Characterization of PWR fuel crud by high resolution transmission electron microscopy

Knowledge is generally lacking about the nature of fuel CRUD in PWRs and its dependence on water chemistry, fuel power history, and other operational conditions. The need for an improved understanding about PWR fuel CRUD becomes obvious when axial offset anomaly, also known as CRUD induced power shift, occurs in an increasing number of PWRs worldwide. This paper contributes to characterization of fuel CRUD scraped and collected on filter papers in pool at a Swedish PWR (Ringhals Unit 4 - using Inconel 600MA as steam generator tubing material). The fuel CRUD particles were captured on TEM grids by removing the filter material, and subjected to high resolution transmission electron microscopy study. The elemental compositions of each CRUD particle were measured by EDX and all collected data presented in a ternary (Cr-Ni-Fe) diagram to show the possible phases in the sample. Some SAED studies were made on some selected particles to identify their phases. In this paper we reported phase compositions of some fuel CRUD samples and their dependences on fuel rod height and fuel burn-up. A new solid phase with a composition of either Ni{sub 1.32}Cr{sub 0.33}Fe{sub 0.78}O{sub 3} or Ni{sub 1.77}Cr{sub 0.45}Fe{sub 1.04}O{sub 4} and tetragonal crystal symmetry (a = 8.67 A, c = 7.82 A) has been preliminarily determined. Results show that with increasing rod height phase fractions of NiO and Ni{sub 1.32}Cr{sub 0.33}Fe{sub 0.78}O{sub 3} (or Ni{sub 1.77}Cr{sub 0.45}Fe{sub 1.04}O{sub 4}) increase while phase fractions of Fe{sub 3}O{sub 4} and (Ni,Cr,Fe){sub 3}O{sub 4} decrease in fuel CRUD. Effect of fuel burn-up on CRUD phase composition was not significant enough to be shown by the TEM method applied, while average nickel concentration of fuel CRUD did decrease with increasing fuel burn-up. (author)