The observation that uniform corrosion of BWR materials may increase at high fluence (burnup) has been introduced. The main factor driving this increase is connected to the initial size distribution of the SPPs and their

4.46 Zirconium oxides (a) away from and (b) near a stainless steel control blade bundle (Adamson et al., 2000).

dissolution during irradiation. This was noted by Cheng and Adamson (1987) and then by Yang and Adamson (1989), in reference to thick uni­form oxide observed in welded regions of Zircaloy-4 having totally dis­solved SPPs. A clear correlation between SPP size and increased corrosion at high fluence was given by Garzarolli et al. (1994) (Fig. 4.47) where it was shown that ‘small SPP sizes’ resulted in relatively thick corrosion films after 3 or 4 cycles in-reactor but not after 1 or 2 cycles. Huang et al. (1996) showed that when SPPs virtually ‘disappeared’ (within the resolution of STEM at that time) corrosion increased, as did hydrogen pickup. Similar results were reported by Tagtstrom etal. (2002), Takagawa etal. (2004) and Ishimoto et al. (2006). It is clear that loss of SPPs affects corrosion performance, and even earlier in fluence, hydrogen pickup.

Zircaloy-4, which does not contain alloying quantities of Ni, is also susceptible to increased corrosion when SPPs dissolve. An example is given in Fig. 4.42 for Zircaloy-4 with a relatively large SPP, about 0.2 pm average size determined by TEM (Garzarolli, et al., 2002). It is seen that when the volume fraction of SPPs gets very low, corrosion increases dramatically.

A compilation of data for Zircaloy-2 and -4 given by Garzarolli in Adamson et al. (2006) and Garzarolli et al. (2011b) illustrates that Zircaloy-4 generally has higher corrosion than Zircaloy-2 (see Fig. 4.49). However, the hydrogen pickup fraction (HPUF) for Zircaloy-4 appears to stay remark­ably low at high burnup, as indicated by the data of Miyashita et al. ( 2006 ) and the correlation given in Fig. 4.50.

In PWRs nodular corrosion in unlikely to occur due to high hydrogen and low oxygen in the water. Accelerated uniform corrosion does occur for Zircaloy-4 in PWRs as seen in Fig. 4.51 (and Fig. 4.48) and the HPUF is rel­atively low, as in BWRs (Fig. 4.52).Those figures also show that M5 (which is basically a ZrlNb alloy with 300-500 ppm Fe) does not undergo accelerated

4.48

I nfluence by irradiation to very high fluences at 290°C (563K) on corrosion and SPP dissolution of Zircaloy-4 with large SPPs. (Source: Reprinted, with permission, from Garzarolli et al. (2002), copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.)

4.49 Oxide thickness of Zircaloy-2 and Zircaloy-4 under isothermal irradiation in different Japanese studies. Particularly important is the data of Miyashita et al. (2006); ZIRAT 11 compilation (Adamson et al., 2006).

uniform corrosion out to exposures equivalent to 70 GWd/mt at the PWR irradiation temperature of 315°C (587K). At that temperature, irradiation effects on SPPs are similar in BWRs and PWRs. Two types of SPPs exist for M5 — pNb and the Laves phase Zr(Fe, Nb)2. At high fluence all the Fe will

204 Materials’ ageing and degradation in light water reactors

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X be dissolved into the matrix, and SPPs of pNb will exist without apprecia­ble dissolution. Again it appears that it is the existence of the SPPs which is important.