Fast-neutron reactor

Shevchenko BN350 nuclear fast reactor and desalination plant situated on the shore of the Caspian Sea. The plant generated 135 MW_e and provided steam for an associated desalination plant. View of the interior of the reactor hall.

A fast neutron reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons. Such a reactor needs no neutron moderator, but must use fuel that is relatively rich in fissile material when compared to that required for a thermal reactor.

Advantages

Actinides				Half-life	Fission products
244Cm	241Pu f	250Cf	243Cmf	10–30 y	137Cs	90Sr	85Kr
232U  f		238Pu	f is for fissile	69–90 y			151Sm nc➔
4n	249Cf  f	242Amf		141–351	No fission product has half-life 102 to 2×105 years
	241Am		251Cf  f	431–898
240Pu	229Th	246Cm	243Am	5–7 ky
4n	245Cmf	250Cm	239Pu f	8–24 ky
	233U    f	230Th	231Pa	32–160
	4n+1	234U	4n+3	211–290	99Tc		126Sn	79Se
248Cm		242Pu		340–373	Long-lived fission products
	237Np	4n+2		1–2 My	93Zr	135Cs nc➔
236U	4n+1		247Cmf	6–23 My		107Pd	129I
244Pu				80 My	>7%	>5%	>1%	>.1%
232Th		238U	235U    f	0.7–12 Gy	fission product yield

• Although it is currently (2010) uneconomic, a fast neutron reactor can reduce the total radiotoxicity of nuclear waste, and dramatically reduce the waste's lifetime. They can also use all or almost all of the fuel in the waste. Fast neutrons have an advantage in the transmutation of nuclear waste. With fast neutrons, the ratio between splitting and the capture of neutrons of plutonium or minor actinide is often larger than when the neutrons are slower, at thermal or near-thermal "epithermal" speeds. The transmuted odd-numbered actinides (e.g. from Pu-240 to Pu-241) split more easily. After they split, the actinides become a pair of "fission products." These elements have less total radiotoxicity. Since disposal of the fission products is dominated by the most radiotoxic fission product, Cesium 137, which has a half life of 30.1 years, the result is to reduce nuclear waste lifetimes from tens of millennia (from transuranic isotopes) to a few centuries. The processes are not perfect, but the remaining transuranics are reduced from a significant problem to a tiny percentage of the total waste, because most transuranics can be used as fuel.
• Fast reactors technically solve the "fuel shortage" argument against uranium-fueled reactors without assuming unexplored reserves, or extraction from dilute sources such as ordinary granite or the ocean. They permit nuclear fuels to be bred from almost all the actinides, including known, abundant sources of depleted uranium and thorium, and light water reactor wastes. On average, more neutrons per fission are produced from fissions caused by fast neutrons than from those caused by thermal neutrons. This results in a larger surplus of neutrons beyond those required to sustain the chain reaction. These neutrons can be used to produce extra fuel, or to transmute long half-life waste to less troublesome isotopes, such as was done at the Phénix reactor in Marcoule in France, or some can be used for each purpose. Though conventional thermal reactors also produce excess neutrons, fast reactors can produce enough of them to breed more fuel than they consume. Such designs are known as fast breeder reactors.
• The fast reactor doesn't just transmute the inconvenient even-numbered transuranic elements (notably Pu-240 and u-238). It transmutes them, and then fissions them for power, so that these former wastes would actually become valuable.

Disadvantages

• The reactor's criticality responds within the flight time of the neutrons across the core. Design of a fast reactor is therefore more demanding, because there is no moderator whose thermal or mechanical behavior can adjust the reactor. Fast reactors cannot be reliably stabilized with control rods, which are too slow. Most designs are stabilized either by doppler broadening or by thermal expansion of the fuel, a neutron poison or a neutron reflector.
• Sodium is often used as a coolant in fast reactors, because it does not moderate neutron speeds much and has a high heat capacity. However, it burns in air, and is very corrosive. It has caused difficulties in reactors (e.g. USS Seawolf (SSN-575), Monju). Although some sodium-cooled fast reactors have operated safely (notably the Superphénix), sodium problems can be prevented by using lead or molten chloride salts as a coolant.

