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The thermal conductivity of ovides of uranium, neptunium and americium at elevated temperatures
Journal of the Less-Common
Metals, 121 (1986)
621 - 630
621
THE THERMAL CONDUCTIVITY OF OXIDES OF URANIUM, NEPTUNIUM AND AMERICIUM AT ELEVATED TEMPERATURES* H. E. SCHMIDT,
C. SARI
and K. RICHTER
Commission of the European Communities, Joint for Transuranium Elements, Karlsruhe (F. R.G.)
Research
Centre,
European
Institute
P. GERONTOPOULOS
Agip ENEA, Rome
(Italy)
Summary Two different methods were used to measure the thermal conductivity of oxides of uranium, americium and neptunium: diffusivities were determined in the temperature range from 1000 to 1800 K by a modulated electron beam technique and converted to conductivities whereas a direct electrical heating method was used to deduce mean conductivities between about 1600 K and the melting temperature of the oxides. The principal result of this investigation is the observation that the thermal conductivity of U-Am dioxide exhibits an upswing above 1800 K as in the case of uranium dioxide and uranium-plutonium dioxide, whereas the conductivity of uranium-neptunium dioxide continues to decrease with temperature, like that of thoria, as one would expect for a pure phonon conductor. The high-temperature rise has been attributed to an electronic contribution which, apparently, is related to the ability of the oxide to accommodate the metals in question in another than the four-valent state.
1. Specimen preparation The thermal properties of mixed uranium-neptunium, uraniumamericium and uranium-americium-neptunium oxides produced via different fabrication routes were investigated in view of a planned irradiation of these materials [ 11. Oxide specimens with different americium, neptunium, uranium contents (see Table 1) were prepared by ammonia precipitation of the actinides dissolved in nitric acid (coprecipitation) as well as by the gelsupported precipitation (GSP) technique [ 21. The thermal treatment of the starting material as well as the pressing and sintering conditions were the same for both fabrication procedures. *Paper
presented
0022-5088/86/$3.50
at Actinides
85, Aix en Provence,
September
@ Elsevier Sequoia/Printed
2 - 6, 1985. in The Netherlands
~0.5Uo.502-X
~0.5u0.502--x Am0.Z5NP0.25U0.502-x
~~0.5fJ0.502 N~o.sUo.502 N~o.5Do.sO2
Am 12.4 Am 13 AmNpl
Np 2.3.4 Np 2.3.5 NP 3
(%TD) 88 95 95.5 95 96 95
Density (g cme3) 10.15 10.50 10.55 10.50 10.60 10.50
GSP, Material produced by the GSP method. O/M: initial, final; O/M ratio after fabrication and after measurements.
Composition
Designation
Sample characterization
TABLE 1
Final 1.81 1.92 2.00 2.00 2.00
1.83 1.83 2.00 2.00 2.00
GSP coprecipitation coprecipitation GSP ex-GSP coprecipitation
OIM Initial
Origin
623
The main preparation steps for the GSP method were as follows. Dissolution of NpOz and/or AmOz in 8M HNOs followed by evaporation of the nitric acid. Mixing with UOz(NO& and other components in the desired proportion to obtain a feed solution of the following composition: total metal content 120 g-r 1; 0.3 M free HNOs; 0.8% Methocal A4C (Cellulose derivate, Dow Chemical); 20% THFA (Tetrahydrofurfurylalcohol); 0.2% Triton-x 100 (Surfactant, C. Erba). Atomization, sol-gel conversion in 8M NH,OH and dehydration by azeotropic distillation of H,O-C!$X, in a remotely operated installation. Calcination at 670 K under argon and oxidation at 1000 to 1200 K in air with subsequent reduction in Ar-5%H,. Pressing and sintering at 1920 K during 6 h under Ar-5%Hz and reduction of the pellet to the desired O/M ratio. The fuel pellets marked ex-GSP in Table 1 were prepared by comminution of the precipitated GSP beads of diameters between 120 to 600 pm to a powder with grains 90 I.trnin diameter and smaller, followed by pressing and sintering. Pellets up to 8 mm in height and 6 mm in diameter were used as-fabricated for the determination of the mean thermal conductivity at high temperatures, whereas disks with the same diameter and 0.8 to 1 mm thick, obtained in the desired shape by pressing and sintering or by slicing from larger pellets, served as specimens for thermal diffusivity measurements between 1100 and 1870 K.
