Abstract
In pebble-bed reactors, the fuel is contained in small grains, which are included in a graphite matrix. Some burnable poison particles may also be present. In this work, an additional ’double heterogeneity’ model is introduced in the DRAGON5 lattice code. The model is based on the legacy work of She (INET) and has been improved to overcome intrinsic limitations. It is based on a simplified physical model whereas the two already existing models were based on the collision probability analysis or on renewal theory. The advantage of this new model is its simplicity to implement. The theory shows that the correction suggested in the original model should not be arbitrary, but a constant equivalent particle fraction of 0.63. Numerical comparison between the models is generally good. However it does not support the theory of a constant equivalent fraction. Additional work is needed to reduce the discrepancy between the models in some cases.
Highlights
In pebble-bed reactors, the fuel is contained in small grains, which are included in a graphite matrix
Several approaches can be found in the literature to take into account properly the double heterogeneity of geometries such as in pebble bed
The model proposed by She is introduced in a subroutine of DRAGON which computes the equivalent cross-sections of a composite
Summary
In pebble-bed reactors, the fuel is contained in small grains, which are included in a graphite matrix. The fuel dispersion reactors is studied and a new ’double heterogeneity’ model is introduced in the DRAGON5 lattice code, based on the legacy work of She (INET). The later is very easy to implement, but some of its extensions to resonance self-shielding and fuel depletion had to be tested. DRAGON5 code already included the first two models, the later was programmed as proposed by She. The advantage of using the same code is that no bias is introduced when comparisons are done. The only differences come from the calculations of the equivalent cross-sections in the composite material, not the resonance treatment or the tracking options for examples.
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