Re-Analysis of Recent Neutron Diffusion and Transmission Measurements in Nuclear Graphite

We revisit the determination by Bowman et al. of unusual neutron transport characteristics for a newly fabricated form of graphite [Nucl. Sci. Eng., 159, 182 (2008); Nucl. Sci. Eng., 161, 68 (2009)]. From MCNP modeling and consideration of data from other experiments, we determine revised values for the neutron transport parameters of this graphite. Our reanalysis gives a coherent scattering cross section coh ˜ 4 b at 50 meV, a small-angle neutron scattering cross section sans ˜ 11 to 13 b at 1 meV, and an effective capture cross section a = 5.8 ± 0.5 mb. Scaled to a graphite reference density of 1.60 g/cm³ , we find a diffusion coefficient [overbar D] = 0.94 ± 0.03 cm and a diffusion length L = 47.7 ± 3.7 cm. Apart from the somewhat larger values of a and [overbar D], these are not untypical parameters for graphite. Based on our investigation, the recent experiments and analysis of Bowman et al. do not give evidence for different transport properties for this newly fabricated graphite.