2. Experimental details The thermal diffusivities of U-Am, U-Np and U-Am-Np oxides were determined in the range of temperatures between 1100 and 1800 K using a previously described modulated electron beam technique [ 31, whereas their mean thermal conductivities from about 1700 K up to their respective melting temperatures were derived from measurements of the electrical power necessary to produce central melting in a direct electrical heating device [ 41. The results of the diffusivity measurements are listed in Table 2. The factors which cause the relatively wide scatter in experimental data are not fully understood. Specific heat capacities C, are needed to convert measured diffusivities into conductivities. Since experimental C, date for the oxides in question are lacking, corresponding specific heat data for U-Pu oxides were taken. These were calculated with the help of a polynomial expression obtained by fitting and interpolating published heat capacity data for hypostoichiometric mixed (U,Pu)-oxides [ 5,6] : C,=a,+a,T+a,T2
2
1093 1163 1198 1323 1448 1383 1583 1498 1713 1113 1313 1193 1273 1323 1383 1523 1623
1133 1213 1313 1398 1493 1593 1693 1498 1593 1673 1643
Am 12.4
Am 13
Sample Designation
3.92 3.69 3.81 3.66 3.65 3.69 3.75 4.17 3.88 3.77 3.89
298 298 298 300 302 306 311 302 306 310 308
298 298 298 298 301 299 306 302 312 298 298 298 298 298 299 303 307
10.50
10.50
9.70
9.79
(g cmw3)
4.13 4.03 3.94 3.95 3.75 3.82 3.79 3.73 3.52 4.64 4.05 4.55 4.22 4.09 4.12 3.57 3.45
d Ws gK-I)
(10e4 m2 s-l)
;?03
a
1.23 1.15 1.19 1.15 1.13 1.18 1.22 1.33 1.25 1.23 1.26
1.20 1.16 1.14 1.14 1.09 1.11 1.12 1.09 1.07 1.34 1.17 1.31 1.22 1.18 1.20 1.05 1.03
(W mK_‘)
x
1590
1840 1920 1970
2690
2690 2690 2690
Measured diffusivity a at temperaLure T, specific heat capacity C,, density d and calculated thermal conductivity h together temperature mean conductivities h measured between Ti and T, for several uranium-neptunium-americium oxides
TABLE
1.87
1.32 2.1 2
(W mK_‘)
x
with high-
Q, 2
1103 1208 1448 1568 1668 1698 1832 1673 1623 1573 1438 1442 1373 1313 1203 1048 1113 1193
1763 1623 1513 1413 1333 1218 1148 1098
AmNpl
Np 2.3.4
1513 1413 1373 1303 1303 1198 1153 1073
5.30 5.96 6.37 6.70 7.31 8.32 9.24 9.28
5.20 4.93 4.49 4.41 4.35 4.23 4.04 4.32 4.40 4.55 4.80 4.65 4.84 4.98 5.31 5.37 3.53 5.23
3.82 4.19 4.32 4.32 4.58 4.75 4.62 4.93
373 365 360 357 356 356 357 358
350 348 349 353 357 358 366 357 355 353 349 349 348 348 348 351 349 348
303 300 299 298 298 298 298 299
10.50
10.10
2.08 2.28 2.41 2.51 2.74 3.11 3.46 3.49
1.84 1.73 1.58 1.57 1.57 1.53 1.50 1.56 1.58 1.62 1.69 1.64 1.70 1.75 1.87 1.90 1.92 1.84
1.22 1.32 1.36 1.35 1.43 1.48 1.45 1.55 2770
2870
1560
1640
(continued)
1.74
1.7
g
Q,
10.27 9.41 9.06 9.02 8.53 a.45 7.96 7.70 6.98
7.94 7.19 6.79 7.66 8.62 9.33 9.50 10.66 5.96 5.47 5.63 6.36 6.85
1123 1193 1253 1333 1413 1473 1573 1613 1698
1423 1553 1773 1553 1378 1243 1248 1133 1563 1773 1723 1623 1443
Np3
(104 m2 s-l)
a
Np 2.3.5
Sample Designation
