4.06.5.1 Introduction and Irradiated Properties Database for W and W Alloys

Despite the recurring interest in the use of tungsten as a structural material for very-high-temperature applications and for use as plasma facing components in fusion devices, the database on irradiated properties is very limited and based primarily on fast neutron irradiation experiments. Similar to all bcc materials, tungsten is susceptible to low-temperature embrit­tlement at T< 0.3 Tm (Tm = 3695 K) for fluences >1 x 1024nm~2, which makes this material even more limited in toughness and ductility.

Improvements in the unirradiated mechanical properties of tungsten are observed with the addi­tion of Re, which is found to increase ductility at elevated temperatures125 as well as fracture tough — ness126 through the reduction in the DBTT. How­ever, as discussed in this section, the gains in performance through added Re content do not nec­essarily hold for irradiated materials.

4.06.5.2 Irradiation-Induced Swelling and Physical Property Changes in W and

W Alloys

Early investigations into the behavior of irradiated tungsten59,125,127-129 examined the defect formation and recovery defects. Much of this initial work was through the examination of electrical resistivity fol­lowing irradiation. Increases in electrical resistivity of pure annealed tungsten of up to 24% following irradiation to 2 x 1022ncm~2 in a fast reactor and ~14% in a mixed spectrum reactor to 1021ncm~ have been reported.59

Keys etal.127,128 examined the recovery of neutron — irradiated tungsten through isochronal resistivity studies following irradiation at 343 K to dose levels of 1.5 x 1021ncm~2 (E > 1 MeV). The beginning of saturation in resistivity observed in their studies appears just below 102°ncm~2 and correlates with the work by Lacefield et a/.130 on the appearance of defect clusters by 2.4 x 1019ncm~ identified through TEM examination. The work by Keys et a/.127,128 identified distinct stages of recovery such as self-interstitial migration occurring near 0.15 Tm, with a less pronounced recovery at 0.22 Tm through divacancy and impurity migration, which was followed by vacancy migration above 0.31 Tm. The residual resistivity, not recovered following anneals above 0.4 Tm after irradiation to fluences >3.3 x 1019 n cm~2, was due to the development of Re in the tungsten from transmutation reactions with thermal neutrons.

Very little data exist on irradiation-induced swell­ing in W and its alloys. Data on pure W are restricted to two reported series of experiments concerning the temperature dependence of swelling. The irradiation — induced swelling measured by Matolich eta/.131 and Wiffen19 using immersion density methods is shown in Figure 23. It should be noted that there is an order of magnitude difference in fluences between the two stud­ies. No other systematic examination of the swelling dependence on fluence and temperature is available.

Though swelling data for W-Re alloys are also lim­ited, work by Matolich eta/.131 for W-25Re irradiated to 5.5 x 1022ncm~2 revealed no significant amount of swelling. The data are also shown in Figure 23. Microstructural examination of W-Re alloys with concentrations of 5%, 11%, and 25%Re showed no cavity formation for fluences between 4.3

Figure 23 Irradiation-induced swelling measured through immersion density methods of W and W-25Re by Matolich etal.131 and Wiffen.19 and 6.1 x 1021 ncm-2 (E > 0.1 MeV) at temperatures between 873 and 1773 K,1 2 while void cavities have been experimentally observed in irradiated pure W over similar fluences.133,134 The effects of increas­ing Re or Os content in W were experimentally shown to decrease the density and radius of dislocation loops and voids in 0.15 dpa proton and neutron — irradiated material by He eta/.135 This reduction in size and number density is the result of the restricted mobility of the radiation-induced defects by the lat­tice dilations from the Re and Os solute.

The transmutation of W to Re and Re to Os during irradiation can have an effect on microstructural, physical, and mechanical properties of the material. The transmutation of the material, which results in the shifting of solute concentrations to higher levels, may result in precipitation in alloys that are nomi­nally in a single-phase region. One example of micro­structural and physical property changes because of irradiation is the decalibration of type-C (W—3%Re/ W-25%Re or W-5%Re/W-26%Re) thermocou­ples, such as that used in fuel element centerline temperature measurements. Reviews of early experi­mental work on W/Re thermocouples and on the dependence of decalibration on the neutron fluence have previously been discussed.136,137 While displa — cive neutron damage may result in material changes such as vacancy clusters or dislocation loops, the maximum theoretical changes expected in emf out­put of the thermocouple is ~1 p. V °C-1,138 whereas the changes associated with transmutation effects can result in more significant decreases. A -300°C drift in temperature following 6000 h irradiation under 2.7 x 10 n cm-2 thermal and 8 x 1021 n cm — fast fluence was reported for a W-3%Re/W-25%Re couple.139

These changes can also be significant in fast reac­tor irradiations. Experimental work by Williams eta/.132 showed that for a W-5%Re/W-25%Re ther­mocouple irradiated to 6.1 x 1021 n cm-2 fast fluence at 1173 K, the precipitation-induced changes in the Seebeck coefficient were -6.6 and -0.02 mV °C-1 for the 5 and 25% Re alloys, respectively. Calculated final compositions following 6.1 x 1021ncm — (14 MeV) irradiation of W—5(wt%)Re produce W-5.130Re-

0. 021Os-0.150Ta alloy, while a W-26Re alloy trans­mutes to W-25.955Re-0.107Os-0.117Ta.140

Postirradiation examination of the microstructure of the irradiated 5, 11, and 25% Re alloys in Williams eta/.132 revealed w-phase precipitation at irradiation temperatures above 1373 K, though unidentifiable precipitation was apparent in the alloys at 1173 K.

The development of the w-phase over the equilibrium а-phase in irradiated samples, but not in the unirradi­ated annealed samples, is the result of irradiation — induced solute segregation to defect sinks. The development of the w-phase was also reported in microstructural studies of W-26Re irradiated up to 11 dpa at temperatures between 646 and 1073 K.141

It should be pointed out that the change or tem­perature shift under irradiation is proportional to the degree of localized transmutation and local tempera­ture gradients and therefore dependent on the pro­files of the temperature and irradiation fields to which the thermocouple is exposed. Therefore, experimental work typically involves the irradiation of the entire cable, while in reactor applications, significant variations in temperature and fluence may result. The changes in thermoelectric power (D) as a function of irradiation fluence can be mod­eled by the following:140

D = 0 for 0 < f < 0.25 x 1021 n cm-2

D = 100[1 — e°’067(°’25-f)] for 0.25 < f < 1 x 1021 n cm-2

D = 100[1 — e°’104(°’52-f)] for f > 1 x 1021 n cm-2 [2]

Though significant radiation-induced decalibration may occur in fission reactors, this effect may not be readily observed in fusion reactors where the thermal flux is much lower. In addition, typical end-of-life estimates of total neutron fluence of <1021ncm-2 suggest that transmutation effects resulting in decali­bration is not an issue.