In order to analyze the performance of a multi-pass shell and tube type heat exchanger used for decay heat removal operation in fast breeder reactor, a computer code has been developed using network model. The model is based on 1-D finite difference method (FDM) formulations. The transient momentum and continuity equations are solved simultaneously to obtain pressure field and flow rates, viz., inlet flow over a distributed inlet window, cross flow between radial zones, and axial flows along the length. Intermixing between radial zones is modeled using cross flow. Transient energy equation is solved to obtain temperatures. The heat transfer capacity of the heat exchanger is estimated as 8.93 MW when the hot pool is at 547 °C. Natural circulation flow rate of primary sodium is found to be 46.3 kg/s. The results are compared with the CFD results and a reasonable agreement is found. A transient study is also carried out and the results are given in this paper.