Activation and transmutation of tungsten boride shields in a spherical tokamak

Abstract

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.

Energetics of intrinsic point defects and hydrogen in tungsten borides: a first-principles study

“…It is known that helium atoms tend to be trapped by vacancies or other excessive volume defects, forming helium clusters or helium bubbles in materials [46,47]. Due to helium production in W borides [27], a large number of helium bubbles will likely form in W borides, leading to swelling and the possibility of microcracking and fracture in W borides under neutron irradiation [28].…”

“…This study is useful for evaluating the response of W borides in a fusion environment. Previous work [27,28] has shown that helium formation in the transmutation of W borides under neutron irradiation could be a challenge to the thermal and mechanical properties of W borides; the helium behavior in W borides and synergetic interaction between helium and hydrogen will be discussed in future publications.…”

“…In addition, W in W borides has a high atomic mass and therefore effectively adsorbs gamma radiation [14], while B has a very high neutron adsorption cross-section resulting in decay products helium, lithium and hydrogen [25]. The use of W boride instead of W carbide in a neutron shield in a spherical tokamak fusion power plant was shown to make an improvement in reducing the power deposition [25,26] and the activation of W boride shields in a spherical tokamak was observed to be lower than either W or WC [27].…”

“…In their recent work, more W borides were considered for the shield materials, such as W 2 B, WB, W 2 B 5 and WB 4 , and they observed that W 2 B 5 gave the most favorable results with a factor of ∼10 or greater reduction in neutron flux and gamma energy deposition as compared to monolithic W [26]. In addition, they investigated the activation and transmutation of these four W borides using the FISPACT-II code, and found that damage production, in terms of the displacements per atom (dpa) tends to increase with boron content and that the production of gaseous isotopes (hydrogen and helium) is problematic in boride shields [27]. To reveal the effect of the stoichiometry on the performance of W borides in a fusion environment, fixed stoichiometric ground state compositions that are on, or near, a convex hull [24], such as W 2 B, WB, WB 2 , W 2 B 5 , WB 3 , and WB 4 , have been studied in this paper.…”

Faster Fusion Power from Spherical Tokamaks with High-Temperature Superconductors

“…Monolithic shields with varying fractions of tungsten and boron were examined [21,22] and shown to give half-attenuation coefficients as low as 40 mm as seen in Elastic scattering (i) of neutrons. Tungsten makes a good reflector as its cross section of ~3 barns of elastic cross section per atom at 14 MeV and its high mass compared to a neutron ensures little recoil energy.…”