The phase relation between actinide and Group IIIA metals depends upon the characteristics of the actinide metals. Table 3 summarizes the phase rela­tions for the Th-related system, which can be divided into five groups. The first group consists of the Th-Sc, Th-Y, Th-Gd, Th-Tb, and Th-Dy systems. The mutual solubility of these systems is very

Pu

good.41-43’47 These Group Ilia metals have two allo — tropes: the low-temperature a-phase (HCP struc­ture) and the high-temperature р-phase (bcc structure). On the other hand, Th has the low — temperature a-phase (fcc structure) and the high-temperature р-phase (bcc structure). The р-phase in these systems is completely soluble as well as the liquid phase. The low-temperature a-phases have a large region of mutual solid solubil­ity, although the crystal structure is different from each other. Regarding the solubility of Th in the a-phase of Sc, Y, Gd, Tb, and Dy, a systematic unlikely tendency is seen in Table 3, which possibly originates from the differences in the method of sample preparation. Figure 7 indicates the Th-Sc phase diagram as a typical example of this group quoted from Okamoto.4 The shape of the phase boundaries suggest that these systems can be mod­eled as a simple regular solution. The second group consists of the Th-La and Th-Ce systems, for which the experimental data were mainly given by Badayeva and Kuznetsova43 and Moffatt.44 Since the low — temperature phase (fcc structure) appears for La and Ce, a complete solubility even for the low — temperature fcc phase as well as the bcc and liquid phases was indicated in these previous studies. Figure 8 shows the Th-La phase diagram as a typical example shown in Kassner and Peterson.1 As for the Th-Ce system, a similar phase diagram was originally proposed by Moffatt.44 According to Okamoto and Massalski,49 however, the shape of the phase bound­aries in the Th-La system is thermodynamically
unlikely (abrupt change in phase boundary between the bcc and fcc phases shown in Figure 8). Also, the recent assessment for the Th-Ce system4 suggests that the separation of the high-temperature bcc-phase region is more likely. These conflicts are due to the difficulty in sample preparation for these systems, and further studies are necessary. The third group consists of the Th-Pr, Th-Nd, Th-Pm, and Th-Sm systems, for which the available data were reported by Moffatt,4 Badayava and Kuznetsova,4 and Norman et a/.46 The low-temperature solid phase structure for these lanthanides is DHCP instead of HCP or fcc. The liquid and bcc phases are completely soluble. The solubility of Pr, Nd, Pm, and Sm in the a-Th phase (fcc) is very large and estimated to be higher than that of the other Group IIIA metals. Figure 9 shows the Th-Pr phase diagram as a typical example quoted from Okamoto.4 The shape of the phase boundaries suggests that these systems can also be explained by the simple regular solution model. The fourth group consists of the Th-Eu and Th-Yb systems. Since Eu and Yb behave as divalent metals, these systems are predicted to be fairly immiscible even for the liquid phase, as shown in Figure 10. The fifth group consists of the Th-Ho, Th-Er, Th-Tm, and Th-Lu systems. Although these lanthanides do not have the bcc allo — trope in the unary system, the wide solid solubility for the bcc phase is seen in these systems. Figure 11 shows the Th-Er phase diagram as a typical example quoted from Okamoto.4 On observing carefully the shape of the liquidus and solidus for the Th-Group IIIa metal system, we can predict a slight positive interaction for the high-temperature solid phase (bcc) in the relation between Th and Sc, Y, or La to Sm; on the other hand, a slight negative interaction between Th and Gd to Ho, and mostly an ideal interaction between Th and Er, Tm, and Lu, can be predicted by assuming that the liquid phase behaves as an ideal solution.

