Pebble bed

The unit Kureha operated at Nakoso to process 120,000 metric tons per year of naphtha produces a mix of acetylene and ethylene at a 1 1 ratio. Kureha s development work was directed toward producing ethylene from cmde oil. Their work showed that at extreme operating conditions, 2000°C and short residence time, appreciable acetylene production was possible. In the process, cmde oil or naphtha is sprayed with superheated steam into the specially designed reactor. The steam is superheated to 2000°C in refractory lined, pebble bed regenerative-type heaters. A pair of the heaters are used with countercurrent flows of combustion gas and steam to alternately heat the refractory and produce the superheated steam. In addition to the acetylene and ethylene products, the process produces a variety of by-products including pitch, tars, and oils rich in naphthalene. One of the important attributes of this type of reactor is its abiUty to produce variable quantities of ethylene as a coproduct by dropping the reaction temperature (20—22). 

For gas-fired systems the state-of-the-art is represented by the preheater described in Reference 69. A pebble bed instead of a cored brick matrix is used. The pebbles are made of alumina spheres, 20 mm in diameter. Heat-transfer coefficients 3—4 times greater than for checkerwork matrices are achieved. A prototype device 400 m in volume has been operated for three years at an industrial blast furnace, achieving preheat temperatures of 1670 to 1770 K. 

The low (ca 2%) yield of NO, the tendency to revert to N2 and O2 if the product stream is not quenched rapidly, the consumption of large (ca 60,000 kWh/1N2 fixed) amounts of electricity, and the concomitant expense to sustain the arc all led to the demise of this process. The related Wisconsin process for oxidising N2 at high temperatures in a pebble-bed furnace was developed in the 1950s (13). Although a plant that produced over 40 t/d of nitric acid was built, the product recovery costs were not economically competitive. 

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). 

The Arbeitsgemeinschaft Versuchsreaktor (AVR) and Thorium High-Temperature Reactor (THTR-300) were both helium-cooled reactors of the pebble-bed design [29,42,43]. The major design parameters of the AVR and THTR are shown in Table 10. Construction started on the AVR in 1961 and full power operation at 15MW(e) commenced in May 1967. The core of the AVR consisted of approximately 100,000 spherical pebble type fuel elements (see Section 5). The pebble bed was surrounded by a cylindrical graphite reflector and structural carbon... 

Fig. 14. HTGR fuel elements (a) prismatic core HTGR fuel element (b) cross section of a spherical fuel element for the pebble bed HTGR. Reprinted from [88], 1977 Ameriean Nuelear Soeiety, La Grange Park, Illinois.

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. 

The rate of reaction of air over a hot MgO pebble bed to form nitric oxide is given by... 

In many poor African states there is no electricity grid or coverage is very limited, but there are often dispersed locations that could use significant amounts of energy—an aluminum smelter in Mozambique, for example. A nuclear power plant could provide electricity, but South African efforts to introduce a new small-scale technology, the Pebble Bed Modular Reactor, which is far safer than previous reactors and can be controlled and shut down remotely, are being hampered by international rejection of older nuclear technologies.12 (See Cohen, this volume, about nuclear power science and politics in the United States.)... 

K. Renun, A New Erafor Nuclear The Development of the Pebble Bed Modular Reactor (Cambridge European Science and Environment Forum [ESEF], 2000). Available atwww.scienceforum.net. 

Cottrell, Frederick G. (1877-1948). American scientist, inventor of an electrostatic precipitator, now known as Cottrell Precipitator, for smoke, dust fumes. Among other inventions are the pebble bed furnace, boiling point apparatus the Cottrell-Daniels process for fixation of atmospheric nitrogen. Cottrell was Director of US Bureau of Mines Director of the Fixed Nitrogen Research Laboratory, and founder of the Research Corporation, a nonprofit organization... 

Pebble-Bed Modular Reactor (PBMR) A nuclear reactor technology that utilizes tiny silicon carbide-coated uranium oxide granules sealed in pebbles about the size of oranges, made of graphite. Helium is used as the coolant and energy transfer medium. This containment of the radioactive material in small quantities has the potential to achieve an unprecedented level of safety. This technology may become popular in the development of new nuclear power plants. 

NuStart made its start in September 2005 with the selection of two potential sites for new reactors Grand Gulf, located near Port Gibson, Mississippi and owned by a subsidiary of Entergy and Bellefont, located near Scottsboro, Alabama and owned by the TVA. Rather than using pebble-bed technology, NuStart is promoting the use of water-cooled reactors. Its next step is to seek COLs for the sites from the NRC. 

The Northern States Power facility experienced significant operational problems with their electrolyzed pebble bed scrubber during tests burning from 7 to 9 percent TDF (mixed with woodwaste) in a retrofitted fluidized combustion bed boiler. The electrostatic voltage dropped to near zero on several occasions on others, the collection efficiency declined continually. Several reasons for this are suggested. First, the ash during the test was more cohesive... 

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. 

TRANSIENT MODELLING OF S-l CYCLE THERMOCHEMICAL HYDROGEN GENERATION COUPLED TO PEBBLE BED MODULAR REACTOR... 

Transient modelling of sulphur-iodine cycle thermochemical hydrogen generation coupled to pebble bed modular reactor... 

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