Energy From Thorium Discussion Forum

This is an extract from ORNL-TM-0728: Molten-Salt Reactor Experiment Design and Operations Reports Part I: Description of Reactor Design (PDF, 40 MB). I'm posting it here because it contains very interesting and valuable design information on the MSRE, and by extension, other fluoride reactors. The document was published in January 1965, about six months before the MSRE achieved criticality (June 1965).


1. INTRODUCTION

The Molten-Salt Reactor Experiment (MSRE) was undertaken by the Oak Ridge National Laboratory to demonstrate that the desirable features of the molten-salt concept could be embodied in a practical reactor that could be constructed and maintained without undue difficulty and one that could be operated safely and reliably. Additional important. objectives were to provide the first large-scale, long-term, high-temperature tests in a reactor environment of the fuel salt, graphite, moderator, and high-nickel-base alloy (INOR-8 [Hastelloy-N]). Operating data from the MSRE should provide important information regarding the feasibility of large-scale molten-salt reactors.

Molten-salt reactors were first investigated as a means of providing a compact high-temperature power plant for nuclear-powered aircraft. In 1954 an Aircraft Reactor Experiment (ARE) was constructed at ORNL which demonstrated the nuclear feasibility of operating a molten-salt-fueled reactor at high temperature. Fuel entered the ARE core at 1200°F (920 K) and left at 1500°F (1100 K) when the reactor power level was 2.5 MW.

Immediately after the successful operation of the ARE, the Aircraft Reactor Test (ART) was started at ORNL as part of the Aircraft Nuclear Propulsion Program (ANP). This test was discontinued in 1957 when the ANP Program was revised, but the high promise of the molten-salt reactor type for achieving low electric power generating costs in central power stations led ORNL to continue parts of the basic study programs. Features of the molten-salt concept which deserve special mention with regard to its future propects are:

1. The fuel is fluid at reactor temperatures, thus eliminating the extra costs associated with the fabrication, handling, and reprocessing of solid fuel elements. Burnup in the fuel is not limited by radiation damage or reactivity loss. The fuel can be reprocessed continuously in a side stream for removal of fission products, and new fissionable material can be added while the reactor is in operation.
   
2. Molten-salt reactors can operate at high temperatures and produce high-pressure superheated steam to achieve thermal efficiencies in the heat-power cycle equal to the best fossil-fuel-fired plants. The relatively low vapor pressure of the salt permits use of low pressure containers and piping.
   
3. The negative temperature coefficient of the reactor and the low excess reactivity are such that the nuclear safety is not primarily dependent upon fast-acting control rods.
   
4. The fuel salt has a low cross section for the parasitic absorption of neutrons, and when used with bare graphite as the moderator, very good neutron economies can be achieved. Molten-salt reactors are thus attractive as highly efficient converters and breeders on the Th-U cycle.
   
5. The fluoride salts used as the fluid fuel mixture have good thermal and radiation stability and do not undergo violent chemical reactions with water or air. They are compatible with the graphite moderator and can be contained satisfactorily in a specially developed high-nickel alloy, INOR-8. The volumetric heat capacity, viscosity, thermal conductivity, and other physical properties are also within desirable ranges.
   
6. Use of relatively high circulation rates and temperature differences results in high mean power density, high specific power, and low fuel inventory.

These attractive features of the molten-salt reactor concept are partially offset by the disadvantages that:

1. The fuel salt mixture melts at about 840°F (720 K), so means must be provided for maintaining all salt-containing portions of the system above this temperature.
   
2. The fluoride salts react with oxygen to precipitate fuel constituents as oxides. Although zirconium tetrafluoride is included in the salt mixture so that ZrO2 will precipitate in preference to UO2, care must be taken to prevent the fuel from being contaminated with air, water, or other oxygen-containing materials.
   
3. The radioactivity in any fluid-fuel system is in a mobile form, and special provisions must be taken for containment and maintenance.

During the period 1957-60, investigations were carried out at ORNL on the fuel salt chemistry, metallurgy of containment materials, the design of salt-circulating pumps, and on remote maintenance techniques. The results of this work lent additional encouragement, and in 1959 studies were made by H. G. MacPherson and L. G. Alexander et al. pertaining to the applicability of the molten-salt concept to central power station reactors. The studies resulted in a proposal to the AEC for construction of a molten-salt experiment to investigate remaining areas of uncertainty that could be resolved only by actually building and operating a molten-salt reactor. In April, 1961, ORNL received a directive from the AEC to design, construct, and operate the Molten-Salt Reactor Experiment (MSRE), the subject of this report.

