Sodium-cooled fast reactor

A sodium-cooled fast reactor is a fast neutron reactor cooled by liquid sodium.

Pool type sodium-cooled fast reactor (SFR)

The acronym SFR particularly refers to two Generation IV reactor proposals, one based on existing liquid metal cooled reactor (LMFR) technology using mixed oxide fuel (MOX), the other based on the metal-fueled integral fast reactor.

Several sodium-cooled fast reactors have been built, some still in operation, and others are in planning or under construction. Bill Gates' TerraPower is planning to build its own reactors named Natrium.[1]

Fuel cycle

The nuclear fuel cycle employs a full actinide recycle with two major options: One is an intermediate-size (150–600 MWe) sodium-cooled reactor with uranium-plutonium-minor-actinide-zirconium metal alloy fuel, supported by a fuel cycle based on pyrometallurgical reprocessing in facilities integrated with the reactor. The second is a medium to large (500–1,500 MWe) sodium-cooled reactor with mixed uranium-plutonium oxide fuel, supported by a fuel cycle based upon advanced aqueous processing at a central location serving a number of reactors. The outlet temperature is approximately 510–550 degrees Celsius for both.

Sodium as a coolant

Liquid metallic sodium may be used as the sole coolant, carrying heat from the core. Sodium has only one stable isotope, sodium-23. Sodium-23 is a very weak absorber of neutrons. When it does absorb a neutron it produces sodium-24, which has a half-life of 15 hours and decays to magnesium-24, a stable isotope.

Pool or loop type
Schematic diagram showing the difference between the Loop and Pool designs of a liquid metal fast breeder reactor

There have been two main design approaches to sodium-cooled reactors.

In the pool type, the primary coolant is entirely contained in the main reactor vessel, which therefore includes not only the reactor core but also a heat exchanger. The US EBR-2, French Phénix and others used this approach, and it is used by India's Prototype Fast Breeder Reactor and China's CFR-600.

In the loop type, the heat exchangers are external to the reactor tank. The French Rapsodie, British Prototype Fast Reactor and others used this approach.

Advantages

The primary advantage of liquid metal coolants, such as liquid sodium, is that metal atoms are weak neutron moderators. Water is a much stronger neutron moderator because the hydrogen atoms found in water are much lighter than metal atoms, and therefore neutrons lose more energy in collisions with hydrogen atoms. This makes it difficult to use water as a coolant for a fast reactor because the water tends to slow (moderate) the fast neutrons into thermal neutrons (though concepts for reduced moderation water reactors exist). Another advantage of liquid sodium coolant is that sodium melts at 371K and boils / vaporizes at 1156K, a total temperature range of 785K between solid / frozen and gas / vapor states. By comparison, the liquid temperature range of water (between ice and gas) is just 100K at normal, sea-level atmospheric pressure conditions. Despite sodium's low specific heat (as compared to water), this enables the absorption of significant heat in the liquid phase, even allowing for safety margins. Moreover, the high thermal conductivity of sodium effectively creates a reservoir of heat capacity which provides thermal inertia against overheating.[2] Sodium also need not be pressurized since its boiling point is much higher than the reactor's operating temperature, and sodium does not corrode steel reactor parts.[2] The high temperatures reached by the coolant (the Phénix reactor outlet temperature was 560 C) permit a higher thermodynamic efficiency than in water cooled reactors.[3] The molten sodium, being electrically conductive, can also be pumped by electromagnetic pumps.[3]

Disadvantages

A disadvantage of sodium is its chemical reactivity, which requires special precautions to prevent and suppress fires. If sodium comes into contact with water it reacts to produce sodium hydroxide and hydrogen, and the hydrogen burns when in contact with air. This was the case at the Monju Nuclear Power Plant in a 1995 accident. In addition, neutrons cause it to become radioactive; however, activated sodium has a half-life of only 15 hours.[2]

Another problem is sodium leaks, regarded by critic of fast reactors M.V. Ramana as "pretty much impossible to prevent".[4]

Design goals
Actinides and fission products by half-life
Actinides[5] by decay chain Half-life
range (a)		 Fission products of 235U by yield[6]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra№ 4–6 a † 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac№ 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[7] 249Cfƒ 242mAmƒ 141–351 a

No fission products
have a half-life
in the range of
100–210 ka ...

241Amƒ 251Cfƒ[8] 430–900 a
226Ra№ 247Bk 1.3–1.6 ka
240Pu 229Th 246Cmƒ 243Amƒ 4.7–7.4 ka
245Cmƒ 250Cm 8.3–8.5 ka
239Puƒ 24.1 ka
230Th№ 231Pa№ 32–76 ka
236Npƒ 233Uƒ 234U№ 150–250 ka ‡ 99Tc₡ 126Sn
248Cm 242Pu 327–375 ka 79Se₡
1.53 Ma 93Zr
237Npƒ 2.1–6.5 Ma 135Cs₡ 107Pd
236U 247Cmƒ 15–24 Ma 129I₡
244Pu 80 Ma

... nor beyond 15.7 Ma[9]

232Th№ 238U№ 235Uƒ№ 0.7–14.1 Ga

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
m  metastable isomer
№  primarily a naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4–97 a: Medium-lived fission product
‡  over 200 ka: Long-lived fission product

