Contact Author:
Mrs Nuria Doncel
ndg@enusa.es
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ENUSA INDUSTRIAS AVANZADAS SA
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Water Chemistry Influence on AOA

N. Doncel (ENUSA INDUSTRIAS AVANZADAS SA -Spain), J. Chen (Studsvik Nuclear AB - Sweden)
Axial Offset Anomaly (AOA) or Crud Induced Power Shift (CIPS) are both names for describing the abnormal power shift on fuel that occurs in some Pressurized Water Reactors (PWR). It has been postulated that this phenomenon is caused by the precipitation of a boron-containing species in fuel CRUD under certain subcooled boiling and water chemistry conditions (pH, Ni concentration). This drove the need to verify the mechanism by which the phenomenon occurs and the speciation of the boron precipitates under proto-typical PWR conditions on a heated rod element. Therefore, ENUSA in collaboration with IBERDROLA GENERACIÓN S.A, ASOCIACIÓN NUCLEAR ASCÓ-VANDELLÓS II A.I.E., A.I.E. CCNN ALMARAZ-TRILLO and later ELECTRIC POWER RESEARCH INSTITUTE, developed a test program at Studsvik Nuclear AB beginning in 2001. This program continues with its last phase in 2006. The purpose of the current paper would be to update with the new information since last Top Fuel 2006 Meeting. The first phase results clearly showed that boron and lithium precipitate over a ZIRLOTM tube specimen under well-defined autoclave experimental conditions. The results indicated that the formation of lithium-borates on the cladding surface is sensitive to boiling and coolant pH conditions. During phase 2, parametric studies to determine the effect of nickel and pH on the formation of the lithium and boron compounds were performed using a window autoclave. The heated rod element was pre-crudded with a nickel-iron-oxide of varying thicknesses and an instant and non-explosive drainage technique was applied to preserve the chemical and physical characteristics of the simulated fuel CRUD. Various advanced analytical techniques such as Raman Spectroscopy, XRD, time-of-flight SIMS, ICP-MS and STEM were used. The STEM examinations on a CRUD sample from one of the last tests has confirmed the presence of tetragonal Li2B4O7 crystals in the crud sample. During the last phase 3, some mitigation solutions for this phenomenon have been evaluated, such as Enriched Boric Acid (EBA), with the intention of generating operational procedures and/or chemistry recommendations to minimize or avoid the presence of the AOA-CIPS. During the course of this project, a number of tests have been conducted under various experimental conditions. It is found that lithium and boron deposition appears to be sensitive to the crud morphology, which is in turn the result of crud deposition under various influencing factors, such as bulk water pH and ECP, elemental compositions of water impurities and subcooled boiling conditions. From the limited number of experimental observations, it appears that more boron would deposit in thicker and denser crud layers under subcooled boiling conditions. If the observation is proven to be true the root cause for AOA must then be sought for among the factors which could cause deposition of the thicker and denser crud on fuel cladding in PWRs. Additionally it is important to examine quantitatively the dependence of boron deposition in crud on crud thickness and density. In this proposal a series of pre-crudded rods with different crud thicknesses and densities are made and exposed to a well simulated PWR water chemistry environment. A quantitative correlation between the content of deposited boron and crud thickness and density can then be depicted in a diagram, which can serve as a calibration diagram for better interpretation of the data discrepancies obtained previously when the crud thicknesses and densities were different. This confirmation could be applied to calibrate an important part of the models to be implemented in the codes that the industry is currently using to evaluate the phenomenology of AOA. Another AOA related issue concerns the zinc injection technology being tested in PWR. Zinc addition in PWR has been reportedly beneficial in retarding materials PWSCC and radioactivity buildup. However, it is unclear whether or not zinc addition would increase the risk of AOA in a PWR. The concern is based on the previous experiences that zinc addition in BWR caused substantial changes in fuel crud morphology and compositions. We propose to study the effect of zinc addition on boron deposition in crud and compare the results to that obtained without zinc addition.