Molten salt reactor

Example of a molten salt reactor scheme

A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. MSRs offer multiple advantages over conventional nuclear power plants, although for historical reasons, they have not been deployed.

MSRs used to be "expensive, required highly enriched fuel, and had a low power density",[1] In comparison, they are now "cleaner, more compact, more affordable",[1] run at higher temperatures, have better thermodynamic efficiency, and perform in low (atmospheric) vapour pressure.[2]

The concept was first established in the 1950s. The early Aircraft Reactor Experiment was primarily motivated by the small size that the technique offered, while the Molten-Salt Reactor Experiment was a prototype for a thorium fuel cycle breeder nuclear power plant. The increased research into Generation IV reactor designs renewed interest in the technology.[3]

History

Aircraft reactor experiment

Aircraft Reactor Experiment building at ORNL. It was later retrofitted for the MSRE.

MSR research started with the U.S. Aircraft Reactor Experiment (ARE) in support of the U.S. Aircraft Nuclear Propulsion program. ARE was a 2.5 MWth nuclear reactor experiment designed to attain a high energy density for use as an engine in a nuclear-powered bomber.

The project included experiments, including high temperature and engine tests collectively called the Heat Transfer Reactor Experiments: HTRE-1, HTRE-2 and HTRE-3 at the National Reactor Test Station (now Idaho National Laboratory) as well as an experimental high-temperature molten salt reactor at Oak Ridge National Laboratory – the ARE.

ARE used molten fluoride salt NaF-ZrF4-UF4 (53-41-6 mol%) as fuel, moderated by beryllium oxide (BeO). Liquid sodium was a secondary coolant.

The experiment had a peak temperature of 860 °C. It produced 100 MWh over nine days in 1954. This experiment used Inconel 600 alloy for the metal structure and piping.[4]

An MSR was operated at the Critical Experiments Facility of the Oak Ridge National Laboratory in 1957. It was part of the circulating-fuel reactor program of the (Pratt & Whitney Aircraft Company (PWAC). This was called Pratt and Whitney Aircraft Reactor-1 (PWAR-1). The experiment was run for a few weeks and at essentially zero power, although it reached criticality. The operating temperature was held constant at approximately 675 °C (1,250 °F). The PWAR-1 used NaF-ZrF4-UF4 as the primary fuel and coolant. It was one of three critical MSRs ever built.[5]

Molten-salt reactor experiment

MSRE plant diagram

Oak Ridge National Laboratory (ORNL) took the lead in researching MSRs through the 1960s. Much of their work culminated with the Molten-Salt Reactor Experiment (MSRE). MSRE was a 7.4 MWth test reactor simulating the neutronic "kernel" of a type of epithermal thorium molten salt breeder reactor called the liquid fluoride thorium reactor (LFTR). The large (expensive) breeding blanket of thorium salt was omitted in favor of neutron measurements.

MSRE's piping, core vat and structural components were made from Hastelloy-N, moderated by pyrolytic graphite. It went critical in 1965 and ran for four years. Its fuel was LiF-BeF2-ZrF4-UF4 (65-29-5-1). The graphite core moderated it. Its secondary coolant was FLiBe (2LiF-BeF2). It reached temperatures as high as 650 °C and achieved the equivalent of about 1.5 years of full power operation.

Oak Ridge National Laboratory molten salt breeder reactor

The culmination of the ORNL research during the 1970–1976 timeframe resulted in a molten salt breeder reactor (MSBR) design. Fuel was to be LiF-BeF2-ThF4-UF4 (72-16-12-0.4) with graphite moderator. The secondary coolant was to be NaF-NaBF4. Its peak operating temperature was to be 705 °C.[6] It would follow a 4-year replacement schedule. The MSR program closed down in the early 1970s in favor of the liquid metal fast-breeder reactor (LMFBR),[7] after which research stagnated in the United States.[8][9] As of 2011, ARE and MSRE remained the only molten-salt reactors ever operated.

The MSBR project received funding until 1976. Inflation-adjusted to 1991 dollars, the project received $38.9 million from 1968 to 1976.[10]

Officially, the program was cancelled because:

  • The political and technical support for the program in the United States was too thin geographically. Within the United States the technology was well understood only in Oak Ridge.[7]
  • The MSR program was in competition with the fast breeder program at the time, which got an early start and had copious government development funds with contracts that benefited many parts of the country. When the MSR development program had progressed far enough to justify an expanded program leading to commercial development, the AEC could not justify the diversion of substantial funds from the LMFBR to a competing program.[7]

Oak Ridge National Laboratory denatured molten salt reactor (DMSR)

In 1980, the engineering technology division at Oak Ridge National Laboratory published a paper entitled "Conceptual Design Characteristics of a Denatured Molten-Salt Reactor with Once-Through Fueling." In it, the authors "examine the conceptual feasibility of a molten-salt power reactor fueled with denatured uranium-235 (i.e. with low-enriched uranium) and operated with a minimum of chemical processing." The main priority behind the design characteristics was proliferation resistance.[11] Although the DMSR can theoretically be fueled partially by thorium or plutonium, fueling solely with low enriched uranium (LEU) helps maximize proliferation resistance.

Other important goals of the DMSR were to minimize R&D and to maximize feasibility. The Generation IV international Forum (GIF) includes "salt processing" as a technology gap for molten salt reactors.[12] The DMSR requires minimal chemical processing because it is a burner rather than a breeder. Both reactors built at ORNL were burner designs. In addition, the choices to use graphite for neutron moderation and enhanced Hastelloy-N for piping simplified the design and reduced R&D.

United Kingdom

The UK's Atomic Energy Research Establishment (AERE) were developing an alternative MSR design across its National Laboratories at Harwell, Culham, Risley and Winfrith. AERE opted to focus on a lead-cooled 2.5 GWe Molten Salt Fast Reactor (MSFR) concept using a chloride.[13] They also researched helium gas as a coolant.[14][15]

The UK MSFR would be fuelled by plutonium, a fuel considered to be 'free' by the program's research scientists, because of the UK's plutonium stockpile.

Despite their different designs, ORNL and AERE maintained contact during this period with information exchange and expert visits. Theoretical work on the concept was conducted between 1964 and 1966, while experimental work was ongoing between 1968 and 1973. The program received annual government funding of around £100,000-£200,000 (equivalent to £2m-£3m in 2005). This funding came to an end in 1974, partly due to the success of the Prototype Fast Reactor at Dounreay which was considered a priority for funding as it went critical in the same year.[13]

Soviet Union

In the USSR, a molten-salt reactor research program was started in the second half of the 1970s at the Kurchatov Institute. It included theoretical and experimental studies, particularly the investigation of mechanical, corrosion and radiation properties of the molten salt container materials. The main findings supported the conclusion that no physical nor technological obstacles prevented the practical implementation of MSRs.[16] A reduction in activity occurred after 1986 due to the Chernobyl accident, along with a general stagnation of nuclear power and the nuclear industry.[17](p381)

Twenty-first century

MSR interest resumed in the new millennium with continuing delays in fusion power and other nuclear power programs.

The LFTR design was strongly supported by Alvin Weinberg, who patented the light-water reactor and was a director of the U.S.'s Oak Ridge National Laboratory. In 2016 Nobel prize winning physicist Carlo Rubbia, former Director General of CERN, claimed that one of the main reasons why research was cut is that thorium is difficult to turn into a nuclear weapon.[18]

Thorium is not for tomorrow but unless you do any development, it will not get there. — Dr Carlo Rubbia, Nobel Laureate and former Director General of CERN, January 2016[18]
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