History, Status, and Potential
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MOLTEN-SALT REACTORS—HISTORY, STATUS, AND POTENTIAL M.W. ROSENTHAL, P.R. KASTEN, and R.B. BRIGGS Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 Received August 4, 1969 Revised October 10, 1969 fuel appeared to offer several advantages, so Molten-salt breeder reactors (MSBR's) are experiments to establish the feasibility of molten- being developed by the Oak Ridge National salt fuels were begun in 1947 on “the initiative of Laboratory for generating low-cost power while V.P. Calkins, Kermit Anderson, and E.S. Bettis. At extending the nation's resources of fissionable fuel. the enthusiastic urging of Bettis and on the The fluid fuel in these reactors, consisting of UF4 recommendation of W.R. Grimes, R.C. Briant and ThF4 dissolved in fluorides of beryllium and adopted molten fluoride salts in 1950 as the main lithium, is circulated through a reactor core line effort of the Oak Ridge National Laboratory's moderated by graphite. Technology developments Aircraft Nuclear Propulsion1 program.” The over the past 20 years have culminated in the fluorides appeared particularly appropriate because successful operation of the 8-MWt Molten Salt they have high solubility for uranium, are among Reactor Experiment (MSRE), and have indicated the most stable of chemical compounds, have very that operation with a molten fuel is practical, that low vapor pressure even at red heat, have the salt is stable under reactor conditions, and that reasonably good heat transfer properties, are not corrosion is very low. Processing of the MSRE fuel damaged by radiation, do not react violently with has demonstrated the MSR processing associated air or water, and are inert to some common with high-performance converters. New fuel structural metals. processing methods under development should A small reactor, the Aircraft Reactor permit MSR's to operate as economical breeders. Experiment, was built at Oak Ridge to investigate These features, combined with high thermal the use of molten fluoride fuels for aircraft efficiency (44%) and low primary system pressure, propulsion reactors and particularly to study the give MSR converters and breeders potentially nuclear stability of the circulating fuel system. The favorable economic, fuel utilization, and safety ARE fuel salt was a mixture of NaF, ZrF4, and UF4, characteristics. Further, these reactors can be the moderator was BeO, and all the piping was In- initially fueled with 233U, 235U, or plutonium. The conel. In 1954 the ARE was operated successfully construction cost of an MSBR power plant is for 9 days at steady-state outlet temperatures estimated to be about the same as that of light- ranging up to 1580°F (1133 K) and at powers up to water reactors. This could lend to power costs 2.5 MWt. No mechanical or chemical problems ~0.5 to 1.0 mill/kWh less than those for light-water were encountered, and the reactor was found to be reactors. Achievement of economic molten-salt stable and self-regulating.2 breeder reactors requires the construction and That molten-salt reactors might be attractive for operation of several reactors of increasing size and civilian power applications was recognized from their associated processing plants. the beginning of the ANP program, and in 1956 H.G. MacPherson formed a group to study the technical characteristics, nuclear performance, and THE HISTORY OF MOLTEN-SALT REACTORS economics of molten-salt converters and breeders.3 After considering a number of concepts over a Investigation of molten-salt reactors started in period of several years, MacPherson and his the late 1940's as part of the United States' program associates concluded that graphite-moderated to develop a nuclear powered airplane. A liquid thermal reactors operating on a thorium fuel cycle would be the best molten-salt systems for and that proceeding to the breeder either directly or producing economic power. The thorium fuel via the converter would achieve reactors with good cycle with recycle of 233U was found to give better fuel conservation characteristics. Since many of performance in a molten-salt thermal reactor than a the features of civilian power reactors would differ uranium fuel cycle in which 238U is the fertile from those of the ARE, and the ARE had been material and plutonium is produced and recycled. operated only a short period, another reactor Homogeneous reactors in which the entire core is experiment was needed to investigate some of the liquid salt were rejected because the limited technology for power reactors. moderation by the salt constituents did not appear The design of the Molten-Salt Reactor to make as good a thermal reactor as one moderated Experiment was begun in 1960. A single-fluid by graphite, and intermediate spectrum reactors did reactor was selected that in its engineering features not appear to have high enough breeding ratios to resembled a converter, but the fuel salt did not compensate for their higher