Space-Energy-Dependent Physics Problems of Plutonium Fuels in HTGRs

The plutonium loadings were determined for a test element for irradiation in the Peach Bottom HTGR. The test element is a graphite circular cylinder with an array of eight fuel rods in a circular pattern. The fuel rods contain dense spherical kernels of PuO₂ or PuO₂/3 ThO₂. Be cause of the strong Pu resonances, the heterogeneity effects in the test element are strongly marked. A code for treating this rather complicated problem rigorously was not available. Thus the problem was simulated within a onedimensional transport code by equating three collision probabilities. The heterogeneities were seen to result in significant grain selfshielding, rod selfshielding, and mutual shielding of the rods. The Pu loading for the central fuel body of the test element was determined to be 19.0 g.