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In [[nuclear physics]], an '''energy amplifier''' is a novel type of [[nuclear power]] reactor, a [[subcritical reactor]], in which an energetic [[Charged particle beam|particle beam]] is used to stimulate a reaction, which in turn releases enough energy to power the [[particle accelerator]] and leave an energy profit for power generation. The concept has more recently been referred to as an '''accelerator-driven system''' (ADS) or [[accelerator-driven sub-critical reactor]].

== History ==

Preceding the concept and the first achieved criticality in [[Chicago Pile-1]], [[Columbia University]] physicists [[Enrico Fermi]] and [[Herbert L. Anderson]] worked at [[Princeton University]] with physicists [[Robert R. Wilson]] and [[Edward Creutz]], developing the A-12 pile, a subcritical experiment similar to an energy amplifier. Nine kilograms of uranium oxide were embedded in a 490 kg graphite column, and spallation of beryllium by protons from the Princeton [[cyclotron]] was used as a neutron source. Most other pile experiments in this period, in both the United States and Germany, used alpha emitters such as radium-beryllium or radon-beryllium sources.<ref name="g235">{{cite journal |last=Reed |first=B. Cameron |date=2021 |title=An inter-country comparison of nuclear pile development during World War II |url=http://arxiv.org/pdf/2001.09971 |journal=The European Physical Journal H |volume=46 |issue=1 |page= |doi=10.1140/epjh/s13129-021-00020-x |issn=2102-6459 |access-date=2025-02-12 |doi-access=free|arxiv=2001.09971 }}</ref>

The concept is credited to Italian scientist [[Carlo Rubbia]],<ref>[https://cdsweb.cern.ch/record/718660 Rubbiatron, il reattore da Nobel], Massimo Cappon, ''CERN docs server: [[Panorama (magazine)|Panorama]]'', 11 giugno 1998. Also: [https://cdsweb.cern.ch/record/718660/files/presscut-98-111.pdf File pdf].</ref> a Nobel Prize particle [[physicist]] and former director of Europe's [[CERN]] international [[nuclear physics]] lab. He published a proposal for a power reactor (nicknamed "Rubbiatron") based on a proton [[cyclotron]] accelerator with a beam energy of 800 [[MeV]] to 1 [[GeV]], and a target with [[thorium]] as fuel and [[lead]] as a coolant. Rubbia's scheme also borrows from ideas developed by a group led by nuclear physicist Charles Bowman of the Los Alamos National Laboratory<ref>{{cite journal|title=Rubbia Floats a Plan for Accelerator Power Plants|url=https://www.science.org/doi/10.1126/science.262.5138.1368|accessdate=6 March 2022|journal=Science|date=Nov 1993|doi=10.1126/science.262.5138.1368 |last1=Aldhous |first1=Peter |volume=262 |issue=5138 |page=1368 |pmid=17736803 |bibcode=1993Sci...262.1368A |url-access=subscription }}</ref>

== Principle and feasibility ==
The energy amplifier first uses a particle accelerator (e.g. [[linac]], [[synchrotron]], [[cyclotron]] or [[FFAG accelerator|FFAG]]) to produce a beam of high-energy (relativistic) protons. The beam is directed to collide with nuclei of a heavy metal target, such as lead, thorium or uranium.  Inelastic collisions between the proton beam and the target results in [[spallation]], which produces twenty to thirty neutrons per event.<ref>{{cite web|url=http://www.psi.ch/bsq/spallation-target |title=Spallation Target &#124; Paul Scherrer Institut (PSI) |website=Psi.ch |date= |accessdate=2016-08-16}}</ref> It might be possible to increase the neutron flux through the use of a [[neutron amplifier]], a thin film of [[fissile]] material surrounding the spallation source; the use of neutron amplification in [[CANDU]] reactors has been proposed. While [[CANDU]] is a critical design, many of the concepts can be applied to a sub-critical system.<ref>http://www.tfd.chalmers.se/~valeri/Mars/Mo-o-f10.pdf {{Bare URL PDF|date=March 2022}}</ref><ref>{{cite web|title=Neutron amplification in CANDU reactors |url=http://canteach.candu.org/library/20041209.pdf |publisher=CANDU |url-status=dead |archiveurl=https://web.archive.org/web/20070929124454/http://canteach.candu.org/library/20041209.pdf |archivedate=2007-09-29 }}</ref> Thorium nuclei absorb neutrons, thus breeding fissile [[uranium-233]], an isotope of uranium which is not found in nature. [[Thermal neutron|Moderated neutrons]] produce U-233 fission, releasing energy.

