/cm2 (>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.
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Table 1--Summary of irradiation matrix for MHTGR reactor vessel materials
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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 (ΔT41) 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 ΔT41 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.
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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
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Fig.1--Effect of irradiation temperature on tensile properties of A 533 grade B class 1 plates
[fluence of 0.7 × 1018 neutrons/cm2 (>1 MeV)].
(a) Plate G, 0.07% Cu, (b) Plate H, 0.14% Cu.
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Fig.2--Effect of irradiation temperature on the Charpy impact energy and the 41-J transition
temperature shift (ΔT41) 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.
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Fig. 4--Effect of irradiation temperature on the Charpy impact energy and the 41-J transition
temperature shift (ΔT41) 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.
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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.
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Table 5--Measured and predicted transition temperature shifts from Charpy impact tests on A 533
grade B class 1 steel after irradiation to 0.7 × 1018 neutrons/cm2 (>1 MeV)
Fig. 8--Miniature tensile specimens [24 specimens packaged equivalent to one Charpy V-notch
specimen (1 × 1 × 5.5cm)].
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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 (ΔT41) and the change in yield strength
due to neutron irradiation.
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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 0.7 × 1018
neutrons/cm2 (>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.
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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.
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HAGGAG ON EFFECT OF IRRADIATION TEMPERATURE
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