The Molten-Salt Reactor Experiment (MSRE) was an experimental molten-salt reactor research reactor at the Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee. This technology was researched through the 1960s, the reactor was constructed by 1964, it went critical in 1965, and was operated until 1969. The costs of a cleanup project were estimated at $130 million.
Initially designed for 15 MWth, the MSRE was operated at 7.4 MWth because of imprecise nuclear cross section data. It was a test reactor simulating the neutronic "kernel" of a type of inherently safer epithermal thorium breeder reactor called the liquid fluoride thorium reactor. It primarily used two fuels: first uranium-235 and later uranium-233. The latter 233UF4 was the result of breeding from thorium in other reactors. Since this was an engineering test, the large, expensive breeding blanket of thorium salt was omitted in favor of neutron measurements.
In the MSRE, the heat from the reactor core was shed via a cooling system using air blown over radiators. It is thought similar reactors could power high-efficiency heat engines such as closed-cycle gas turbines. The MSRE's piping, core vat and structural components were made from Hastelloy-N, and its moderator was a pyrolytic graphite core. The fuel for the MSRE was LiF-BeF2-ZrF4-UF4 (65-29.1-5-0.9 mole %). The secondary coolant was FLiBe (2LiF-BeF2), and it operated as hot as 650 °C and operated for the equivalent of about 1.5 years of full power operation.
The result promised to be a simple, reliable reactor. The purpose of the Molten-Salt Reactor Experiment was to demonstrate that some key features of the proposed molten-salt power reactors could be embodied in a practical reactor that could be operated safely and reliably and be maintained without excessive difficulty. For simplicity, it was to be a fairly small, one-fluid (i.e. non-breeding) reactor operating at 10 MWth or less, with heat rejection to the air via a secondary (fuel-free) salt.
Reactor description
Core
The pyrolytic graphite core, grade CGB, also served as the moderator.Molten-Salt Reactor Experiment 1965-1972 ORNL, Oct 2015 (8 MB) [Archive March 2016]Molten Salt Reactor Experiment, Oct 2015 (2 MB) Before the MSRE development began, tests had shown that salt would not permeate graphite in which the pores were on the order of a micrometer. However, graphite with the desired pore structure was available only in small, experimentally prepared pieces, and when a manufacturer set out to produce a new grade (CGB) to meet the MSRE requirements, difficulties were encountered..
Fuel The fuel was 7LiF-BeF2-ZrF4-UF4 (65-29.1-5-0.9 mole %). The first fuel was 33% 235U; later a smaller amount of 233UF4 was used. By 1960 a better understanding of fluoride salt based molten-salt reactors had emerged from earlier molten salt reactor research for the Aircraft Reactor Experiment. Fluoride salts are strongly ionic, and when melted they are stable at high temperatures, low pressures, and high radiation fluxes. Stability at low pressure permits less robust reactor vessels and increases reliability. The high reactivity of fluorine traps most fission reaction byproducts. It appeared that the fluid salt would permit on-site chemical separation of the fuel and wastes.
The fuel system was located in sealed cells, laid out for maintenance with long-handled tools through openings in the top shielding. A tank of LiF-BeF2 salt was used to flush the fuel circulating system before and after maintenance. In a cell adjacent to the reactor was a simple facility for bubbling gas through the fuel or flush salt: H2-hydrogen fluoride mixture, in roughly 10:1 ratio, to remove oxide, fluorine to remove uranium as uranium hexafluoride. The secondary coolant was LiF-BeF2 (66–34 mole %).
Pump The bowl of the fuel pump was the surge space for the circulating loop, and here about of fuel was sprayed into the gas space to allow xenon and krypton to escape from the salt. Removing the most significant neutron poison xenon-135 made the reactor safer and easier to restart. In solid-fuel reactors, on restart the 135Xe in the fuel absorbs neutrons, followed by a sudden jump in reactivity as the 135Xe is burned out. Conventional reactors may have to wait hours until xenon-135 decays after shutting down and not immediately restarting (so-called iodine pit).
Also in the pump bowl was a port through which salt samples could be taken or capsules of concentrated fuel-enriching salt (UF4-LiF or PuF3) could be introduced.
Air-cooled heat exchangers
At the time, the high temperatures were seen almost as a disadvantage because they hampered use of conventional steam turbines. Now, such temperatures are seen as an opportunity to use high-efficiency closed-cycle gas turbines. After two months of high-power operation, the reactor was down for 3 months because of the failure of one of the main cooling blowers.
Neutronics and thermal-hydraulics The reactor experienced stable neutronic operation. If temperatures increased or bubbles formed, the volume of the fluid fuel salts would increase and some fluid fuel salts would be forced out of the core, thereby reducing the reactivity. The MSRE development program did not include reactor physics experiments or heat transfer measurements. There was enough latitude in the MSRE that deviations from predictions would not compromise safety or accomplishment of the objectives of the experimental reactor.