This design is entirely plausible with currently available technology, but requires more study before it can be declared both practical and economical.

OMEGA project ({{nihongo|option making of extra gain from actinides and fission products|[[:ja:オメガ計画|オメガ計画]]|}}) is being studied as one of methodology of accelerator-driven system (ADS) in Japan.<ref>{{cite web|url=http://jolisfukyu.tokai-sc.jaea.go.jp/fukyu/gihou/pdf2/5913.pdf|script-title=ja:大電流電子線加速器の性能確認試験|date=December 2000|publisher=[[Japan Atomic Energy Agency]]|location=[[Ōarai, Ibaraki]]|language=Japanese|trans-title=Performance of Ｈigh Power CW Electron Linear Accelerator|accessdate=2013-01-21}}</ref>

[[Richard Garwin]] and [[Georges Charpak]] describe the energy amplifier in detail in their book "[[Megawatts and Megatons|Megawatts and Megatons: A Turning Point in the Nuclear Age?]]" (2001) on pages 153-163.

Earlier, the general concept of the energy amplifier, namely an [[accelerator-driven sub-critical reactor]], was covered in "The Second Nuclear Era" (1985) pages 62–64, by [[Alvin M. Weinberg]] and others.

== Advantages ==

The concept has several potential advantages over conventional nuclear [[fission reactor]]s:

* Subcritical design means that the reaction could not run away — if anything went wrong, the reaction would stop and the reactor would cool down. A [[nuclear meltdown|meltdown]] could however occur if the ability to cool the core was lost.
* [[Thorium]] is an abundant element — much more so than [[uranium]] — reducing strategic and political supply issues and eliminating costly and energy-intensive [[isotope]] [[Enriched uranium|separation]]. There is enough thorium to generate energy for at least several thousand years at current consumption rates.<ref>{{Cite web|url=http://www.inference.org.uk/withouthotair/c24/page_166.shtml|title=Ch 24 Page 166: Sustainable Energy - without the hot air &#124; David MacKay|website=www.inference.org.uk}}</ref>
* The energy amplifier would produce very little [[plutonium]], so the design is believed to be more [[Nuclear proliferation|proliferation]]-resistant than conventional nuclear power (although the question of [[uranium]]-233 as [[nuclear weapon]] material must be assessed carefully).
* The possibility exists of using the reactor to consume plutonium, reducing the world stockpile of the very-long-lived element.
* Less long-lived [[radioactive waste]] is produced — the waste material would decay after 500 years to the radioactive level of [[coal]] ash.
* No new science is required; the technologies to build the energy amplifier have all been demonstrated. Building an energy amplifier requires only [[engineering]] effort, not fundamental research (unlike [[nuclear fusion]] proposals).
* Power generation might be economical compared to current nuclear reactor designs if the total [[Nuclear fuel cycle|fuel cycle]] and [[Nuclear decommissioning|decommissioning]] costs are considered.
* The design could work on a relatively small scale, and has the potential to load-follow by modulating the proton beam, making it more suitable for countries without a well-developed [[power grid]] system.
* [[Inherent safety]] and safe fuel transport could make the technology more suitable for [[developing countries]] as well as in densely populated areas.
* Desired [[nuclear transmutation]] could be employed deliberately (rather than as an unavoidable consequence of nuclear fission and neutron irradiation) either to transmute [[high level waste]] (such as [[long-lived fission products]] or [[minor actinides]]) into less harmful substances, for producing radionuclides for use in [[nuclear medicine]] or to produce [[precious metal]]s from low-priced feedstocks.
* The lower fraction of [[delayed neutrons]] in the fission of {{chem|239|Pu}} compared to {{chem|235|U}}, which hampers the use of plutonium-containing fuels in critical reactors (which need to operate in the narrow band of neutron flux between [[prompt critical]] and [[delayed criticality|delayed critical]]), is of no concern as no criticality of any kind is achieved or needed
* While [[nuclear reprocessing]] runs into the problem that [[MOX-fuel]] can not be further recycled for use in current [[light-water reactor]]s as the [[reactor-grade plutonium]] concentration of fissile isotopes is not achieved due to {{chem|240|Pu}} impurities exceeding acceptable levels, all fissile and [[fertile material|fertile]] isotopes of actinoids can be "burned" in a subcritical reactor, thus closing the [[nuclear fuel cycle]] without the need for [[fast breeder reactor]]s

