International Conference on Mathematics, Computional Methods & Reactor Physics 2009
Multigroup (MG) nodal and pin power reconstruction methods based on the source expansion formulation are established for analyzing transient problems by extending the corresponding steady-state formulation. It was tried to retain the original form of the steady-state routines as much as possible with an elaborated treatment of the transient specific source terms. In addition, a simple formulation for removing the rod cusping effect is also introduced. The MOX core transient benchmark is analyzed to examine the MG effects on the predicted evolution of the rod ejection accident using the RENUS MG nodal code. The core power, 3-D peak pin power and the hottest pin enthalpy behaviors obtained with three sets of homogenized group constants (2, 4 and 8 groups) are compared. The effect of the proper delayed neutron spectrum is also investigated together with the effect of the replacement of the effective delayed neutron fractions with the physical ones. The results indicate that the core power behavior obtained more energy group shows a considerably earlier and higher peak, but the resulting hottest pin enthalpy behaviors do not differ significantly, leading to only about 7% error in the two group case.