TABLE 2 (continued}
358 362 374 362 357 356 356 357 362 368 365 361 358
357 356 356 356 357 359 363 364 369
(lo3WsgK-")
5
10.50
10.45
(gcm-')
d
2.98 2.73 2.67 2.91 3.23 3.49 3.55 3.99 2.27 2.11 2.24 2.41 2.56
3.83 3.50 3.37 3.36 3.18 3.17 3.07 2.93 2.69
(WmK-')
x
1600
-
x
-
1.53
-
2870
(WmK-I)
627
with a0 = -13000 a, = -27
+ 12720 (0~~) - 3000 (O/M)’
+ 28 (O/M) - 7.3 (O/M)*
a2 = 0.019 - 0.019 (O/M) + 0.005 (O/M)2 where T is in kelvin and C, in joules per gramme per kelvin. The above expression has been verified experimentally for 1.9 < O/M < 2. C, values calculated with the aid of the above equation and determined under the assumption that C, remained practically unchanged for O/M ratios between 1.8 and 1.9, are included in Table 2, together with calculated conductivities. 3. Discussion In the case of the uranium-americium oxides investigated (Am 12.4 and Am 13), powder preparation procedures apparently have no effect on the measured thermal properties. The difference between the two sets of curves in Fig. l(a) is due to oxidation which occurred during the measurement of sample Am 13. The situation is less obvious for the U-Np oxides studied, where the GSP material exhibited a somewhat lower conductivity than the samples produced from the ex-GSP and coprecipitated powders. In a second run, however, the Np 3-values (coprecipita~d~ were lower than in the first run. It remains to be clarified whether this is due to a change in structure or in the composition. The data for the U-Am-Np oxide sample coincide surprisingly well with values obtained for (U, Pu) mixed oxide specimens with an O/M ratio of 1.94. In the case of oxides coning neptunium, low-tempe~ture (converted) conductivity data and high-temperature mean conductivities correspond well. Apparent discrepancies in the case of the U-Am oxides between high- and low-temperature data can be attributed to the fact that during the high-temperature conductivity measurement a radial O/M gradient builds up, leading to higher O/M ratios at the periphery than in the centre. This shift in stoichiometry affects a considerable volume fraction of the sample and leads to higher apparent conductivities than one might expect from the starting O/M. In Fig. l(a), smoothed conductivity data for (U, Am) oxides are compared with the conductivities of stoichiometric and substoichiometric (Uo.s~o.~) oxides from ref. 7. In the same manner, Fig. l(b) shows smoothed conductivities of U-Np oxides obtained by the two methods and compares them with the thermal conductivity of (U, Th)O, from ref. 8. Figure l(c) demonstrates the case of (U, Am, Np) oxide in comparison with hypostoichiometric (U, Pu) oxide.
626
4
3
2
1
O/M = l.St - 1.92
0 4
/’
b)
3 z 5 4-z
1
0 4
3
2
1
0
2500
_.a
3OUOr,K
Fig. 1. Smoothed thermal conductivities of (a) (U, Am) oxides compared with conductivities of stoichiometric (U, Pu) oxides, (b) (U, Np) oxides compared with ThOz, (c) (U, Np, Am) oxide compared with substoichiometric (U, Pu) oxide.
