Thermomechanics of solid breeder and Be pebble bed materials

The thermomechanical interaction of solid breeder and beryllium pebble beds with structural material (BSMI) has been identified as a critical issue for solid breeder blanket designs. For example, the expansion of pebble beds restrained by blanket structure exerts stresses on the pebbles as well as the blanket structure wall, which might cause the pebbles to break and jeopardise the blanket operation. However, at elevated temperatures thermal creep will reduce these stresses and might compensate for stress build-up due to irradiation-induced swelling. A significant influence of irradiation on the pebble bed properties is expected. Computationally, the BSMI can be assessed in two ways: (i) by applying appropriate finite element codes combined with the description of modules for the pebble beds. As input, these modules require data on characteristic pebble bed properties determined in different standard-type tests; (ii) by numerical simulations based on a discrete numerical model. Here, the stress profiles are calculated while the effective modulus and bed thermal expansion coefficients are back estimated. In this paper, recent experimental results on thermomechanical pebble bed properties for ceramic breeder (metatitanate and orthosilicate) pebble beds and beryllium pebble beds are presented, including data on the moduli of deformation, thermal creep, inner friction angle, and thermal conductivity of deformed pebble beds. Furthermore, modelling results of the BSMI for simple geometries are reported based both on homogeneous and discrete models and are compared with experimental results.