Pebble bed reactor design

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. 

In the design of the reactor core, the reduction of the number of subcomponents received priority over the use of designs with maximum operating experience. Unlike previous pebble bed reactor designs, ACACIA lacks the on-line refuelling system. Refuelling will be a short off-line operation, in which a mobile refuelling system is mounted on top of the reactor vessel and the whole core is replaced at once. 

Figure 3.1 Pebble bed reactor design and prismatic reactor design.

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),... 

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. 

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. 

Figure 11.10 Design of a spherical fuel element ( pebble ) of a very high-temperature (VHTR) gas-cooled pebble bed reactor (PBR).

For cogeneration, a 60 MW(th) helium cooled pebble bed reactor is coupled with a secondary nitrogen cycle through a He/N2 heat exchanger. If the application is electricity production only, a combined cycle of a gas turbine and a steam turbine is used. Major design and operating characteristics of the ACACIA plant are summarized in Tables XIX-1 and XIX-2. 

The HTR-PM shown in Fig. 3.9 contains two parallel trains of nuclear steam supply system (NSSS) of identical design, each consisting of a 250-MWth pebble bed reactor and a steam generator. The two NSSS systems have independent primary loops but share auxiliary facilities, such as fuel handling system and helium purification system. The two trains jointly supply superheated steam to a common steam turbine power generator rated at 200 MWg. 

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). 

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... 

Several reactors are candidates for use as a high temperature heat source for the S-I cycle. Candidates include the modular helium reactor (MHR) and pebble bed modular reactor (PBMR). One of the most thoroughly investigated candidates is the PBMR. Recent work has been performed in benchmarking the THERMIX code to the PBMR-268 design (Reitsma, 2004 Seker, 2005). 

SCHULTEN, R., et al.. The Pebble-Bed High-Temperature Reactor as a Source of Nuclear Process Heat, Vol 1 Conceptual Design, Report Jul-1113-RG, Research Center Julich (1974). 

In 1982, the Research Center Jiilich presented the conceptual design of a 50 MW(th) nuclear process heat plant with a pebble-bed HTGR, named AVR-II, for which a safety-related study has been conducted [29]. Its characteristic features are a slim steel pressure vessel, no separate decay heat removal system, shutdown and control system via reflector rods, surface cooling system, and a simplified containment. The safety of the reactor is principally based on passive system feamres. 

Air Plow Data. The reactor is. designed for normal Operation at a power level of 30,000 kw. The design aii flow of -2000 Ib/min and pressure drop of 55 in. of water through the reactor cooling-air system are based on maximum temperatures of 570°F in the graphite pebble bed and 500°F in the permanent graphite at 45,0OO- kw operation of the reactor. The. .2000-lb/niin... 

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