Editing Pebble-bed reactor (section)

==History==
[[Farrington Daniels]] originated the concept and the name in 1947 at Oak Ridge.<ref>{{cite web |url=http://www.ornl.gov/info/ornlreview/v36_1_03/article_01.shtml |title=ORNL Review Vol. 36, No. 1, 2003 - Nuclear Power and Research Reactors |publisher=Ornl.gov |access-date=2013-09-05 |url-status=dead |archive-url=https://web.archive.org/web/20130701145044/http://www.ornl.gov/info/ornlreview/v36_1_03/article_01.shtml |archive-date=2013-07-01 }}</ref> [[Rudolf Schulten]] advanced the idea in the 1950s. The crucial insight was to combine fuel, structure, containment, and neutron moderator in a small, strong sphere. The concept depended on the availability of engineered forms of silicon carbide and pyrolytic carbon that were strong.

===AVR===
{{Multiple issues|{{update section|date=December 2023}}
{{refimprove section|date=December 2023}}}}{{Main|AVR reactor}}
[[Image:Hogetemperatuurreactor.JPG|right|250px|thumb|AVR in Germany.]]A 15 [[MWe#MWe, MWt|MW<sub>e</sub>]] demonstration reactor, Arbeitsgemeinschaft Versuchsreaktor (''experimental reactor consortium''), was built at the [[Jülich Research Centre]] in [[Jülich]], [[West Germany]]. The goal was to gain operational experience with a high-temperature gas-cooled reactor. Construction costs of AVR were 115 million Deutschmark (1966), corresponding to a 2010 value of 180 million €. The unit's first criticality was on August 26, 1966. The facility ran successfully for 21 years. 

In 1978, the AVR suffered from a water/steam ingress accident of {{convert|30|MT}}, which led to contamination of soil and groundwater by strontium-90 and by tritium.{{cn|date=October 2021}} The leak in the steam generator leading to this accident was probably caused by high core temperatures (see criticism section). A re-examination of this accident was announced by the local government in July 2010.{{Citation needed|date=January 2021}}

The AVR was originally designed to breed [[uranium-233]] from [[thorium-232]]. A practical thorium [[breeder reactor]] was considered valuable technology. However, the AVR's fuel design contained the fuel so well that the transmuted fuels were uneconomic to extract&mdash;it was cheaper to use mined and purified uranium.{{Citation needed|date=January 2021}}

The AVR used helium [[coolant]], has a low [[neutron cross-section]]. Since few neutrons are absorbed, the coolant remains less radioactive. It is practical to route the primary coolant directly to power generation turbines. Even though the power generation used primary coolant, it was reported that the AVR exposed its personnel to less than 1/5 as much radiation as a typical light water reactor.{{Citation needed|date=January 2021}}

==== Decommissioning ====
It was decommissioned on December 1, 1988, in the wake of the [[Chernobyl accident|Chernobyl disaster]] and operational problems. During removal of the fuel elements it became apparent that the neutron reflector under the pebble-bed core had cracked during operation. Some hundred fuel elements remained stuck in the crack. During this examination it was revealed that the AVR was the world's most heavily beta-contaminated ([[strontium-90]]) nuclear installation and that this contamination was present as dust (the worst form).<ref>{{Cite web |title=E. Wahlen, J. Wahl, P. Pohl (AVR GmbH): Status of the AVR decommissioning project with special regard to the inspection of the core cavity for residual fuel. WM’00 Conference, February 27 - March 2, 2000, Tucson, AZ |url=http://www.wmsym.org/archives/2000/pdf/36/36-5.pdf}}</ref>

Localized fuel temperature instabilities resulted in heavy vessel contamination by [[Cs-137]] and [[Sr-90]]. The reactor vessel was filled with light concrete in order to fix the radioactive dust and in 2012 the reactor vessel of {{convert|2100|MT}} was to be moved to intermediate storage until a permanent solution is devised. The reactor buildings were to be dismantled and soil and groundwater decontaminated. AVR dismantling costs were expected to far exceed its construction costs. In August 2010, the German government estimated costs for AVR dismantling without consideration of the vessel dismantling at 600 million € ( $750 million, which corresponded to 0.4 € ($0.55) per kWh of electricity generated by the AVR. A separate containment was erected for dismantling purposes, as seen in the AVR-picture.{{Citation needed|date=January 2021}}

