Pebble type fuel elements

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... 

Decision to cancel the HHT project in 1981 and to concentrate instead on a shorter term feasible HTR with the steam cycle Thermal power 3000 MJ/s with block-type fuel elements and later with pebble bed core. 

The fabrication cost of a core based on pebble bed of micro fuel elements is 40% lower than that of a traditional core based on rod-type fuel elements [X-4]. The reason behind this is that the fabrication of a pebble bed core eliminates costly welding and air-tightness control operations, as well as mechanical treatment of uranium dioxide kernels. 

Loading and unloading of the THTR reactor core was carried out while the bumup of the pebble-shaped fuel elements was monitored by means of a bumup measuring system. The main component of this system was the bumup measuring reactor (Solid Moderated Reactor) in which a reactivity effect is caused by operating elements as they pass through the reactor. Evaluation of this effect permits determination of the type of element and, in case of fuel elements, the bumup of the element. 

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. 

The main design variants of this type of reactor have depended on whether fuel is of the pebble bed type or of the prismatic type. The pebble bed type consists of a large number of spherical fuel elements. The fuel element matrix is graphite and the fiael kernels are imbedded within the inner layer of the matrix. A fuel free graphite reflector shell is located inside the RPV. 

TRISO type UO2 fuel as a bed of micro fuel elements in direct contact with coolant or in HTGR-type pebble or compact configuration for thermal spectmm reactors. 

No operations with fuel are performed during the entire reactor lifetime therefore, there is no storage capacity for fresh or spent fuel elements on the site. Different from pebble bed high temperature gas cooled reactors (HTGRs) and previous high temperature molten salt cooled reactors with HTGR type fuel (abbreviated as VTRS in Russian), the MARS concept incorporates no pebble transport. 

More long-lived radionuclides have to be considered for final disposal of the waste. Another problem is the dust production in the HTGR-pebble bed core -type from the circulation of the spherical fuel elements. The expected dust mass in the AVR primary circuit is about 70 kg which is expected to be evenly distributed on the primary system surfaces. Reactor designers in the early 60ies did not properly take into account the aspects of decommissioning and waste disposal. Specifications of the impurities in graphite and carbon can reduce these problems. 

The VHTR has two typical reactor configurations, namely the pebble bed type and the prismatic block type. Although the shape of the fuel element for two configurations are different, the technical basis for both configuration is same, such as the TRISO-coated particle fuel in the graphite matrix, foil ceramic (graphite) core structure, helium coolant, and low power density, in order to achieve high outlet temperature and the retention of fission production inside the coated particle under normal operation condition and accident condition. The VHTR can support alternative fuel cycles such as U—Pu, Pu, mixed oxide (MOX), and U—thorium (Th). 

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