Two broken control rods and a large number of rods with cracks were found at the inspection carried out during the refueling outage of the twin reactors Oskarshamn 3 and Forsmark 3 in the fall of 2008. As a part of an extensive damage investigation, time dependent CFD simulations of the flow and the heat transfer in the annular region formed by the guide tube and control rod stem were carried out, [1]. The simulations together with metallurgical and structural analyses indicated that the cracks were initiated by thermal fatigue. The knowledge assembled at this stage was sufficient to permit the restart of both reactors at the end of year 2008 conditioned to that further studies to be carried out for clarifying all remaining matters. Additionally, all control rods were inserted 14% to protect the welding region of the stem. Unfortunately, this measure led to new cracks a few months later. This matter will be explained in the second part of this work, [2]. As a part of the accomplished complementary work, new CFD models were developed in conformity with the guidelines of references [3] and [4]. The new results establish the simulation requirements needed to accomplish accurate conjugate heat transfer predictions. Those requirements are much more rigorous than the ones needed for flow simulations without heat transfer. In the present case, URANS simulations, which are less resource consuming than LES simulations, seem to rather accurately describe the mixing process occurring inside the control rod guide tube. Structure mechanics analyses based on the CFD simulations show that the cracks are initiated by thermal fatigue and that their propagation and growth are probably enhanced by mechanical vibrations.

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