An analogy in the variation of thermal conductivity with temperature between (U, Am) and (U, Pu) oxide on the one hand and (U, Np) and (U, Th) oxide on the other is obvious. This analogy is underlined by the fact
629
4
a)
3
2
1
0 c b)
3 2 5 x-2
1
0 c
3
2
1
0
1500 2000 1000 2500 3000T Fig. 2. Recommended data for the thermal conductivities Np) oxide, (c) (U, Am, Np) oxide.
t
( ) (U, Am) oxide,
(b)
(U,
that in the oxygendeficient state of a mixed oxide, neptunium, like thorium, remains four-valent up to high temperatures [9], whereas americium, like plutonium, may become three-valent even at lower temperatures.
630
On the basis of the above arguments, “recommended” conductivities between 1000 K and their melting temperatures are given in Figs. Z(a) - Z(c) for the three materials investigated. These recommendations may still be subject to errors and should be used with appropriate caution. Efforts are presently being made to confirm these predictions.
4. Conclusion The experimental evidence described above may help to clarify the nature of a possible electronic contribution to the thermal condudtivity of mixed (U, Pu)-oxides and of UOz. This contribution, which in the case of UOz has been attributed to the presence of 5f electrons [ 10 1, appears to be related to the ability of some of the oxides in question to accomodate metals in another than the four-valent state.
References 1 L. Koch, Costs and Benefits of Minor Actinide Recycling, to be published. 2 G. Cogliati, P. Gerontopoulos and K. Richter, Trans. Am. Nucl. Sot., 31 (1979) 175. 3 H. E. Schmidt, M. van den Berg and L. van der Hoek, High Temp.-High Press., 1 (1969) 309. 4 C. Sari and F. Quik, J. Nucl. Mater., 47 (1973) 219. 5 A. E. Ogard, J. A. Leary, Rep. LADC 5622 (1967). 6 C. Affortit, J. P. Marcon, Rev. Ink Hautes Temp. Refract., 7 (1970) 236. 7 H. E. Schmidt, High Temp.-High Press., 3 (1971) 345. 8 D. A. Himes, Trans. Am. Nucl. Sot., 30 (1978) 174. 9 C. Sari, unpublished, 1984. 10 G. J. Hyland and J. Ralph, High Temp.-High Press., 15 (183) 179.
Metals, 121 (1986)
621 - 630
621
THE THERMAL CONDUCTIVITY OF OXIDES OF URANIUM, NEPTUNIUM AND AMERICIUM AT ELEVATED TEMPERATURES* H. E. SCHMIDT,
C. SARI
and K. RICHTER
Commission of the European Communities, Joint for Transuranium Elements, Karlsruhe (F. R.G.)
Research
Centre,
European
Institute
P. GERONTOPOULOS
Agip ENEA, Rome
(Italy)
Summary Two different methods were used to measure the thermal conductivity of oxides of uranium, americium and neptunium: diffusivities were determined in the temperature range from 1000 to 1800 K by a modulated electron beam technique and converted to conductivities whereas a direct electrical heating method was used to deduce mean conductivities between about 1600 K and the melting temperature of the oxides. The principal result of this investigation is the observation that the thermal conductivity of U-Am dioxide exhibits an upswing above 1800 K as in the case of uranium dioxide and uranium-plutonium dioxide, whereas the conductivity of uranium-neptunium dioxide continues to decrease with temperature, like that of thoria, as one would expect for a pure phonon conductor. The high-temperature rise has been attributed to an electronic contribution which, apparently, is related to the ability of the oxide to accommodate the metals in question in another than the four-valent state.
1. Specimen preparation The thermal properties of mixed uranium-neptunium, uraniumamericium and uranium-americium-neptunium oxides produced via different fabrication routes were investigated in view of a planned irradiation of these materials [ 11. Oxide specimens with different americium, neptunium, uranium contents (see Table 1) were prepared by ammonia precipitation of the actinides dissolved in nitric acid (coprecipitation) as well as by the gelsupported precipitation (GSP) technique [ 21. The thermal treatment of the starting material as well as the pressing and sintering conditions were the same for both fabrication procedures. *Paper
presented
0022-5088/86/$3.50
at Actinides
85, Aix en Provence,
September
@ Elsevier Sequoia/Printed
2 - 6, 1985. in The Netherlands
~0.5Uo.502-X
~0.5u0.502--x Am0.Z5NP0.25U0.502-x
~~0.5fJ0.502 N~o.sUo.502 N~o.5Do.sO2
Am 12.4 Am 13 AmNpl
Np 2.3.4 Np 2.3.5 NP 3
(%TD) 88 95 95.5 95 96 95
Density (g cme3) 10.15 10.50 10.55 10.50 10.60 10.50
GSP, Material produced by the GSP method. O/M: initial, final; O/M ratio after fabrication and after measurements.
