Radiation shield analyses in support of the FS design for the ITER ECRH launcher

Radiation shield analyses have been performed to guide the Front Steering (FS) design concept of the Electron Cyclotron Resonance Heating (ECRH) launcher in ITER. Such a launcher is proposed for integration into an upper port to stabilize neoclassical tearing modes (NTM) of the plasma. The microwave beams are transmitted through long and narrow waveguide channels. They are injected into the plasma by steerable mirrors placed in the front shield of the port plug. This allows a well-focused beam injection over a wide steering range. The physics concept of the FS launcher has been elaborated by CRPP-EPFL, Lausanne. The engineering design is under development in co-operation with other EURATOM associations in the frame of EFDA tasks. The FS design is the present reference concept for the upper port ECRH launcher in ITER. This paper is devoted to the neutronics analyses of the radiation shield for the FS design concept. Neutronics calculations have been performed with the Monte Carlo code MCNP using the standard 3D ITER torus sector model with an ECRH-FS launcher integrated in the upper port. The computational model of the ECRH launcher was generated by converting the 3D CAD model into the geometry representation of the MCNP code using the interface programme McCAD. A fusion power of 500 MW was used for the normalization of the calculated nuclear responses. The nuclear heating was calculated in the vicinity of the steering mechanism considering different candidate materials, comprising the ECRH mirrors, steering mechanism, shielding material and port plug. For example, the values obtained are less than 1.5 W/cm3 for the copper used on the reflective surface of the mirror. The neutron induced radiation damage has been calculated in terms of displacements per atom (dpa) for the materials to be used in the steering mirror structures. A peak value of 1 dpa per full power year was found for the SS316L steel at the lower side of the flexure pivot. The influence of neutron displacements on the tensile strength and on cyclic fatigue of the candidate materials for the steering mechanism is discussed in the paper. The results of the neutronics analyses confirm the compliance of the FS design concept with the ITER design and safety rules. (author)