The Molten-Salt Reactor Experiment

Molten

salt

reactor

s for electrical power production were studied at Oak Ridge National Laboratory from 1957 to 1960. Design studies and technological advances clearly indicated that

molten

salt

breeder

reactor

s could be developed that operated on a cycle of thorium to produce low-cost electricity and conserve our uranium resources. However, an in-reactor

experiment

was necessary to demonstrate the feasibility of this unique high-temperature fluid fuel reactor concept. As a result, the United States Atomic Energy Commission in 1960 authorized the laboratory to design, build and operate the

molten

salt reactor

experiment

, also known as MSRE. Preliminary design of the reactor began in May 1960. Technology developed by the laboratory's previous nuclear aircraft propulsion program was used as a basis for designing the MSRE.

The molten salt fuel for this reactor is radically different from that used in solid fuel reactors. Here, salt flows from a pipe that is heated to keep the salt molten. At 1200 degrees Fahrenheit. Salt does not react with air. It flows like water, but does not boil. The molten salt reactor experiment uses a fuel system that includes the reactor core, in which heat is generated. A heat exchanger to transfer heat from the fuel cooling salt, a centrifugal sump pump to circulate the fuel, three salt storage tanks, and connecting pipes. The MSRE cooling system consists of a circulation pump, a salt-to-air radiator, a salt storage tank and the connecting pipe.

Both systems were designed for a temperature of 1300 degrees Fahrenheit and a pressure of 50 PSIG. MSRE is a single-region, graphite-moderated, circulating fuel reactor capable of generating 7,500 kilowatts of heat. During reactor operation, fuel, which is almost as fluid as water at an elevated temperature of 1170 degrees Fahrenheit and twenty PSIG, enters the flow distributor near the top of the vessel and spirals downward in a flow turbulent through the space between the container. and central cam. The fuel is then directed upward in a laminar flow through channels machined into graphite rods of the core matrix where heat is generated by the fission of uranium-235.

The fuel leaves the reactor 40 degrees hotter than it enters. A simple control system of three flexible control bars helps regulate power and temperature. The fuel pump, which runs at 1,200 gallons per minute, draws its suction directly from the reactor. At full power, the salt, which is now at 1,210 degrees Fahrenheit, is discharged by the fuel pump and flows through the primary heat exchanger and then back to the reactor. Gaseous fission products that interfere with the chain reaction are removed from the circulating salt stream in the pump tank. The cooling salt is circulated through a sump-type centrifugal pump, similar to the fuel pump.

It flows through the heat exchanger and exits at 1070 degrees Fahrenheit after a 60 degree temperature rise. The heat from the cooling salt is transferred to the air in the radiator. Cooling air, supplied by axial flow blowers, is forced through the radiator and up the chimney. The major components of the fuel and coolant salt circulation systems are connected by five-inch pipes and smaller fill and drain lines. During reactor operation, molten salts are kept throughout the system by special freezing valves. The fuel, solid at room temperature, is composed of lithium fluoride salts, beryllium, zirconium and uranium. One third of uranium is fissile U-235.

The coolant is a mixture of fluoride salts like the fuel, but does not contain uranium or zirconium. These mixtures were produced in the laboratory using techniques developed during the nuclear aircraft propulsion program. Salt-containing pipes and components are manufactured from Hastelloy-N, a special nickel-molybdenum alloy developed at Oak Ridge that has good resistance to corrosion attack by fluoride salts up to temperatures of 1,500 degrees. The MSRE was designed to use the aircraft reactor experimental facilities of 1954. The ARE was the first molten salt reactor to use circulating molten fluorides as fuel. It ran at 2500 kilowatts in this building. Modifications to the building extended from 1961 to mid-1962.

The MSRE fuel storage cell and other areas are new, but most of the previous facilities were only slightly modified. It was necessary to lengthen the fuel circuit's steel containment vessel, which is 24 feet in diameter. The tension was then relieved by gas heating. The container is located in another cylindrical steel protection tank. For biological protection, the lower portion of the annular space between the tank and the vessel was filled with magnetite sand and water, and the upper end with magnetite concrete. The containment vessel cover consists of two layers of 3 1/2-foot-thick magnetite concrete blocks separated by a welded stainless steel membrane.