Composition
Designation
Sample characterization
TABLE 1
Final 1.81 1.92 2.00 2.00 2.00
1.83 1.83 2.00 2.00 2.00
GSP coprecipitation coprecipitation GSP ex-GSP coprecipitation
OIM Initial
Origin
623
The main preparation steps for the GSP method were as follows. Dissolution of NpOz and/or AmOz in 8M HNOs followed by evaporation of the nitric acid. Mixing with UOz(NO& and other components in the desired proportion to obtain a feed solution of the following composition: total metal content 120 g-r 1; 0.3 M free HNOs; 0.8% Methocal A4C (Cellulose derivate, Dow Chemical); 20% THFA (Tetrahydrofurfurylalcohol); 0.2% Triton-x 100 (Surfactant, C. Erba). Atomization, sol-gel conversion in 8M NH,OH and dehydration by azeotropic distillation of H,O-C!$X, in a remotely operated installation. Calcination at 670 K under argon and oxidation at 1000 to 1200 K in air with subsequent reduction in Ar-5%H,. Pressing and sintering at 1920 K during 6 h under Ar-5%Hz and reduction of the pellet to the desired O/M ratio. The fuel pellets marked ex-GSP in Table 1 were prepared by comminution of the precipitated GSP beads of diameters between 120 to 600 pm to a powder with grains 90 I.trnin diameter and smaller, followed by pressing and sintering. Pellets up to 8 mm in height and 6 mm in diameter were used as-fabricated for the determination of the mean thermal conductivity at high temperatures, whereas disks with the same diameter and 0.8 to 1 mm thick, obtained in the desired shape by pressing and sintering or by slicing from larger pellets, served as specimens for thermal diffusivity measurements between 1100 and 1870 K.
2. Experimental details The thermal diffusivities of U-Am, U-Np and U-Am-Np oxides were determined in the range of temperatures between 1100 and 1800 K using a previously described modulated electron beam technique [ 31, whereas their mean thermal conductivities from about 1700 K up to their respective melting temperatures were derived from measurements of the electrical power necessary to produce central melting in a direct electrical heating device [ 41. The results of the diffusivity measurements are listed in Table 2. The factors which cause the relatively wide scatter in experimental data are not fully understood. Specific heat capacities C, are needed to convert measured diffusivities into conductivities. Since experimental C, date for the oxides in question are lacking, corresponding specific heat data for U-Pu oxides were taken. These were calculated with the help of a polynomial expression obtained by fitting and interpolating published heat capacity data for hypostoichiometric mixed (U,Pu)-oxides [ 5,6] : C,=a,+a,T+a,T2
2
1093 1163 1198 1323 1448 1383 1583 1498 1713 1113 1313 1193 1273 1323 1383 1523 1623
1133 1213 1313 1398 1493 1593 1693 1498 1593 1673 1643
Am 12.4
Am 13
Sample Designation
3.92 3.69 3.81 3.66 3.65 3.69 3.75 4.17 3.88 3.77 3.89
298 298 298 300 302 306 311 302 306 310 308
298 298 298 298 301 299 306 302 312 298 298 298 298 298 299 303 307
10.50
10.50
9.70
9.79
(g cmw3)
4.13 4.03 3.94 3.95 3.75 3.82 3.79 3.73 3.52 4.64 4.05 4.55 4.22 4.09 4.12 3.57 3.45
d Ws gK-I)
(10e4 m2 s-l)
;?03
a
1.23 1.15 1.19 1.15 1.13 1.18 1.22 1.33 1.25 1.23 1.26
