Corrosion resistance in zirconium alloys is intimately related to the presence of second phase particles (SPPs) formed in the zirconium matrix by delib­erate additions of alloying elements. The precipitates are usually incoherent crystalline intermetallic compounds, meaning that their physical structure is unrelated to the Zr matrix in which they are imbedded. In as-fabricated Zircaloy-4 the most common SPP is Zr(Fe, Cr)2, while in Zircaloy-2 they are Zr(Fe, Cr)2 and Zr2(Fe, Ni). For the ZrNb type alloys the most common is pNb (which is not an intermetallic) and for the ZrSnNbFe alloy types are Zr(Nb, Fe)2 and PNb. Table 4.6 gives a more complete description, also indi­cating some neutron irradiation effects.

At normal LWR temperatures (270-370°C, 543-643K) the SPPs change under irradiation in a combination of two ways — amorphization and dissolution.

Amorphization means that the original SPP crystalline structure is con­verted to an amorphous structure. Amorphization is a complex process, described in some detail by Griffiths et al. (1987); Yang (1989);Motta (1997); Bajaj et al. (2002); and Taylor et al. (1999). It occurs when an intermetallic compound accumulates enough irradiation-induced defects to cause it to thermodynamically favour an amorphous rather than a crystalline structure. The rate of amorphization depends on the relative rates of damage creation and damage annealing in the SPP; therefore important parameters are neu­tron flux, irradiation temperature and SPP chemistry. A critical tempera­ture exists above which the annealing processes are fast enough to prevent the damage accumulation of defects needed for transformation. For typical reactor irradiations amorphization of both Zr(FeCr)2 and Zr2(Fe, Ni) occurs readily at temperatures near 100°C (373K) (although Fe is not related from the SPPs into the Zr matrix, as discussed later). At typical (LWR) tem­peratures (300°C, 573K) and neutron flux, Zr(Fe, Cr)2 becomes amorphous but Zr2(Fe, Ni) does not. Above about 330°C (603k) neither SPP becomes amorphous.

The amorphization process begins at the outside surface of the SPP and works its way inward with increasing fluence. This is illustrated in Fig. 4.11 (Etoh & Shimada, 1993) where the SPP on the left has an amorphous rim (dark area) and the one on the right, at higher fluence, is fully amorphous. There appears to be an incubation period prior to amorphization initiation, with the incubation fluence decreasing with temperature in the range 270­330 ° C (543-603K).

Reference Material As- fabricated Moderate burnup <330°C >330°C High burnup <330°C >330°C Adamson, 2000; Zircaloy-2 or Zircaloy-4 Zr( Fe, Cr)2 PA X A X Griffiths et al., 1996 PD PD D D Garzarolli, eta!., 1996(a); Zr2(Fe, Si) X X X X Adamson & Rudling, 2002 PD PD D D Garzarolli etal, 2002(a) Zr3Fe X X X X s PD Griffiths et ai, 1996; Zircaloy-2 Zr2(Fe, Ni) X X X X Etoh &Shimada, 1993 PD PD D D Zr-1 Nb Shishov et a!., 1996 E110 (3N b X Bossis et al., 2002 M5 (3N b X Gilbon et al., 2000 IE PN b Doriot etal., 2004/2005 M5 (RX) (3Nb Zr(Fe, Nb)2 X, PD, IE X, PD Shishov et al., 2007 E110(RX)(0.1 Fe) |3Nb Zr(Fe, Nb)2 X, PD, IE X, D E110(RX) (0.01 Fe) PNb X, PD ,IE Zr-2.5Nb Urbanic & Griffiths, 2000 (27% CW) o(in p phase) X, PD IE PNb (in a phase) Averin et al., 2000; E125 (RX, PRX) PNb X, PD IE PNb X, PD IE PNb Shishov et al., 1996 ZrSnNbFe Nystrand & Bergquist, 1997; ZIRconium Low Oxidation (ZIRLO) ZrFeNb PNb X, PD, PNb X Comstock et al., 1996 (Stress Relieved (SR)) Averin et al., 2000; E635 (RX, PRX) Zr(Nb, Fe)2 — PA, PD X, PD IE PNb A, D X, D IE PNb Shishov et al., 1996 Nikulina et al., 1996 (Zr, Nb)2Fe X, PD, IEZr3-4Fe X X X, IEZr3-4Fe Shishov et al., 2002 E635,RX(0.15Fe) Zr(Nb, Fe)2 PNb X, PD, PNb D, lEpNb Shishov et al., 2007 E635(RX) Zr( Fe, Nb)2 D, IE

Notes: All are crystalline (X) as-fabricated.

