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In no case wear indication has been detected up to now ( insertion time up to 14 years, more than 264 accumulated cycles) on any of the coated control rods during routine eddy current testing or visual inspection [12]. 7. Optimization of RCCA Insertion The objective of optimizing the RCCA insertion strategy is to minimize the total number of RCCAs used during the specified life of a PWR. In addition, the need for optimization arises also from the problem of disposal of control rods [19]. In plants with more pronounced wear, coated RCCAs showing no wear are used for reloads.

The main advantages of this material are : high neutron worth, high melting point (2450°C), very low radioactive waste easy to reprocess, low cost in case of natural B4C. However it presents some drawbacks, the main one being the swelling due to the formation of helium atoms. For this reason, in the French 1300 MW PWRs, B4C is only used in the upper part of the control rods where the neutron flux is low enough to minimize the swelling rate. If we look ahead for a wider use of boron carbide in the control rods it is necessary to improve our knowledge concerning the phenomena which limit the life time of this absorber.

This leads to the choice of different or improved materials and designs. In this frame, a material such as Silver-Indium-Cadmium (AIC) should be eliminated, for the following reasons : low mechanical properties, inducing creep and premature absorbercladding interaction ; weak melting temperature (« 800 °C), which means that AIC is the first core material which melts in the case of LOCA. In order to satisfy the new requirements of safety, life time, efficiency, of the control rods materials, we can either consider new concepts or new materials.

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Advances in Control Assembly Mtls for Water Reactors (IAEA TECDOC-813)

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