The paper addresses the design optimization of the radiation chamber of Sludge Hygenization Research Irradiator (SHRI). The distribution and placement of source assembly are the parameters of optimization. The physical parameters that are associated with radiation effects are the absorbed dose rate, the dose distribution and radiation quality. The objective of the present study is to achieve increased average absorbed dose rate with improved distribution by minimal alteration in the geometry of the existing SHRI that was designed by the Bhabha Atomic Research Centre and installed in Vadodara, India. A new approach is proposed to evaluate the time averaged absorbed dose rate and is validated. A better design for the arrangement of source assembly inside the irradiator vessel of SHRI has been proposed.