A — amorphous; D — dissolved; P — partially…; X — crystalline; S — stable; IE — irradiation-enhanced precipitation Source: A. N.T. International (2011).

On/m2	7x1025n/m2	1.2x1026n/m2
Amorphous state ‘%
Crystalline state
Amorphous state — шгтк. ‘M
Crystalline state
200 nm

4.10 The fluence dependence of the amorphous transformation of Zr(Fe, Cr)2 precipitate in recrystallized annealed (RXA) Zircaloy-2, neutron irradiated at 288°C (561K). Diffraction patterns indicate stages of the transformation (Etoh & Shimada, 1993).

Amorphization rate increases as temperature decreases, as neutron flux increases and as SPP size decreases. Literature evaluation therefore needs to consider reactor and material conditions of specific interest.

The fluence required to produce complete amorphization depends on neutron flux, temperature and SPP size, but for typical Zr(Fe, Cr)2 SPPs of initial size near 0.1 pm and the entire SPP is amorphous by the end of bundle life burnups <50 MWd/KgU (1 x 1022 n/cm2, E > 1 MeV). Interestingly, under well controlled conditions of flux and temperature, the amorphization rate of Zr(Fe, Cr)2 in Zircaloys can be used to estimate the neutron fluence (Motta & Lemaignan, 1992; Taylor et al, 1999; Bajaj et al, 2002).

For the Zr-Nb type alloys neither the pNb nor Zr(Nb, Fe)2 SPPs become amorphous for irradiation temperature >330°C (603K). However, at 60°C (333K) Zr(Nb, Fe)2 does become amorphous at high fluences.

SPP amorphization in itself does not appear to affect material behaviour; however, dissolution of both amorphous and crystalline SPPs does influence corrosion, growth and mechanical properties, to be discussed later. At typi­cal LWR operating temperatures, SPP dissolution occurs relentlessly until the SPP essentially disappears.

As SPPs dissolve, the zirconium matrix becomes enriched (well beyond the normal solubility limit) in the dissolving element. For instance in Zircaloy-2, Fe leaves both Zr(Fe, Cr)2 and Zr2(Fe, Ni) SPPs as schemat­ically illustrated in Fig. 4.12 (Mahmood et al., 2000). This process is given in more detail by Takagawa et al. (2004) and in Fig. 4.13 . Here it is seen

4.11 Evolution of a Zr-Fe-Cr particle under BWR irradiation. Upper figures: TEM micrographs; middle diagrams: schematic illustration of amorphization; lower figures: schematic illustrations of the chemical compositions. (Source: Reprinted, with permission, from Takagawa et al. (2004), copyright AsTm International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.)

that Fe rapidly diffuses from the amorphous rim into the matrix, while Cr diffusion is sluggish. At high fluence (~1 x 1022 n/m2, E > 1 MeV) complete amorphization and Fe-depletion has occurred, while the Cr level is still high. Only at very high fluence (~1.5 x 1022 n/m2, E > 1 MeV) is the Cr dispersed into the matrix, and the SPP essentially disappears.

The rate of dissolution depends on the SPP size (higher rate for smaller sizes), and the extent of dissolution depends on size and fluence. It has been demonstrated in a BWR that small (<.04 pm) SPPs can completely dissolve at low to moderate burnups, (Huang et al, 1996). Also in a PWR, but at temper­ature near 290°C, SPPs with an average size of 0.2 pm were >80% dissolved at moderate burnup (1 x 1026 n/m2, E > 1 MeV) (Garzarolli et al., 2002).

