It is rather surprising that although the most stable loops are the prismatic loops, basal loops are also observed in zirconium alloys. Moreover, these loops are of the vacancy character. According to the usual rate theory, vacancy loops should not grow as a result of the bias of edge dislocation toward SIAs.

Figure 7 High density of c-component loops in the vicinity of the precipitates in a Zy-4 sample irradiated to 6 x 1025 n m~2; at 585 K. The arrow shows the diffracting vector [0002]. Adapted from De Carlan, Y.; Regnard, C.;

Griffiths, M.; Gilbon, D. Influence of iron in the nucleation of (c) component dislocation loops in irradiated zircaloy-4.

In Eleventh International Symposium on Zirconium in the Nuclear Industry, 1996; Bradley, E. R., Sabol, G. P., Eds.; pp 638-653, ASTM STP 1295.

The reason for the nucleation and growth of the (c) component loops in zirconium alloys has been ana­lyzed and discussed in great detail by Griffiths and co-workers.46,56,57,74 The most likely explanation for their appearance46 is that they nucleate in collision cascades, as shown recently by De Diego.66 Their stability is dependent to a large extent on the pres­ence of solute elements, which probably lower the stacking-fault energy of the Zr lattice, making the basal (c) component loops more energetically stable. It is also possible that small impurity clusters, espe­cially iron in the form of small basal platelets, could act as nucleation sites for these loops.74,76 However, according to Griffiths,46 this cannot account for the very large vacancy (c) component loops observed, since the growth of vacancy loops is not favorable considering the EID discussed previously. In order to understand the reason for the important growth of the (c) component loops, another mechanism must occur. As discussed by Woo,44 the growth of (c) component loops is well understood in the frame of the DAD model. Indeed, because ofthe higher mobil­ity of SIAs in the basal plane rather than along the (c) axis (and the isotropic diffusion of vacancies), dislo­cations parallel to the (c) axis will absorb a net flux of SIAs whereas dislocations in the basal plane will absorb a net flux of vacancies. This can therefore explain why the basal vacancy loops can grow. The incubation period before the appearance of (c) component loops can be explained, according to Griffiths eta/.,73 by the fact that the (c) loop formation is dependent on the volume of the matrix containing a critical interstitial solute concentration. This volume increases as the interstitial impurity concentration is gradually supplemented by the radiation-induced dis­solution of elements such as iron from intermetallic precipitates (or р-phase in the case of Zr-Nb alloys).

4.01.1.3.2 Void formation

Early studies failed to show any cavity in Zr alloys after irradiation.77 From all the obtained data, it is seen that zirconium is extremely resistant to void formation during neutron irradiation (Figure 8).46,52 The effect of very low production of helium by (n, a) reactions during irradiation was mentioned as a possible reason for this absence of voids. But most probably, the fact that in zirconium alloys vacancy type loops are easily formed can be the reason for the absence of void.52 To favor the formation of voids, various studies per­formed, especially on model alloys, have shown that stabilization of voids can occur when impurities are present in the metal. Helium coming from transmu­tation of boron on Zr sponge67 as well as impurities located near Fe-enriched intermetallics are found to favor the stability of voids.54 Irradiations with elec­trons give better conditions to stabilize voids: the main reason is that irradiation doses can be very high — hundreds of displacements per atom can be reached after few hours.19 Moreover, electron irradi­ation on Zr samples preimplanted with He at various concentrations showed the nucleation and growth of voids only for the samples doped with at least 100 ppm of He.78