Analysis of the stresses in a side reflector block of the pebble bed reactor

The side reflector of the pebble bed reactor is exposed to a high fast neutron dose during operation. At the end of its service life maximum doses of approximately 3-4x1022 n/cm-2 (EDN) are reached. The shrinkage induced in the graphite by the neutron flux results in internal stresses in the individual reflector blocks which may cause damage of the side reflector, e.g. chipping of major fragments. For calculating the internal stresses in graphite structures the SIGMA code was developed. This is a two-dimensional finite-element-program permitting the calculation of time-dependent stresses and deformations of graphite reactor components. The loads may be exerted by primary loads, temperatures, and shrinkage induced by irradiation doses. The SIGMA program permits to take into account the particular material properties of the graphite such as shrinkage and creep under the effect of fast neutrons as well as the anisotropy of the mechanical properties and thermal expansion specific to numerous materials. In all calculations the horizontal middle plane of the block was considered and generalized plane strain conditions were assumed