Pebble bed thermal-mechanical theoretical model Application at the geometry of test blanket module of ITER-FEAT nuclear fusion reactor

摘要

This paper deals with a research activity on the test breeding blanket modules (TBM) performed in the framework of ITER-FEAT project. In particular, a solid breeder (a lithium compound) and a solid neutron multiplier (beryllium) in the form of pebble bed are taken into consideration. Several nuclear, thermo-hydraulic and thermo-mechanical problems need a better knowledge in order to technologically develop these pebble bed blankets. These problems concern mainly the theoretical modelling of the thermal-mechanical behaviour of the pebble bed. This paper illustrates a theoretical and numerical model to simulate the thermal-mechanical behaviour of the pebble bed. This model is based on the analysis of the elastic-plastic interactions between the pebbles. The mechanical (stiffness) and thermal (conductivity) characteristics of the bed is deduced, considering it made up of regular lattices of pebbles (simple cubic, cubic centred body and cubic centred face) and applying the combination rules of series or parallel systems. The combination of these regular lattices depends on a packing factor (the ratio between the bed density and the density of the pebble material). Moreover, with this model, it is possible to simulate the steady state creep under static pressure and high temperature. In order to simulate complex geometries as the module of the breeding blanket of the nuclear fusion reactor, the theoretical model has been used for implementing a new finite element in a finite element methods (FEM) code. This finite element simulates the thermal-mechanical behaviour of an assembly of regular lattices. In order to assess the theoretical model and to gain useful information for the implementation of the finite element, a numerical model of pebble bed made up of beam elements has been elaborated. The numerical results obtained with this mesh have been compared with experimental ones, found in the technical literature. The comparison shows a good agreement between the numerical and experimental results in the simulations of uniaxial compressive tests, creep tests and heat transfer.