Regarding the U-Group IIIa metal systems, the U-Sc phase diagram is the only exception in which several percent of mutual solid solubility and the com­plete liquid solubility were observed by Holcombe and Chapman50 and Terekhov and Sinyakova.51 Figure 12 shows the U-Sc phase diagram quoted from Okamoto.4 The unique features of the U-Sc system are a miscibility gap for the liquid phase and a steep temperature variation on the phase boundary between the a-Sc and р-Sc phases near the Sc terminal. The latter feature needs further confirmation because it is thermodynamically

Element Th Sc Y La Ce Pr Nd Pma Sm Allotrope a-Th (fcc) a-Sc (HCP) a-Y (HCP) b-La (fcc) g-Ce (fcc) a-Pr (DHCP) a-Nd (DHCP) a-Pm (DHCP) b-Sm (DHCP) b-Th (bcc) b-Sc (bcc) b-Y (bcc) g-La (bcc) S-Ce (bcc) b-Pr (bcc) b-Nd (bcc) b-Pm (bcc) g-Sm (bcc) System — Th-Sc Th-Y Th-La Th-Ce Th-Pr Th-Nd Th-Pm Th-Sm Maximum solid — ~20at.%Th ~30at.%Th Completely Completely ~18at.%Th ~18at.%Th ~18at.%Th ~20at.%Th solubility between in Sc ~60 in Y ~50 soluble soluble in Pr ~80 in Nd ~80 in Pm ~80 in Sm ~60 low-temperature at.% Sc at.%Y in Th at.%Pr at.% Nd at.% Pm at.% Sm phases in Th in Th in Th in Th in Th Maximum solid — Completely Completely Completely Completely Completely Completely Completely Completely solubility between soluble soluble soluble soluble or soluble soluble soluble soluble bcc phases miscibility gap References 41 42 43 44 45 45 44 44 4 46 Element Eu Gd Tb Dy Ho Er Tm Yb Lu Allotrope — a-Gd (HCP) a-Tb (HCP) a-Dy (HCP) a-Ho (HCP) a-Er (HCP) a-Tm (HCP) b-Yb (fcc) a-Lu (HCP) a-Eu (bcc) b-Gd (bcc) b-Tb (bcc) b-Dy (bcc) — — — g-Yb (bcc) — System Th-Eu Th-Gd Th-Tb Th-Dy Th-Ho Th-Er Th-Tm Th-Yb Th-Lu Maximum solid Immiscible ~13at.%Th ~24at.%Th ~10at.%Th ~20at.%Th ~8at.%Th in ~10at.%Th Immiscible ~25at.%Th solubility between in Gd ~60 in Tb in Dy in Ho Er ~55at. in Tm in Lu ~40 at. low-temperature at.% Gd ~60at.% ~57 at.% ~60at.% % ErinTh ~55at.% % LuinTh phases in Th Tb in Th Dy in Th Ho in Th Tm in Th Maximum solid Immiscible Completely Completely Completely ~99at.% Ho ~99at.% Er ~20at.%Th Immiscible ~6at.%Th in solubility between soluble soluble soluble in Th in Th in Tm Lu ~90at.% bcc phases ~63at.% Tm in Th Lu in Th References 43 43 47 43 47 47 44 43 48 43 aPhase relation for the Th-Pm system is estimated from the Th-Pr and Th-Nd systems.

90 100
1600 1541°C
1755 °C
1360 °C
800
600
400
200
0 Eu	30 40 50 60 70 80 90 100 Atomic percent thorium Th
10 20
0 10 20 Sc	100 Th
80	90
Figure 10 Th-Eu phase diagram taken from Okamoto.4
Figure 7 Th-Sc phase diagram taken from Okamoto.4
Weight percent thorium 0 10 20 30 40 50 60 70 80 90 100 Figure 11 Th-Er phase diagram taken from Okamoto.4
Weight percent thorium 0 10 20 30 40 50 60 70 80 90 100 Figure 8 Th-La phase diagram taken from Okamoto.4
Weight percent uranium 0102030 40 50 60 70 80 90
Weight percent thorium 0 10 20 30 40 50 60 70 80 90 100 Figure 9 Th-Pr phase diagram taken from Okamoto.4
100
135 °C
Figure 12 U-Sc phase diagram taken from Okamoto.

