Molten-Salt Reactor Experiment
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The Molten-Salt Reactor Experiment (MSRE) was an experimental molten-salt reactor
research reactor Research reactors are nuclear fission-based nuclear reactors that serve primarily as a neutron source. They are also called non-power reactors, in contrast to power reactors that are used for electricity production, heat generation, or maritim ...
at the
Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a federally funded research and development centers, federally funded research and development center in Oak Ridge, Tennessee, United States. Founded in 1943, the laboratory is sponsored by the United Sta ...
(ORNL) in
Oak Ridge, Tennessee Oak Ridge is a city in Anderson County, Tennessee, Anderson and Roane County, Tennessee, Roane counties in the East Tennessee, eastern part of the U.S. state of Tennessee, about west of downtown Knoxville, Tennessee, Knoxville. Oak Ridge's po ...
. This technology was researched through the 1960s, the reactor was constructed by 1964, it went
critical Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine * Critical juncture, a discontinuous change studied in the social sciences. *Critical Software, a company specializing i ...
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 The nuclear cross section of a nucleus is used to describe the probability that a nuclear reaction will occur. The concept of a nuclear cross section can be quantified physically in terms of "characteristic area" where a larger area means a larg ...
data. It was a test reactor simulating the neutronic "kernel" of a type of inherently safer epithermal
thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...
breeder reactor A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. These reactors can be fueled with more-commonly available isotopes of uranium and thorium, such as uranium-238 and thorium-232, as opposed to the ...
called the liquid fluoride thorium reactor. It primarily used two fuels: first
uranium-235 Uranium-235 ( or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nat ...
and later
uranium-233 Uranium-233 ( or U-233) is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a Nuclear fuel, reactor fuel. It has been used successfully ...
. 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
radiator A radiator is a heat exchanger used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...
s. It is thought similar reactors could power high-efficiency
heat engine A heat engine is a system that transfers thermal energy to do mechanical or electrical work. While originally conceived in the context of mechanical energy, the concept of the heat engine has been applied to various other kinds of energy, pa ...
s 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 A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corr ...
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) rchive March 2016/ref>''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 It is also known as the "Year of Africa" because of major events—particularly the independence of seventeen African nations—that focused global attention on the continent and intensified feelings of Pan-Africanism. Events January * Janu ...
a better understanding of
fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...
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 Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
mixture, in roughly 10:1 ratio, to remove oxide,
fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...
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 Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
and
krypton Krypton (from 'the hidden one') is a chemical element; it has symbol (chemistry), symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace element, trace amounts in the Earth's atmosphere, atmosphere and is of ...
to escape from the salt. Removing the most significant
neutron poison In applications such as nuclear reactors, a neutron poison (also called a neutron absorber or a nuclear poison) is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable ef ...
xenon-135 Xenon-135 (135Xe) is an Isotope#Radioactive, primordial, and stable isotopes, unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and it is the most powerful known neutron-absorbing nuclear poison ...
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 The iodine pit, also called the iodine hole or xenon pit, is a temporary disabling of a nuclear reactor due to the buildup of short- lived nuclear poisons in the reactor core. The main isotope responsible is 135Xe, mainly produced by natural d ...
''). 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 turbine A steam turbine or steam turbine engine is a machine or heat engine that extracts thermal energy from pressurized steam and uses it to do mechanical work utilising a rotating output shaft. Its modern manifestation was invented by Sir Charles Par ...
s. 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 Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
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 Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...
,
nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...
molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...
alloy—was used in the MSRE and proved compatible with the fluoride salts FLiBe and FLiNaK. 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 Nuclear propulsion includes a wide variety of propulsion methods that use some form of nuclear reaction as their primary power source. Many aircraft carriers and submarines currently use uranium fueled nuclear reactors that can provide propulsio ...
conditions and the availability of much of the required metallurgical data. Development for the MSRE generated the further data required for
ASME The American Society of Mechanical Engineers (ASME) is an American professional association that, in its own words, "promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe" via "continuing edu ...
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 Paducah ( ) is a List of cities in Kentucky, home rule-class city in the Upland South, and the county seat of McCracken County, Kentucky, United States. The most populous city in the Jackson Purchase region, it is located in the Southeastern Unit ...
. At the time that design stresses were set for the MSRE, the data that was available indicated that the
strength Strength may refer to: Personal trait *Physical strength, as in people or animals *Character strengths like those listed in the Values in Action Inventory *The exercise of willpower Physics * Mechanical strength, the ability to withstand ...
and creep rate of Hastelloy-N were hardly affected by
irradiation Irradiation is the process by which an object is exposed to radiation. An irradiator is a device used to expose an object to radiation, most often gamma radiation, for a variety of purposes. Irradiators may be used for sterilizing medical and p ...
. After the construction was well along, the stress-rupture life and
fracture Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...
strain were found to be drastically reduced by
thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium wit ...
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 Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
test program was carried out for Hastelloy-N, which indicated extremely low corrosion rates at MSRE conditions. Capsules exposed in the
Materials Testing Reactor A materials testing reactor (MTR) is a high-flux research nuclear reactor aimed at studying materials exposed to a high neutron flux. Examples Materials testing reactors include: * SCK_CEN#BR2, BR2, the Belgian Reactor 2, moderated with a parabo ...
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 Radiolysis is the dissociation of molecules by ionizing radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux. The radiation in this context is associated with ionizing radiation; radiolysis is ...
of frozen salts, but only at temperatures below about . Components that were developed especially for the MSRE included
flange A flange is a protruded ridge, lip or rim (wheel), rim, either external or internal, that serves to increase shear strength, strength (as the flange of a steel beam (structure), beam such as an I-beam or a T-beam); for easy attachment/transfer o ...
s 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 pump Centrifugal pumps are used to transport fluids by the Energy transformation, conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are ...
s 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 Brazing is a metal-joining process in which two or more metal items are joined by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal. Brazing differs from welding in ...
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 Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine * Critical juncture, a discontinuous change studied in the social sciences. *Critical Software, a company specializing i ...
. 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, 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. The 233U zero-power experiments and dynamics tests confirmed the predicted
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
ic characteristics. An unexpected consequence of processing the salt was that its physical properties were altered slightly so that more than the usual amount of gas was entrained from the fuel pump into the circulating loop. The circulating gas and the power fluctuations that accompanied it were eliminated by operating the fuel pump at slightly lower speed. Operation at high power for several months permitted accurate measurement of the capture-to- fission ratio, for 233U in this reactor, completing the objectives of the 233U operation. In the concluding months of operation, xenon stripping, deposition of fission products, and
tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...
behavior were investigated. The feasibility of using
plutonium Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is a silvery-gray actinide metal that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four ...
in molten-salt reactors was emphasized by adding PuF3 as makeup fuel during this period. After the final shutdown in December 1969, the reactor was left in standby for nearly a year. A limited examination program was then carried out, including a moderator bar from the core, a control rod thimble, heat exchanger tubes, parts from the fuel pump bowl, and a freeze valve that had developed a leak during the final reactor shutdown. The radioactive systems were then closed to await ultimate disposal.


