**1. Introduction**

Electricity production largely depends on the production of steam using coal, gas, or nuclear fission of uranium as heat sources. To produce steam, it is necessary to heat the water to its boiling point and then to provide a sufficient amount of heat to change the boiling water into steam. Steam production and utilization techniques are therefore important aspects of engineering technology. The steam generator is one of the means used to produce steam. A steam boiler plays an important role in all types of industries; it is one of the key components of a thermal installation. The main function of the steam boiler is to produce steam for the purpose of using it for industrial reasons such as the production of electrical energy, petro-chemistry, district heating, and others [1]. In general, the steam boilers can be classified in two categories: water-tube and fire-tube steam boilers. The choice of the type of industrial steam boiler to be selected can be made according to several criteria, the main one being the thermal power to be supplied or its equivalent in production of steam.

In the steam boiler, several problems can occur during its service because it works in severe conditions (high pressure, high temperature, corrosive environment, and continuous operation). These problems have an influence on steam boiler operation and sometimes lead to serious consequences such as explosions. Indeed, accidental transient was already observed during normal operation of the installation [2]; the most important and the most frequent are loss of feedwater, loss of flow, pipeline ruptures, loss of electrical power, equipment failures, and others. Early detection of such faults under operation is of great importance. Therefore, it is very necessary to perform an accident analysis to evaluate causes and make an assessment of the accidents' consequences [3]. Finally, it is important to consider the safety aspects and analysis of the steam boiler to guarantee the reliability and stability.

Steam boiler is a complex equipment considering the nonlinear, phase change, and inverse response behavior (shrink and swell). However, the operating conditions of the steam boiler are very difficult to control because all the parameters are interrelated. In addition, the steam boiler has very high manufacturing, operating, and maintenance cost. Hence, it is very difficult to take measurement and carry out tests directly on steam boiler. However, modeling and simulation are also effective tools for safety assessment and prediction of installation behavior of real process under transient conditions. The usefulness of numerical simulation tools is mainly based on the development of numerical methods, the progress of programming, and the provision of powerful computing resources [4].

The power plants' safety is largely based on simulation [5]. Nowadays, the bestestimate nuclear system codes such as TRAC [6], RETRAN [7], RELAP5 [8, 12], ATHLET [9], CATHARE [10], and APROS [11] are widely used to investigate the thermal-hydraulic characteristics of nuclear power plants either during steady-state operation or accidental transients and simulate the overall behavior of the installation (pumps, piping, heat exchangers, tanks, valves, control loops, etc.).

They are mainly produced to simulate the behavior of nuclear installations, but they can also be used to study the normal and accidental operation of conventional thermal, industrial, and solar installations [13, 14, 1, 2].

RELAP5 code used to carry out the present study is a thermal-hydraulic analysis system code of a realistic estimation level (best estimate). It is used to simulate the thermal-hydraulic transient of light water systems during postulated accidents [15]. RELAP5 is widely used in nuclear safety studies; its scope extends to energy systems using water and its vapor. Research work in this direction is very limited to the nuclear field. Extrapolation of the code scope is possible for the thermal-hydraulic behavior study of an industrial boiler [1, 2, 16].

In this chapter, realistic simulation of the global behavior of an industrial natural circulation steam boiler during normal and accidental operation is performed using RELAP5/Mod3.2 system code with a thermal-hydraulic performance analysis of the main equipment of the installation. A better understanding of the physical phenomena occurring during all phases of a hypothetical accident is necessary for the safety of an installation. The accidental transient simulated in this chapter is the loss of feedwater (pump stop) with and without protective operations. The chapter is divided into the following sections:


*Numerical Simulation of the Accidental Transient of an Industrial Steam Boiler DOI: http://dx.doi.org/10.5772/intechopen.86129* 