unlikely.19’49 In the other U-Group IIIa metal sys­tems, an extremely limited solubility was observed even for the liquid phase. Table 4 summarizes the solubility data for the liquid phase. Generally, it may
be said that the mutual solubility between U and light lanthanides is a little larger than that between U and heavy lanthanides. As for the solid solubility, the solubility of Gd and Ho in a-U was reported to

be <0.08 and <0.2 at.%, respectively.56 Figure 13 shows the U-Ce phase diagram calculated in the present work by assuming a regular solution model for each phase. The interaction parameter is estimated to be 53 kJ mol~ by fitting the mutual solubility data.52 This preliminary estimation is prac­tically useful not only to predict the phase diagrams but also to evaluate the safe performance of metallic nuclear fuels.

As for the Np-Group Ilia metal systems, the phase relation of the Np-La, Np-Nd, and Np-Lu systems was already studied by thermal analysis.57 The melting points or the transformation tempera­tures are depressed by several degrees compared to those of the pure elements. This suggests that there is no intermetallic compound and only a small percentage of mutual solubility in these systems. Figure 14 shows the Np-La phase diagram calcu­lated in the present study by assuming a regular solution model for each phase. When the estimated interaction parameters for the liquid and bcc phases are ^42 and 52kJmol_1, respectively, the depres­sions for the melting points of Np and La, which are of the order of 4 K, and those for the transforma­tion temperature between p-La and g-La, which are of the order of 13 K, are in good agreement with the experimental observations. Considering the system­atic variation, the phase relation between Np and Y, Ce, Pr, Pm, Sm, Gd, Tb, Dy, Ho, Er, or Tm is expected to be similar to that of the Np-La system. Better miscibility is expected for the Np-Sc system from the comparison to the U-Sc system.

500

Table 5 summarizes the solubility data for the Pu-Group Ilia metal systems. The systematic varia­tion in the Pu-related system is mostly similar to that in the Th-related system, with some exceptions. As for the first group, the shape of the previously reported Pu-Sc phase diagram44 is different from that of the Pu-Y, Pu-Gd, Pu-Tb, and Pu-Dy phase diagrams.44,58,62,64 There are several similarities between the Pu-Sc and Th-Sc systems, such as the complete solubility for the liquid and bcc phases, several tens of percent of solubility even for
the low-temperature solid phase, etc. However, an intermediate Z-phase appears in the Pu-Sc phase diagram, which suggests some degree of stabilization for the mixing between Pu and Sc in the low — temperature region. On observing the phase bound­ary between the liquid and bcc phases, on the other hand, we can conclude that these phases obey the simple regular solution model. Due to this conflict, the previously reported Pu-Sc phase diagram could not be modeled with a reasonable set of thermody­namic functions.65 This indicated that some of the phase boundaries need substantial modifications.1 Regarding the P-Y system, the shape of the phase boundary near the Y terminal is different from that for Gd, Tb, and Dy. Avery thin monophase region for p-Y appears and steeply depresses with increasing Pu Concentration in the previously reported Pu-Y phase diagram.58 This feature is thermodynamically unlikely. The phase relations for the Pu-La system are quite similar to those for the Pu-Gd, Pu-Tb, and Pu-Dy systems. Figure 15 shows the Pu-La phase diagram as a typical example for this group, which is quoted from Okamoto.4 The phase diagram was orig­inally reported in Ellinger et al.59 It appears from the phase diagram that there are miscibility gaps for the liquid phase and large regions of solid solubility of Pu in the p-La (fcc) and g-La (bcc) phases (^20 at.% at the maximum). The solid solubility of La in 8-Pu (fcc) is negligibly small, and that in e-Pu (bcc) is estimated to be about 1 at.%. As for the third group, the Pu-Pr, Pu-Nd, Pu-Pm, and Pu-Sm systems have similar features,44,62,61 as well as the Th-related sys­tem. Figure 16 shows the Pu-Nd phase diagram as a typical example, quoted from Okamoto.4 The phase relations shown in the Pu-Nd system are quite similar to those in the Pu-La system, with a few exceptions. Although the crystal structures for the low-temperature solid phase are different from each other, the shape of the phase boundaries around a-La and a-Nd are quite similar. A few per­cent of solid solubility in the 8-Pu phase (fcc) was observed not in the Pu-La system but in the Pu-Nd system, although pure La does take the fcc allo — trope whereas Nd does not. Regarding the heavy lanthanides beyond Ho as well as Y, such as the Pu-Ho, Pu-Er, Pu-Tm, and Pu-Lu systems, the mis­cibility gap for the liquid phase is expected to disap — pear.44,64 Figure 17 shows the Pu-Er phase diagram as a typical example quoted from Okamoto.4 How­ever, the experimental information is limited and confirmation is necessary, for instance, by thermal arrest measurement for the high-temperature region.

