Good neutron economy

One of the distinctive features of a fast reactor is its good neutron economy. Utilizing the excess of neutrons enables us to constract flexible cores such that they breed or bum plutonium in consideration of plutonium balance, incinerate MA and long lived fission products for reducing radio toxicity and improve safety. 

The LSPR has a good neutron economy, resulting in a breeding ratio of about unity and an enhanced transmutation capability. When operated in a closed fuel cycle with a nuclear energy park (see Fig. XXV-5), the total system is fissile self-sustainable and ensures that the radiotoxicity of disposed waste is comparable to that of the extracted natural uranium. Moreover, the transmutation requirements to a nuclear energy park can be reduced because all actinides could be effectively recycled in the LSPR. 

A new reactor bumup strategy CANDLE was proposed, in which relative distribution shapes of neutron flux, nuclide densities and power density are constant over reactor lifetime but move in the axial direction with a velocity proportional to the power rate during the whole life of reactor operation. This strategy requires a special pattern of the core s infinite neutron multiplication factor changes with bumup, and can be applied to a fast reactor with good neutron economy. 

Because of the small reactivity margin available for breeding in a thermal reactor, the use of the thorium cycle has mainly been associated with reactors with very good neutron economy based on low parasitic absorption, such as the high-temperature gas-cooled reactor, where graphite is used in place of metal for the fuel cladding, or heavy water reactors, with very low moderator absorption. A special case is the molten salt breeder reactor, where circulation of the fissile and fertile materials allows continuous removal not only of Pa but also of fission products. 

Because of its good neutron economy, the heavy water reactor is an obvious candidate for the role of thermal near-breeder or advanced converter, using the fuel cycle. The value of the cycle depends on... 

The quite good neutron economy allows use of low initial enrichment in comparison with traditional water-cooled reactors, guaranteeing better utilization of the primary energy source. 

HWRs with a good neutron economy (such as the Swedish BHWRs) have very low fuel cycle costs. Pig, 10 shows the capitalized cost advantages earning from the fuel cycle for BHWRs optimized for natural uranium compared to LWRs. It will be seen that this Is substantially Independent of reactor output, but quite strongly dependent on uranium price, normalized fabrication costs per kg and reprocessing costs per kg. 

Two-region UOa-PuOa-DzO power reactors [11]. The major advantage associated with two-region reactors is that good neutron economy can be combined wdth relatively low inventory requirements. None of the uranium-plutonium fueled aqueous homogeneous reactors appear... 

Gas solubilities in bismuth. The question of the solubility of the fission-product gases xenon and krypton in bismuth is of extreme im-poi ttmce. In particular, Xe , a strong neutron poison, must be removed from the sy.stem as fast as it is formed in order to have a good neutron economy. 

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