Analyses of fuel crud and coolant-borne corrosion products in normal water chemistry BWRs
Highlights
► Corrosion products formed in boiling water reactors were examined. ► Samples were obtained from three reactors operating at normal water chemistry. ► Phase composition of samples was determined using 57Fe Mössbauer spectroscopy. ► Oxidation form of iron in coolant and on fuel rods is discussed.
Introduction
Over the years, the nuclear power industry has experienced some adverse behaviors associated with deposition of corrosion products (crud) on nuclear fuel. In particular, some BWRs have suffered crud and corrosion-induced fuel failures with a significant impact on plant performance and fuel economy [1], [2], [3], [4], [5]. The fuel failures are generally attributed to a combination of water chemistry, fuel duty, and cladding materials. To mitigate intergranular stress corrosion cracking (IGSCC) of core internals and other components and to minimize the activity transport the water chemistry in BWRs evolved from essentially pure normal water chemistry (NWC) to hydrogen water chemistry (HWC) for IGSCC mitigation, depleted zinc oxide (DZO) addition to minimize shutdown dose rates, and noble metal chemical application (NMCA, NobleChem™ or On-Line NobleChem™ (OLNC) to reduce stress corrosion cracking in stainless steel piping. Crud deposits that form under NWC are normally porous and structurally very different from tenacious crud formed under HWC operation. Under ongoing BWR Chemistry Monitoring and Assessment effort, EPRI collects and compiles information on plant design, operating practices, chemistry control, IGSCC mitigation and monitoring strategies, along with detailed chemistry and radiation dose rate data [5].
The general objective of research in this area is to mitigate the impact of fuel crud deposition and cladding corrosion on fuel reliability by studying the roles of various factors on crud deposition and cladding corrosion and associated deleterious effects. Another very important issue of water chemistry in BWR plants is the activity build-up caused by activated corrosion products in the core. Improving iron control and minimizing the crud transport and its deposition on fuel requires the study of characteristics of tenacious crud and the conditions that form types of crud carrying the most risk, including the role of specific chemistry impurities, such as Fe, Cu, Zn and SiO2. However, fuel crud sampling campaigns and especially thorough analyses of samples are difficult and are seldom performed, and the results are even less often published in journals, e.g. [6], [7], [8], [9], [10], [11], [12], [13].
In this work the fuel crud and corrosion product transport have been investigated in three General Electric BWRs in the United States that operated under NWC for about 15 years. The primary objectives were to (i) examine the solids that were collected during post-cycle fuel crud sampling in one of such BWRs, and especially to determine the chemical form and oxidation state of iron-bearing oxides and other compounds incorporated in these deposits; (ii) examine the concentration and speciation of iron-bearing particles typically transported in cooling system at steady-state full power in two similar units, and especially in the condensate downstream and upstream of water polishers and in reactor feed water.
Section snippets
Fuel crud samples
The fuel crud formed under NWC operation is usually soft and porous and therefore can be prone to physical and chemical changes after extraction of fuel assemblies during refueling and their subsequent storage in fuel bay. However, most of the characterization work on BWR fuel crud was done in the past on fuel rods that had been exposed to wet or dry storage, sometimes for several years. During prolonged storage the crud can be susceptible to oxidation, contamination, dissolution and
Mössbauer spectroscopy analyses and discussion
Mössbauer spectroscopy of 14.4 keV gamma rays in 57Fe is one of the most suitable techniques at present to determine phase composition of various iron oxides present in such small-size radioactive crud samples. The advantage of Mössbauer spectroscopy over electron microscopy techniques is that the volume of the investigated sample is quite large, usually about 1 mm3 – compared to μm3 – which allows determining the average volume fractions of different iron-bearing phases present in such sample.
Evolution of iron speciation in condensate, feed water and fuel crud
The primary source of crud in BWR is the corrosion products of stainless steel and carbon steel in the feedwater system. Despite of the fact that the total surface area of zirconium alloys exposed to coolant is very large it is known that the contribution of zirconium to corrosion products is very small. Under NWC the whole coolant and steam circuit operates with fully demineralized water without any addition of chemicals. Water radiolysis in the radiation field of the reactor generates
Summary and conclusions
The Mössbauer spectroscopy of 14.4 keV gamma rays in 57Fe nuclei has been used to examine radioactive fuel crud samples removed from the surface of fuel rods in a boiling water reactor operating at normal water chemistry. The examined fuel crud represented a highly oxidized, mostly ferric form of iron oxides and oxyhydroxides, namely hematite and goethite, similarly as final feed water crud filters, but unlike the feed water samples, the fuel crud did not show any significant presence of
Acknowledgements
Early part of this work was funded by Electric Power Research Institute, BWR feed water iron reduction and removal Projects. The samples have been analyzed at Atomic Energy of Canada Limited Chalk River Laboratories. The author would like to acknowledge valuable discussions with Drs. P.L. Frattini (EPRI) and J. Sundberg (GE Nuclear).
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