Activation and transmutation of tungsten boride shields in a spherical tokamak

The FISPACT-II code is used to compute the levels of activation and transmutation of tungsten borides for shielding the central High Temperature Superconductor (HTS) core of a spherical tokamak fusion power plant during operations at 200 MW fusion power for 30 years and after shutting down for 10 years. The materials considered were W2B, WB, W2B5 and WB4 along with a sintered borocarbide B0.329C0.074Cr0.024Fe0.274W0.299, monolithic W and WC. Calculations were made within shields composed of each material, for five reactor major radii from 1400 to 2200 mm, and for six 10B isotope concentrations and at five positions across the shield. The isotopic production and decay in each shield is detailed. The activation of boride materials is lower than for either W or WC and is lowest of all for W2B5. While isotopes from tungsten largely decay within 3 years of shut-down, those from boron have a much longer decay life. An acceptable 70% of the absorbing 10B isotope will remain after 30 years of operations behind the first wall for a 1400 mm radius tokamak. Gaseous production is problematic in boride shields, where 4He in particular is produced in quantities 3 orders of magnitude higher than in W or WC shields. The FISPACT-II displacements per atom (dpa) tend to increase with boron content, although they decrease with increased 10B isotopic content. The dpa ranges of boride shields tend to lie between those of W and WC. Overall, the results confirm that the favourable fusion reaction shielding properties of W2B5 are not seriously challenged by its irradiation and transmutation properties, although helium gas production could be a challenge to its thermal and mechanical properties.

Demixing in the plasma created in capillary discharges with polymeric wall

The results of spectral diagnostics of erosion plasma obtained in a pulsed discharge in a capillary with an evaporating wall made of hydrogen-carbon and fluorine-carbon polymers - polymethylmethacrylate and polytetrafluoroethylene - are presented. It was found that in both cases the distribution of chemical elements along the discharge radius is highly inhomogeneous, and their concentration ratio differs significantly from that in the capillary wall. The mass of particles is a common sign characterizing the demixing degree of chemical elements and direction of diffusion flows in fluorine-carbon and hydrogen-carbon plasmas. In both cases, lightweight particles are concentrated in the central high-temperature region, while heavy ones run away onto the low-temperature peripheral region of the discharge. Estimates show that the thermal diffusion mechanism is quite capable for providing the observed demixing degree of chemical elements. Favorable conditions for thermal diffusion processes are formed in the layer adjacent to the capillary wall, where the intense dissociation of radicals occurs, and the temperature gradient reaches up to ΔT∼10 eV/mm.