The concrete serves as a biological shield and as a support structure for the pressure-tight cell. Considerable excavation was required in the fuel storage cell to allow gravity drainage of fuel from the reactor to the storage tanks. The bottom and sides of the cells are stainless steel backed by heavily reinforced concrete. This pressure-tight cell is also covered with two layers of reinforced concrete blocks, separated by a welded stainless steel membrane joint. The domed heads of the reactor vessel were cold-pressed from 1 1/8-inch-thick Hastelloy-N alloy plates. The reactor vessel fuel inlet flow distribution channel was also cold formed, as were most Hastelloy-N alloy parts.

Some development work was needed to manufacture the main components. Hastelloy-N was generally found to present no more problems than other high nickel alloys or stainless steel. Assembly of the reactor vessel began in mid-1962. By the fall of 1963, special graphite, with low salt absorption and low gas permeability, had been commercially produced and precision machined. The reactor core, made up of nearly 600 vertical graphite rods, was carefully assembled on a graphite lattice inside a clean room. A removable control rod thimble assembly was manufactured separately for the reactor. The five-foot diameter reactor with core and control rod assembly weighs about nine tons.

This special transport device kept the reactor upright during its journey from the workshop to the reactor site. The horizontal shell and tube primary heat exchanger is approximately eight feet long. Its U-shaped tubes, half an inch in diameter, were fusion welded and furnace brazed in a hydrogen atmosphere to the tube sheet. The package contains spacer bars firmly attached in two directions through spaces between the triangular camber tubes to eliminate clutter and vibration. The fuel salt pump container serves as the pump sump and normal volume to allow for salt expansion. This rotating fuel pump element with water-cooled drive motor, bearing assembly, bolt extensions for remote replacement and impeller circulated molten salt at 1200 degrees during non-nuclear run-in test.

The salt-to-air radiator tubes, each three-quarters of an inch in diameter, are arranged in banks of 12 tubes each. Special mounting fixtures were required to accurately position the tube banks and sub-directories for welding and assembly. The radiator housing and coil were manufactured separately and then assembled after removing one end of the housing. The enclosure that supports the radiator coil is an oven that contains electric heaters to prevent uncontrolled freezing of the salt. Adjustable doors and dampers control air flow through the radiator to regulate heat removal. The tanks provide safe storage of salt mixtures when the fuel and coolant salt circulation systems are not operating.

These include two fuel storage tanks with bayonet tubes and a steam dome in which the decay heat of the fission product is removed by converting water to steam. Several thousand welds of high-integrity reactor-grade Hastelloy-N alloys were made during the manufacturing of major components. The history of each was recorded for future reference. The installation of the fuel circuit equipment in the reactor cell began in January 1963 with the placement of the base of the heat shield and the cylindrical section. The heat shield is a double-walled stainless steel container that surrounds the reactor and protects the cell equipment from neutron fields.

The ring is filled with water and carbon steel balls for primary shielding. The shield also provides a support for the reactor vessel. One of the objectives of the MSRE was to ensure the remote replacement of all reactor cell equipment. To locate the equipment during installation and replacement, a large assembly device was built outside the cell in which the reactor, fuel salt circulation pump, primary heat exchanger and pipelines were precisely located. connection and disconnections using optical tool methods. After connecting the connecting pipes and auxiliary equipment by using the fitting, the reactor was lowered into the heat shield.

The connecting pipe bends and welds were normalized to a temperature 450 degrees higher than the normal operating temperature of the salt to minimize movement of the pipe disconnects after the fuel salt had been circulated. The same procedure was followed for the fuel salt circulation pump vessel and the furnace. The pump installation was completed with the addition of the pump rotating element, drive motor, auxiliary piping and electrical connections. Special freeze flanges on the fuel salt circulation lines allow important components such as the heat exchanger to be removed and replaced. All components and connecting pipes of salt circulation systems are electrically heated to keep the system above the salt freezing point of 840 degrees.

