(1)
Vanadium-based alloys are considered promising candidate structural materials for fusion in-vessel first wall and blanket applications. Although the data base is limited compared to other leading candidate structural materials, such as austenitic and martensitic steels, vanadium-based alloys exhibit several attractive properties for the fusion environment.
The relatively high thermal conductivity and low thermal expansion coefficient of vanadium-based alloys result in lower thermal stresses for a given heat flux compared with that of most other candidate materials, which enhances the lifetime and allows for higher stresses. Since the mechanical strength is retained at fairly high temperatures, higher operating temperatures are projected for these alloys than for austenitic or martensitic steels. Also, activation levels following neutron irradiation are low.
The refractory metals, which also include vanadium exhibit good characteristics relating to corrosion resistance in purified liquid metals. The vanadium alloys also exhibit useful neutronic properties which include lower parasitic neutron absorption cross section leading to better tritium generating performance, lower neutron heating rates and lower helium generation rates, compared to the steels.
Major concerns regarding the use of these alloys relate primarily to their chemical reactivity with non-metallic elements such as oxygen and nitrogen, for example as impurities in liquid metal systems, during accidential exposure to air at high temperature and comtamination during welding or fabrication. Other concerns relate to the lack of industrial experience with fabrication and welding.
The V-15Cr-5Ti alloy is considered representative of a partially optimized vanadium-based alloy and is discussed in more detail. General features are as follows:
* This is a solid solution strengthened alloy.
* Mechanical properties of the V-Cr-Ti alloys are relatively insensitive to sizable compositional variations, which should decrease the sensitivity to solute segregation induced by radiation or chemical effects.
* Relatively high concentrations of interstitial impurities (> 5000 appm of either O, N, C or H) are required to raise the DBTT above room temperature.
* This alloy exhibits superior creep and fatigue properties compared with most other alloys tested.
* Titanium (>= 1%) substantially improves the resistance to irradiation-induced swelling.
* Titanium improves the fabricability.
* Chromium improves the creep strength and oxidation resistance.
* This alloy exhibits fine-grained weld microstructure.
* Vanadium, chromium and titanium meet low activation criteria for material disposal, ref [1].
Resources/considerations
The resources and availability of vanadium are important considerations for a fusion reactor structural material since relatively large quantities will be required for a fusion economy. Vanadium is a moderately abundant material in comparison with other transition metals that are typically used as structural alloys. On results based from the Blanket Comparison and Selection Study, ref [1], the vanadium needs for 1,000 GWe for 40 years (assuming 5 year blanket life and 40 year reactor life with no material recycle) would be approximately 1% of the known resources in the United States, which is ~20x109 Kg, ref [2].
Corrosion/Compatability
In general it has been considered that vanadium-based alloys could not be used in pressurised water-cooled systems because of excessive corrosion but recently, scoping data suggest that selected vanadium-based alloys may be acceptable for use in pressurized water environments. Corrosion rates observed at 288 C in water for 500 hours are greater, but within an order of magnitude, than those observed in austenitic stainless steels, ref [3].
Vanadium will react with air and oxygen at elevated temperatures therefore the vanadium alloy structure must be protected from air during normal operations at elevated temperatures. However the possibility of the accidental exposure of the alloy structure to air must be considered. Scoping studies indicate that a short term exposure to air in the event of an accident would not prove severe at temperatures below 650 C, ref [4]. The lower oxides like VO, V2O3 and VO2 all melt at ~1700 C but the higher oxide V2O5 melts at the relatively low temperature of 670 C, ref [5]. The possible consequences of formation and melting of this oxide under accident conditions could be of concern.
The primary concerns related to hydrogen's compatibility with vanadium-based alloys are related to hydrogen embrittlement, excessive tritium inventory and tritium permeation. Data on the effect of hydrogen solubility, permeability and embrittlement on the ductile to brittle transition temperature of vanadium are reasonably well established and indicate that the properties of V-15Cr-5Ti alloy do not differ greatly from those of unalloyed vanadium.
In general the hydrogen permeation rate in vanadium is relatively high as compared to that in stainless steels, therefore tritium containment is an important consideration when vanadium is used as the structural material.
GENERAL PROPERTIEs - V-15Cr-5Ti
Chemical analysis of V-15Cr-5Ti
Titanium Chromium Carbon Nitrogen Oxygen
5.0 15.3 0.017 0.052 0.023
Physical properties
Melting temperature (C) : 1880
Nuclear characteristicsa
dpa/MW-a/m2 : 11
appm He/MW-a/m2 : 57
appm H/MW-a/m2 : 240
Nuclear heating rate (W/cm3) : 25
Tritium generating ratio : 1.28
Thermal stress factor
MW/m2-mm (500 C) : 9.8
Max. surf. heat flux (MW/m2)b : 1.8
Design stress limitc
Sm (MPa) 500 C : 220
Sm (MPa)500 C : 235
Smt (MPa) (2x04 hr, 100 dpa)
500 C : 165
600 C : 165
700 C : 165
a For lithium blanket
b Idealized flat plate 5 mm thick with 50 C film coefficient, Tout = 400 C
c Stress criteria defined in refs [1] and [6].
