Maximum allowable fluid velocity and concern on piping stability of ITER Tokamak cooling water system

Abstract

The ITER facility is an international research project that is under construction in Cadarache, France. US ITER is responsible for the design, engineering, and procurement of the Tokamak Cooling Water System (TCWS). TCWS piping size varies from DN40 to DN500. A design guideline specifies that piping smaller than DN150 must have a fluid velocity below 6 m/s, while a fluid velocity of up to 9 m/s is permitted for piping larger than DN150. Fluid velocity affects pipe size, pressure drop, pump size, piping support, etc. Factors considered when determining maximum allowable fluid velocity include erosion and corrosion, pump capacity, cavitation, hydrodynamic instability, and actual nuclear plant experience. The previous study performed to support the ITER piping design indicated that the hydrodynamic instability was a reason to limit maximum allowable fluid velocity below 6 m/s regardless of the piping size. However, the later study found that, when properly supported, pipe buckling and fluttering from hydrodynamic force would not cause instability. Instead, cavitation erosion—a pipeline phenomenon that forms vapor cavities in pipe—may limit allowable fluid velocity. Cavitation is a pipeline phenomenon that forms vapor cavities, which then go unstable and collapse violently in low-pressure, turbulent, flow-separation regions inside valves, elbows, pipe expansions, and other fluid handling components. Considering the cavitation issue, pressure drop, and a typical practice in a nuclear plant system, fluid velocity is to limit below 6 m/s for piping smaller than DN150 and 9 m/s for piping larger than DN150. This paper describes a hydraulic stability analysis, the fluid velocity generally accepted in nuclear plants and the cavitation-erosion limit in fluid velocity.