  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. Of all these 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.
