Liquid metal cooled reactor

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A liquid metal cooled nuclear reactor, or LMR is a type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for breeder reactor power generation. They have also been used to power nuclear submarines.

Due to their high thermal conductivity, metal coolants remove heat effectively, enabling high power density. This makes them attractive in situations where size and weight are at a premium, like on ships and submarines. Most water-based reactor designs are highly pressurized to raise the boiling point (thereby improving cooling capabilities), which presents safety and maintenance issues that liquid metal designs lack. Additionally, the high temperature of the liquid metal can be used to drive power conversion cycles with high thermodynamic efficiency. This makes them attractive for improving power output, cost effectiveness, and fuel efficiency in nuclear power plants.

Liquid metals, being electrically highly conductive, can be moved by electromagnetic pumps.[1] Disadvantages include difficulties associated with inspection and repair of a reactor immersed in opaque molten metal, and depending on the choice of metal, fire hazard risk (for alkali metals), corrosion and/or production of radioactive activation products may be an issue.

Applications

Liquid metal coolant has been applied to both thermal- and fast-neutron reactors.

To date, most fast neutron reactors have been liquid metal cooled and so are called liquid metal cooled fast reactors (LMFRs). When configured as a breeder reactor (e.g. with a breeding blanket[definition needed]), such reactors are called liquid metal fast breeder reactors (LMFBRs).

Coolant properties

Suitable liquid metal coolants must have a low neutron capture cross section, must not cause excessive corrosion of the structural materials, and must have melting and boiling points that are suitable for the reactor's operating temperature.

Liquid metals generally have high boiling points, reducing the probability that the coolant can boil, which could lead to a loss-of-coolant accident. Low vapor pressure enables operation at near-ambient pressure, further dramatically reducing the probability of an accident. Some designs immerse the entire core and heat exchangers into a pool of coolant, virtually eliminating the risk that inner-loop cooling will be lost.

Liquid metal coolants
Metal Coolant Melting point Boiling point
Sodium 97.72 °C, (207.9 °F) 883 °C, (1621 °F)
NaK
−11 °C, (12 °F) 785 °C, (1445 °F)
Mercury −38.83 °C, (−37.89 °F) 356.73 °C (674.11 °F)
Lead 327.46 °C, (621.43 °F) 1749 °C, (3180 °F)
Lead-bismuth eutectic 123.5 °C, (254.3 °F) 1670 °C, (3038 °F)
Tin 231.9 °C, (449.5 °F) 2602 °C, (4716 °F)

Mercury

neutron cross-section
, it has fallen out of favor.

Sodium and NaK

Sodium and NaK (a

Experimental Breeder Reactor-1
, in 1951.

Sodium and NaK do, however, ignite spontaneously on contact with air and react violently with water, producing hydrogen gas. This was the case at the Monju Nuclear Power Plant in a 1995 accident and fire. Sodium is also the coolant used in the Russian BN reactor series and the Chinese CFR series in commercial operation today.[citation needed] Neutron activation of sodium also causes these liquids to become intensely radioactive during operation, though the half-life is short and therefore their radioactivity does not pose an additional disposal concern.

There are two proposals for a sodium cooled

Gen IV LMFR, one based on oxide fuel, the other on the metal-fueled integral fast reactor
.

Lead

Lead has excellent neutron properties (reflection, low absorption) and is a very potent radiation shield against

degrees Celsius above normal operating conditions. However, because lead has a high melting point and a high vapor pressure, it is tricky to refuel and service a lead cooled reactor. The melting point can be lowered by alloying the lead with bismuth, but lead-bismuth eutectic is highly corrosive to most metals[4][5]
used for structural materials.

Lead-bismuth eutectic

eutectic point: 123.5 °C / 255.3 °F).[4][6]

Beside its highly corrosive character,

radiotoxicity, above that of plutonium
).

Tin

Although tin today is not used as a coolant for working reactors because it builds a crust,[7] it can be a useful additional or replacement coolant at nuclear disasters or loss-of-coolant accidents.

Further advantages of tin are the high boiling point and the ability to build a crust even over liquid tin helps to cover poisonous leaks and keeps the coolant in and at the reactor. It has been tested by

Fukushima Daiichi nuclear disaster into liquid tin cooled reactors.[8]

Propulsion

Submarines

The Soviet November-class submarine K-27 and all seven Alfa-class submarines used reactors cooled by lead-bismuth eutectic and moderated with beryllium as their propulsion plants. (VT-1 reactors in K-27; BM-40A and OK-550 reactors in others).

The second nuclear submarine, USS Seawolf was the only U.S. submarine to have a sodium-cooled, beryllium-moderated nuclear power plant. It was commissioned in 1957, but it had leaks in its superheaters, which were bypassed. In order to standardize the reactors in the fleet,[citation needed] the submarine's sodium-cooled, beryllium-moderated reactor was removed starting in 1958 and replaced with a pressurized water reactor.

Nuclear aircraft

Liquid metal cooled reactors were studied by

nuclear aircraft as part of the Aircraft Nuclear Propulsion program.[9]

Power generation

The Sodium Reactor Experiment was an experimental sodium-cooled graphite-moderated nuclear reactor (A Sodium-Graphite Reactor, or SGR) sited in a section of the Santa Susana Field Laboratory then operated by the Atomics International division of North American Aviation.

In July 1959, the Sodium Reactor Experiment suffered a serious incident involving the partial melting of 13 of 43 fuel elements and a significant release of radioactive gases.[10] The reactor was repaired and returned to service in September 1960 and ended operation in 1964. The reactor produced a total of 37 GW-h of electricity.

SRE was the prototype for the Hallam Nuclear Power Facility, another sodium-cooled graphite-moderated SGR that operated in Nebraska.

fast breeder reactor that operated from 1963 to 1972. It suffered a partial nuclear meltdown
in 1963 and was decommissioned in 1975.

At

Prototype Fast Reactor
, which operated from 1974 to 1994 and used liquid sodium as its coolant.

The Soviet

Integral Fast Reactor
.

Many Generation IV reactors studied are liquid metal cooled:

References

  1. ^ Bonin, Bernhard; Klein, Etienne (2012). Le nucléaire expliqué par des physiciens.
  2. ^ Bunker, Merle E. "Early Reactors From Fermi’s Water Boiler to Novel Power Prototypes" a chapter in Los Alamos Science - Winter/ Spring 1983 Edition Page 128. Published by Los Alamos National Laboratory and available here: http://library.lanl.gov/cgi-bin/getfile?00416628.pdf
  3. ^ "Neutron Scattering Lengths and cross sections". www.ncnr.nist.gov.
  4. ^
    OSTI 4803122
    .
  5. ^ .
  6. ^ Fazio, Concetta; Li, Ning; Na, Byung-Chan (2005-07-01). Handbook on heavy liquid metal technology. Prepared in the frame of the OECD/NEA working party of the fuel cycle. Retrieved 2022-06-05.
  7. ^ Atmospheric corrosion of tin and tin alloy[dead link]
  8. ^ Ukraine advises Japan to use tin to cool Fukushima reactor Kyivpost
  9. ^ "40 Curious Nuclear Energy Facts You Should Know". December 9, 2019.
  10. ^ Ashley, R.L.; et al. (1961). SRE Fuel Element Damage, Final Report of the Atomics International Ad Hoc Committee (PDF). NAA-SR-4488-supl. Archived from the original (PDF) on 2009-04-10.