Study on the impact of non-silver coated area in the ITER vacuum vessel thermal shield

Abstract

In the ITER tokamak, a thermal shield (TS) is installed to reduce the heat oad radiated to the superconducting magnet system from vacuum vessel (VV) and cryostat. TS has a thin silver surface to keep its emissivity below 0.05. The silver surface is achieved by electroplating. However, some vacuum vessel thermal shield (VVTS) may be imperfectly silver coated due to their complex geometry. When a segment of the VVTS is immersed in silver coating bath, electron is concentrated to the sharp edge of segment and doesn’t reach to the deep area. Also, solution in a segment surface for pre-treatment may not be removed in some point due to its geometry, and it disturbs the silver coating. In this paper, the impact of non-silver coating on heat loads from TS to magnet system is researched as following process; 1) Identification of area where the VVTS may be imperfectly silver coated – it is obtained from actual silver coating of ITER VVTS. 2) FE model/analysis condition construction of VVTS – VVTS solid element / cooling tube beam element is combined, and silver coated / non-coated area shell element with respective emissivity is applied. 3) Thermal analysis of VVTS with non-silver coated area – heat load via non-coated area of VVTS is calculated, and its impact to magnet system is estimated. According to thermal analysis of VVTS, the essential silver coating points to reduce heat load are identified. As remedial action is applied to the essential points, the heat load from VVTS to magnet system is minimized.