Editing Pebble-bed reactor (section)

== Safety ==
When the reactor temperature rises, the atoms in the fuel move rapidly, causing [[Doppler broadening]]. The fuel then experiences a wider range of neutron speeds. [[Uranium-238]], which forms the bulk of the uranium, is much more likely to absorb fast or [[Neutron temperature|epithermal neutrons]] at higher temperatures. This reduces the number of neutrons available to cause fission, and reduces power. Doppler broadening therefore creates a negative feedback: as fuel temperature increases, reactor power decreases. All reactors have reactivity feedback mechanisms. The pebble-bed reactor is designed so that this effect is relatively strong, inherent to the design, and does not depend on moving parts. This negative feedback creates passive control of the reaction process.

Thus PBRs passively reduce to a safe power-level in an accident scenario. This is the design's main passive safety feature.

The reactor is cooled by an inert, fireproof gas, which has no phase transitions&mdash;it is always in the gaseous phase. The moderator is solid carbon; it does not act as a coolant, or move, or change phase.

Convection of the gas, driven by the heat of the pebbles, ensures that the pebbles are passively cooled.{{citation needed|date=January 2019}}

Even in the event that all supporting machinery fails, the reactor will not crack, melt, explode or spew hazardous wastes. It heats to a designed "idle" temperature, and stays there. At idle, the reactor vessel radiates heat, but the vessel and fuel spheres remain intact and undamaged. The machinery can be repaired or the fuel can be removed. 

In a safety test using the German AVR reactor, all the control rods were removed, and coolant flow was halted. The fuel remained undamaged.<ref name="ISR">[http://www.fz-juelich.de/isr/2/tint-a_e.html] {{webarchive|url=https://web.archive.org/web/20060613225502/http://www.fz-juelich.de/isr/2/tint-a_e.html|date=June 13, 2006}}</ref>

PBRs are intentionally operated above the {{convert|250|C}} [[Annealing (metallurgy)|annealing]] temperature of graphite, so that [[Wigner energy]] does not accumulate. This solves a problem discovered in the [[Windscale fire]]. One reactor (not a PBR) caught fire because of the release of energy stored as crystalline dislocations (Wigner energy) in the graphite. The dislocations are caused by neutron passage through the graphite. Windscale regularly annealed the graphite to release accumulated Wigner energy. However, the effect was not anticipated, and since the reactor was cooled by ambient air in an open cycle, the process could not be reliably controlled, and led to a fire.

Berkeley professor [[Richard A. Muller]] described PBRs as "in every way ... safer than the present nuclear reactors".<ref>{{cite book | title = Physics for Future Presidents | publisher = Norton Press| year = 2008| page = 170| isbn = 978-0-393-33711-2 | url = http://www.google.com/products/catalog?q=physics+for+future+presidents&safe=on&um=1&ie=UTF-8&tbm=shop&cid=15719616496879467605&sa=X&ei=RkhOT7zhCtOUtwe1j5GmCA&ved=0CDQQ8wIwAQ | author = Richard A. Muller| author-link = Richard A. Muller}}</ref>

===Containment===
Most PBR designs include multiple reinforcing levels of containment to prevent contact between the radioactive materials and the biosphere:
*Most reactors are enclosed in a [[containment building]] designed to resist aircraft crashes and earthquakes.
*The reactor is usually in a room with two-meter-thick walls with doors that can be closed, and cooling [[Plenum space|plenums]] that can be filled with water.
*The reactor vessel is typically sealed.
*Each pebble, within the vessel, is a {{convert|60|mm|in}} hollow sphere of pyrolytic graphite, wrapped in fireproof silicon carbide.
*Low density porous pyrolytic carbon, high density nonporous pyrolytic carbon
*The fission fuel is in the form of metal oxides or carbides.
Pyrolytic graphite is the main structural material in pebbles. It sublimates at {{convert|4000|C}}, more than double the design temperature of most reactors. It slows neutrons effectively, is strong, inexpensive, and has a long history of use in reactors and other high temperature applications. For example, pyrolytic graphite is also used, unreinforced, to construct missile reentry nose-cones and large solid rocket nozzles.<ref>{{cite web
| title = Fabrication of pyrolytic graphite rocket nozzle components| access-date = 2009-10-06
| url = http://issuu.com/glass4/docs/the_cooling_jacketed_reactor/=html&identifier=AD0270153
| website = issuu.com
}}</ref> Its strength and hardness come from its anisotropic crystals.

Pyrolytic carbon can burn in air when the reaction is catalyzed by a hydroxyl radical (e.g., from water).{{Citation needed|date= February 2009}} Infamous examples include the [[list of nuclear accidents|accidents]] {{citation needed inline|at Windscale|reason=at Windscale, no graphite fire occured|date=February 2025}} and Chernobyl&mdash;both graphite-moderated reactors. However, PBRs are cooled by inert gases to prevent fire. All designs have at least one layer of silicon carbide that serves as a fire break and seal.

===Fuel production ===
All kernels are precipitated from a [[sol-gel]], then washed, dried and calcined. U.S. kernels use [[uranium carbide]], while German (AVR) kernels use [[uranium dioxide]]. German-produced fuel-pebbles release about 1000 times less radioactive gas than the U.S. equivalents, due to that construction method.<ref name=Key-diff-2002>{{Cite web |url=http://www.iaea.org/inis/aws/htgr/fulltext/htr2002_201.pdf |title=''Key Differences in the Fabrication of US and German TRISO-COATED Particle Fuel, and their Implications on Fuel Performance '' Free, accessed 4/10/2008 |access-date=February 25, 2004 |archive-date=September 21, 2004 |archive-url=https://web.archive.org/web/20040921163426/http://www.iaea.org/inis/aws/htgr/fulltext/htr2002_201.pdf |url-status=dead }}</ref><ref name=Key-diff-2003>
{{cite journal|author1= D. A. Petti |author2= J. Buongiorno |author3= J. T. Maki |author4= R. R. Hobbins |author5= G. K. Miller |title= Key differences in the fabrication, irradiation and high temperature accident testing of US and German TRISO-coated particle fuel, and their implications on fuel performance|journal= Nuclear Engineering and Design|year= 2003
|volume= 222|pages= 281–297|doi= 10.1016/S0029-5493(03)00033-5|issue= 2–3|url= https://digital.library.unt.edu/ark:/67531/metadc882322/ }}</ref>
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