Very-high-temperature reactor description

This appendix provides a brief description of the Fast Flux Test Facility (FFTF) fuel handling system and its operation as described by Cabell (1980) and in the Fast Flux Test Facility System Design Description (FFTF, 1983). The description is limited to those system features that are potentially relevant to the refueling of a liquid-salt very high-temperature reactor (LS-VHTR). Because the FFTF was designed as a reactor to test fuel, it has additional capabilities and equipment compared with a sodium-cooled fast reactor designed only to produce electricity. 

Description Ammonia and carbon dioxide react at 155 bar to synthesize urea and carbamate. The reactor conversion rate is very high under the N/C ratio of 3.7 with a temperature of 182-185°C. Unconverted materials in synthesis solution are efficiently separated by C02 stripping. The milder operating condition and using two-phase stainless steel prevent corrosion problems. Gas from the stripper is condensed in vertical submerged carbamate condenser. Using an HP Ejector for internal synthesis recycle, major synthesis equipment is located on the ground level. 

Description Ammonia and carbon dioxide react at 150 bar to yield urea and ammonia carbamate. The conversion in the reactor is very high due to favorable NH3/CO2 ratio of 3.5 1 and operating temperature of 185°C to 190°C. These conditions prevent corrosion problems. Carbamate is decomposed in three stages at different pressures in the stripper at the same pressure as the reactor, in the medium-pressure decomposer at 18 bar and in the low-pressure decomposer at 4.5 bar. 

Since no detailed description of the experimental techniques used in the cited works will be given in the main text, we would like to note that most experiments in the 1930s, as well as experiments at very high (thousands of atmospheres) pressures [46—48] were performed in static reactors. In later studies, predominantly flow reactors have been used, which are more suited to the conditions of the practical implementation of the process. Experiments in flow reactors have been carried out over a wide pressure range (1—300 bar) and initial temperatures of 300—600 °C and above, with the reaction time ranging from a second to tens of minutes. In addition, more exotic reactors, such a cylinder of a high-compression internal combustion engine [49] or a rapid compression machine [50] were used. 

In such systems modeling meets very serious difficulties, since the problem of formulation of a kinetic description of reactions in the adsorbed layer on the active metal is added and interferes with other problems stated above. One of the first attempts to suggest such a description was done by Hickman and Schmidt (1992, 1993). Analyzing a nearly 10-year period of development in the area, Schmidt (2001) concluded that ... these apparently simple processes are in fact far more complicated than the usual packed bed catalytic reactor assumptions used for typical modeling. First, the temperatures are sufficiently high that some homogeneous reaction may be expected to occur, even at very... 

A description of the fuel basin is given to help in drawing conclusions from the results. The basin was built during the last reconstruction of the reactor, i.e. between 1986 and 1992. The reconstructed reactor went critical for the first time in December 1992. Regular operations started in November 1993, and the basin has been in use since then, i.e. the first spent fuel elements were removed from the reactor core and immediately put into the basin in 1994. The fuel elements are normally used up to a relatively high burnup of 60%. They are exposed to temperatures of up to 60°C in the reactor. The temperature of the storage pool is about 20°C, with very limited seasonal variations. 

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