Statistics

Parameters and operational statistics: Power: 8 MW (thermal)
output: 92.8 GWh
equivalent full-power: 11,555 h Fuel salt:
fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s: 65% Li-7, 29.1% Be, 5% Zr, 0.9% U
weight: 11,260 lbs (5,107 kg)
melting temp: 813 F (434 C)
inlet temp: 1175 F (635 C)
outlet temp: 1225 F (663 C)
flow rate: 400 gal/min (1514 l/min)
fuel pump circulating: 19,405 h Coolant salt: fluoride
cations: 66% Li-7, 34% Be
weight: 15,300 lbs (6,940 kg)
coolant pump circulating: 23,566 h Moderator: nuclear graphite Container: Hastelloy-N First fuel:
U-235 Uranium-235 ( or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nat ...

first critical: 1 June 1965
thermal output: 72,441 MWh
critical hours: 11,515 h
full-power output equivalent: 9,006 h Second fuel: U-233
critical: 2 October 1968
thermal output: 20,363 MWh
critical hours: 3,910 h
full-power output equivalent: 2,549 h Shutdown: December 1969


Results

The broadest and perhaps most important conclusion from the MSRE experience was that a molten salt fueled reactor concept was viable. It ran for considerable periods of time, yielding valuable information, and maintenance was accomplished safely and without excessive delay. The MSRE confirmed expectations and predictions. For example, it was demonstrated that: the fuel salt was immune to radiation damage, the graphite was not attacked by the fuel salt, and the corrosion of Hastelloy-N was negligible. Noble gases were stripped from the fuel salt by a spray system, reducing the 135Xe poisoning by a factor of about 6. The bulk of the
fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the releas ...
elements remained stable in the salt. Additions of uranium and plutonium to the salt during operation were quick and uneventful, and recovery of uranium by fluorination was efficient. The neutronics, including critical loading, reactivity coefficients, dynamics, and long-term reactivity changes, agreed with prior calculations. In other areas, the operation resulted in improved data or reduced uncertainties. The 233U capture-to- fission ratio in a typical MSR neutron spectrum is an example of basic data that was improved. The effect of fissioning on the
redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
of the fuel salt was resolved. The deposition of some elements ("
noble metals A noble metal is ordinarily regarded as a metallic element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium, palladium, osmium, i ...
") was expected, but the MSRE provided quantitative data on relative deposition on graphite, metal, and liquid-gas interfaces.
Heat transfer coefficient In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the Proportional (mathematics), proportionality constant between the heat flux and the thermodynamic driving force for the Heat transfer, flow of heat ...
s measured in the MSRE agreed with conventional design calculations and did not change over the life of the reactor. Limiting oxygen in the salt proved effective, and the tendency of fission products to be dispersed from contaminated equipment during maintenance was low. Operation of the MSRE provided insights into the problem of
tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...
in a molten-salt reactor. It was observed that about 6–10% of the calculated 54 Ci/day (2.0  TBq) production diffused out of the fuel system into the containment cell atmosphere and another 6–10% reached the air through the heat removal system. The fact that these fractions were not higher, indicated that something partially negated the transfer of tritium through hot metals. One unexpected finding was inter-granular cracking in all metal surfaces exposed to the fuel salt. The cause of the embrittlement was
tellurium Tellurium is a chemical element; it has symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...
, a fission product generated in the fuel. This was first noted in the specimens that were removed from the core at intervals during the reactor operation. Post-operation examination of pieces of a control-rod thimble, heat-exchanger tubes and pump bowl parts revealed the ubiquity of the cracking and emphasized its importance to the MSR concept. The crack growth was rapid enough to become a problem over the planned 30-year life of a follow-on thorium breeder reactor. This cracking could in short-term be reduced by adding small amounts of
niobium Niobium is a chemical element; it has chemical symbol, symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and Ductility, ductile transition metal. Pure niobium has a Mohs scale of mineral hardness, Mohs h ...