DATA AND CORRELATIONS
The data for the thermal and structural properties with temperature are presented in Table 1, refs [7, 8]. Polynomial correlations of the thermal and structural properties of V-15Cr-5Ti as functions of temperature using the data of Table 1 are as follows:
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(2)
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(4)
with T in degrees Celcius and Eqs 1-4 valid in the range 20-800 C. The following equations represent polynomial fits to data of Table 2 for the yield stress and the ultimate tensile strength of the following vanadium alloys:
V-15Cr-5Ti (CAM 834).
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(6)
V-15Cr-5Ti (ANL 204).
(7)
(8)
V-15Cr-5Ti (ANL 101).
(9)
(10)
with T in degrees Celcius and Eqs 5-10 valid in the range 25-700 C. Fig 6 shows the yield stress versus the Larson-Miller parameter, ref [8], which gives the stress to rupture in terms of temperature and time to rupture, with V-5Cr-5Ti also included for comparison purposes. The following expressions are curve fits to the data:
(11)
(12)
(13)
with P the Larson-Miller parameter P, T in degrees K, and time t in hours.
TABLE 1 Thermal and structural properties of V-15Cr-5Ti
T C [[rho]] E GPa [[nu]] k W/m-K c J/kg-K [[alpha]]
kg/m3 (10-6)
m/m-K
20 6100.0 126.00 0.360 21.00 450.00 9.30
100 125.12 22.22 468.61 9.57
200 123.73 23.69 491.43 9.83
300 122.02 25.15 513.80 10.03
400 120.00 26.80 535.00 10.20
500 117.66 28.00 560.00 10.30
600 115.00 29.50 575.00 10.50
700 112.02 31.00 600.00 10.66
800 108.73 32.47 618.97 10.90
TABLE 2 Yield stress and the ultimate tensile strength of various V-15Cr-5Ti alloys
CAM 834 ANL 204 ANL 101
T C [[sigma]]y [[sigma]]u [[sigma]]y [[sigma]]u [[sigma]]y [[sigma]]u
MPa MPa MPa MPa MPa MPa
25 588.0 688.0 570.0 674.0 579.0 682.0
100 483.0 589.0 495.0 599.0 485.0 593.0
225 407.0 548.0 399.0 525.0 370.0 478.0
325 368.0 522.0 348.0 500.0 317.0 449.0
420 379.0 571.0 346.0 524.0 341.0 518.0
520 358.0 567.0 350.0 538.0 326.0 502.0
600 384.0 610.0 335.0 560.0 342.0 555.0
650 373.0 573.0 356.0 575.0 342.0 559.0
700 355.0 519.0 339.0 564.0 337.0 544.0
k (W/m-K) c (J/kg-K)
Temperature (C)
Figure 1 : Thermal conductivity and specific heat of vanadium alloy V-15Cr-5Ti.
E (GPa) [[alpha]] (10-6 m/m-K)Temperature (C)
Figure 2 : Elastic modulus and coefficient of thermal expansion of the vanadium alloy V-15Cr-5Ti.
Stress (MPa)
Temperature (C)
Figure 3 : Yield stress ([[sigma]]y) and ultimate tensile strength ([[sigma]]u) of V-15Cr-5Ti (CAM 834).
Stress (MPa)
Temperature (C)
Figure 4 : Yield stress ([[sigma]]y) and ultimate tensile strength ([[sigma]]u) of V-15Cr-5Ti (ANL 204)
Stress (MPa)
Temperature (C)
Figure 5 : Yield stress ([[sigma]]y) and ultimate tensile strength ([[sigma]]u) of V-15Cr-5Ti (ANL 101).
Stress (MPa)
Larson-Miller Parameter (P/1000)
Figure 6 : Yield stress ([[sigma]]y) and ultimate tensile strength ([[sigma]]u) of V-15Cr-5Ti (ANL 101).
References
- R. Conn et al., Report of the DOE Panel on low Activation Materials for Fusion Applications, University of California-Los Angeles Report, PPG-728 (June, 1983).
- Vanadium Supply and Demand Outlook, National Academy of Science Report, NMAB-346, Washington, DC (1978).
- D. Diercks and D.L. Smith, Alloy Development for Irradiation Performance Progress Report: Period Ending September 30, 1984, DOE/ER-0045/13 (March 1985) pp. 209-213.
- S. Piet, EG &G. Idaho (unpublished data).
- C. Wicks and F. Block, U.S. Bureau of Mines Bulletin 605 (1963).