Building grounds
Construction of the primary system components and alterations of the old Aircraft Reactor Experiment building (which had been partly remodeled for a proposed 60 MWth aircraft reactor) were started in 1962. Installation of the salt systems was completed in mid-1964. ORNL was responsible for quality assurance, planning, and management of construction. The primary systems were installed by ORNL personnel; subcontractors modified the building and installed ancillary systems.
Structural alloy Hastelloy-N Hastelloy-N—a low chromium, nickel–molybdenum alloy—was used in the MSRE and proved compatible with the fluoride salts FLiBe and FLiNaK.DeVan, Jackson H. "Effect of Alloying Additions on Corrosion Behavior of Nickel–Molybdenum Alloys in Fused Fluoride Mixtures." Thesis. University of Tennessee, 1960. Web. <>. All metal parts contacting salt were made of Hastelloy-N. The choice of Hastelloy-N for the MSRE was on the basis of the promising results of tests at aircraft nuclear propulsion conditions and the availability of much of the required metallurgical data. Development for the MSRE generated the further data required for ASME code approval. It also included preparation of standards for Hastelloy-N procurement and for component fabrication.
Almost 200,000 lb (90,000 kg) in a variety of shapes of material for the MSRE were produced commercially. Requests for bids on component fabrication went to several companies in the nuclear fabrication industry, but all declined to submit lump-sum bids because of lack of experience with the new alloy. Consequently, all major components were fabricated in U.S. Atomic Energy Commission-owned shops at Oak Ridge and Paducah, Kentucky..
At the time that design stresses were set for the MSRE, the data that was available indicated that the strength and creep rate of Hastelloy-N were hardly affected by irradiation. After the construction was well along, the stress-rupture life and fracture strain were found to be drastically reduced by thermal neutron irradiation. The MSRE stresses were reanalyzed, and it was concluded that the reactor would have adequate life to reach its goals. At the same time a program was launched to improve the resistance of Hastelloy-N to the embrittlement.
An out-of-pile corrosion test program was carried out for Hastelloy-N,. which indicated extremely low corrosion rates at MSRE conditions. Capsules exposed in the Materials Testing Reactor showed that salt fission power densities of more than 200 W/cm3 had no adverse effects on compatibility of fuel salt, Hastelloy-N, and graphite. Fluorine gas was found to be produced by radiolysis of frozen salts, but only at temperatures below about ..
Components that were developed especially for the MSRE included flanges for lines carrying molten salt, freeze valves (an air-cooled section where salt could be frozen and thawed), flexible control rods to operate in thimbles at , and the fuel sampler-enricher.. Centrifugal pumps were developed similar to those used successfully in the aircraft reactor program, but with provisions for remote maintenance, and including a spray system for xenon removal. Remote maintenance considerations pervaded the MSRE design, and developments included devices for remotely cutting and brazing together pipe, removable heater-insulation units, and equipment for removing specimens of metal and graphite from the core.
Development and construction Most of the MSRE effort from 1960 through 1964 was devoted to design, development, and construction of the MSRE. Production and further testing of graphite and Hastelloy-N, both in-pile and out, were major development activities. Others included work on reactor chemistry, development of fabrication techniques for Hastelloy-N, development of reactor components, and remote-maintenance planning and preparations..
Operation
The MSRE operated for 5 years. The salt was loaded in 1964, and nuclear operation ended in December 1969, and all the objectives of the experiment were achieved during this period.
Checkout and prenuclear tests included 1,000 hours of circulation of flush salt and fuel carrier salt. Nuclear testing of the MSRE began in June 1965, with the addition of enriched 235U as UF4-LiF eutectic to the carrier salt to make the reactor critical. After zero-power experiments to measure rod worth and reactivity coefficients, the reactor was shut down and final preparations made for power operation. Power ascension was delayed when vapors from oil that had leaked into the fuel pump were polymerized by the radioactive offgas and plugged gas filters and valves. Maximum power, which was limited to 7.4 MWth by the capability of the heat-rejection system, was reached in May 1966.
After two months of high-power operation, the reactor was down for three months because of the failure of one of the main cooling blowers. Some further delays were encountered because of offgas line plugging, but by the end of 1966 most of the startup problems were behind. During the next 15 months, the reactor was critical 80% of the time, with runs of 1, 3, and 6 months that were uninterrupted by a fuel drain. By March 1968, the original objectives of the MSRE had been accomplished, and nuclear operation with 235U was concluded.
By this time, ample 233U had become available, (see PDF page 10) "The MSRE was fueled with 39 kilograms of 233U that contained ~220 parts per million (ppm) of 232U [...that was bred in] various Light Water Reactors that had operated on 235U (such as the Indian Point PWR)" so the MSRE program was extended to include substitution of 233U for the uranium in the fuel salt, and operation to observe the new nuclear characteristics. Using the on-site processing equipment the flush salt and fuel salt were fluorinated to recover the uranium in them as UF6. 233UF4-LiF eutectic was then added to the carrier salt, and in October 1968, the MSRE became the world's first reactor to operate on 233U.