Neutron absorption profile in a reactor moderated by different mixtures of light and heavy waters

• We studied neutron absorption spectra in a mixed water moderated reactor.
• Changing D2O% in moderator induced neutron energy spectral shift.
• Most of the neutrons absorbed in control rods were epithermal.
• Control rods worth changes were not proportional to changes of D2O% in moderator.
• Control rod arrangement influenced the neutronic behavior of the reactor.

A Monte-Carlo parametric study was carried out to investigate the neutron absorption profile in a model of LR-0 reactor when it is moderated by different mixtures of heavy/light waters at molecular ratios ranging from 0% up to 100% D2O at increments of 10% in D2O. The tallies included; neutron absorption profiles in control rods and moderator, and neutron capture profile in 238U. The work focused on neutron absorption in control rods entailing; total mass of control rods needed to attain criticality, neutron absorption density and total neutron absorption in control rods at each of the studied mixed water moderators. The aim was to explore whether thermal neutron poisons are the most suitable poisons to be used in control rods of nuclear reactors moderated by mixed heavy/light water moderators.