4.15.4.1 Introduction

One of the issues that need to be addressed is the thermal and mechanical behavior of constrained peb­ble beds under (cyclic) nuclear loading conditions experienced in a tritium breeding blanket.

The way a pebble bed responds to a thermal load depends primarily on the thermal transport proper­ties of the bed, such as the packing factor of the bed, and the thermal conductivities of the pebble material and the surrounding purge gas. In addition, due to differences in the thermal expansion coefficients between the pebble-bed material and its surrounding structure, stresses will be induced in both the pebble bed and the structural material, and the contact
areas between pebbles and pebbles and walls become an important parameter. Also, during longer term operation in the neutron-irradiation environment, swelling of the breeder material will generate stresses. Furthermore, the pebble-bed thermal transfer prop­erties may deteriorate with irradiation dose and lithium burnup.

These induced stresses may directly or indirectly affect the functional operation if the mechanical integrity of the blanket element is endangered or if heat or tritium removal is significantly deterio­rated due to pebble fracture, sintering, or melting. Various creep phenomena will affect the actual evo­lution of stresses, like thermal creep and irradiation creep. In addition, chemical reactions in between pebbles and between structures and pebbles may be enhanced under high contact pressures and the compositional changes arising from long-term oper­ation by lithium burnup and other transmutation reactions.

When the (constrained) pebbles experience stres­ses, by either compression due to thermal expansion or irradiation-induced swelling, these stresses will be (partially) relieved through thermal creep, that is, irreversible deformation of the pebbles. Under high stresses and at high temperatures, typically 0.6-0.8 times the melting temperature TM, these relaxation processes include sintering of the material. This irre­versible strain and deformation in the material can lead to embrittlement and, ultimately, to fragmenta­tion of the pebbles.

The fracture of pebbles results in an inhomoge­neous pebble-bed density and contact area and would lead to an inhomogeneous temperature distribu­tion that is hard to model or predict. The lack of predictability of the temperature distribution can be a major safety issue. A mechanically stable pebble bed with a high packing factor is desired.