Editing Pebble-bed reactor (section)

==Design==
{{More citations needed section|date=January 2021}}
A pebble-bed power plant combines a gas-cooled core<ref>{{Cite web|url=https://www.hashdoc.com/documents/42279/the-frame-characteristics-of-jacketed-reactor|archiveurl=https://web.archive.org/web/20150503014959/https://www.hashdoc.com/documents/42279/the-frame-characteristics-of-jacketed-reactor|url-status=usurped|title=Pebble Bed Modular Reactor - What is PBMR?|archivedate=May 3, 2015}}</ref> and a novel fuel packaging.<ref>{{Cite web|url=http://www.pbmr.com/download/FuelSystem.pdf|archiveurl=https://web.archive.org/web/20080309163451/http://www.pbmr.com/download/FuelSystem.pdf|url-status=dead|title=How the PBMR Fueling System Works|archivedate=March 9, 2008}}</ref>

The [[uranium]], [[thorium]] or [[plutonium]] [[nuclear fuel]]s are in the form of a [[ceramic]] (usually [[oxide]]s or [[carbide]]s) contained within spherical pebbles a little smaller than the size of a tennis ball and made of pyrolytic graphite, which acts as the primary [[neutron moderator]]. The pebble design is relatively simple, with each sphere consisting of the nuclear fuel, fission product barrier, and moderator (which in a traditional water reactor would all be different parts). Grouping sufficient pebbles in the correct geometry creates [[Critical mass|criticality]].

The pebbles are held in a vessel, and an [[inert gas]] (such as helium, nitrogen or carbon dioxide) circulates through the spaces between the fuel pebbles to carry heat away from the reactor. Pebble-bed reactors must keep the pebbles' [[graphite]] from burning in the presence of air if the reactor wall is breached (the flammability of the pebbles is [[pebble-bed reactor#Containment|disputed]]). The heated gas is run directly through a [[turbine]]. However, if the gas from the primary [[coolant]] can be made radioactive by the [[neutron]]s in the reactor, or a fuel defect could contaminate the power production equipment, it may be brought instead to a [[heat exchanger]] where it heats another gas or produces steam. The turbine exhaust is warm and may be used to heat buildings or in other applications.

Pebble-bed reactors are gas-cooled, sometimes at low pressures. The spaces between the pebbles replace the piping in conventional reactors. Since there is no actual piping in the core and the coolant contains no hydrogen, embrittlement is not a failure concern. The preferred gas, helium, does not easily absorb neutrons or impurities. Therefore, compared to water, it is both more efficient and less likely to become radioactive.

Much of the cost of a [[Pressurized water reactor|conventional, water-cooled nuclear power plant]] is due to cooling system complexity, which is not a factor in PBRs. Conventional plants require extensive safety systems and redundant backups. Their reactor cores are dwarfed by cooling systems. Further, the core irradiates the water with neutrons causing the water and impurities dissolved in it to become radioactive. The high-pressure piping in the primary side eventually becomes [[Hydrogen embrittlement|embrittled]] and requires inspection and replacement.

Some designs are throttled by temperature rather than [[control rod]]s. Such reactors do not need to operate well at the varying neutron profiles caused by partially withdrawn control rods.{{citation needed|date=August 2019}}

PBRs can use fuel pebbles made from various fuels in the same design (though perhaps not simultaneously). Proponents claim that pebble-bed reactors can use thorium, plutonium and natural unenriched uranium, as well as [[enriched uranium]]. 

In most stationary designs, fuel replacement is continuous. Pebbles are placed in a bin-shaped reactor. Pebbles travel from the bottom to the top about ten times over a period of years, and are tested after each pass. Expended pebbles are removed to the nuclear-waste area, replaced by a new pebble.