We proceed with sections describing fundamental metallurgical properties and phenonema which ultimately affect core component behavior.

4.3.1 Basic irradiation damage

In structural materials like Zircaloy, the overwhelming majority of defects are caused by neutrons, and the most important type of defect is the dislo­cation loop. Two types of loops predominate: <a> and <c> loops. The <a> loop lies on a prism plane and has a Burgers vector in the <a> direction of the HCP lattice. Table 4.4 lists some important characteristics. Both vacancy and interstitial loops exist, but more than half have vacancy character. They are very small (100 nm ‘black spots’) and are difficult to analyze even with the transmission electron microscopy (TEM) (see Fig. 4.7).

Table 4.3 Commercial Zr base materials currently used for zirconium alloy fuel components in PWRs, BWRs, VVERs and RBMKs (Cox et a!., 2006) Alloy Sn % Nb % Fe % Cr % Ni % 0 % Fuel vendor BWRs Zircaloy-2 (SRAa/(RXAb) 1.2-1.7 — 0.07-0.2 0.05-0.15 0.03-0.08 0.1-0.14 All fuel vendors Zr-Linerb Sponge — — 0.015-0.06 — — 0.05-0.1 Only used in Japan and Russia ZrSn 0.25 — 0.03-0.06 — — 0.05-0.1 W ZrFe — — 0.4 — — 0.05-0.1 AREVA ZrFe — — 0.10 — — 0.05-0.1 GNFC PWRs Zircaloy-4 (SRA) 1.2-1.7 — 0.18-0.24 0.07-0.13 — 0.1-0.14 ZIRLO (SRA) 1 1 0.1 — — 0.12 W Optimized ZIRLO (SRA/pRXAd) 0.7 1 0.1 — — 0.12 W M5 (RXA) — 0.8-1.2 0.015-0.06 — — 0.09-0.12 AREVA HPA-49 (SRA/RXA) 0.6 — Fe+V — — 0.12 AREVA NDA’ (SRA) 1 0.1 0.3 0.2 0.12 N FI3 MDAh (SRA) 0.8 0.5 0.2 0.1 0.12 МНР VVER, RBMK E110 (RXA) — 0.9-1.1 0.014 <0.003 0.0035 0.05-0.07 Fuel cladding Alloy E125 (SRA) — 2.5 — — — 0.06 Structural components

aStress relieved annealed. bRecrystallized annealed. cGlobal nuclear fuel. d Partially recrystallized condition. eHigh performance alloy.

‘New developed alloy.

9 Nuclear fuel industries. h Mitsubishi developed alloy. ‘Mitsubishi heavy industries. Source: A. N.T International (2011).

These <a> loops form early in the irradiation and the number density reaches a saturation value at a fuel burnup below 5 GWd/MT (1 x 1021 n/ cm2, E > 1 MeV). The size of the loops increases with irradiation temper­ature, and the loops become unstable (start to disappear) at about 673K (400°C). As will be discussed later they have a strong effect on mechanical properties and dimensional stability.

The <c> type of loop lies on the basal plane and has its Burgers vector, or at least a strong component of it, in the c-direction of the HCP cell. As indi­cated in Table 4.5, and unlike the <a> loop, it is strictly a vacancy-type loop, is relatively large (100 nm) and does not form until considerable irradiation effects have occurred. In Zircaloy, <c> loops are first observed by TEM at a burnup of around 15 GWd/MT (~3 x 1025 n/m2, E > 1 MeV) and increase in density for the remainder of the fuel lifetime. They are thermally stable

to high temperature (>833K). It is thought that <c> loops strongly influ­ence irradiation growth and creep behaviour and probably do not affect mechanical properties. Figure 4.8 shows TEM images of a high density of <c> loops in highly irradiated Zircaloy. Such <c> loops, unlike <a> loops, do not appear to form in all zirconium alloys, particularly in those having additions of Nb, or Nb and Fe (Shishov et al., 2002), until high fluences are experienced.

As outlined in Tables 4.4 and 4.5, the formation kinetics of <a>- and <c>- type loops differ. The density of <a> type dislocation builds up quickly and saturates at a fluence less than 1 x 1025 n/m2, E > 1 MeV, as illustrated in Fig. 4.9. It appears that a fluence-incubation period exists before <c> type loops begin to form at about 3 x 1025 n/m2, E >1 MeV for typical reactor temperatures, as illustrated in Fig. 4.10 .

4.9 Variation of <a> type dislocation loops as a function of fluence in various reactors at 250-290°C (523-563K). (Source: Reprinted, with permission, from Davies et al. (1994), copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.) For a straightforward review of the relationship between irradiation — induced microstructure and Zircaloy properties, see Adamson (2000). More technical details and references can be found there.