For developing the simulation techniques applicable to engineering system, the modeling of a vertical inverted U–tube steam generator (UTSG) operating in a Nuclear Power Plant (NPP) has been carried out for finding the temperature distribution, optimum boiling length in the tube and addressing the interaction of technical and environmental factors involved in the system. Modeling of a system consists of three stages: (i) Development of computer code, (ii) Application of the model in the steam generator, (iii) Evaluation of the model in the power plant. Computer model has the ability for optimization of steam generator parameters to be used in simulation. The steam generator modeling for the temperature distributions comprises of development of a specific mathematical model considering the real engineering constraints. The modeling approach used for the simulation of a conventional boiler has to be revised, since the heat transfer regime in each tube can not be fixed by the equipment design. General equations have to be used for each tube of the boiler and the actual heat transfer conditions in the tube has to be identified. The solution of the model has been discussed analytically under steady state conditions. Empirical equations are available to predict the saturation temperature in each region of the steam generator. The computed results show the temperature distribution of the primary fluid along the whole length of the tube and the point of saturation temperature, where steam formation starts. The behaviours of important parameters involved in the process are studied by the comparison of simulated results. The analytical solution is based on certain simplifying assumptions, which to some extent limits the scope of its applicability. A numerical simulation technique has been developed by adopting Euler–Cauchy method to obtain a solution independent of the assumptions made in the analytical solution. This makes the model more realistic and flexible and enables to find the relative temperature distribution behaviours of the primary and secondary fluids in UTSG and an optimum boiling length of the tube (Lb). The main advantage of the proposed method is that it permits a better understanding of the influence of the design parameters on the cycle performance. A boundary condition needs to be prescribed along the tube to start calculations.7 The solution of the model has also been discussed under unsteady (Transient) conditions. To simulate the transients, the mathematical model is discretised in terms of time and space coordinates. The flow model in transient state is used to derive a time dependent finite difference simulation technique which gives the relative temperature distributions of the fluids and an optimum boiling length (Lb) with respect to time from 0.0% Reactor Power (RP) level to 100% RP level. For further analysis, the graphs of the temperature distributions of secondary and primary fluids are drawn at specified intervals of time. These results can be used for a multi–objective optimization of the steam generator in future. The model provides a significant analytical capabilities for the specialists working in the field of NPP safety. The purpose of the work is to predict the behaviour of steam generator working in NPP and to help correctly for defining the operator action validation and verification of Emergency Operating Instruction (EOIs). The model is practically feasible under prevailing constraints. Evaluation of the model in terms of calibration, sensitivity and verification yields a good agreement between observed and computed results (hydrograph) of the steam generator and its components. The work on unsteady (transient) heat flow in a UTSG is very scarce in the literature. Mostly, the steady flow problems have been discussed. Few attempts have been made regarding the transient flows. Keeping the above facts in view, the entire work of the present thesis is organized as follows: Chapter 1 consists of some introductory remarks, complete modeling process, explanation of computer codes, the physical model and general features of Nuclear Power Plant (NPP) and Nuclear Steam Generator (NSG). Objectives, scope of study and previous work are also mentioned in this chapter. Chapter 2 studies the basic preliminaries relevant to the laws for system analysis and energy balance equations. Parallel and counter flow heat exchangers and their temperature distributions with length are explained. Shell and tube and multipass shell and tube heat exchangers are also discussed. Heat exchangers effectiveness against the number of transfer units is described at the end of this chapter. Chapter 3 describes modeling and simulation of the steam generator in steady state – a case study. The formulation of the mathematical model and its analytical solution are given. Equation which governs the primary side incompressible fluid flow in8 a UTSG is also modeled here. Model layout and solution procedure is also discussed in this chapter. One fortunate outcome of the graphical approach works best to find percentage changes in the outlet temperature of the primary fluid corresponding to any desired changes in the mass flow rate (m) and the factor Rh. The results of this chapter are published in “Pakistan Journal of Scientific Research, Vol. 60, pp. 40–43, (2008)”. Chapter 4 is concerned with the formulation of numerical models for primary and secondary of fluids. The numerical scheme evaluates the temperature distribution of the primary fluid relative to the temperature distribution of the secondary fluid and vice– versa for the user specified input data. An iterative procedure is followed until the program converges for an optimum boiling length of the tube using relative error criteria. A reasonable agreement between experimental observations and numerical solutions is presented through graphs. These results are published in “Pakistan Journal of Engineering and Applied Sciences, Vol. 4, pp. 74–79, (2009)”. Chapter 5 deals with the solution of the model of UTSG in transient condition. The flow model in transient state is used to derive a time dependent finite difference simulation technique which gives the relative temperature distributions of the fluids and an optimum boiling length (Lb) with respect to time from 0.0% Reactor Power (RP) level to 100% RP level. The results obtained from the suggested models have been found to be in good agreement with the experimental data. The contents of this chapter are published in “Pakistan Journal of Engineering and Applied Sciences, Vol. 5, pp. 10–15, (2009)”. Some observations from chapter 5 have also been accepted for publication in “International Journal of Mathematical Modeling, Simulation and Applications, (2009)”. To upgrade the existing control system, an improved control strategy can be developed. The numerical simulation techniques show that the procedures are effective and can be used for a multi–objective optimization of the steam generator in future.