Development of a Nodal Expansion Method based space-time kinetics code for hexagonal geometry

A two and three dimensional, multigroup, neutron space-time kinetics theory based computer code IQSHEX (Improved Quasi Static method based kinetics code for HEXagonal geometry) is developed in order to carry out reactivity initiated transient in large size thermal (i.e. neutronically loosely coupled reactor core) and fast breeder reactors with hexagonal lattice. For such reactors, neutron flux is considered as function of both space and time and neutron space-time kinetics equations are solved for obtaining the dependence of flux on space and time. This code uses Improved Quasi Static (IQS) method to solve the equations. In this method, it is assumed that shape function changes slowly relative to amplitude function and product of these two functions gives the complete behavior of neutron flux both in space- and time. Multigroup neutron diffusion equation is solved over macro (or larger) time step by advanced Nodal Expansion Method (NEM) to find out shape function, and point kinetics equations are solved over micro (or smaller) time step by Piecewise Constant Approximation (PCA) method to find out amplitude function. The code has been validated against several international benchmark problems with and without temperature feedback. Results corroborate well with that of the benchmark problems.