Self-diffusion of U and Pu in (U, Pu)C and (U, Pu)N

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Self-diffusion of U and Pu in (U, Pu)C and (U, Pu)N

Hj. Matzke

To cite this version:

Hj. Matzke. Self-diffusion of U and Pu in (U, Pu)C and (U, Pu)N. Journal de Physique Colloques,

1979, 40 (C4), pp.C4-24-C4-25. �10.1051/jphyscol:1979407�. �jpa-00218803�

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Colloque C4, supplkment au no 4, Tome 40, avril 1979, page C4-24

Self-diffusion of U and Pu in (U, Pu)C and (U, Pu)N

Hj. Matzke

Commission of the European Communities, Joint Research Centre, European Institute for Transuranium Elements, Postfach 2266, D-7500 Karlsruhe 1, F.R.G.

R6sum6. - Des mesures trbs compl&tes sur la diffusion d'U et de Pu dans UC, (U, Pu)C, UCN, UN et (U, Pu)N ont 6tC faites. En enlevant C de MC, les taux de diffusion du metal augmentent, indiquant que les atomes de carbone en donnant des Clectrons renforcent la liaison mBtal-mttal. Pour des nitrures, au contraire, la rtduction du contenu de I'azote diminue les taux de diffusion du mCtal, indiquant aussi des conditions diffkrentes de liaison dans les nitrures.

Abstract. - Very extensive measurements on U and Pu diffusion in UC, (U, Pu)C, UCN, UN and (U, Pu)N were performed. Removing C from MC increased metal diffusion rates indicating that the carbon atoms strengthen the metal-metal bond by donating electrons. The reverse is true for nitrides where removing nitrogen from the lattice decreased metal diffusion rates indicating different bonding conditions in nitrides.

1. Introduction.

-

A detailed knowledge of the self-diffusion of the metal atoms U and Pu as the slower moving species in MX compounds (X = C or N, M = U

+

Pu) is important for understanding and predicting high temperature kinetic processes such as grain growth, sintering, pore mobility and swell- ing during reactor operation of these advanced LMFBR fuel candidates. Since these diffusion processes are likely to proceed via a vacancy mecha- nism in the metal sublattice, additional direct infor- mation can be obtained on the bonding conditions of these materials. The activation enthalpy of diffusion, AH, consists of two terms, one for vacancy formation, AH,, and one for vacancy migration, AH,. For vacancy formation, all bonds must be broken, and for vacancy migration, the moving atom must pass through the saddle point in the potential, a process which again depends on bond strength.

Because of these reasons extensive measurements were performed on the diffusion of U and Pu in UC,,,, (U, PuIC,,,, UCN, (U, Pu)CN, U N and (U, Pu)N with different nitrogen contents.

2. Experimental.

-

UC,,, compounds of different C/U-ratios were used as either single crystals or as arc-cast pellets, whereas UCN and the Pu- containing materials (Uo,,Puo,~Cl,,, (Uo.8Puo.2)CN and (U,,Pu,,)N were used as sinters. As starting condition, a thin tracer layer of U-233 or Pu-238 was chosen to study metal diffusion. The non- destructive method of a -energy degradation [I, 21 was employed to obtain tracer concentration profiles following diffusion since with this method, both temperature and time dependence of diffusion processes can easily be followed. In this way, any perturbing phenomena like surface effects or contri- butions of grain boundaries to the overall tracer

penetration can easily be observed and corrected for [3, 41. The volume diffusion coefficients obtained in this way are thus reliable.

Annealing of MC and MCN compounds was done in vacuum, whereas MN-compounds were annealed in nitrogen. T o vary C/M-ratios, different specimens with pre-selected C/M-ratio were used. To vary N/M-ratios, specimens from the same batch were pre-annealed at different nitrogen pressures in the range of 1 to 1 000 torr with subsequent chemical analysis to check the N/M-ratio established during the pre-anneal. Diffusion anneals were subsequently performed at the same nitrogen pressure.

3. Results.

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The diffusion coefficients of U and Pu could be shown to depend strongly on metallic impurity content [5] and added fission products [6]

both of which increased diffusion rates. Diffusion rates were also increased at lower temperatures by fission events (radiation enhanced diffusion [7]).