Element Pu Sca yb La Ce Pr Nd Pmc Sm Allotrope S-Pu (fcc) a-Sc (HCP) a-Y (HCP) p-La (fcc) g-Ce (fcc) a-Pr (DHCP) a-Nd (DHCP) a-Pm (DHCP) p-Sm (DHCP) £-Pu (bcc) p-Sc (bcc) p-Y (bcc) g-La (bcc) S-Ce (bcc) p-Pr (bcc) p-Nd (bcc) p-Pm (bcc) g-Sm (bcc) System — Pu-Sc Pu-Y Pu-La Pu-Ce Pu-Pr Pu-Nd Pu-Pm Pu-Sm Maximum solid — ~48at.% Pu ~15at.%Pu ~19at.% Pu ~34at.% Pu ~29at.% Pu ~27 at.% Pu ~28at.% Pu ~29at.% Pu solubility between in Sc in Y ~0at. in La ~0at. in Ce in Pr ~2 at. in Nd ~2 at. in Pm ~2 at. in Sm ~2 at. low-temperature ~22at. % Y in Pu % La in Pu ~24at. % Pr in Pu % Nd in Pu % Pm in Pu % Sm in Pu phases % Sc in Th % Ce in Pu Maximum solid — Completely ~17at.%Pu ~20at.%Pu ~18at.% Pu ~30at.% Pu ~33at.% Pu ~35at.% Pu ~33at.% Pu solubility between soluble in Y ~0at. in La ~1 at. in Ce in Pr ~2 at. in Nd ~2 at. in Pm ~2at. in Sm ~2 at. bcc phases % Y in Pu % La in Pu ~15at. % Pr in Pu % Nd in Pu % Pm in Pu % Sm in Pu % Ce in Pu References 44 58 59 60 61 61 44 61 Element Eu Gd Tb Dy Ho Er Tm 62 Yb Lu Allotrope — a-Gd (HCP) a-Tb (HCP) a-Dy (HCP) a-Ho (HCP) a-Er (HCP) a-Tm (HCP) p-Yb (fcc) a-Lu (HCP) a-Eu (bcc) p-Gd (bcc) p-Tb (bcc) p-Dy (bcc) — — — g-Yb (bcc) — System Pu-Eu Th-Gd Th-Tb Th-Dy Th-Ho Th-Er Th-Tm Th-Yb Th-Lu Maximum solid Immiscible ~28at.% Pu ~28at.%Pu ~28at.%Pu ~28at.% Pu ~20at.% Pu ~20at.% Pu Immiscible ~20at.% Pu solubility between in Gd ~0at. in Tb ~0at. in Dy ~0at. in Ho ~0at. in Er ~0at. in Tm ~0at. in Lu ~0 at. low-temperature % Gd in Pu % Tb in Pu % Dy in Pu % Ho in Pu % Er in Pu %Tm in Pu % Lu in Pu phases Maximum solid Immiscible ~30at.% Pu ~30at.%Pu ~30at.%Pu ~0at.% Pu in ~0at.% Pu ~0at.% Pu in Immiscible ~0at.% Pu in solubility between in Gd ~2 at. in Tb ~2at. in Dy ~2 at. Ho ~2 at. in Er ~2at. Tm ~2at. Lu ~2at. bcc phases %Gd in Pu % Tb in Pu % Dy in Pu % Ho in Pu % Er in Pu %Tm in Pu % Lu in Pu References 63 44 44 44 44 44 44 44 44 64 64 62 64 64 64 64 64 aPhase relation for the Pu-Sc system need substantial modification.65 bPhase boundary of p-Y in the Pu-Y system shows thermodynamically unlikely feature. cPhase relation for the Pu-Pm system is estimated from the Pu-Pr and Pu-Nd systems.

