Environmental Engineering Reference
In-Depth Information
4.11
The fl uence 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 fl ux
increases and as SPP size decreases. Literature evaluation therefore needs
to consider reactor and material conditions of specifi c interest.
The fl uence required to produce complete amorphization depends on
neutron fl ux, temperature and SPP size, but for typical Zr(Fe,Cr)
2
SPPs of
initial size near 0.1 µm and the entire SPP is amorphous by the end of bundle
life burnups <50 MWd/KgU (1 × 10
22
n/cm
2
,
E
> 1 MeV). Interestingly, under
well controlled conditions of fl ux and temperature, the amorphization rate of
Zr(Fe,Cr)
2
in Zircaloys can be used to estimate the neutron fl uence (Motta
& Lemaignan, 1992; Taylor
et al
., 1999 ; Bajaj
et al
., 2002 ).
For the Zr-Nb type alloys neither the
Nb 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 fl uences.
SPP amorphization in itself does not appear to affect material behaviour;
however, dissolution of both amorphous and crystalline SPPs does infl uence
corrosion, growth and mechanical properties, to be discussed later. At typi-
cal 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 Zr
2
(Fe,Ni) SPPs as schemat-
ically 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
β
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