The ARIES team is presently studying a fusion power plant based on the sphe
rical tokamak (ST) concept. This paper addresses the key nuclear issues for
spherical tokamaks and illustrates the impact of the neutronics factors on
the ARIES-ST design. A three-dimensional analysis was carried out for an i
nterim design to determine the key nuclear parameters. Preceding the 3-D an
alysis, a series of parametric I-D analyses were performed to guide the des
ign toward the final configuration. Comparing the I-D and 3-D results, impo
rtant differences were identified and attributed mainly to the angular dist
ribution of the incident source neutrons on the first wall. Those differenc
es are unique to spherical tokamaks.