The most interesting parameter, however, was the

t yp~cal -scatter in

N I M determ

C I U -ratlo N I M -ratio

Fig. 1. -Dependence of Du in UC,,, and D, in (U, PU)NI-, on non-metal to metal ratio at constant temperatures.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979407

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SELF-DIFFUSION OF U AND Pu IN (U, Pu)C AND (U, Pu)N C4-25

aconstan t pressure

o ~ U , P u l C 1 ~ ,

Fig. 2. -Dependence of activation enthalpy, A H , for U- diffusion in UCl,, and for Pu-diffusion in (U, Pu)NI-,, on non- metal to metal ratio, measured at temperatures high enough to ensure the existence of a single phase of the NaC1-type.

ratio of non-metal (X = C and/or N) to metal (M = U and/or Pu) atoms. Figure 1 shows the results for UC,,, and for (U, Pu)N. Whereas D, in UC,,, increases drastically with decreasing non -metal atom content, the reverse behaviour is observed for Dm in MN : diffusion rates decrease with decreasing non-metal atom content. Correspondingly, figure 2 shows that the activation enthalpy of diffusion, A H , increases with increasing C-content in UC,,, whereas it decreases in nitrides. To prepare figure 2, literature data were included [8-lo]. Besides this reverse trend in MC and MN, absolute diffusion rates are lower in MN than in MC, with a change in rates being indicated at a N/(C

+

N)-ratio of about 0.5 to 0.8 (see Fig. 3). The data included in figure 3 for MN are dependent on nitrogen partial pressure [I 1-13].

The conclusions of the present data for the ques- tion of bonding in MX-type compounds are :

Fig. 3. -Dependence of carbon and uranium diffusion in UCI-,N, and of plutonium diffusion in (U, Pu)C,-,N, on nitrogen/carbon ratio for T = 1 700 "C.

i) bonding conditions in carbides are different from those in nitrides, ii) a change occurs at about 50 % N-content in MCN, iii) the non-metal atoms contri- bute essentially to the metal-metal bond. For UC,,,, the carbon atoms are suggested to donate electrons to the metal-metal bond thus increasing the binding energy (see also [l4]). Band structure calculations for off-stoichiometric MC, MN and MCN are still missing, but published results for e.g. ThC [I51 as well as calculations for UN [16] indicate charge transfer between metal and metalloid atoms and are thus in agreement with the above statements.

Thanks are due to Drs. M. H. Bradbury and H. Matsui (now University of Nagoya, Japan) for providing data for MN and MCN.

References

[I] SCHMITZ, F. and LINDNER, R., J. Nucl. Mater. 17 (1965) 259.

[2] HOH, A. and MATZKE, Hj., Nucl. Instrum. Methods 114 (1974) 459.

[3] MATZKE, Hj., in Plutonium and Other Actinides 1975 (North Holland Publ. Co.) 1976, p. 801.

[4] MATZKE, Hj., J. Physique Colloq. 37 (1976) C7-452.

[5] ROUTBORT, J. L. and MATZKE, Hj., J. Nucl. Mater. 54 (1974) 1.

161 BRADBURY, M. H. and MATZKE, Hj., J. Nucl. Mater., to be submitted.

[7] H ~ H , A. and MATZKE, Hj., J. Nucl. Mater. 48 (1973) 157.

181 HIRSCH, H. J. and SCHERFF, H. L., J. Nucl. Mater. 45 (1973) 123.

[9] SARIAN, S. and DALTON, J. T., J. Nucl. Mater. 48 (1973) 351.

[lo] NITZKI, V. PhD Thesis, University of Karlsruhe (1975).

[11] BRADBURY, M. H. and MATZKE, Hj., Euratom Report EUR- 5905E (1978).

[12] ROUTBORT, J. L., personal communication (1974).

[I31 HOLT, J. B. and ALMASSY, M. T., J. Am. Ceram. Soc. 52 (1969) 631.

1141 WEBER, W., Phys. Rev. B 8 (1973) 5082.

[I51 IMOTO, S., ADACHI, H. and HORI, T., J. Nucl. Sci. Technol.

12 (1975) 711.

[16] IMOTO, S., personal communication (1977).