== Disadvantages ==
* Each reactor needs its own facility ([[particle accelerator]]) to generate the high energy proton beam, which is very costly. Apart from [[linear particle accelerator]]s, which are very expensive, no proton accelerator of sufficient power and energy {{nowrap|(> ~12 MW}} at {{nowrap|1 GeV)}} has ever been built.  Currently, the [[Spallation Neutron Source]] utilizes a {{nowrap|1.44 MW}} proton beam to produce its neutrons, with upgrades envisioned to {{nowrap|5 MW.}}<ref>http://accelconf.web.cern.ch/AccelConf/e04/PAPERS/TUPLT170.PDF {{Webarchive|url=https://web.archive.org/web/20060518083638/http://accelconf.web.cern.ch/accelconf/e04/PAPERS/TUPLT170.PDF |date=2006-05-18 }} {{Bare URL PDF|date=March 2022}}</ref>  Its {{nowrap|1.1 billion USD}} cost included research equipment not needed for a commercial reactor. [[Economies of scale]] might come into play if particle accelerators (which are currently only rarely built to the above mentioned strengths and then only for research purposes) become a more "mundane" technology. A similar effect can be observed when comparing the cost of the [[Manhattan Project]] up to the construction of [[Chicago Pile-1]] to the costs of subsequent research or power reactors.
* The fuel material needs to be chosen carefully to avoid unwanted nuclear reactions. This implies a full-scale [[nuclear reprocessing]] plant associated with the energy amplifier.<ref>[http://ppd.fnal.gov/experiments/e907/raja/energy_amplifier/cer-0210391.pdf ''Conceptual design of a fast neutron operated high power energy amplifier''], [[Carlo Rubbia]] et al., CERN/AT/95-44, pages 42 ff., section ''Practical considerations''</ref>
* If, for whatever reason, neutron flux exceeds design specifications enough for the assembly to reach [[Criticality (status)|criticality]], a [[criticality accident]] or power excursion can occur. Unlike a "normal" reactor, the [[scram]] mechanism only calls for the "switching off" of the neutron source, which wouldn't help if more neutrons are constantly produced than consumed (i.e. Criticality), as there is no provision to rapidly increase neutron consumption e.g. via the introduction of a [[neutron poison]].
* Using lead as a coolant has similar disadvantages to those described in the article on [[lead cooled fast reactor]]s
* Many of the current spallation-based neutron sources used for research are "pulsed" i.e. they deliver very high [[neutron flux]]es for very short durations of time. For a power reactor a smaller but more constant neutron flux is desired. The [[European Spallation Source]] will be the strongest neutron source in the world (measured by peak neutron flux) but will only be capable of very short (on the order of milliseconds) pulses.

== See also ==

* [[Accelerator-driven sub-critical reactor]]
* [[Alternative energy]]
* [[Thorium fuel cycle]]
* [[Breeder reactor]], another type of nuclear reactor that aims for an energy profit by creating more fissile material than it consumes.
* [[Thorium-based nuclear power]]
* [[Muon capture]]
* [[Nuclear transmutation]]

== References ==
{{Reflist}}
{{Refbegin}}
*[https://cds.cern.ch/record/297967/files/lhc-96-001.pdf A PRELIMINARY ESTIMATE OF THE ECONOMIC IMPACT OF THE ENERGY AMPLIFIER] – An in-depth review of the Energy Amplifier co-authored by Rubbia (pdf download available from the CERN document server)
* Christoph Pistner, [http://www.inesap.org/sites/default/files/inesap_old/bulletin17/bul17art25.htm Emerging Nuclear Technologies: The Example of Carlo Rubbia's Energy Amplifier], International Network of Engineers and Scientists Against Proliferation
{{Refend}}

== External links ==
{{Commons|Thorium}}
{{Wiktionary|thorium}}
*[https://web.archive.org/web/20060903071404/http://www.cosmosmagazine.com/node/348 New Age Nuclear: article on energy amplifiers | ''Cosmos Magazine'']

[[Category:Nuclear technology]]
[[Category:Accelerator physics]]
[[Category:Nuclear reactors]]
[[Category:CERN]]