Shielding Material Performance in a Preconcept Spherical Tokamak Power Plant

Spherical tokamaks present uniquely challenging neutronics requirements owing to very limited space on the inboard side to shield the magnets in the center column from radiation and heat. Since the shielding efficacy of the center column shield directly impacts the feasibility of a spherical tokamak–based fusion power plant concept, extensive integrated neutronics analyses are required for the shielding concept design. These include assessments of neutron flux and nuclear heating to the magnets, thermal management of the shield, shielding material damage, activation of shield components, and radioactive waste inventories.

In this study, advanced neutronics calculations are performed on a representative concept of a fusion power plant to compare a set of shielding materials of interest: tungsten borides, tungsten carbide (WC), tungsten heavy alloy (AEM-18C), and hafnium hydride (HfH₂). To rapidly cycle through tokamak design iterations, a workflow was developed that coupled Tokamak Energy’s in-house system code, PyTOK, to an in-house application of the Geant4 radiation-transport toolkit, G4Tokamak. All the analyses are performed on a computer-aided design model of the tokamak using G4Tokamak.

The first assessment included high-fidelity three-dimensional maps of the neutron fluxes and nuclear heating in the center column components, focusing on the high-temperature superconducting magnets. In the second assessment, two-step activation calculations were performed in conjunction with FISPACT-II to calculate displacement rates, gas production, and activation of the center column shielding material options to quantify radioactive waste.

The shielding materials assessed in this study, starting with the best radiation and nuclear heating attenuation performance, were tungsten tetraboride (WB₄), HfH₂, tungsten boride (W₂B), WC, and AEM-18C. Hydrogen production rates were the highest in AEM-18C, followed by the borides, while helium production rates were highest in W₂B, going down by the tungsten content of the material. WC had the lowest long-term activity, posting a 10-year operation period.

A neutronics workflow was also thus successfully developed and used to rapidly assess early-stage power plant concepts for feasibility based on shielding efficacy and the resulting magnet system lifetime.