The criterion used to select neutron shielding was the amount of hydrogen it contained. Lithium hydride was found most suitable. The hydrogen number density is 5.9 E 22 atoms/cm3 and the bulk density of the material is 0.775 g/cm3. This shield is intended to stop neutrons with energy above 1 eV. Power generation in the shield due to the nuclear reaction 6Li (n, a) 3H was examined, since 6Li is 7.5% naturally abundant. Calculations using DOT3.5 showed that power generation was negligible and need not be considered in the design. Helium generated in this nuclear reaction will accumulate over the life of the reactor and cause a pressure buildup in the shield. The shielding material was divided into ten discs and covered with container material in order to lessen the stress applied to the container. Tungsten was chosen as the gamma shield since high Z elements are appropriate in this application. The shape of the shield was taken as a frustrum. Shield dimensions were a function of power level, cone half angle and distance between payload and shield. Total five year radiation dose at the payload location 25 m from the reactor was calculated assuming thermal power of 6.2 MWt. Dose as a function of radial location is shown in Fig. 7(a, b).
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