In 1998, a leak occurred in the main mixing zone of the residual heat removal systems (RHR) of the EDF CIVAUX nuclear plant unit 1. The crack is attributed to high-cycle thermal fatigue due to the fluid thermal turbulences. An research and development (R&D) program is now being conducted at EDF to understand the incident and to assess the risks of cracks in other mixing zones. This program includes thermal-hydraulic tests on mock-ups and their numerical interpretation, material testing in high-cycle fatigue, structure tests on mock-ups submitted to high-cycle thermal stresses and their interpretations, and developments in non destructive evaluations in the presence of crazing zones (thermal striping). After a brief presentation of the R&D program, the paper presents a new high-cycle thermal fatigue bench test, named INTHERPOL, for pipe structures. The thermal cycles consist of periodic controlled cold shocks by water sprays and hot shocks by infrared radiations on the inner surface of part of the structure. The structure under test, the water spray devices and infrared modules are inserted into a tank to allow constant control of the environment. The types of pipe structures tested include plain or welded structures and various industrial surface finishes. The first tests results and their numerical interpretations are presented.

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