1.20 1.16 1.14 1.14 1.09 1.11 1.12 1.09 1.07 1.34 1.17 1.31 1.22 1.18 1.20 1.05 1.03
(W mK_‘)
x
1590
1840 1920 1970
2690
2690 2690 2690
Measured diffusivity a at temperaLure T, specific heat capacity C,, density d and calculated thermal conductivity h together temperature mean conductivities h measured between Ti and T, for several uranium-neptunium-americium oxides
TABLE
1.87
1.32 2.1 2
(W mK_‘)
x
with high-
Q, 2
1103 1208 1448 1568 1668 1698 1832 1673 1623 1573 1438 1442 1373 1313 1203 1048 1113 1193
1763 1623 1513 1413 1333 1218 1148 1098
AmNpl
Np 2.3.4
1513 1413 1373 1303 1303 1198 1153 1073
5.30 5.96 6.37 6.70 7.31 8.32 9.24 9.28
5.20 4.93 4.49 4.41 4.35 4.23 4.04 4.32 4.40 4.55 4.80 4.65 4.84 4.98 5.31 5.37 3.53 5.23
3.82 4.19 4.32 4.32 4.58 4.75 4.62 4.93
373 365 360 357 356 356 357 358
350 348 349 353 357 358 366 357 355 353 349 349 348 348 348 351 349 348
303 300 299 298 298 298 298 299
10.50
10.10
2.08 2.28 2.41 2.51 2.74 3.11 3.46 3.49
1.84 1.73 1.58 1.57 1.57 1.53 1.50 1.56 1.58 1.62 1.69 1.64 1.70 1.75 1.87 1.90 1.92 1.84
1.22 1.32 1.36 1.35 1.43 1.48 1.45 1.55 2770
2870
1560
1640
(continued)
1.74
1.7
g
Q,
10.27 9.41 9.06 9.02 8.53 a.45 7.96 7.70 6.98
7.94 7.19 6.79 7.66 8.62 9.33 9.50 10.66 5.96 5.47 5.63 6.36 6.85
1123 1193 1253 1333 1413 1473 1573 1613 1698
1423 1553 1773 1553 1378 1243 1248 1133 1563 1773 1723 1623 1443
Np3
(104 m2 s-l)
a
Np 2.3.5
Sample Designation
TABLE 2 (continued}
358 362 374 362 357 356 356 357 362 368 365 361 358
357 356 356 356 357 359 363 364 369
(lo3WsgK-")
5
10.50
10.45
(gcm-')
d
2.98 2.73 2.67 2.91 3.23 3.49 3.55 3.99 2.27 2.11 2.24 2.41 2.56
3.83 3.50 3.37 3.36 3.18 3.17 3.07 2.93 2.69
(WmK-')
x
1600
-
x
-
1.53
-
2870
(WmK-I)
627
with a0 = -13000 a, = -27
+ 12720 (0~~) - 3000 (O/M)’
+ 28 (O/M) - 7.3 (O/M)*
a2 = 0.019 - 0.019 (O/M) + 0.005 (O/M)2 where T is in kelvin and C, in joules per gramme per kelvin. The above expression has been verified experimentally for 1.9 < O/M < 2. C, values calculated with the aid of the above equation and determined under the assumption that C, remained practically unchanged for O/M ratios between 1.8 and 1.9, are included in Table 2, together with calculated conductivities. 3. Discussion In the case of the uranium-americium oxides investigated (Am 12.4 and Am 13), powder preparation procedures apparently have no effect on the measured thermal properties. The difference between the two sets of curves in Fig. l(a) is due to oxidation which occurred during the measurement of sample Am 13. The situation is less obvious for the U-Np oxides studied, where the GSP material exhibited a somewhat lower conductivity than the samples produced from the ex-GSP and coprecipitated powders. In a second run, however, the Np 3-values (coprecipita~d~ were lower than in the first run. It remains to be clarified whether this is due to a change in structure or in the composition. The data for the U-Am-Np oxide sample coincide surprisingly well with values obtained for (U, Pu) mixed oxide specimens with an O/M ratio of 1.94. In the case of oxides coning neptunium, low-tempe~ture (converted) conductivity data and high-temperature mean conductivities correspond well. Apparent discrepancies in the case of