Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 100

that Fe rapidly diffuses from the amorphous rim into the matrix, while Cr
diffusion is sluggish. At high fl uence (~1 × 10 22 n/m 2 , E > 1 MeV) complete
amorphization and Fe-depletion has occurred, while the Cr level is still high.
Only at very high fl uence (~1.5 × 10 22 n/m 2 , 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 fl uence. It has been
demonstrated in a BWR that small (<.04 µm) SPPs can completely dissolve at
low to moderate burnups, (Huang et al ., 1996). Also in a PWR, but at temper-
ature near 290°C, SPPs with an average size of 0.2 µm were >80% dissolved at
moderate burnup (1 × 10 26 n/m 2 , E > 1 MeV) (Garzarolli et al ., 2002 ).
Modelling of the dissolution process gives insight into the alloying con-
centration 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 µm) 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 µm.
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 , fl uence
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 × 10 26 n/m 2 ) 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 illus-
trate 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 amor-
phous (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), depend-
ing 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

￿ ￿ ￿ ￿ ￿ ￿

Nb and
the (Laves phase) (L) Zr(Nb,Fe) 2 . Also observed is the T-phase (Zr,Nb) 2 Fe.
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 simplifi ed diagram is pre-
sented in Fig. 4.17 (Garzarolli in Nikulina et al ., 2006 ). Such phase diagrams

β