Measurement of Thermal Neutron Spectra in High Temperature Graphite Bulk Poisoned with Boron

Thermal neutron spectra in the graphite bulk of slab geometry with boron carbide were measured at temperatures from 20 to -800°C by the time-of-flight method, using a 100 MeV electron linear accelerator. The results were compared with calculation based on the multi-group S 8 method using Young-Koppel's scattering law. Fairly good agreement is obtained between the calculated and measured spectra except in a limited range of energy just above the peak of the thermal spectra. Considering the uncertainty in the spectrum calculations, the scattering kernel of graphite based on the scattering law will be accurate sufficiently to evaluate the infinite multiplication factor of a graphite moderated reactor within an error of about 0.15%. It is also emphasized from the two-dimensional S 4 calculations that the effect of reentrant hole perturbation should be so significant in the graphite bulk as to decrease the peak of the thermal spectra by 10-12%.