HOURS 30 HOURS 70 SAMPLE

^iigS^Ftr ^rSW'pT'1

:-1

2 3

9.9 10.51 9.62 10.17 10.78 10.7 10.56 10.8

10.26 10.34 10.37

10.4

10.53

These are precisely the pores that shrink and contribute to density decrease together with the annihilating smaller pores. The homogeneity of this sample has to do with his lower diameter and consequently his facility to be reduced faster.

Either solarization will not take place or its value will be small in so far as porosity may be eliminated before big enough intergranular pores are formed. The annealing profile should be slow enough so as to prevent the formation of sufficiently big intergranular pores.

Conversely, steep profiles contribute to fast grain growth, thus sweeping out small porosity and giving rise to intergranular porosity. Kingery and Francois [8] observed this phenomenon in UO² using a steep slope annealing profile up to 1700°C.

The same type of porosity may be seen in sintering under oxidative atmosphere (CO²) after one hour at 1500°C (Fig.4 ).

Differences in porosity between the central zone and the border may be due to:

• overstoichiometry in the powders (samples 1 and 2) , and the presence of U³O^g as additive (sample 1), both involving a gradual reduction from the external part of the pellet towards the center. This entails a pseudo-oxidative sintering at the center (let us recall that this type of process involves a dramatic increase of the U self-diffusion coefficient [4, 9] and, as a consequence, the growth of the grain), thus yielding the beginning of the big intergranular pores.

• differences in green density due to pressure gradients within the pellet.

If differences between zone densities increase, pellets may even break down (Fig. 5) when the first plateau in the sintering (Fig. 1) is not enough to homogenize the stoichiometry of the pellet. Rupture is a consequence of differences in density: from the external, denser zone and the inner zone, less dense because of the porosity derived from solarization.

Premature growing of intergranular pores, as in sample 1 , my be due also to the use of powders ex-AUC with excessive quantity of intraparticle pores, of the order of 0.1 micrometers.

FIG. 4. Porosity of a sintered pellet annealed in CO² atmosphere 1 h at 1500 °C magnification x770

FIG. 5.

46X

Broken pellet due to differences in density between central and border zone

3. Solarization in oxidative sintering

Oxidative sintering (at low temperature, i.e 1100 to 1300°C) properly carried out, with the first plateau acting as normalizer of the intergranular porosity, yields pellets with normal porosity. In order to bring pellets to stoichiometric values, annealing in H² atmosphere is

needed. Although solarization is also observed, in this case, (Fig.6) decrease in density should be attributed to a lower lattice parameter after oxidative annealing than after reductive annealing [10]. That is why the annealing earned out at lower ppO² involves less decrease in density.

A Density/density = Am/m - 3Aa/a

Am/m - 3Aa/a * -0.47 % - 3 x 0.15 % « - 0.9%

The porcentual variation of density is calculated for a stoichiometry of 2.08 , where Am/m is the fractional variation of mass in the unit cell due to the change in stoichiometry and Aa/a is the change in lattice parameter due to the change in stoichiometry. This figures gives a variation of the order of magnitude as observed.

S 10.66

§10.64 •

-1150 1300°C 1150 1300°C

FIG. 6. Density after oxidarive and reduced anneal: 1st. oxidative CO/CO2 = 1/1000, reduced at 1150°C, 2nd oxidarive CO/CO, = 7/700, reduced at 1300°C.

CONCLUSIONS

Solarization in UO2 pellets is a consequence of the growth of coarse pores in grain boundaries. The growth of such coarse pores occurs in the third stage of sintering and is a general result of the tendency to equilibrium of intergranular pores. Incomplete reduction of the pellet at the beginning of sintering and the consequent activation of cation diffusion yields a fast grain growth that sweeps out the smaller porosity into coarser intergranular pores in the central zone of the pellet at lower temperatures. Such grain growth may also occur in a reduced pellet through a fast arrival to a high enough sintering temperature. In order to prevent excessive solarization, slower slopes and not too high sintering temperatures are recommended.

Differences in density between the center of the pellet and the periphery, due to solarization, may produce its rupture.

In oxidative sintering, after the reducing annealing, solarization occurs due to the change in the lattice parameter.

ACKNOWLEDGEMENTS

To A. Benitez for the ceramographies and useful discussions and G. Them for the termogravimetries.

REFERENCES

[1] AMATO, R. COLOMBO, A.M. PROTTI, On a case of solarization during steam sintering of UO² pellets. JMN,8, No. 2 (1963) 271-272.

[2] AMATO, R. COLOMBO,A.M. PROTTI, Pore growth during solarization of sintered UO2. JMN 13, No.2 (1964), 265-267

[3] AMATO, R. COLOMBO, The influence of organic additions on the solarization and grain growth of sintered UO2. JMN 11,3 (1964) 348-351

[4] ASSMANN,H. DORR W. AND PEEKS. N. JMN 140(1986)1-6. Control of UO2 microstructure by oxidative sintering.

[5] LAVAGNINO, CARLOS, Sistema para Analisis cuantitativo de imagenes. Asociacidn Arg. deTecnologfa Nuclear.Cordoba 1995.

[6] KINGERY,W.D., BOWEN H.K., y UHLMANN D.R., Introd. to Ceramics. John Wiley and Sons(1976)

[7] NICOLAESCU I.V. and GLODEANU F.. Pore Geometry and interfacial energy.

JMN 113 (1983)253-255

[8] KINGERY W.D. and FRANCOIS B., Grain growth in porous compacts. J. of The Am. Cer. Soc. Vol 48 No. 10 546-547

[9] MATZKE HJ.,On Uranium self-diffusion in UO2 and UO2+X. JMN 30 (1969) 26-35 [10] Uranium dioxide. Ed. J. Belle. 1962.Naval Reactor Dept.

DISCUSSION

(Questions are given in italics)

The presentation illustrates the necessity to properly handle the results of the thermal resinter test specified as QC in the fabrication of fuel. It is not sufficient to specify a maximum densification limit: a densification (increase in volume of the pellet) limit must also be considered, as well as a modification of the O/U ratio during the test.

I agree that a scale test would be necessary. Under hydrogen atmosphere, measuring of O/M will not be necessary. The oxidative sintering, yes.

GRAIN SIZE DISTRIBUTION IN SEEDED XA9847844