inventory of fuel. contain thorium and thus was similar to the fuel salt Studies of fast spectrum molten-salt reactors4,5 for a two-fluid breeder. The MSRE fuel salt is a indicated that good breeding ratios could be mixture of uranium, lithium-7, beryllium, and obtained, but very high power densities would be zirconium fluorides. Unclad graphite serves as the required to avoid excessive fissile inventories. moderator (the salt does not wet graphite and will Adequate power densities appeared difficult to not penetrate into its pores if the pore sizes are achieve without going to novel and untested heat small). All other parts of the system that contact removal methods. salt are made from the nickel-base alloy, INOR-8 Two types of graphite-moderated reactors were (also called Hastelloy-N), which was specially considered by MacPherson’s group—single-fluid developed in the aircraft program for use with reactors in which thorium and uranium are molten fluorides. The maximum power is ~8 MWt, contained in the same salt, and two-fluid reactors in and the heat is rejected to the atmosphere. which a fertile salt containing thorium is kept Construction of the MSRE began in 1962, and separate from the fissile salt which contains the reactor was first critical in 1965. Sustained uranium. The two-fluid reactor had the advantage operation at full power began in December 1966. that it would operate as a breeder; however, the Successful completion of a six-month run in March single-fluid reactor appeared simpler and seemed to of 1968 brought to a close the first phase of offer low power costs, even though the breeding operation during which the initial objectives were ratio would be below 1.0 using the technology of achieved. The molten fluoride fuel was used for that time. The fluoride volatility process6, which many months at temperatures > 1200ºF (920 K) could remove uranium from fluoride salts, had without corrosive attack on the metal and graphite already been demonstrated in the recovery of parts of the system. The reactor equipment uranium from the ARE fuel and thus was available operated reliably and the radioactive liquids and for partial processing of salts from either type of gases were contained safely. The fuel was reactor. completely stable. Xenon was removed rapidly The results of the ORNL studies were from the salt. When necessary, radioactive considered by a US Atomic Energy Commission equipment was repaired or replaced in reasonable task force that made a comparative evaluation of time and without overexposing maintenance fluid-fuel reactors early in 1959. One conclusion of personnel. the task force7 was that the molten-salt reactor, The second phase of MSRE operation began in although limited in potential breeding gain, had August 1968 when a small processing facility “the highest probability of achieving technical attached to the reactor was used to remove the feasibility.” original uranium by treating the fuel salt with By 1960, more complete conceptual designs of fluorine gas. A charge of 233U fuel was added to molten-salt reactors had emerged. Although em- the same carrier salt, and on October 2 the MSRE phasis was placed on the two-fluid concept because was made critical on 233U. Six days later the power of its better nuclear performance,8 the single-fluid was taken to 100 kW by Glenn T. Seaborg, reactor was also studied.9 ORNL concluded that Chairman of the US Atomic Energy Commission, either route would lead to low-power-cost reactors, bringing to power the first reactor to operate on service life, and to plan on replacement of the core 233U. at fairly frequent intervals.14 Moreover, During the years when the MSRE was being complexities in the assembly of the core seemed to built and brought into operation, most of the de- require that the entire core and reactor vessel be velopment work on molten-salt reactors was in replaced whenever a graphite element reached its support of the MSRE. However, basic chemistry radiation limit or developed a leak. Under such studies of molten fluoride salts continued through- circumstances, many years of operation of a out this period. One discovery10 during this time prototype reactor would be required to prove was that the lithium fluoride and beryllium fluoride convincingly that the two-fluid core is practicable. in a fuel salt can be separated from rare earths by At about the time the problems associated with vacuum distillation at temperatures near 1000°C. long graphite exposure became evident, a chemical This was a significant discovery, since it provided processing development occurred15 that greatly an inexpensive, on-site method for recovering these improved the prospect for a single-fluid breeder. valuable materials. As a consequence, the study To obtain good breeding performance in a single-- effort looking at future reactors focused on a two- fluid reactor, 233Pa (27.4 day half-life) must be held fluid breeder in which the fuel salt would be up outside the core until it decays to 233U.