In the field of energy engineering, it is essential to have a thorough understanding of radioactivity and radiation, as they play a crucial role in nuclear power systems. This subchapter aims to provide engineers with a comprehensive overview of these concepts, their properties, and their applications in the field of nuclear power. Radioactivity refers to the spontaneous emission of radiation from the nucleus of an atom. This emission occurs due to the instability of certain atomic nuclei, known as radionuclides. Radionuclides can emit different types of radiation, including alpha particles, beta particles, and gamma rays. Each type of radiation possesses distinct characteristics and poses varying levels of risk to human health and the environment. Alpha particles are positively charged particles consisting of two protons and two neutrons, similar to a helium nucleus. Due to their large size and positive charge, alpha particles have a limited range and can be easily stopped by a sheet of paper or a few centimeters of air. Beta particles, on the other hand, are high-speed electrons or positrons emitted by certain radionuclides. They have a smaller mass and can penetrate further than alpha particles but can be stopped by a few millimeters of aluminum or plastic. Gamma rays are electromagnetic radiation of high energy and frequency. Unlike alpha and beta particles, gamma rays have no mass or charge, enabling them to penetrate several centimeters of lead or even meters of concrete. Due to their penetrating ability, gamma rays pose the greatest risk to human health and require appropriate shielding measures. Understanding the properties and behavior of radiation is of paramount importance when designing and operating nuclear power systems. Engineers must ensure the safety of both workers and the general public by implementing robust shielding measures, monitoring radiation levels, and minimizing exposure. Furthermore, engineers need to be familiar with the various applications of radiation in energy engineering. Nuclear power systems rely on controlled nuclear reactions to generate heat, which is then converted into electricity. Radioactive isotopes are used in the production of nuclear fuels, such as uranium and plutonium, while radiation is employed in the inspection and testing of materials, such as welds and pipelines, to ensure their integrity and reliability.