Nuclear reactor design

Coolant

Water, the most common coolant in thermal reactors, is generally not a feasible coolant for a fast reactor, because it acts as a neutron moderator. However the Generation IV reactor known as the supercritical water reactor with decreased coolant density may reach a hard enough neutron spectrum to be considered a fast reactor.

All current fast reactors are liquid metal cooled reactors. The early Clementine reactor used mercury coolant and plutonium metal fuel. NaK coolant is popular in test reactors due to its low melting point. Molten lead cooling has been used in naval propulsion units as well as some other prototype reactors. All large-scale fast reactors have used molten sodium coolant.

Another proposed fast reactor is a Molten Salt Reactor, one in which the molten salt's moderating properties are insignificant. This is typically achieved by replacing the light metal fluorides (e.g. LiF, BeF₂) in the salt carrier with heavier metal chlorides (e.g., KCl, RbCl, ZrCl₄).

Gas-cooled fast reactors have been the subject of research as well.

Nuclear fuel

In practice, sustaining a fission chain reaction with fast neutrons means using relatively highly enriched uranium or plutonium. The reason for this is that fissile reactions are favored at thermal energies, since the ratio between the Pu239 fission cross section and U238 absorption cross section is ~100 in a thermal spectrum and 8 in a fast spectrum. Therefore it is impossible to build a fast reactor using only natural uranium fuel. However, it is possible to build a fast reactor that will breed fuel (from fertile material) by producing more fissile material than it consumes. After the initial fuel charge such a reactor can be refueled by reprocessing. Fission products can be replaced by adding natural or even depleted uranium with no further enrichment required. This is the concept of the fast breeder reactor or FBR.

So far, most fast neutron reactors have used either MOX (mixed oxide) or metal alloy fuel. Soviet fast neutron reactors have been using (high U-235 enriched) uranium fuel. The Indian prototype reactor has been using uranium-carbide fuel.

Control

Like thermal reactors, fast neutron reactors are controlled by keeping the criticality of the reactor reliant on delayed neutrons, with gross control from neutron-absorbing control rods or blades.

They cannot, however, rely on changes to their moderators because there is no moderator. So Doppler broadening in the moderator, which affects thermal neutrons, does not work, nor does a negative void coefficient of the moderator. Both techniques are very common in ordinary light water reactors.

Doppler broadening from the molecular motion of the fuel, from its heat, can provide rapid negative feedback. The molecular movement of the fissionables themselves can tune the fuel's relative speed away from the optimal neutron speed. Thermal expansion of the fuel itself can also provide quick negative feedback. Small reactors such as those used in submarines may use doppler broadening or thermal expansion of neutron reflectors.

Shevchenko BN350 desalination unit. View of the only nuclear-heated desalination unit in the world

History

A 2008 IAEA proposal for a Fast Reactor Knowledge Preservation System notes that:

during the past 15 years there has been stagnation in the development of fast reactors in the industrialized countries that were involved, earlier, in intensive development of this area. All studies on fast reactors have been stopped in countries such as Germany, Italy, the United Kingdom and the United States of America and the only work being carried out is related to the decommissioning of fast reactors. Many specialists who were involved in the studies and development work in this area in these countries have already retired or are close to retirement. In countries such as France, Japan and the Russian Federation that are still actively pursuing the evolution of fast reactor technology, the situation is aggravated by the lack of young scientists and engineers moving into this branch of nuclear power.

List of fast reactors

USA

• CLEMENTINE, the first fast reactor, built in 1946 at Los Alamos National Laboratory. Plutonium metal fuel, mercury coolant, power 25 kW thermal, used for research, especially as a fast neutron source.
• EBR-I at Idaho Falls, which in 1951 became the first reactor to generate significant amounts of electrical power. Decommissioned 1964.
• Fermi 1 near Detroit was a prototype fast breeder reactor that began operating in 1957 and shut down in 1972.
• EBR-II Prototype for the Integral Fast Reactor, 1965-1995?.
• SEFOR in Arkansas, a 20 MWt research reactor which operated from 1969 to 1972.
• Fast Flux Test Facility, 400MWt, Operated flawlessly from 1982 to 1992, at Hanford Washington, now deactivated, liquid sodium is drained with argon backfill under care and maintenance.

