Pebble fuel

A two-zone core design is adopted, with one central movable column of graphite spheres surrounded by pebble fuel elements. Tlie purpose of using the two-zone core design is to increase the power output of a single reactor module while maintaining the passive decay heat removal capability. 

UO2 pebble fuel with 16.76% enrichment Core equivalent diameter = 1.25m Core H/D from 0.8 to 1.4 C/U-235 from 5 630 to 11 120 Water simulated by plastic inserts... 

There is currently a lot of interest worldwide in the development and deployment of modular high temperature gas cooled reactors. Besides in China, similar design and technology development activities are underway in France, Japan, Russia, South Africa, the USA, etc. [XVII-5-7]. Main differences between the programmes are related to fuel technology (pebble fuel versus prismatic fuel, uranium versus plutonium) and to power conversion technology (steam turbine versus gas turbine). 

The performance test for the pebble fuel handling system,... 

The components and systems of the HTR-Module are simple and can he produced in a modular way. There are only two shghtiy complex systems from the point view of the system arrangement and the number and requirement of system components, which are the pebble fuel handling system and the helium purification system. Other systems are very conventional and are easy to install. 

The GFR also has a rapid heat up on loss of cooling, which as noted previously cannot be maintained unless power to gas circulators (pumps) is available, which then means high-temperature-resistant fuel is needed. This has led to the adoption of so-called particle or pebble fuel. In addition, RPV or primary system failure is difficult, and so attention has even been given in the past to Class G using prestressed concrete vessels. 

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. 

Hum gas is inert, so leaks are not radioactive. The heat could also be used to refine shale oil or desalinate water. Each day about 3,000 pebbles are removed from the bottom as some fuel is spent from the 360,000 pebbles, so there is no need to shut down the reactor to replace fuel. The pebbles are fireproof and extremely difficult to turn into weapons. If the fuel gets too hot, it begins absorbing neutrons, shutting down the reactor. 

In terms of safety, two issues are regularly debated. First, the issue of nuclear waste and, second, concerns over potential terrorist attacks on nuclear power plants. The first objection may be overcome through the introduction of new types of power plants, such as the pebble-bed modular reactor.This type of reactor uses graphite balls flecked with tiny amounts of uranium, rather than conventional fuel rods. With the fuel encased in graphite and impermeable silicon carbide, the theory is that the waste should be relatively easy to dispose of.The terrorism fears are less easily addressed and may ultimately stall the construction of new plants in countries such as the U.S., where these worries are greatest. 

A probabilistic safety analysis has been conducted at the Research Center Jiilich for the process heat variant of the German modular HTGR with the purpose to identify differences compared with the electricity generating HTR-MODUL [55]. The process heat HTR-MODUL consists of a pebble bed core with 360,(KX) spherical fuel elements to produce a thermal power of 170 MW. Helium coolant gas inlet / outlet temperatures are 300 and 950 °C, respectively. The system pressure is 4 MPa. The connection to the secondary circuit is given by a He / He intermediate heat exchanger. 

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