Comparison of two- and three-dimensional calculations of super prompt critical excursions

Calculations of super prompt critical excursions induced by ramp variation of absorption cross sections are performed in various geometries (r, xy, rz, xyz). The time behavior of neutron flux and fuel temperature is analyzed, especially with regard to their dependence on geometry and rod velocity. The evaluations establish the need for genuine three-dimensional calculations to correctly predict peak flux and temperature distributions for situations where space-dependent feedback effects are essential.