Therefore the neutron reflectors are installed in PWR and BWR reactor cores. On the other hand, common water volume cannot be used as a reflector in radial directionВ because it is of the highest importance to maintain high flow rates in the core and not bypass fuel assemblies. This axial reflector does not form any special device, and the neutrons are simply reflected by the core inlet and outlet coolant. In pressurized water reactors, water serves as an axial reflector. Water, heavy water, beryllium, or graphite are commonly used as reflectors. Therefore the material should not get oxidized.Д®ssentially, for thermal reactors, a good moderator is also a good reflector because most moderators also possess the properties mentioned above of a good reflector. Resistance to Oxidation вЂ“ The reflector is often situated in a chemically aggressive environment (especially in fast reactors).Radiation stability вЂ“ Radiation stability is a very important property of reflectors because the reflector material will be exposed to high levels of radiation (especially gamma radiation and fast neutrons).

Its material must possess the following properties to be an effective reflector. Since the moderation of neutrons is undesirable in fast reactors, they use only high Z materials.

Since main differences among reactor types arise from differences in their neutron energy spectra, we must specify whether we mean reflectors in fast reactors or thermal reactors. In general, reflector materials are normally not fissionableВ and have high scattering cross-sectionВ and low absorption cross-section. There are a variety of materials that are used as a reflecting medium for neutrons in nuclear reactors. The lower support structure, the neutron reflector, and 11 fuel assemblies. Materials and Types of Neutron Reflectors Top view of the WWER-1000 nuclear core. Where D refl is the diffusion coefficient in the reflector, and the L refl is the reflectionвЂ™s diffusion length. For sufficiently thick reflectors, it can be derived that albedo becomes: Still, in all practical cases, the geometry of a reflector must conform to the geometry of a reactor core and a reactor pressure vessel. An infinite reflector will have the maximum albedo. The value of the albedo will depend on the composition and thickness of the reflector. The albedo can be expressed in terms of neutron currents as: This ratio is called the albedoВ or the reflection coefficient. The reflectorвЂ™s efficiency is measured by the ratio of the number of neutrons reflected back into the reactor to the number entering the reflector.

Neutron reflectors serve as a thermal and radiation shield of a reactor core.

Neutron reflectors reduce a coolant flow bypass of a core.Neutron reflectors reduce neutron leakage, i.e., to reduce the neutron fluence on a reactor pressure vessel.The decrease in the critical size of a core required is known as reflector savings. Alternatively, if the core size is maintained, the reflector makes additional reactivity available for higher fuel burnup. This means that the minimum critical size of the reactor is reduced. The neutrons reflected back into the core are available for a chain reaction. The neutron reflector scatters back (or reflects) into the core many neutrons that would otherwise escape.In the outer regions of the core, this fuel now contributes much more to the total power production. Because of the higher flux at the edge of the core, there is much better utilization in the peripheral fuel assemblies.Therefore reflectors reduce the non-uniformity of the power distribution. The neutron flux distribution is вЂњ flattenedвЂќ, i.e., the ratio of the average flux to the maximum flux is increased. Neutron reflector inside a reactor core of LWR.In LWRs, the neutron reflector is installed for the following purposes: By reducing neutron leakage, the reflector increases k eff and reduces the amount of fuel necessary to maintain the reactor critical for a long period. The neutron reflector scatters back (or reflects) into the core many neutrons that would otherwise escape (i.e., reduces the neutron leakage). The neutron reflector is a non-multiplying medium, whereas the reactor core is a multiplying medium. The reflector reduces the non-uniformity of the power distribution in the peripheral fuel assemblies, reduces neutron leakage, and reduces a coolant flow bypass of the core. It is well known that each reactor core is surrounded by a neutron reflector or reactor core baffle. In nuclear reactors, a neutron reflector is a component that reflects neutrons, reduces a coolant flow bypass, and serves as a thermal and radiation shield of a reactor core.

Reflector definition