==== Thorium high-temperature reactor ====
{{Main|THTR-300}}

Following the experience with the AVR, Germany constructed a full scale power station (the thorium high-temperature reactor or [[THTR-300]] rated at 300 MW), using thorium as the fuel. THTR-300 suffered technical difficulties, and owing to these and political events in Germany, was closed after four years of operation. An incident on 4 May 1986, only a few days after the Chernobyl disaster, allowed a release of part of the radioactive inventory into the environment. Although the radiological impact was small, it had a disproportionate impact. The release was caused by a human error during a blockage of pebbles in a pipe. Trying to restart the pebbles' movement by increasing gas flow stirred up dust, always present in PBRs, which was then released, unfiltered, into the environment due to an erroneously open valve.{{Citation needed|date=January 2021}}

In spite of the limited amount of radioactivity released (0.1 GBq [[Cobalt|{{chem|60|Co}}]], [[Caesium|{{chem|137|Cs}}]], [[Protactinium|{{chem|233|Pa}}]]), a commission of inquiry was appointed. The radioactivity in the vicinity of the THTR-300 was finally found to result 25% from Chernobyl and 75% from THTR-300. The handling of this minor accident severely damaged the credibility of the German pebble-bed community, which lost support in Germany.<ref>Der Spiegel (German news magazine), no. 24 (1986) p. 28–30</ref>

The overly complex design of the reactor, which is contrary to the general concept of self-moderated thorium reactors designed in the U.S., also suffered from the unplanned high destruction rate of pebbles during the test series and the resulting higher contamination of the containment structure. Pebble debris and graphite dust blocked some of the coolant channels in the bottom reflector, as was discovered during fuel removal after final shut-down. A failure of insulation required frequent reactor shut-downs for inspection, because the insulation could not be repaired. Metallic components in the hot gas duct failed in September 1988, probably due to thermal fatigue induced by unexpected hot gas currents.<ref>R. Baeumer, THTR-300 Erfahrungen mit einer fortschrittlichen Technologie, Atomwirtschaft, May 1989, p. 226.</ref> This failure led to a long shut-down for inspections. In August, 1989, the THTR company almost went bankrupt, but was rescued by the government. The unexpected high costs of THTR operation and the accident ended interest in THTR reactors. The government decided to terminate the THTR operation at the end of September, 1989. This particular reactor was built despite criticism at the design phase. Most of those design critiques by German physicists, and by American physicists at the National Laboratory level, went ignored until shutdown. Nearly every problem encountered by the THTR 300 reactor was predicted by the physicists who criticized it as "overly complex".{{Citation needed|reason=What physicists?|date=October 2015}}

=== China ===
In 2004 China licensed the AVR technology and developed a reactor for power generation.<ref>{{cite news |date=2004-10-05 |title=China leading world in next generation of nuclear plants |work=[[South China Morning Post]] |url=http://daga.dhs.org/daga/readingroom/newsclips/2004/wto/41005scmp03.htm |url-status=dead |access-date=2006-10-18 |archive-url=https://web.archive.org/web/20120211094120/http://daga.dhs.org/daga/readingroom/newsclips/2004/wto/41005scmp03.htm |archive-date=2012-02-11}}</ref> The 10 megawatt prototype is called the [[HTR-10]]. It is a conventional helium-cooled, helium-turbine design. In 2021 the Chinese then built a 211&nbsp;MW<sub>e</sub> gross unit [[HTR-PM]], which incorporates two 250&nbsp;MW<sub>t</sub> reactors.<ref name="HTR-PM-2021">{{Cite web |title=China's HTR-PM reactor achieves first criticality : New Nuclear - World Nuclear News |url=https://www.world-nuclear-news.org/Articles/Chinas-HTR-PM-reactor-achieves-first-criticality |access-date=2021-09-28 |website=www.world-nuclear-news.org}}</ref> {{as of|2021}}, four sites were being considered for a 6-reactor successor, the HTR-PM600.<ref name="HTR-PM-2021" /> The reactor entered service in December 2023.<ref>{{Cite web |last=Wang |first=Brian |date=2023-12-13 |title=China's Pebble Bed Reactor Finally Starts Commercial Operation {{!}} NextBigFuture.com |url=https://www.nextbigfuture.com/2023/12/chinas-pebble-bed-reactor-finally-starts-commercial-operation.html |access-date=2023-12-15 |language=en-US}}</ref>