to the Hastelloy-N. However, further studies were needed to assess the effects of longer exposure times and some interaction parameters for the used mixtures. The operation experience gained with the MSRE showed that the following areas require further investigation for the successful operation of a commercial MSR: * Maintaining the salt as a liquid in all parts of primary system, particularly in extremities far from the core. * Tight control of
tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...
production and transport from the core (only <20% could be removed due to diffusion and heat removal system in the MSRE). * Reduction in growth of inter-granular cracks in exposed metal surfaces (due to
tellurium Tellurium is a chemical element; it has symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...
, a
fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the releas ...
of uranium). * Decommissioining and disposal of the reactor structure and waste salt (approx. costs in 2019 are $10mil/yr ).


Decommissioning

As of 2019, the MSRE is in a SAFESTOR state, meaning it still intact but shut down and actively monitored and maintained. After shutdown, the salt was believed to be in long-term safe storage. At low temperatures, radiolysis can free fluorine from the salt. As a countermeasure, the salt was annually reheated to about until 1989. But beginning in the mid-1980s, there was concern that radioactivity was migrating through the system, reported by an ORNL employee who was among 125 people working above the reactor, which had not been decontaminated or decommissioned. Department of Energy Oak Ridge Operations Manager Joe Ben LaGrone ordered evacuation of 125 employees, based on findings reported to him inspector William Dan DeFord, P.E. Sampling in 1994 revealed concentrations of uranium that created a potential for a nuclear
criticality accident A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the uninten ...
, as well as a potentially dangerous build-up of fluorine gas: the environment above the solidified salt was approximately one atmosphere of fluorine. The ensuing decontamination and decommissioning project was called "the most technically challenging" activity assigned to Bechtel Jacobs under its environmental management contract with the U.S. Department of Energy's Oak Ridge Operations organization. In 2003, the MSRE cleanup project was estimated at $130 million, with decommissioning expected to be completed in 2009. Removal of uranium from the salt was completed in March 2008, however still leaving the salt with the fission products in the tanks. Much of the high cost was caused by the unpleasant surprise of fluorine and uranium hexafluoride evolution from cold fuel salt in storage that ORNL did not defuel and store correctly, but this has now been taken into consideration in MSR design. A potential decommissioning process has been described;Evaluation of the U.S. Department of Energy's Alternatives for the Removal and Disposition of Molten Salt Reactor Experiment Fluoride Salts
(1997), Commission on Geosciences, Environment and Resources
uranium is to be removed from the fuel as the hexafluoride by adding excess fluorine, and plutonium as the
plutonium dioxide Plutonium(IV) oxide, or plutonia, is a chemical compound with the chemical formula, formula plutonium, Puoxygen, O2. This high melting-point solid is a principal compound of plutonium. It can vary in color from yellow to olive green, depending on ...
by adding
sodium carbonate Sodium carbonate (also known as washing soda, soda ash, sal soda, and soda crystals) is the inorganic compound with the formula and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water ...
.


See also

*
Thorium fuel cycle The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, , as the fertile material. In the reactor, is transmuted into the fissile artificial uranium isotope which is the nuclear fuel. Unlike natural uranium, natural ...
* Fuji MSR * Thorium-based nuclear power


References

*


Further reading


MSRE Safety analysis


External links

*, a film published by Atomic Energy Commission *
An Account of Oak Ridge National Laboratory’s Thirteen Nuclear Reactors
(from ORNL; includes a section on the MSRE)
2015 Workshop on Molten Salt Reactor Technologies
("Commemorating the 50th Anniversary of the Startup of the MSRE"), including
50th anniversary brochureposters
and
history of the ORNL molten salt program
{{Nuclear fission reactors 1965 in science Molten salt reactors Nuclear reactors Oak Ridge National Laboratory Science experiments es:Reactor de sal fundida