Moderator materials

The moderator material and optimum sizes, the extraction channel configuration, as well as the converter and reflector material and sizes, were determined using the MCNP-4B code and the steepest descent method to attain a maximum flux density of thenual neutrons at the position of an object to be studied. The calculated data were experimentally verified, which showed good agreement. 

Radiolytic oxidation is important to the design and operation of reactors because it adversely affects key graphite properties and, by removing moderator material, may bring about the need for increased fuel enrichment. As mentioned earlier, an inhibitor (methane) is added to the coolant to reduce radiolytic oxidation to acceptable levels. However, access of the inhibitor to the inner portions of the moderator brick must be assured. Two approaches have been adopted in the AGRs to provide this access. Vertical methane access holes are provided in the fuel bricks and in the later stations, Heysham II and Tomess, a pressure drop from outside to inside the brick was established to cause an enhanced flow through the brick. The amount of inhibitor added must be restricted, however, because the carbon inhibition reaction product deposits on the fuel pin and restricts heat transfer to the coolant, thus reducing reactor efficiency. 

Kelly, B.T., Nuclear reactor moderator materials. In Materials Science and Technology Nuclear Materials, Part 1 (VCH Weinheim, 1994) pp. 365-417. 

Moderate. Materials their packaging, both of which will contribute fuel to fire Low. Materials which, in themselves, will not normally ignite, but which in combination with their packaging will contribute fuel to fire... 

The cross sections for (n,y) reactions common in reactor thermal neutron activation generally decrease with increasing neutron energy with the exception of resonance-capture cross section peaks at specific energies. This reaction is, therefore, not important in most 14 MeV activation determinations. However, some thermalization of the 14 MeV flux may always be expected due to the presence of low Z elements in the construction materials of the pneumatic tubes, sample supports, sample vial, or the sample itself (particularly when the sample is present in aqueous solution). The elements Al, Mn, V, Sn, Dy, In, Gd, and Co, in particular, have high thermal neutron capture cross sections and thermal capture products have been observed in the 14 MeV neutron irradiation of these elements in spite of care taken to reduce the amount of low Z moderating materials in the region of the sample irradiation position 25>. 

In Figure 6b, a multibed reaction system is shown in which only part of the reactant is fed to each bed. The reactant fed to each bed is essentially completely converted and the reaction products from that conversion become part of the temperature-moderating material for each succeeding bed. In such a reaction system the methanol feed to each succeeding bed would be mixed with aromatics made in the preceding beds - - an arrangement that tends to increase durene formation. 

Fissionable Nuclides (includes fission) Moderating Materials ... 

Capability of burning. Flammable. The relative combustibility of materials in storage is defined as hazardous - materials that by themselves or in combination with their packaging, are easily ignited and will contribute to the intensity and rapid spread of a fire moderate -materials and their packaging both of which will contribute fuel to a fire noncombustible - materials and their packaging that will neither ignite nor support combustion. 

T. S. Lundy, E. E. Gross, An Evaluation of Solid Moderating Materials, U.S. AEC Report ORNL-2891, Oak Ridge National Laboratory, Oak Ridge, TN, April 1960. 

Reactors of this kind, in which the fuel is physically separated from the moderator, are said to be of the heterogeneous type, while in homogeneous reactors the fuel is directly dissolved in the moderator material. Homogoieous reactors have only been built for experimental purposes. 

Principles of Nuclear Power TABLE 19.3. Physical constants of some moderator materials... 

The cross sections for neutron capture increase for all atoms for thermal energy neutrons. As a result, even though low cross section materials are used some neutrons are captured by the structural and moderator materials. The probability for the non-capture of thermal neutrons in this fashion is signified by/, the thermal utilization factor, which in our case can be assumed to be 0.9. Thus of the original N neutrons 112 thermal neutrons remain in the second generation to cause fission in the nuclear fuel. 

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