- 21. -


neutron spectrum in graphite can be used to obtain averaged uranium cross sections, even though the uranium contains no graphite. The neutron spectrum of graphite will actually be slightly modified in the fuel region due to epithermal and thermal absorptions in uranium. The temperature of the graphite will essentially determine the distribution of thermal neutrons. The influence of uranium absorptions on the thermal neutron spectrum is through second order effects, since the absorption mean free path for thermal neutrons in the fuel region of a
                                     a
zero power plasma reactor is about Xu  = 1700 cm.  For this value, a 78 meter thickness of uranium plasma would be required to attenuate the thermal neutron intensity by one per cent. The epithermal neutron spectrum of graphite will be modified at energies coinciding with large uranium resonance cross sections. From the above arguments, the zero power plasma reactor is seen to be well moderated, with an epithermal neutron spectrum determined by slowing down collisions in graphite, and a thermal spectrum determined by the graphite temperature.

        Extensive one and two dimensional transport theory calculations have been used to study the effects of materials, temperature and dimensions on the criticality of gaseous core reactors (22). For the purpose of calculating detailed design parameters, it will probably be necessary to use the more exact computational methods