**Abstract**

Numerical simulation allows a better understanding of thermal-hydraulic phenomena that can take place in thermal installations. It is a capital contribution, especially in accident situations. The code RELAP5/Mod3.2 makes it possible to predict the thermal-hydraulic behavior of these installations during the normal and accidental operations. The present chapter focuses on accidental transient modeling and simulation of an industrial steam boiler by the code RELAP5/Mod3.2. This steam boiler is radiant type, high power, natural circulation, and a single drum. The model of the boiler developed for the RELAP5/Mod3.2 code encompasses the entire installation. The control loop of the water level in the steam drum and the superheated steam temperature are also included in the model. The qualification process of the steam boiler model is based on the steam boiler operation data under steady-state operating conditions. The comparative study shows that the theoretical results of the code RELAP5 are in good agreement with the operating data of the installation. To evaluate the behavior and response of the boiler in accident situations, the loss of feedwater following pump power loss, with and without protective operations, was simulated.

**Keywords:** industrial steam boiler, natural circulation, safety, modeling and simulation, RELAP5/Mod3.2, accidental transient
