Performance characteristics and waste implications of WC materials used as neutron shielding materials in fusion

Abstract

The nuclear fusion of deuterium and tritium (D-T) produces a high flux of 14 MeV neutrons. While these neutrons are the primary source of energy, they also cause damage to the structural and functional materials of fusion devices. Tungsten carbide (WC), with its excellent neutron attenuation properties and high density, is being proposed as a radiation shield to protect critical components, such as the high temperature superconducting (HTS) magnets used to confine plasmas in a spherical tokamak (ST). Due to its brittle nature, WC needs binder materials, such as cobalt, iron, or iron‑chromium, to form a ductile matrix for the WC grains. These binder materials enhance thermomechanical properties but subsequently contribute to an additional radioactive waste hazard and high decay heat that complicates maintenance and radioactive waste management strategies. In the present work, an evaluation of the neutronics properties of various grades of WC has been conducted, containing cobalt (3 wt% and 6 wt%), iron (2.76 wt%), and iron‑chromium (2 wt%) as binder materials. Additionally, a comparative analysis has been made with predictions for pure WC without any binder materials and impurities. A generic ST model producing 1 GW of fusion power has been implemented in the Monte Carlo N-Particle (MCNP) code, with neutron spectra at varying depths within the centre column shield calculated. These neutron spectra are used in the FISPACT-II code to predict the gas production, displacement per atom (dpa), induced activity, decay dose, and decay heat in various WC grades. This paper examines how different binder materials affect neutron attenuation, gas production, displacements per atom (dpa), and radioactive waste production, including waste categorisation. The choice of binder material has little effect on neutron attenuation, dpa, and helium production. However, it significantly affects hydrogen production, induced activity, and gamma dose rate after shutdown. The paper further discusses the impact of binder materials on the production of intermediate level radioactive waste, as well as the major contributing radionuclides (to activity) that are produced during operation. The paper also compares some crucial mechanical properties – flexural strength, fracture toughness, elastic modulus, grain size, and grain size distribution of these grades at room temperature.