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Formation of loading cones in the PBMR-core
Published in Heinz Konietzky, Numerical Modeling in Micromechanics via Particle Methods, 2017
A reactor following these principals is the Pebble Bed Modular Reactor (PBMR), a 302 MWthermal reactor. The core is built like a silo with 36 vertical graphite walls and a 30° discharging cone at the bottom. To achieve the goal of self-acting afterheat removal, the core is power optimized. The core is composed of spherical elements (diameter = 6 cm). These elements are divided into two zones, the inner zone comprising graphite spheres and the outer, annular zone comprising fuel spheres. The graphite spheres are introduced to the center of the core by means of a central fuel line; the fuel spheres are introduced through 9 satellite fuel lines. The total number of spheres is 425,000 (75% fuel located in the annulus at the theoretical location of radius >87.5 cm). The flow through the core is gravity driven, spheres are removed from the bottom of the core by means of a core unloading device and the core settles under gravity. The burn-up of the extracted fuel elements is measured and those reaching the burn-up limit are removed from the cycle, while the others are recycled (returned to the core) together with the fresh fuel elements. The graphite elements are also recycled.
High-Temperature Reactors
Published in William J. Nuttall, Nuclear Renaissance, 2022
HTGR systems make use of fuel/moderator composites usually in the form of Triso fuel. Lyman focused on the risk of a graphite fire, but in so doing he may have over-stated the problem. Nuclear graphite cannot ignite or maintain self-sustaining combustion, even at very high temperatures [113]. In pebble bed reactors, the Triso fuel is assembled in spherical fuel elements of a few centimetres diameter coated in pyrolytic graphite. Perhaps the concern of the critics should be regarded as the possibility of graphite oxidation, but if that is the case the risks and consequences are not so severe. Strictly graphite does not burn, as there is no flame, but one might say that: at its worst graphite does not burn, it smoulders. The PBMR is designed to operate at approximately 900°C with the graphite-clad fuel elements in an inert helium atmosphere. Despite the lack of conventional fire risk, it would be a serious problem if air or water were to encounter a bare graphite surface at such temperatures. Nevertheless, it is important to note that oxidation of the outer graphite cladding of the fuel pebbles probably would not in itself lead to a breakdown of the SiC-clad Triso fuel or the release of radioactive materials. The designers are confident that the ingress of oxidisers (air or water) would always be inhibited by the nature of their design and that the barriers to release could cope with the gaseous and sooty products of oxidation. Experience at the Jülich AVR points to concerns about hot spots in the reactor core and the physical integrity and quality of the fuel pebbles as constructed and handled at that time. Defects in Triso fuel construction, especially poor integrity of the SiC shell, might lead to the possibility of the release of radioactive fission products and minor actinides in the event of a serious accident. Defects would also relate to the cleanliness of the internal reactor components. The structural integrity of pebble bed Triso fuel is of particular importance, given the high temperature anticipated in some HTGR accident scenarios.
Review of the Fluid Dynamics and Heat Transport Phenomena in Packed Pebble Bed Nuclear Reactors
Published in Nuclear Science and Engineering, 2023
Rahman S. Almusafir, Ahmed A. Jasim, Muthanna H. Al-Dahhan
The pebble bed nuclear reactor gets its name from the type of nuclear fuel it consumes, and it offers many advantages over conventional reactors. A pebble bed type of very high-temperature gas-cooled reactor (VHTR) is one of the most probable solutions4 and promising concepts5 of the six classes of Gen IV advanced nuclear technologies. The PBR concept has been adopted by many test and demonstration reactors, including the modular pebble bed reactor (MPBR) in the United States6 and the prototype reactor of the pebble bed modular reactor (PBMR) in South Africa5,7; the 10-MW(thermal) high-temperature reactor (HTR-10) in China8,9; and the prototype PBR at Jülich research center in Germany that is known as Arbeitsgemeinschaft Versuchsreaktor (AVR), which translates to experimental reactor consortium, from the early 1960s (Refs. 10, 11, and 12). Recently, Xe‐100, a HTGR is being developed by X-Energy under the U.S. Department of Energy’s Advanced Reactor Demonstration Program. It is designed to provide a secure future for the global energy and process heat markets.
Neutron poison distribution in the central reflector to reduce the DLOFC temperature of a Th-LEU fueled OTTO PBMR DPP-400 core
Published in Journal of Nuclear Science and Technology, 2018
Marius Tchonang Pokaha, Dawid E. Serfontein
Neutron poison was successfully used in combination with a mixture of LEU and thorium to reduce the maximum DLOFC temperature of a medium-sized annular core pebble-bed reactor (PBMR-400). Thorium in the fuel helped to reduce the peaking factor, while the neutron poison distribution shaped the axial power profile.
Numerical Investigation and Parametric Study on Thermal-Hydraulic Characteristics of Particle Bed Reactors for Nuclear Thermal Propulsion
Published in Nuclear Technology, 2020
Yu Ji, ZeGuang Li, Jun Sun, ErSheng You, MingGang Lang, Lei Shi
Nuclear thermal propulsion was first proposed in the early 1950s and was aggressively developed from the late 1950s to the early 1970s. During this period, the principal research activities were performed in the United States and the Union of Soviet Socialist Republics,5 and the most representative effort was the Rover and later the Nuclear Engine Rocket Vehicle Application (NERVA) Project.6 NERVA was a hardware-oriented program that achieved many accomplishments, such as fuel design and fabrication, system design, and the testing of 20 reactors or engines. However, the NERVA program was terminated in 1973 due to recessionary ambitions for space exploration and changed national economic priorities.4 Afterward, some space technologists continued to reconsider the benefits of space nuclear systems, and several different nuclear system concepts for propulsion were conceived and studied. The particle bed reactor (PBR) was one of these novel concepts.4 Similar to other established or planned pebble bed reactors or facilities, such as the HTR-10 (Ref. 7) and the HTR-PM (Ref. 8) in China, the PBMR (Ref. 9) in South Africa, and the SANA experiments facility10 in Germany, the PBR was also a gas-cooled reactor that utilized nuclear fuel particles to pack a bed. The PBR is different, however, as this bed has been transformed into an annular region within two concentric porous frits from a cylindrical region in the common pebble bed reactor and the coolant, mostly the hydrogen, flows through the fuel bed radially rather than axially to reduce the pressure drop,11 as shown in Fig. 1. As a variant of the pebble bed reactor, the operating temperature in the PBR was significantly higher than that of ground pebble bed reactors due to the adoption of fuel and other structural materials with higher melting points. In addition, the power density in the PBR core was also increased because of the reduction in fuel particle size and the subsequent enhancement of its heat removal capability, thus leading to a compact, lightweight, and efficient reactor design. Compared to the NERVA reactor, the PBR fit better to an advanced space nuclear propulsion system. Unfortunately, the development of PBR systems lasted only 7 years, i.e., 1987 to 1994, and the demonstration was never initiated. In recent years, the U.S. National Aeronautics and Space Administration has responsored the NTP research and has chosen it as the primary option for cargo and crew transfer in outer space.12 It is estimated in the Design Reference Architecture 5.0 that the use of clustered, lower-thrust NERVA engines is capable of human exploration missions to Mars at a reduced risk,13 and that the exploration capability could be enhanced further if the PBR systems are realized.