NEMSQR: A 3-D multi group diffusion theory code based on nodal expansion method for square geometry

•A three dimensional diffusion theory code, based on Nodal Expansion Method (NEM), is developed.•The code uses quartic flux expansion with quadratic transverse leakage approximation.•The code is mainly used for calculation of keff, neutron flux and integral kinetics parameters.•The code is tested against several static benchmark problems.•Results corroborate well with the results available in literature.

A three dimensional, multigroup, neutron diffusion theory based computer code NEMSQR (Nodal Expansion Method for SQuare geometry) is developed for square geometry in order to perform reactor core calculation. The code is based on Nodal Expansion Method (NEM). In this method, an inhomogeneous matrix equation, which involves spatial moments of nodal flux distribution and surface averaged partial currents across the nodal surfaces, is derived using fourth order polynomial approximation to spatial dependence of nodal flux. Discontinuity factor is incorporated into the code to reduce homogenization error. This code is used for calculation of effective multiplication factor, neutron flux (both direct and adjoint flux), integral kinetics parameters and subcritical count in presence of external neutron source. The numerical studies reported here for several benchmark problems related to light water reactor as well as fast breeder reactor demonstrate the accuracy of the code.