the U-Am oxides between high- and low-temperature data can be attributed to the fact that during the high-temperature conductivity measurement a radial O/M gradient builds up, leading to higher O/M ratios at the periphery than in the centre. This shift in stoichiometry affects a considerable volume fraction of the sample and leads to higher apparent conductivities than one might expect from the starting O/M. In Fig. l(a), smoothed conductivity data for (U, Am) oxides are compared with the conductivities of stoichiometric and substoichiometric (Uo.s~o.~) oxides from ref. 7. In the same manner, Fig. l(b) shows smoothed conductivities of U-Np oxides obtained by the two methods and compares them with the thermal conductivity of (U, Th)O, from ref. 8. Figure l(c) demonstrates the case of (U, Am, Np) oxide in comparison with hypostoichiometric (U, Pu) oxide.
626
4
3
2
1
O/M = l.St - 1.92
0 4
/’
b)
3 z 5 4-z
1
0 4
3
2
1
0
2500
_.a
3OUOr,K
Fig. 1. Smoothed thermal conductivities of (a) (U, Am) oxides compared with conductivities of stoichiometric (U, Pu) oxides, (b) (U, Np) oxides compared with ThOz, (c) (U, Np, Am) oxide compared with substoichiometric (U, Pu) oxide.
An analogy in the variation of thermal conductivity with temperature between (U, Am) and (U, Pu) oxide on the one hand and (U, Np) and (U, Th) oxide on the other is obvious. This analogy is underlined by the fact
629
4
a)
3
2
1
0 c b)
3 2 5 x-2
1
0 c
3
2
1
0
1500 2000 1000 2500 3000T Fig. 2. Recommended data for the thermal conductivities Np) oxide, (c) (U, Am, Np) oxide.
t
( ) (U, Am) oxide,
(b)
(U,
that in the oxygendeficient state of a mixed oxide, neptunium, like thorium, remains four-valent up to high temperatures [9], whereas americium, like plutonium, may become three-valent even at lower temperatures.
630
On the basis of the above arguments, “recommended” conductivities between 1000 K and their melting temperatures are given in Figs. Z(a) - Z(c) for the three materials investigated. These recommendations may still be subject to errors and should be used with appropriate caution. Efforts are presently being made to confirm these predictions.
4. Conclusion The experimental evidence described above may help to clarify the nature of a possible electronic contribution to the thermal condudtivity of mixed (U, Pu)-oxides and of UOz. This contribution, which in the case of UOz has been attributed to the presence of 5f electrons [ 10 1, appears to be related to the ability of some of the oxides in question to accomodate metals in another than the four-valent state.
References 1 L. Koch, Costs and Benefits of Minor Actinide Recycling, to be published. 2 G. Cogliati, P. Gerontopoulos and K. Richter, Trans. Am. Nucl. Sot., 31 (1979) 175. 3 H. E. Schmidt, M. van den Berg and L. van der Hoek, High Temp.-High Press., 1 (1969) 309. 4 C. Sari and F. Quik, J. Nucl. Mater., 47 (1973) 219. 5 A. E. Ogard, J. A. Leary, Rep. LADC 5622 (1967). 6 C. Affortit, J. P. Marcon, Rev. Ink Hautes Temp. Refract., 7 (1970) 236. 7 H. E. Schmidt, High Temp.-High Press., 3 (1971) 345. 8 D. A. Himes, Trans. Am. Nucl. Sot., 30 (1978) 174. 9 C. Sari, unpublished, 1984. 10 G. J. Hyland and J. Ralph, High Temp.-High Press., 15 (183) 179.