Europe

• DFR (Dounreay Fast Reactor, 1959–1977, 14MWe) and PFR (Prototype Fast Reactor, 1974–1994, 250MWe), in Caithness, in the Highland area of Scotland.
• Rhapsodie in Cadarache, France, (20 then 40 MW) between 1967 and 1982.
• Superphénix, in France, 1200MWe, closed in 1997 due to a political decision and very high costs of operation.
• Phénix, 1973, France, 233 MWe, restarted 2003 at 140 MWe for experiments on transmutation of nuclear waste for six years, ceased power generation in March 2009, though it will continue in test operation and to continue research programs by CEA until the end of 2009. Stopped in 2010.
• KNK-II, Germany

USSR

• Small lead-cooled fast reactors used for naval propulsion, particularly by the Soviet Navy.
• BN-350, constructed by the Soviet Union in Shevchenko (today's Aqtau) on the Caspian Sea, 130MWe plus 80,000 tons of fresh water per day.

Never operated

• Clinch River Breeder Reactor, USA
• Integral Fast Reactor, USA. Design emphasized fuel cycle based on on-site electrolytic reprocessing. Cancelled 1994 without construction.
• SNR-300, Germany

Currently operating

• Jōyō (常陽), 1977–1997 and 2003-, Japan
• Monju reactor, 300MWe, in Japan. was closed in 1995 following a serious sodium leak and fire. It was restarted May 6, 2010
• BN-600, 1981, Russia, 600 MWe, scheduled end of life 2010 but still in operation.
• FBTR, 1985, India, 10.5 MWt
• China Experimental Fast Reactor, 65 MWt, planned 2009, critical 2010

Under construction

• PFBR, Kalpakkam, India, 500 MWe. Planned to open 2011
• BN-800, Russia, planned operation in 2012

In design phase

• BN-1800, Russia, build starting in 2012, operation in 2018-2020
• Toshiba 4S being developed in Japan and is planned to be shipped to Galena, Alaska (USA) in 2012 (see Galena Nuclear Power Plant)
• KALIMER, 600 MWe, South Korea, projected 2030
• Generation IV reactor (Gas·Sodium·Lead cooled) US-proposed international effort, after 2030
• JSFR, Japan, project for a 1500 MWe reactor begin in 1998->2010
• ASTRID, France, project for a 600 MWe sodium-cooled reactor. Planned experimental operation in 2020.

Chart

Fast reactors

	U.S.	Russia	Europe	Asia
Past	Clementine, EBR-I/II, SEFOR, FFTF	BN-350	Dounreay, Rhapsodie, Superphénix, Phénix (stopped in 2010)
Cancelled	Clinch River, IFR		SNR-300
Operating		BN-600	Jōyō, FBTR, CEFR
Under construction		BN-800		Monju, PFBR,
Planned	Gen IV (Gas·Sodium·Lead)	BN-1800		4S, JSFR, KALIMER

See also

• Nuclear fuel cycle
• Breeder reactor
• Liquid Fluoride Reactor
• Molten-Salt Reactor Experiment
• Lead-cooled fast reactor
• Gas-cooled fast reactor
• Generation IV reactor
• Energy amplifier

External links and references

• ANL report on EARLY SOVIET FAST REACTORS
• Article on recent work on fast neutron reactors in Scientific American, December, 2005
• IAEA Fast Reactor Database
• Fast Reactor Data Retrieval and Knowledge Preservation seeks to establish a comprehensive, international inventory of fast reactor data and knowledge, which would be sufficient to form the basis for fast reactor development in 30 to 40 years from now.
• World Nuclear Association: Fast Neutron Reactors