Reducing Parasitic Thermal Neutron Absorption in Graphite Reactors by 30%

Abstract

The results of two experiments combined show that the diffusion length D for thermal neutrons in the graphite studied is 24% larger than expected from classical experiments and that the boron equivalent absorption is smaller than expected and consistent with zero. Taken together, the results indicate a reduction in parasitic thermal neutron absorption in heterogeneous graphite reactors by about 30%. The first experiment measured the z-dependence of thermal neutron flux in a column of 12 t of granular graphite with a neutron source at the bottom. A second measurement was made by pulsing the column with a neutron source at its center and measuring the neutron decay rate as a function of time after a pure exponential decay had been established. The diffusion coefficient D adjusted to a density of 1.60 g/cm³ is 1.05 ± 0.03 cm compared with the commonly accepted value of 0.85 ± 0.013 cm. The absorption in our graphite owing to impurities was found to be <10% of that from carbon alone. The parameter Σ_a/D that measures neutron loss was determined to be 0.000235 ± 0.000026 cm⁻² for a density of 1.60 g/cm³ and may be compared with the commonly accepted value of 0.000340. The performance of graphite thermal spectrum reactors constructed using our graphite would be significantly enhanced over present expectations because neutron loss to graphite is a major factor in the neutron economy of graphite-moderated thermal reactors.