The operating temperature should not exceed the melting temperature of the fuel. Fuel-to-cladding chemical interaction (FCCI) has to be designed against. FCCI is eutectic melting between the fuel and the cladding; uranium, plutonium, and lanthanum (a fission product) inter-diffuse with the iron of the cladding. The alloy that forms has a low eutectic melting temperature. FCCI causes the cladding to reduce in strength and could eventually rupture. The amount of transuranic transmutation is limited by the production of plutonium from uranium. A design work-around has been proposed to have an inert matrix. Magnesium oxide has been proposed as the inert matrix. Magnesium oxide has an entire order of magnitude smaller probability of interacting with neutrons (thermal and fast) than elements like iron.[10]

The SFR is designed for management of high-level wastes and, in particular, management of plutonium and other actinides. Important safety features of the system include a long thermal response time, a large margin to coolant boiling, a primary system that operates near atmospheric pressure, and intermediate sodium system between the radioactive sodium in the primary system and the water and steam in the power plant. With innovations to reduce capital cost, such as making a modular design, removing a primary loop, integrating the pump and intermediate heat exchanger, or simply find better materials for construction, the SFR can be a viable technology for electricity generation.[11]

The SFR's fast spectrum also makes it possible to use available fissile and fertile materials (including depleted uranium) considerably more efficiently than thermal spectrum reactors with once-through fuel cycles.

Reactors

Sodium-cooled reactors have included:

Model Country Thermal power (MW) Electric power (MW) Year of commission Year of decommission Notes
BN-350  Soviet Union 135 1973 1999 Was used to power a water de-salination plant.
BN-600  Soviet Union 1470 600 1980 Operational Together with the BN-800, one of only two commercial fast reactors in the world.
BN-800  Soviet Union/ Russia 2100 880 2015 Operational Together with the BN-600, one of only two commercial fast reactors in the world.
BN-1200  Russia 2900 1220 2036 Not yet constructed In development. Will be followed by BN-1200M as a model for export.
CEFR  China 65 20 2012 Operational
CRBRP  United States 1000 350 Never built Never built
EBR-1  United States 1.4 0.2 1950 1964
EBR-2  United States 62.5 20 1965 1994
Fermi 1  United States 200 69 1963 1975
Sodium Reactor Experiment  United States 20 65 1957 1964
S1G  United States United States naval reactors
S2G  United States United States naval reactors
PFR  United Kingdom 500 250 1974 1994
FBTR  India 40 13.2 1985 Operational
PFBR  India 500 2020 Under construction Under construction
Monju  Japan 714 280 1995/2010 2010 Suspended for 15 years. Reactivated in 2010, then permanently closed
Jōyō  Japan 150 1971 Operational
SNR-300  Germany 327 1985 1991
Rapsodie  France 40 24 1967 1983
Phénix  France 590 250 1973 2010
Superphénix  France 3000 1242 1986 1997 Largest SFR ever built. Suffered a terrorist attack during its construction.

Most of these were experimental plants, which are no longer operational. On November 30, 2019, CTV reported that the 3 Canadian provinces of New Brunswick, Ontario and Saskatchewan are planning an announcement about an interprovincial plan to cooperate on small sodium fast modular nuclear reactors from New Brunswick-based ARC Nuclear Canada.[12]

Related:

See also

References
  1. https://www.reuters.com/article/us-usa-nuclearpower-terrapower/bill-gates-nuclear-venture-plans-reactor-to-complement-solar-wind-power-boom-idUSKBN25N2U8
  2. Fanning, Thomas H. (May 3, 2007). "Sodium as a Fast Reactor Coolant" (PDF). Topical Seminar Series on Sodium Fast Reactors. Nuclear Engineering Division, U.S. Nuclear Regulatory Commission, U.S. Department of Energy. Archived from the original (PDF) on January 13, 2013.
  3. Bonin, Bernhard; Klein, Etienne (2012). Le nucléaire expliqué par des physiciens.
  4. Martin, Richard (2015-10-21). "TerraPower Quietly Explores New Nuclear Reactor Strategy". Technology Review. Retrieved 2020-09-20. "The problem with sodium is that it has been pretty much impossible to prevent leaks," says nuclear physicist M.V. Ramana, a lecturer at Princeton University’s Program on Science and Global Security and the Nuclear Futures Laboratory.
  5. Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
  6. Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  7. Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
    "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 [years]. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
  8. This is the heaviest nuclide with a half-life of at least four years before the "Sea of Instability".
  9. Excluding those "classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.
  10. Bays SE, Ferrer RM, Pope MA, Forget B (February 2008). "Neutronic Assessment of Transmutation Target Compositions in Heterogeneous Sodium Fast Reactor Geometries" (PDF). Idaho National Laboratory, U.S. Department of Energy. INL/EXT-07-13643 Rev. 1. Archived from the original (PDF) on 2012-02-12.
  11. Lineberry MJ, Allen TR (October 2002). "The Sodium-Cooled Fast Reactor (SFR)" (PDF). Argonne National Laboratory, US Department of Energy. ANL/NT/CP-108933.
  12. https://www.ctvnews.ca/politics/three-premiers-plan-to-fight-climate-change-by-investing-in-small-nuclear-reactors-1.4709865
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