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High-Temperature Gas-Cooled Thermal Reactors
Published in Kenneth D. Kok, Nuclear Engineering Handbook, 2016
In a pebble-bed reactor, the fuel particles are contained in billiard ball-sized spherical fuel elements (i.e., pebbles), as shown in Figure 5.7 (PBMR 2005). The fuel is cooled by helium flowing downward through a close-packed bed of the spherical fuel elements. These pebbles are removed continuously from the core during reactor operation, measured for radionuclide content, and returned to the core or replaced with a fresh fuel element depending on the amount of fuel depletion. With this continuous online refueling approach, there is no need for refueling outages.
Nuclear and Hydropower
Published in Roy L. Nersesian, Energy Economics, 2016
The primary safety feature of a pebble bed reactor is its low fuel density with a power density only one-thirtieth that of a PWR. The reactor is inherently safe for a total loss of coolant—no core meltdown occurs in a PWR as there is no core. A loss of coolant heats the pebbles up to a maximum temperature of 1,600°C, well below the 2,000°C needed to melt the ceramic coating surrounding each bit of fissionable fuel. As pebbles heat up, the frequency of fissions drops, which lowers the power output of a pebble reactor to a level where more heat escapes through the reactor wall than is produced by nuclear reactions. The reactor cannot crack, explode, melt, or spew hazardous materials: it simply remains at an idle temperature with pebbles intact and undamaged. Known as passive nuclear safety, the reactor’s low fuel density allows more heat to escape than is generated in the absence of coolant rather than having to depend on an active nuclear safety feature such as inserting control rods and maintaining coolant. A pebble bed reactor is inherently safer than traditional reactors. It is impossible to have a runaway reaction as occurred at Chernobyl by a sudden withdrawal of control rods that caused the reactor to go supercritical or to have a partial core meltdown as at Three Mile Island or a total core meltdown as at Fukushima by a loss of coolant.
Energy and Environment
Published in T.M. Aggarwal, Environmental Control in Thermal Power Plants, 2021
Indeed the idea of modularity can be taken even further Toshiba a large Japanese firm is planning something known as nuclear batteries factory made sealed units with an output of 10 megawatts and a lifetime of 15–30 years. When they stop working you simply send them back to the factory for disposal. The acme of modular factory built passively safe reactor design however is found in south Africa People there have been experimenting with so called pebble-bed reactors for decades. They hope to start building one for real in 2010. A pebble-bed reactor is filled by small spheres that are in essence tiny reactors in their own right they are made of Uranium oxide (the fuel) and graphite (a substance that slows down the flying neutrons that cause nuclear fusion) Pile enough pebbles together and a chain reaction will start Nor is any complicated pipework required to extract the heat. All you need to do is run an inert gas such as helium through the pebbles and it will collect the heat for you. The design also looks like the ultimate in passive safety because a phenomenon called he Doppler broadening which changes the speed of neutrons and make them less likely to cause fusion shuts it down automatically. If it overheats though critics argue that the graphite in the pebbles is a fire hazard and that helium is so leaky that there is a risk of air getting into the system and starting a fire None of these ideas deals with the question of nuclear waste. But that is largely a political problem not a technical one Though it sounds like a copout the best answer really is to bury the stuff for the time being. That should be done in places where it can be easily be recovered for reprocessing one day when technology has caught up. But it is also worth noting that buried unprocessed waste cannot be used to make bombs.
FHR, HTGR, and MSR Pebble-Bed Reactors with Multiple Pebble Sizes for Fuel Management and Coolant Cleanup
Published in Nuclear Technology, 2019
Charles W. Forsberg, Per F. Peterson
A pebble-bed reactor with two or more sizes of pebbles creates options that have not been previously explored. In pebble-bed reactors, the greater heat transfer per unit volume of the smaller pebbles enables higher power densities relative to the large pebbles that enables higher power densities in the reactor core (smaller reactor) and creates a new pathway to actinide burning. The small pebbles could be used for coolant cleanup, from tritium to noble metals. In MSRs, multiple pebble sizes may address the challenge of replacing graphite damaged by neutron irradiation, a design option that translates into smaller reactor cores that implies less salt and a lower reactor fissile fuel inventory.
A Better Method to Calculate Fuel Burnup in Pebble Bed Reactors Using Machine Learning
Published in Nuclear Technology, 2023
Carlos X. Soto, Odera Dim, Yonggang Cui, Warren Stern
Pebble bed reactors (PBRs) operate with spherical fuel elements (the “pebbles”) roughly the size of a tennis ball, which contain thousands of submillimeter-sized TRIstructural ISOtropic (TRISO) fuel particles. The TRISO particles are composed of minute fuel kernels of uranium dioxide or uranium oxycarbide surrounded by multiple layers of highly durable and impermeable coating. Key to PBR operation is the fact that the TRISO-impregnated pebbles circulate through the reactor vessel multiple times before final discharge. During normal operation, ejected pebbles are assessed for physical condition and fuel burnup to determine whether the pebbles may be returned to the reactor or discharged.