Melting Temperature and Evaporation

Materials for NREs should have, along with high melting points, low evaporation rates and should weakly interact with hydrogen. Changes in the material composition caused by evaporation or interaction with hydrogen should not remove the compo­sition from the homogeneity region during a specified operation time resource.

Melting temperature of carbides in the homogeneity range does not change monotonously. Typically, the maximum melting temperature is demonstrated by the non-stoichiometric phases with composition close to C/Me ratio of 0.8. The melting temperature of solid monocarbide solutions is expected to change similarly in the

Compound formula

Structure

typea

Density (g-cm 3)

Melting points (TmK)

Linear expansion coefficient (10-6 K-

Heat conduction ‘) (Wm-1)

Elastic

(GPa)

modulus Vickers hardness (GPa)

Fuel materials

UC

C

12.9

2,500

10.4

19

220

9.0

UN

C

14.4

3,074

9.3

18

265

8

ZrC + 5%UC

C

6.9

3,380

11.8

30

380

25

ZrC + 5%UC + C

C

6.6

3,250

11

32

350

20

ZrC + 5%UC + Nbc

C

7.6

3,520

11

22

320

28

Construction

materials

ZrC

C

6.73

3,690

8.6

30

390

27

ZrC + 5% C

C

6.5

3,180

5.5

52

230

18

NbC

C

7.8

3,870

7.7

20-30

500

20

ZrC+50% NbC

C

7.3

3,620

5.9

25

470

28

ZrH19 є-phase

T

5.6

2,470b

7.0

30

69

0.16

Pyrographite

H

1.7

4,000c

8.5

70

48

0.1

Table 4.1 Averaged physical characteristics of reactor core materials in the temperature range from 300 to 700 K [14]

aC cubic structure; T tetragonal structure, H hexagonal structure bValue for the hydrogen pressure 100MPa cSublimation temperature

4.1 Thermodynamic and Structural Characteristics of Materials 31

homogeneity range [12]. The melting temperatures of fuel compositions based on ZrC, NbC, and ZrN decrease as the UC content increases (Figs. 4.1 and 4.2). The measured absolute values of melting temperatures for nuclear fuel rods are close to solidus line for the ZrC-UC and ZrC-NbC-UC solid solutions and average around 3,570 and 3,520 K, respectively, being significantly higher than carbon nitride melting temperature. Eutectic compounds from the typically used range of carbide-carbon compounds have the following melting temperatures: ZrC + C: 3,180; NbC + C: 3,580; TaC + C: 3,715K; the said melting temperatures being lower than that of pure carbides. Evaporation processes have a considerable effect on the performance of the HGA parts and on the behavior of these materials at high temperatures under processes involving mass transfer through vapor phase. In considering evaporation rates of alloys and compounds one should take into account not only the integrated evaporation rates, but also the partial evaporation rates. This is particularly impor­tant for such compounds as carbides, nitrides, etc., that demonstrate broad scatter of partial component evaporation rates [13]. The methods for determining the par­tial thermodynamic functions of two — and three-component interstitial phases, based on statistic and thermodynamic approach are developed in [2]. These methods can determine changes of gas pressure in the homogeneity range of congruently evaporat­ing phase compositions (i. e. the compositions which remain practically unchanged during evaporation) (Table4.2).

Table 4.2 Superficial V and linear V1 speeds of evaporation of carbides

Material

V (g/sm2s) 2,500K

3,100K

V1 (sm/s) 2,500 K

3,100K

ZrC0.85

1.21 • 10-6

8.37-10-4

1.824-10-7

1.257-10-4

NbCo.77

2.99-10-9

5.03-10-6

3.83-10-10

6.5-10-7

TaC0.5

6.15T0-9

9.83-10-6

4.16-10-10

6.64-10-7

UC-ZrC

3-10-6(2,270K)

1.9-10-4

UC-UN

8.2-10-7(2,800K)

1.8-10-4

Predicted values for NbC are in good agreement with the experimental data; for ZrC one has to take into account the concentration dependence of the vacancy formation energy [2, 13].

As uranium has higher thermodynamic activity in the U1-y MeyC solid carbide solutions, compared to that of Zr, Nb, or Ta, and as there are no congruently evapo­rating compositions in ternary systems, consequently, the uranium loss is expected to prevail in evaporating from an open surface, with corresponding surface enrichment in Zr, Nb, or Ta atoms. Simultaneously, the condensate deposited on a cold wall should have higher U/Me ratio than the one in the initial carbide.

Noncongruent nature of U1-y MeyC solid solution evaporation makes rather prob­lematic their prolonged use at high temperatures in vacuum. Special protection mea­sures are required. In vacuum metallic component of NbCx evaporates much slower than that of zirconium carbide and, despite larger cross-section values of thermal neutrons absorption in Nb (as compared to Zr), NbC may be effectively used in carbide-graphite compositions, in binary or ternary carbide solutions, and most par­ticularly, as protective coatings on graphite parts.

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