Pebble Bed, reactor

SiHcon carbide s relatively low neutron cross section and good resistance to radiation damage make it useful in some of its new forms in nuclear reactors (qv). SiHcon carbide temperature-sensing devices and stmctural shapes fabricated from the new dense types are expected to have increased stabiHty. SiHcon carbide coatings (qv) may be appHed to nuclear fuel elements, especially those of pebble-bed reactors, or siHcon carbide may be incorporated as a matrix in these elements (153,154). 

Alternative reactor types are possible for the VHTR. China s HTR-10 [35] and South Africa s pebble bed modular reactor (PBMR) [41] adopted major elements of pebble bed reactor design including fuel element from the past German experience. The fuel cycles might be thorium- or plutonium-based or potentially use mixed oxide (MOX) fuel. 

Lee, )-)., et al. (2007), Numerical Treatment of Pebble Contact in the Flow and Heat Transfer Analysis of a Pebble Bed Reactor Core , Nucl. Eng. and Design, 237, 2183-2196. 

Either to reduce the size so much that core melt accidents almost certainly can be contained by the vessel used (this involves maximum unit sizes of 50-100 MW in a traditional design, while the pebble-bed reactor may circumvent this limitation, if the integrity of the pebbles can be guaranteed),... 

High temperature rectors were very successful in Germany as pebble bed reactors (PBR s) with the AVR-reactor in Jiilich over 20 years, 50 MW l/h, and with the THTR 300 in Hamm-Ueontrop with 300 MW, which was tested electrically for 3 years. 

A 10 MW(th) HTGR test module, HTR-10 [89], is under construction since 1995 by the Institute of Nuclear Energy Technology (INET) of Tsinghua University in Beijing. It is a pebble bed reactor with 27,000 spherical fuel elements and a coolant outlet temperature of 700 °C (later 950 °C). Its operation is expected to start end of 1999. 

The prototype pebble bed reactor THTR-300 [50] has demonstrated the expected large-scale applicability of HTGR technology [10] and the relatively low level of radioactivity in the HTGR plant. The measured generator power of 303 MW(e) translates into an efficiency of 39.7 %. The early shutdown after 423 efpd was not related to the reactor concept neither to any safety concerns, but primarily associated with technical and economic problems. 

FIG. 20.3. Schematic drawir of the German pebble bed reactor fTHTR). 

A rather unique design is the pebble bed reactor, see Figure 20.3. It is a helium cooled graphite moderated reactor with a core consisting of a bed of spheres each about 6 cm in diameter. The fuel is initially based on a mixture of carbides as microspheres... 

There is some continuing interest in gas-cooled reactors in South Afi ica, China, and Russia. ESKOM, the South African state-operated utility, is interested in a hi -temperature, gas-cooled reactor combined with a direct cycle gas turbine for powering rural areas that are currently without electricity. They have developed a preliminary design for a system based on the pebble bed reactors developed in Germany. General Atomic and Russia, in cooperation with others, have also completed a study on design of a gas-cooled gas turbine plant for use in Russia. China has operated a 5 MWt gas-cooled reactor and has plans for construction of a 200 MWt which is intended for process heat applications. 

FIGU RE 21.20 Fuel spheres used In a hIgh-temperature pebble-bed reactor. 

A Figure 21.20 Fuel spheres used in a high-temperature pebble-bed reactor. The image on the right is an optical microscope image of a fuel particle. 

In a high-temperature pebble-bed reactor, the fuel elements are spheres ( pebbles ) roughly the size of an orange (A Figure 21.20). The spheres are made of graphite, which acts as the moderator, and thousands of tiny fuel particles are embedded in the interior of each sphere. Each fuel particle is a kernel of fissionable material, typically... 

Pebble bed. In a pebble-bed LS-VHTR, the core is filled with pebbles, which flow through the core over a period of time. The gas-cooled variant of this option was developed in Germany. A gas-cooled pebble-bed test reactor is operating in China, and a precommercial gas-cooled pebble-bed reactor is currently being built in South Africa. Refueling in a liquid-salt-cooled pebble reactor would be relatively easy and would be done online, as is the case with the gas-cooled reactors. 

Unlike other fuel forms, the ratio of fuel and moderator to liquid coolant is fixed in a pebble-bed reactor. This places major constraints on the choice of coolant (it most likely will require the use of a salt with enriched lithium-7 and beryllium) and other core design parameters. While this salt is more expensive, it has very low parasitic neutron capture, which combined with the very small excess reactivity and large cylindrical core, would provide high fuel utilization. Initial studies on a liquid-salt-cooled pebble-bed reactor have been conducted at Delft University in the Netherlands. 

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