Abstract
The favourable properties of tungsten borides for shielding the central high temperature superconductor (HTS) core of a spherical tokamak fusion power plant are modelled using the MCNP code. The objectives are to minimize the power deposition into the cooled HTS core, and to keep HTS radiation damage to acceptable levels by limiting the neutron and gamma fluxes. The shield materials compared are W2B, WB, W2B5 and WB4 along with a reactively sintered boride B0.329C0.074Cr0.024Fe0.274W0.299, monolithic W and WC. Five shield thicknesses between 253 and 670 mm were considered, corresponding to plasma major radii between 1400 and 2200 mm. W2B5 gave the most favourable results with a factor of ∼10 or greater reduction in neutron flux and gamma energy deposition as compared to monolithic W. These results are compared with layered water-cooled shields, giving the result that the monolithic shields, with moderating boron, gave comparable neutron flux and power deposition, and (in the case of W2B5) even better performance. Good performance without water-coolant has advantages from a reactor safety perspective due to the risks associated with radio-activation of oxygen. 10B isotope concentrations between 0% and 100% are considered for the boride shields. The naturally occurring 20% fraction gave much lower energy depositions than the 0% fraction, but the improvement largely saturated beyond 40%. Thermophysical properties of the candidate materials are discussed, in particular the thermal strain. To our knowledge, the performance of W2B5 is unrivalled by other monolithic shielding materials. This is partly as its trigonal crystal structure gives it higher atomic density compared with other borides. It is also suggested that its high performance depends on it having just high enough 10B content to maintain a constant neutron energy spectrum across the shield.
Highlights
Spherical tokamaks present a unique opportunity to accelerate the delivery of safe, carbon-free, abundant, baseload fusion power [1]
The neutron is absorbed to form a tungsten isotope with one higher atomic weight with the emission of a gamma ray of significant energy, (vi) The lower energy neutrons produced by moderation or inelastic scattering become increasingly likely to be absorbed by isotopes such as 10B with a high neutron absorption cross section which normally increases with decreasing energy as 1/v where v is the neutron velocity, as illustrated in figure 1
The plot illustrates the local minimum power deposition for W2B5, shown previously in Figures 4, 6 and 12, is maintained at all 10B enrichment levels. It shows that the optimal material, i.e. W2B5 with 60% 10B or more, shows a factor of ~20 lower gamma power deposition compared to monolithic W
Summary
Spherical tokamaks present a unique opportunity to accelerate the delivery of safe, carbon-free, abundant, baseload fusion power [1]. They have the plasma closer to the current-carrying central column and so make more efficient use of the magnetic field, which decreases with distance from the column. The total power deposition determines the costs of the cryogenic plant needed to keep the HTS at operating temperatures of say 20 K Calculations suggest that it would be a relatively low fraction of the cost of a 200 MW fusion power plant [3]. Detailed computations of the heat generated within the shield and the resulting shield temperatures are planned in further studies
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