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The surfaces of fuel pin cladding and spacer wire wound round it in the JOYO Mark-I and EBR-II test fuel were found to carry wear marks. Out-of-pile experiments indicated that simple hydraulic vibration could not constitute the cause of marking. Assuming the alternative factor of thermohydraulic vibration to be the cause, a model has been devised, applying which a method is proposed for estimating the ranges in which wear and contact marks can respectively be expected to occur. The two ranges are determined in terms of two parameters combining variables commonly used in practical fuel assembly design, and which are adopted as coordinates for a diagram in which data from actual fuel design are plotted. Data from actual reactor designs plotted in this diagram have proved to fall into two distinct domains respectively covering cases where wear and contact marks had been occasioned.

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