Fast spectrum vs. Thermal spectrum

Neutron cross-section for fission.
Fast spectrum vs. Thermal spectrum

Nuclear Power -> Nuclear Power Plant -> Types of Reactors -> Fast Neutron Reactor

A fast neutron reactor is a nuclear reactor in which the fission chain reaction is sustained by fast neutrons. That means the neutron moderator (slowing down) in such reactors is undesirable. This is a key advantage of fast reactors, because fast reactors have a significant excess of neutrons (due to low parasitic absorbtion), unlike PWRs (or LWRs).

Sodium-cooled Fast Reactor (SFR).

Sodium-cooled Fast Reactor (SFR).

On the other hand such reactors must compensate for the missing reactivity from neutron moderator efect. They use fuel with higher enrichment when compared to that required for a thermal reactor. Fast reactors require enrichments about 10%, or more. Most fast reactors use a hexagonal lattice cells (as VVER reactors) in order to reach smaller volume ratios of coolant to fuel. Generally, fast reactors have to utilize much more compact nuclear cores than thermal reactors (PWRs or BWRs) in order to reach required core reactivity. This implies the fast reactor cores achieve higher power densities. As a consequence, they cannot use water as coolant, because of its moderating properties and insufficient thermal properties. The solution given this problem is to use another coolant as liquid sodium or lead.

Lead-cooled Fast Reactor (LFR)

Lead-cooled Fast Reactor (LFR).

Fast reactor fuel may be metal or a ceramic, encapsulated in metal cladding, unlike the PWR’s zirconium cladding. Liquid metals are the most widely used coolant because they have excellent heat transfer properties and can be employed in lowpressure systems. Sodium-cooled fast reactors (SFRs) are the most common designs. Because sodium reacts violently with water, however, SFRs require the placement of an intermediate heat exchanger between the reactor core and the steam generator. This hi-tech technology requires a lot of experience, therefore only few countries have developed their own fast reactor design (e.g. Russia, USA, France, Japan, ). Especially Russians continue in fast reactor developement program with their BN reactors.

Breeder reactor

Neutrons can breed fuel

Free neutrons can “breed” more fuel from otherwise non-fissionable isotopes.

A breeder reactor is essentially a particular configuration of a fast reactor (but not only FBR can be used as a breeder). Fast reactors generally have an excess of neutrons (due to low parasitic absorbtion), the neutrons given off by fission reactions can “breed” more fuel from otherwise non-fissionable isotopes or can be used for another purposes (e.g.transmutation of spent nuclear fuel). The most common breeding reaction is an absorbtion reaction on uranium-238, where a plutonium-239 from non-fissionable uranium-238 is produced. A key parameter of breeder reactors is a breeding ratio, although this ratio describes also thermal reactors fuel cycle.

The term “breeder” refers to the types of configurations which can be the breeding ratio higher than 1. That means such reactors produce more fissionable fuel than they consume (i.e. more fissionable Pu-239 is produced from non-fissionable uranium-238, than consumed initial U-235+Pu-239 fuel).

See also: Breeder Reactor


  • FBRs have improved neutron economy
  • FBRs can recycle nuclear waste
  • FBRs can produce fuel for thermal reactors
  • FBRs liquid metals have superior heat transfer properties
  • FBRs do not use pressure vessel


  • FBRs must use superior control system
  • FBRs can have positive reactivity feedback from void coefficient
  • Liquid metals require special technology and handling
  • Fast reactor technology can be more expensive

Generation IV reactors

In 2003 the Generation IV International Forum (GIF) representing ten countries announced the selection of six reactor technologies which they believe represent the future shape of nuclear energy. These were selected on the basis of being clean, safe and cost-effective means of meeting increased energy. Three of the six reactors are fast reactors and one can be built as a fast reactor, one is described as epithermal. Only two operate with slow neutrons like today’s plants.


Liquid Metal cooled Fast Reactors designs. Integral (pool) design vs. Loop design

Liquid Metal cooled Fast Reactors designs. Integral (pool) design vs. Loop design