Numerically investigation of the neutron irradiation swelling on the mechanical and structural evolution of beryllium pebble bed

Pebble bed is employed in the blanket system to achieve neutron multiplication and tritium breeding. Neutron irradiation among the pebble bed is inevitable when the tokamak device is in normal operation. Particles could be swelled and the formation of bubbles or pore on the surface could aggravate the roughness of particles the surface under irradiation. It has great impact of pebble bed structure. In this paper, the mechanical and structure characteristics of the Beryllium (Be) pebble bed in the Helium Coolant Ceramic Breeding (HCCB) blanket were investigated. Furthermore, the evolution process of the pebble bed with mesoscale is analyzed as well. The Discrete Element Method (DEM) with Particle Flow Code (PFC) software is utilized to simulate the variation of particles under neutron irradiation with different roughness of particle surfaces and irradiation doses. Meanwhile, 0, 1%, 2%, 4% and 6% irradiation of single particle in radius direction with 0, 0.1, 0.3 and 0.5 friction coefficients were assumed. The evolution of the local packing factor, coordination number, contact force and force chain are given, and the fabric analysis is also given. The results show that the irradiation swelling of the particles and the deterioration of the surface have great impacts on the structure and mechanical properties of the pebble bed under neutron irradiation. Particle swelling could improve the packing factor and coordination number of the pebble bed, and the contact force distribution. The deterioration of the particle surface reduces the coordination number and shortens the force chain length. This paper presents the working condition under neutron irradiation which could provide reference for the design and evaluate of the blanket pebble bed.