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The decrease in the critical size of core required is known as the reflector savings. Neutron reflectors reduce neutron leakage, i.e., to reduce the neutron fluence on a reactor pressure vessel. Neutron reflectors reduce a coolant flow bypass of a core. Neutron reflectors serve as a thermal and radiation shield of a reactor core.
A neutron supermirror is a highly polished, layered material used to reflect neutron beams. Supermirrors are a special case of multi-layer neutron reflectors with varying layer thicknesses. [1] The first neutron supermirror concept was proposed by Ferenc Mezei, [2] inspired by earlier work with X-rays.
Moderation is the process of the reduction of the initial high speed (high kinetic energy) of the free neutron. Since energy is conserved, this reduction of the neutron speed takes place by transfer of energy to a material called a moderator. The probability of scattering of a neutron from a nucleus is given by the scattering cross section. The ...
In all of these cases, the use of a neutron reflector like beryllium can substantially drop this amount, however: with a 5 centimetres (2.0 in) reflector, the critical mass of 19.75%-enriched uranium drops to 403 kilograms (888 lb), and with a 15 centimetres (5.9 in) reflector it drops to 144 kilograms (317 lb), for example.
Flattop is a benchmark critical assembly that is used to study the nuclear characteristics of uranium-233, uranium-235, and plutonium-239 in spherical geometries surrounded by a relatively thick natural uranium neutron reflector. Flattop assemblies are used to measure neutron activation and reactivity coefficients. Since the neutron energies ...
The pits of the first nuclear weapons were solid, with an urchin neutron initiator in their center. The Gadget and Fat Man used pits made of 6.2 kg of solid hot pressed plutonium-gallium alloy (at 400 °C and 200 MPa in steel dies – 750 °F and 29,000 psi) half-spheres of 9.2 cm (3.6 in) diameter, with a 2.5 cm (1 in) internal cavity for the initiator.
Any material with a positive neutron optical potential can reflect UCN. The table on the right gives an (incomplete) list of UCN reflecting materials including the height of the neutron optical potential (V F) and the corresponding critical velocity (v C). The height of the neutron optical potential is isotope-specific.
A subcritical reactor —regardless of neutron spectrum— can also "breed" fissile nuclides from fertile material, allowing in principle the consumption of very low grade actinides (e.g. Spent MOX fuel whose plutonium-240 content is too high for use in current critical thermal reactors) without the need for highly enriched material as used in ...