HTGR Unit Fuel Pebble k-infinity Results Using Chord Length Sampling

There is considerable interest in transport models that will permit the simulation of neutral particle transport through stochastic mixtures. Chord length sampling techniques that simulate particle transport through binary stochastic mixtures consisting of spheres randomly arranged in a matrix have been implemented in several Monte Carlo Codes [1-3]. Though the use of these methods is growing, the accuracy and efficiency of these methods has not yet been thoroughly demonstrated for an application of particular interest – a high temperature gas reactor (HTGR) fuel pebble element. This paper presents comparison results of k∞ calculations performed on a LEUPRO-1 pebble cell. Results are generated using a chord length sampling method implemented in a test version of MCNP version 4C [3]. This Limited Chord Length Sampling (LCLS) method eliminates the need to model the details of the microheterogeneity of the pebble. Results are also computed for an explicit pebble model where the TRISO fuel particles within the pebble are randomly distributed. Finally, the heterogeneous matrix region of the pebble cell is homogenized based simply on volume fractions. These three results are compared to results reported by Johnson et al. [4], and duplicated here, using a cubic lattice representation of the TRISO fuel particles. Figures of Merit for the four k∞ calculations are compared to judge relative efficiencies.