Very high temperature reactor fuel cycle

Although the reductions in plant costs from the magnox to the AGR and high-temperature reactors have been and will continue to be important factors in the development of economic gas-cooled reactors in the power range of 500 to 1000 MWe, it is to be expected that future improvements in capital costs of very large plants will be less significant than possible improvements in fuel cycle economics. For example, the differ-... 

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. 

The eommereial HTS and LTS eatalysts require activation by careful pre-reduetion in situ and, once activated, lose aetivity very rapidly if they are exposed to air. Further, the HTS eatalyst is inactive at temperatures <300°C, while the LTS eatalyst degrades if heated to temperatures >250°C. The automotive application, because of its highly intermittent duty cycle, requires alternative water-gas shift catalysts that (1) eliminate the need to sequester the eatalyst during system shutdown (2) eliminate the need to aetivate the eatalyst in situ (3) inerease toleranee to temperature exeursions and (4) reduee the size and weight of the shift reactors. Another desirable property for an automotive WGS catalyst is tolerance to ppm levels of sulfur in the feed stream because sulfur species are present as contaminants or additives in conventional fuels (30 parts per million weight [ppmw] in future gasoline gets converted to 3 ppmv H2S in reformate). 

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