Modelling of the dissolution process gives insight into the alloying con­centration of the matrix (Mahmood et al, 1997). Figure 4.14 illustrates the model for release of solute into the matrix for various size SPPs. For the small SPPs (1R, 2R, 3R) all the Fe is released by moderate burnup.

For the channel material with very large (0.6 pm) SPPs only a small amount of Fe would be released even at high burnups. However, modern materials have SPPs with an average size < 0.3 pm.

In another study, experimental measurement of Fe released from Zr (Fe, Nb) 2 SPP in an E635 alloy containing 0.35% Fe during irradiation at 330-350°C is shown in Fig. 4.15 (Shishov et al, 2002). (In Fig. 4.15, fluence has been converted from E > 0.1 MeV to E > 1.0 MeV by dividing by 4.) Here it is seen that the Fe has diffused from the SPP to the alpha Zr matrix such that all of the Fe is in the matrix by moderate burnup. Extending to high burnup (2 x 1026 n/m2) in this case may only increase the probability of re-precipitation of Fe in the matrix. It should be noted that the ‘normal’ solubility of Fe in unirradiated Zr is <0.02 wt%.

Table 4.6 outlines changes in SPPs to be expected at moderate (50 MWd/ kgU) to high (100 MWd/kgU) burnup for various alloys now in use. To illus­trate interpretation of the table, consider the as-fabricated crystalline (X), Zr(Fe, Cr) 2 SPP for Zircaloy-2 or -4.

For moderate burnup at <330°C, the SPP would become partially amor­phous (PA) and partially dissolved (PD), depending on its initial size. At >330°C it would remain crystalline (X) and become PD, the extent of which would depend on initial size. At high burnup for <330°C it would very likely become totally amorphous (A) and could completely dissolve (D), depend­ing on its initial size. At >330°C it would remain crystalline (X), although it would become strongly fragmented as it eventually totally dissolved (D).

For the ZrNb and ZrSnNb alloys the most common SPPs are pNb and the (Laves phase) (L) Zr(Nb, Fe)2. Also observed is the T-phase (Zr, Nb)2Fe. Details of the SPPs present in the Nb-Fe corner of the phase diagram are presented in Fig. 4.16 (Shishov et al., 2007). Also, a simplified diagram is pre­sented in Fig. 4.17 (Garzarolli in Nikulina et al, 2006). Such phase diagrams

4.12

Modelling predictions for solute release to the matrix as a function of fluence for Zircaloy-2 (SPP size: 1R = 0.026 pm; 2R = 0.042 pm;

3R = 0.056 pm and Zircaloy-4 channel= 0.6 pm) irradiated near 300°C:

(a) Fe, (b) Cr and (c) Ni (Mahmood et al., 1997). Copyright 1997 by the American Nuclear Society, La Grange Park, Illinois.

are only approximate, and may vary because the heat treatments used may not achieve equilibrium conditions. Figure 4.16 points out that there is a region, not reported in Fig. 4.17 , where only the Laves phase exists. This tends to be for 1%Nb and 0.2-0.4 Fe; for example in the ‘normal’ E635 alloy Zr-1.2Sn-1.0Nb-0.4Fe.

None of the SPPs become amorphous at normal reactor temperatures, but at high burnup at 60°C (333K) the Laves phase does become at least partially amorphous. However, all of the SPPs undergo irradiation-induced dissolution. The PNb SPP loses Nb to the matrix, but the excess Nb then re-precipitates as a very fine PNb. The Laves phase Zr(Fe, Nb)2 transforms to a fine PNb SPP, with essentially all Fe ending up in the matrix (see Fig. 4.15). Behaviour of the T-phase (Zr, Nb)2Fe is more complicated, with Fe diffusing

4.16 Zr-Nb-Fe ternary alloy phase diagram, zirconium corner at 580°C (853K), non-equilibrium conditions. (Source: Reprinted, with permission, from Shishov et al. (2005), copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.)

to the matrix, and Nb and Sn concentrating in the outer shell of the SPP. The core remains a T-phase. Details can be obtained in the following references: Shishov et al. (2002, 2007) and Shishov (2011) and Doriot et al. (2004).