Nuclear Energy: Reactors, Fuel Cycle, Radiation and Alternatives

Item 7: Energy from Atomic Nuclei

Chemical vs Nuclear Reactions

The energy that comes from atomic nuclei differs from chemical energy. In chemical reactions, changes occur in the distribution of electrons orbiting the nuclei, while nuclear reactions involve changes to the nuclei themselves. Matter cannot be created or destroyed; it is only transformed — in nuclear reactions this transformation releases energy.

Controlling Nuclear Reactions

A common system used to control nuclear chain reactions is the use of a moderator, which slows neutrons and helps regulate the fission process.

Technical Achievements of Nuclear Energy

Reactor Classification

Nuclear reactors are classified according to several criteria:

• By neutron speed

  Thermal reactors and fast reactors — classification depends on the speed of the neutrons produced in fission reactions.

• By fuel used

  Reactors use either natural uranium or enriched uranium as fuel.

• By moderator

  Common moderators include light water, heavy water, and graphite.

• Coolant and heat exchangers

  Coolants and heat exchangers transfer heat from the reactor core to the turbine or secondary circuit. Some reactors use water as the primary coolant.

• Pressurized water reactors

  Pressurized water reactors (PWRs) use water as both moderator and coolant under high pressure; they typically operate with slightly enriched uranium as fuel.

• Boiling water reactors

  Boiling water reactors (BWRs) generate steam directly in the reactor vessel; this technology has been widely developed in the United States, Sweden, and Germany.

• Graphite-moderated boiling reactors

  Graphite-moderated boiling water reactors use graphite as the moderator while boiling water acts as the coolant.

Nuclear Fuel Cycle

Many nuclear power plants operate with enriched uranium. The fuel cycle includes extraction of radioactive minerals, processing of the fuel, and subsequent treatment of waste. The stages of this process are:

Stages of the fuel cycle

• Obtaining and preparing nuclear fuel

  Mining of uranium and thorium ores, converting the ore into the appropriate chemical form, and enrichment to increase the concentration of fissile isotopes when required.

• Irradiation

  The fuel at this stage is irradiated by neutrons, causing fission of atomic nuclei and producing energy.

• Recovery, processing and storage

  Reprocessing can recover still-fertile materials to produce more energy; radioactive waste is processed and stored or conditioned for long-term management.

Types of Radiation

• Alpha (α): Particles with positive electric charge, similar to helium ions — two protons and two neutrons.
• Beta (β): Particles with negative (or positive, in beta-plus decay) electric charge; beta particles have mass and charge similar to electrons (or positrons).
• Gamma (γ): Electromagnetic radiation waves with no associated rest-mass particle.

Half-Life Concept

To describe the average life of a radioactive element, the concept of half-life is used: it is the time required for a given number of radioactive atoms to decrease by half.

Alternatives and Research Directions

Science and technology address problems by researching ideas and developing solutions. Current and proposed research directions include:

• Development of low-residue fission processes: Research aims to design fission processes that generate fewer high-activity wastes — a field that could promote "smart" nuclear energy rather than high-waste solutions.
• International research on safe waste storage: Experts often conclude that confining waste underground in geologically stable locations is a viable long-term solution; international research focuses on identifying the safest sites and methods.
• Research on transmutation: Investigating methods to transmute long-lived radionuclides into shorter-lived or stable isotopes.
• Investigation of fusion processes: Continued research into fusion seeks long-term energy solutions with different waste and safety profiles compared with fission.