Fuel element, design fast reactors

Kazakhstan has a nuclear scientific-industrial complex which was set up as a part of a nuclear infrastructure of the former USSR. More than 50% of the uranium resources of the former Soviet Union are in Kazakhstan, with seven uranium mines. Two UO2 plants produced up to 35% of the total uranium in the USSR in 1990. There are extensive facilities for producing UO2 pellets for VVER fuel elements from Russian enriched uranium. Kazakhstan has several research reactors and one operating nuclear power plant, the BN-350 fast reactor, which started operation in 1973 with a design life of 20 years. Work on its lifetime extension has the intention of bringing it into compliance with current safety standards. 1995 and 1996 were devoted to this work. In October 1996. experimental investigation on accident-proofdecay heat removal by natural circulation was carried out. The reactor BN-350 was restarted in February 4, 1997 at a power level of 420 MW(th). 

Stainless-steel-clad UOj-PuO fuel elements have been assumed for the reference design of the gas-cooled fast reactor cores, although comparative evaluations of stainless-steel-clad carbide fuel elements have been made 14). In either case, the type of fuel element envisioned for the gas-cooled fast reactors is an assembly of approximately 1-cm-diam, stainless-steel-clad fuel rods. The subdivision of the fuel must be very great in order to achieve the high core power densities and fuel ratings required for reasonable performance with a fast-neutron-spectrum reactor. 

The VKR-MT fuel assembly was designed with the use of a certain experience in the design of assemblies with pebble beds of micro fuel elements and lateral coolant flow, available for gas cooled fast reactors [X-4]. 

Thermal analysis is a major feature of the design of liquid-metal cooled fuel elements and assemblies in a fast reactor core. A brief discussion of a typical thermal design approach is therefore useful for the identification of many of the heat transfer questions in the core area. 

The calculated elemental composition, radioactivity, and decay-heat rate for discharge fuel are shown in Table 8.7 for the uranium-fueled PWR (cf. Fig. 3.31), in Table 8.8 for the liquid-metal fast-breeder reactor (LMFBR) (cf. Fig. 3.34), and in Table 8.9 for the uranium-thorium-fueled HTGR (cf. Fig. 3.33). These quantities, expressed per unit mass of discharge fuel, are useful in the design of reprocessing operations. For the purpose of comparison, all quantities are calculated for 150 days of postirradiation cooling. 

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