Types of Nuclear Reactors

The Nuclear Power industry classifies Nuclear Reactors based on the:

• Type of Nuclear Reaction,
• Nature of the Moderator Material,
• Coolant Type, and
• Reactor Design Types

 

Types of Nuclear Reaction

Nuclear fission – All commercial nuclear reactors operate under the principles of nuclear fission, for which they generally use uranium. Fission reactors can be further categorized based on the energy of the neutrons that sustain the fission chain reaction:

• Thermal reactors, representing almost all current nuclear fission reactors, use slowed or thermal neutrons. In principle, these reactors use neutron moderator materials that slow neutrons until their neutron temperature is “thermalized,” that is, until their kinetic energy approaches the average kinetic energy of the surrounding particles.
• Fast neutron reactors use fast neutrons to cause fission in their fuel. They do not have a neutron moderator, and use less-moderating coolants. Maintaining a chain reaction requires the fuel to be more highly enriched in fissile material (about 20 percent or more).

Nuclear fusion – Fusion power is an experimental technology envisioned to use hydrogen as fuel.

Radioactive decay – Examples include radioisotope thermoelectric generators as well as other types of atomic batteries. These devices generate heat and power by exploiting passive radioactive decay.

Nature of Moderator Material

Nuclear reactors are moderated by various materials and methods, categorized one of the following:

• Graphite moderated reactors
• Water moderated reactors, which include:
  • Light water moderated reactors (LWRs) – Light water reactors use ordinary water to moderate and cool the reactors. When at operating temperature, as the water temperature increases, its density drops, resulting in the passing through of fewer neutrons that are slowed enough to trigger further reactions, thus stabilizing the reaction rate.
  • Heavy water reactors – Graphite and heavy water reactors tend to be more thoroughly thermalized than light water reactors, allowing for the use of natural uranium fuel.
• Light element moderated reactors – These reactors are moderated by lithium or beryllium.
  • Molten salt reactors (MSRs) are moderated by a light elements such as lithium or beryllium, which are constituents of the coolant/fuel matrix salts LiF and BeF2.
  • Liquid metal cooled reactors, such as one whose coolant is a mixture of Lead and Bismuth, may use BeO as a moderator.
• Organically moderated reactors (OMR) use biphenyl and terphenyl as a moderator and coolant.

Coolant Type

In thermal nuclear reactors, the coolant acts as a moderator that must slow down the neutrons before they can be efficiently absorbed by the fuel. The various coolant reactor types include:

• Water cooled reactors, which are categorized as:
  • Pressurized water reactor (PWR) – Representative of the majority of commercial naval reactors, the primary characteristic of PWRs is a specialized pressure vessel, typically referred to as a “pressurizer.” During normal operation, a pressurizer is partially filled with water, and a steam bubble is maintained above it by heating the water with submerged heaters. The pressurizer is connected to the primary reactor pressure vessel (RPV) and the pressurizer “bubble” provides an expansion space to handle the changes in water volume in the reactor.
  • Boiling water reactor (BWR) – BWRs are characterized by boiling water around the fuel rods in the lower portion of primary reactor pressure vessel. During normal operation, pressure control is achieved by controlling the amount of steam flowing from the reactor pressure vessel to the turbine.
• Liquid metal cooled reactors – Since water is a moderator, it cannot be used as a coolant in a fast reactor. Therefore, liquid metal coolants (i.e. sodium, NaK, lead, lead-bismuth eutectic, and in early reactors, mercury) have been used.
• Gas cooled reactors are cooled by a circulating inert gas (i.e. typically helium, though nitrogen and carbon dioxide have also been used). Depending on the particular reactor, utilization of the residual heat varies. But some run hot enough that the gas can directly power a gas turbine.
• Molten Salt Reactors (MSRs) are cooled by circulating a molten salt, typically a eutectic mixture of fluoride salts, such as LiF and BeF2. In a typical MSR, the coolant is also used as a matrix in which the fissile material can be dissolved.

Reactor Design Types

The nomenclature for reactor designs, describing four ‘generations’, was proposed by the U.S. Department of Energy (US DOE) when it introduced the concept of Generation IV reactors. Reactors:

• Generation I Reactors, developed in the 1950-1960s time frames, are only found operating in the United Kingdom.
• Generation II Reactors comprise about 85 percent of the world’s nuclear power, referring to the class of nuclear reactors constructed up to the end of the 1990s.
• Generation III Reactors represent the newest and evolutionary designs developed during the operating era of the Generation II reactor designs.
• Generation IV Reactor designs are still on the drawing board and will not be operational before 2020.

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