The accountability and safeguard offissile material like plutonium (Pu) is very important for the nuclear fuel cycle programs. As per the IAEA guidelines for limit for disposal of alpha bearing nuclear material waste is 4000 Bq/gm. A monitoring system based on the gamma spectrometry technique employing NaI(TI) detector, for the assessment of ²³⁹PU activity in the waste drum, was developed and installed. In the present work the simulation of the detector and the assay system was carried out using Monte Carlo techniques. The sensitivity of the assay system for plutonium was evaluated by assuming homogeneous distribution of activity within the matrix material. The sensitivity factors obtained by simulation were 11.28 mg/cps and 11.3 mg/cps for cotton and rubber medium respectively. The simulation results were compared with the experimental data generated by distributing standard sources in the matrix material. This paper describes the details of the assay system and the Monte Carlo simulation techniques adopted for the sensitivity calibration.