Spectrum, thermal neutron, hardening

As the absorption becomes stronger, the reactor becomes less thermal, and the spectrum of neutrons in the thermal energy region becomes hardened with respect to an equilibrium Maxwellian distribution. This hardening is frequently described in terms of a neutron temperature which exceeds the moderator temperature, but this description has no theoretical basis. For small l/v absorption, the hardening is best described in terms of the function G v) defined in Equation (13). This function depends on the model used for thermalization calculations, but not on the absorption. The sensitivity of this function to scattering model has not been well explored. 

The subcadmium activation distributions were used in conjunction with cross sections computed by Westcott to calculate values of the thermal utilization f and the thermal migration area L in the usual way. A base value of V was calculated from Westcott values, assuming the neutron flux spectrum in the moderator to be Maxwellian at 2(PC. This value was then modified for flux hardening effects >y comparing the ratios of the 1/v activations (Cu and Mn) and the U-235 activations at various locations. Values of the fast fission factor < were obtained by comparing the fission product activities of natural and depleted uranium foils according to the technique described by Futch . The neutron age r was measured to indium resonance from isolated fuel assemblies in DjO. Corrections were calculated for the age to thermal energy and for lattice effects. 

Thermal expansion of the sodium results in a net positive reactivity feedback. Thermal expansion results in fewer sodium atoms within and surrounding the core. The reduced density surrounding the core results in fewer neufrons being scattered back into the core and produces a small negative feedback effecf by increasing fhe leakage around the periphery. However, the dominant effect is the reduchon of collisions between neutrons and sodium atoms, which hardens the neutron energy spectrum and yields a net positive reactivity feedback effect. 

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