Crud induced corrosion

With regard to CICL, crud induced corrosion involves failures of heat treated cladding not susceptible to nodular corrosion. The corrosion characteristics are different and do not involve nodular corrosion, even if the root cause may be similar.

Recently, four US BWRs experienced fuel failure due to accelerated cladding corrosion; these involved different fuel suppliers. About 14 months after the startup of cycle 11 at River Bend, an off-gas activity increase was noted. During core off-load at EOC-11, six failed one cycle ATRIUM™10 fuel assemblies were identified, with burnups in the range of 14.6 to 19.0 GWd/MTU [5.43]. Examinations performed during the refuelling outage and after startup of cycle 12 indicated that Span 2 (the axial location between the second and third spacers from assembly bottom) failed in both, and one cycle assemblies which did not fail had an unusually thick tenacious crud accumulation on the peripheral rods. An example of heavy crud in lower Span 2 before and after brushing is given in Fig. 5.21. It was observed that the thickest tenacious crud was very specific to a particular region of the River Bend core and axial location on new fuel rods. The failed assemblies were located in core locations which formed a ring, positioned seven assemblies from the core centre. Visual examinations after rod brushing/cleaning showed that the peripheral rods had accumulated thick tenacious crud mainly on the side of the rod facing outward toward the fuel channel, while interior rods were generally unaffected by the thick crud. With this asymmetric crud deposition and oxide formation, excessive rod bow toward the fuel channel was experienced by higher power rods (failed and unfailed), and some of these rods may have bowed enough to touch the inside surface of the fuel channel at or near the location of failure. Several crud flakes were successfully acquired and evaluated and these indicated high levels of copper and zinc. The cause of failure in River Bend rods during cycle 11 was determined to be accelerated oxidation of the cladding in Span 2, resulting from unusually heavy deposits of insulating tenacious crud. Detailed reviews pointed out the River Bend water chemistry was within EPRI guidelines and no specific events of concern were noted. However, it was noted that River Bend was unique relative to other units due to its combination of high copper, zinc, and iron. The most probable cause of insulating tenacious crud was that copper and zinc were available in sufficient quantity to plug either normal wick boiling paths within the crud or any delamination within the crud or clad oxide, resulting in diminished heat transfer in local areas of the cladding surface.

The majority of other failures were localized in Browns Ferry Unit 2 where, during cycle 12, which operated from April 2001 to March 2003, 63 GE13 fuel assemblies in their second cycles of operation had fuel rods that failed from accelerated corrosion [5.44]. The entire second cycle reload, first loaded in May 1999, exhibited accelerated corrosion peaking towards the top of the bundle. The first and third cycle bundles operating in the same cycle, as well as previously discharged fuel in the fuel pool, were normal, well inside GNF’s experience base.

Despite a significant effort, including detailed reviews of reactor water chemistry, fuel manufacture and fuel operation, no root cause of the fuel corrosion failure at Browns Ferry has been determined. It is likely that a combination of conditions existed which resulted in the fuel failure. The combined failures at Units 2 and 3, and their timing, seem to indicate an unusually aggressive water chemistry condition, perhaps related, during or immediately after one or both of the affected reloads’ first cycle of operation. Cladding material which was lower in alloy content and higher in duty was most affected by this environment. A hot cell exam to provide further insight is in progress.

Enhanced clad corrosion due to crud deposit was also reported in a few US PWR plants between 1995 and 2000. In particular, during cycle 10 in 1995, TMI-1 experienced nine failed rods owing to unusual buildup of corrosion products in the upper spans on the outer surface of peripheral fuel rods [5.45]. This crud buildup and the following fuel failures occurred in the first cycle with highly enriched fuel of 4.75% (in a 24 month cycle). The distinctive crud pattern (DCP) of the failed rod was characterized by a mottled appearance, specifically a dark, nearly black surface with jagged patches of white showing through and occurring around 2.8 metres from the bottom of the fuel rod as shown in Fig. 5.23.

Poolside and hot cell investigations to delineate the cause of fuel failures were performed [5.46]. Results indicated greater corrosion and hydrogen pickup than predicted in the upper spans, regions of cladding recrystallization in areas of unusual crud pattern as shown in Fig. 5.24, and hydrogen migration away from those areas. These observations indicated that higher than expected temperatures occurred in DCP areas. Cladding oxidation and hydrogen pickup were also higher than predicted at the elevation of DCP regions on rods, consistent with high cladding temperatures.

Localized cladding penetration due to crud induced localized corrosion was identified as the failure mechanism, see Figs 5.25 and 5.26. Evaluations attributed the problem to a combination of high temperature due to higher power and some flux tilt with fresh assemblies, and a low pH value because of the applied high level of boron poison used to transition from 18 month to 24 month cycles [5.46, 5.47].

FIG. 5.22. Failure location noted from bubbles escaping at a defect in a peripheral rod [5.43].

FIG. 5.23. Distinctive crud pattern (DCP) in TMI fuel assemblies [5.46].

FIG. 5.24. Grain structure of the zircaloy cladding [5.46].

FIG. 5.25. TMI 1, Through-wall defect on a rod O11 [5.46].

FIG. 5.26. TMI 1, Cladding corrosion on a rod O11 at an elevation of 118.5 inches [5.46].