Editing Pebble-bed reactor (section)

== Design criticisms ==

===Graphite combustion===
The primary criticism of pebble-bed reactors is that encasing the fuel in graphite poses a hazard. Graphite can burn in the presence of air, which could happen if the reactor vessel is compromised. Fire could vaporize the fuel, which could then be released to the surroundings. Fuel kernels are coated with a layer of silicon carbide to isolate the graphite. While silicon carbide is strong in abrasion and [[compression (physical)|compression]] applications, it has less resistance to expansion and shear forces. Some [[Nuclear fission|fission]] products such as {{Chem|133|Xe}} have limited absorbance in carbon, so some fuel kernels could accumulate enough gas to rupture the silicon carbide.{{Citation needed|reason=Is it not the case that "The layer of *porous* pyrolytic graphite right next to the fissionable ceramic absorbs the radioactive gases"|date=May 2008}}

===Containment building===
Some designs do not include a containment building, leaving reactors more vulnerable to attack. However, most are surrounded by a reinforced concrete containment structure.<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=NRC: Speech - 027 - "Regulatory Perspectives on the Deployment of High Temperature Gas-Cooled Reactors in Electric and Non-Electric Energy Sectors"|archivedate=May 3, 2015}}</ref> 

===Waste handling===
PBR waste volumes are much greater, but have similar [[radioactivity]] measured in [[becquerel]]s per [[kilowatt-hour]]. The waste tends to be less hazardous and simpler to handle.{{Citation needed|date=February 2009}} Current US [[legislation]] requires all waste to be safely contained, requiring waste storage facilities. Pebble defects may complicate storage. Graphite pebbles are more difficult to reprocess due to their construction.{{Citation needed|date=February 2009}} 

===2008 report===
In 2008, a report<ref>{{Cite journal|author=Rainer Moormann|year=2008|title=A safety re-evaluation of the AVR pebble bed reactor operation and its consequences for future HTR concepts|series=Berichte des Forschungszentrums Jülich |publisher=Forschungszentrum Jülich, Zentralbibliothek, Verlag|id=Berichte des Forschungszentrums Jülich JUEL-4275|hdl = 2128/3136}}</ref><ref>{{cite news|url=http://www.neimagazine.com/story.asp?storyCode=2052589|title=PBR safety revisited|author=Rainer Moormann|publisher=Nuclear Engineering International|date=1 April 2009|access-date=2009-04-02|url-status=dead|archive-url=https://archive.today/20120530042951/http://www.neimagazine.com/story.asp?storyCode=2052589|archive-date=30 May 2012}}</ref> about safety aspects of Germany's [[AVR reactor]] and general PBR features drew attention. The claims are contested.<ref>{{cite news|url=http://www.neimagazine.com/story.asp?sectioncode=76&storyCode=2053102|title=Pebble Bed Reactor - Safety in perspective|author=Albert Koster|publisher=Nuclear Engineering International|date=29 May 2009|url-status=dead|archive-url=https://web.archive.org/web/20100626054414/http://www.neimagazine.com/story.asp?sectioncode=76&storyCode=2053102|archive-date=26 June 2010}}</ref> The report cited:
* Impossible to place standard measurement equipment in the reactor core{{Citation needed|date=January 2021}}
* The cooling circuit can be contaminated with metallic fission products ([[Strontium-90|{{SimpleNuclide|strontium|90}}]], [[Caesium-137|{{SimpleNuclide|caesium|137}}]]) due to limited pebble retention capabilities for metallic fission products. The report claimed that even modern fuel elements do not sufficiently retain [[strontium]] and [[caesium]].
* Elevated core temperatures (>{{convert|200|C-change|disp=or}} above calculated values)
* Dust formation from pebble friction under pebble breach (Dust acts as a mobile fission product carrier, if fission products escape the fuel particles.)

Report author [[Rainer Moormann]], recommended that average hot helium temperatures be limited to {{convert|800|C}} minus the uncertainty of the core temperatures (about {{convert|200|C-change|disp=or}}).