Weight percent plutonium 0 10 20 30 40 50 60 70 80 90 100 Figure 15 Pu-La phase diagram taken from Okamoto.4
Figure 18 Calculated Pu-Ce phase diagram, and experimental data taken from Selle and Etter.60 Data from Shirasu, N.; Kurata, M. Private communication.
Weight percent plutonium 60 70
Table 6 Calculated interaction parameters for the Pu-Ce system GO(Pu, liq), GO(Pu, bcc), GO(Pu, fcc): given in Dinsdale67 GO(Ce, liq), GO(Ce, bcc), GO(Ce, fcc): given in Dinsdale67 GO(U, HCP) = 5000 + GO(U, a-U) GO(Zr, b-U), GO(Zr, a-U) = 5000 + GO(Zr, HCP) GO(S-UZr2) = -13394 + 21.484T Gex(Ce-Pu, liq) = xPu(1 — xPu) (15080 + 0.2T-1000 (XCe — Xpu)) Gex(Ce-Pu, bcc) = xPu(1 — xPu) (16483 — 1595(xCe-xPu)) Gex(Ce-Pu, fcc) = xPu(1 — xPu) (10076 + 6.0T + (5745 — 7.744T) (xCe — xPu))
Figure 16 Pu-Nd phase diagram taken from Okamoto.4
Weight percent plutonium 10 20 30 40 50 60 70 80
90	Source: Shirasu, N.; Kurata, M. Private communication. Note: other allotropes for Pu are neglected.
	100
system shows a unique feature near the Pu termi­nal.60 The solid solubility of Ce in 8-Pu (fcc struc­ture) is far larger compared to the other lanthanides and attains 24 at.% at the maximum. Also, the 8-Pu region is enlarged at lower temperature. This feature is observed in the Pu-Al or Pu-Ga system. Although La also has a p-La phase with fcc structure, this stabilization of 8-Pu phase was observed only in the Pu-Ce system among the Pu-lanthanide systems. Figure 18 indicates the Pu-Ce phase diagram calcu­lated by the CALPHAD approach,66 in which only the liquid, bcc, and fcc phases were modeled and other phases were omitted. Experimental data points, however, fit reasonably well with the calculated values when using the assessed interaction para­meters indicated in Table 6.
In the Pu-Eu and Pu-Yb systems, the mutual solu­bility is considered to be very low. The solid solubility of Pu in Eu was reported to be 0.74 at.%, and vice versa <0.02 at.%.63 The phase relation in the Pu-Ce

Regarding the Am-Group Ilia metal systems, wide regions for the solid solubility were expected from the experimental observation by Kurata,68 in which the phase relation of an annealed alloy con­taining U, Pu, Zr, Np, Am, Y, Ce, Nd, and Gd was studied by scanning electron microscopy/wavelength dispersive X-ray (SEM/WDX). Two and three phases were observed in the samples annealed at 973 and 773 K, respectively. These phases were identified: (1) A bcc phase rich in U, Pu, Zr, and Np and (2) a rare-earth phase rich in Pu, Am, Y, Ce, Nd, and Gd were detected in the samples annealed at 973 K; and (3) the Z — and (4) 8-phases rich in U, Pu, Zr, and Np and (5) the rare-earth phase rich in Pu, Am, Y, Ce, Nd, and Gd were detected in the samples annealed at 773 K. The Pu and Am con­centrations in those rare-earth phases were roughly 8 and 30 at.%, respectively, at both temperatures. The Pu concentration agrees reasonably well with the phase diagrams described earlier. Perhaps the Cm- related system systematically has features similar to those of Am.