The equipment is preheated before adding salt. The fuel storage tanks and related equipment were installed using mounting hardware and the same procedure was used for the main fuel circuit. Fuel storage tanks, with steam cooling systems, were located inside the electric furnaces. Soft piping and all ancillary services, including instrumentation and controls, were thoroughly checked for leaks and electrical continuity. Since the heaters and thermocouples, as well as the coil, were installed in the shop radiator enclosure, no additional work was required before placing the radiator in the coolant cell. The installation of the radiator, salt storage tank and circulation pump completed the cooling circuit.

The MSRE incorporates some novel features and components, designed and developed specifically for molten salt reactors. The data on the operation of these elements makes it possible to evaluate ideas and principles that could be used in breeder-type molten salt reactors. This on-site processing facility plans to remove impurities and recover uranium from the fuel salt. The sampling enricher is a unique device for taking samples and adding fuel during full power operation of the reactor. A power-operated cable remotely lowers a small bucket into the liquid salt of the bomb container and then raises the bucket to a handling area for transfer to a shielded container.

More than 300 samples have been routinely obtained using this method. When necessary, the fuel is enriched by lowering a bucket of enriched U-235 and dissolving it in the circulating fuel salt stream. This eliminates the need for excess fuel reactivity. The fuel has been enriched three times by adding 114 cubes of enriching salt. Another feature of the MSRE is its high-speed digital computer. The computer does not exercise direct control over the reactor, but rather collects and displays data and performs online calculations for immediate and long-range purposes. With the final addition of 680 grams of enriched uranium to the fuel stream through sample enrichment, the molten salt reactor experiment became critical on June 1, 1965.Low-power nuclear experiments lasted about a month.

After final preparations, electrical operations began in early 1966. During the year ending January 1968, the reactor was critical more than 75 percent of the time, producing nearly 41,000 megawatt hours of heat. By then, many of the test's objectives had been achieved. It has been demonstrated that the MSRE can be operated safely and reliably, and that maintenance of radioactive equipment has been achieved with minimal difficulty. The nuclear characteristics were extremely close to the predicted values ​​and the system was dynamically stable at all power levels. Samples of graphite and Hastelloy-N similar to these have been removed from the fission fuel stream in the reactor core.

Examination of the samples in hot cells indicated no damaging corrosion or structural changes after the salt had circulated in the fuel system for more than 6,000 hours. At that time the fuel proved to be stable to radiation and heat. The reactor, with a core exit temperature of 1,200 degrees, is one of the highest temperature reactors operating today. As a result of this success, ORNL proposes to develop a thermal breeder reactor. This is one that produces more fissile material than it consumes. Thermal breeders have many practical advantages in construction and operation, and the molten salt reactor appears to be a very attractive thermal breeder.

Molten salt thermal breeders should have a low inventory of fissile material, meaning that the amount of uranium needed to fuel the reactor is low. In the design of a two-fluid molten salt generator, the mixture of fuel salts of lithium, beryllium and uranium fluorides is pumped through graphite tubes into the reactor core, where heat is generated by the fission of the uranium, and then through the primary heat exchanger. , where heat is extracted. Fertile salt or blanket surrounds the core, where additional heat is generated. It is then pumped through the blanket heat exchanger, where this heat is extracted.

The salt mixture contains thorium, which is converted into fissile uranium atoms faster than it is consumed in the reactor fuel. The heat produced in the fuel and blanket streams is transferred by an intermediate salt heat transfer system to a supercritical steam generating electrical system. In an alternative design, a salt containing thorium and uranium circulates through the reactor vessel and heat exchangers. This simpler, single-fluid reactor is also a promising generator, if the problems of operational fuel processing can be solved. ORNL studies indicate that molten salt breeder reactors should be able to produce electricity at low cost and with the high efficiency of modern steam plants.

Thus, the important objectives of the laboratory in the development of nuclear reactors converge in the molten salt breeder reactor: the search for a stable fuel at high temperature, the demand for fluid fuels and the ability to provide an infinite supply of electrical energy at low cost.