Early in the design phases it was decided that the MSRE was to have as its primary purpose the investigation of the practicality of the molten-salt concept for central power station applications. As such, the MSRE was envisioned as a straightforward-type of installation, uncomplicated by the inclusion of experimental apparatus which might jeopardize reliable, long-term operation. It was also necessary that the MSRE be of a large-enough capacity for the experimental findings to be meaningfully extrapolated to the full-scale plants. It was decided that a reactor of 10 MW thermal output would satisfy the criterion.

Conversion of the 10 MW of heat to useful electricity was not considered to be necessary to demonstrate the concept, so existing blowers and stack were used to dissipate the heat to the atmosphere. Containment requirements dictated a double barrier between the highly radioactive fuel salt and the environment, and a salt very similar to the fuel salt in composition and physical properties was chosen to transport the heat from the fuel salt to air-cooled surfaces.

An expanded plant layout was adopted in order to provide access to equipment and to facilitate maintenance operations. The MSRE was installed in an existing building in the 7503 Area at ORNL that was constructed specifically for the ARE and ART. This arrangement provided some savings and expedited construction in that the building included a containment vessel which, with modification, was suitable for the MSRE. A significant amount of usable auxiliary equipment was also on hand, including air blowers and a stack for dissipation of heat to the atmosphere, emergency diesel-electric power supply, heavy-duty cranes, etc. Shop, office, washroom, and control room spaces were also available, and some of the heavy concrete shielding was adaptable to the MSRE. Fitting the MSRE design to the existing facilities required numerous design compromises, but no extreme difficulties were encountered.

Construction of the MSRE officially started in July, 1961, although much of the advance thinking and preliminary design work were well under way by that time. Major building modifications were started in 1961 and were completed by the end of 1962. Lack of funds and late delivery of the graphite moderator delayed installation of major equipment until early 1964. The installation was scheduled for completion in the early summer of 1964, and the target date to achieve criticality was set for the end of that year.

2. GENERAL DESCRIPTION

2.1 Type

The Molten-Salt Reactor Experiment (MSRE) is a single-region, unclad, graphite-moderated, fluid-fuel type of reactor with a design heat generation rate of 10 MW. The circulating fuel is a mixture of lithium, beryllium, and zirconium fluoride salts that contains uranium or thorium and uranium fluorides. Reactor heat is transferred from the fuel salt to a similar coolant salt and is then dissipated to the atmosphere.

2.2. Location

The Experiment is located in the 7503 Area of the Oak Ridge National Laboratory, Oak Ridge, Tennessee. The site is about one-half mile south of the main laboratory buildings, in a wooded, secluded bend of the Clinch River that is reserved for special reactor installations.

2.3 Fuel and Coolant Salts

The composition and physical properties of various fuel and coolant salts are given in Table 2.1. Favorable neutron absorption and chemical and physical properties were important requirements for the compositions selected. Beryllium fluoride is used to obtain a low melting point. Lithium fluoride (99.99% lithium-7 in both fuel and coolant salts) imparts good fluid flow properties to the mixture. Zirconium fluoride protects the fuel salt against precipitation of UO2 from contamination by air and moisture. Fuel salts containing throium are of interest for future large-scale thorium breeder reactors. The first experiments in the MSRE will be run with partially enriched uranium because there are fewer uncertainties concerning the chemical behavior of that fuel. Later the reactor will be operated with the highly enriched uranium fuel and then with the thorium-uranium fuel.

2.4 Equipment and Process

The major items of equipment in the MSRE are shown on a simplified flowsheet in Fig. 2.1. The fuel-salt circulating system is the reactor primary system. It consists of the reactor vessel where the nuclear heat is generated, the fuel heat exchanger in which heat is transferred from fuel to coolant, the fuel circulating pump, and the interconnecting piping. The coolant system is the reactor secondary system. It consists of the coolant pump, a radiator in which heat is transferred from coolant salt to air, and the piping between the pump, the radiator, and the fuel heat exchanger. There are also drain-tank systems for containing the fuel and coolant salts when the circulating systems are not in operation.

2.4.1 Reactor

The reactor vessel is a 5-ft-diam by 8-ft-high tank that contains a 55-in-diam by 64-in-high graphite core structure. A cutaway drawing of the reactor is shown in Fig. 2.2. Under design conditions of 10 MW of reactor heat, the fuel salt enters the flow distributor at the top of the vessel at 1175°F and 20 psig. The fuel is distributed evenly around the circumference of the vessel and then flows turbulently downward in a spiral path through a 1-in. annulus between the vessel wall and the core can. The wall of the vessel is thus cooled to within about 50°F of the bulk temperature of the entering salt. The salt loses its rotational motion in the straightening vanes in the lower plenum and turns and flows upward through the graphite matrix in the core can.

The graphite matrix is an assembly of vertical bars, 2 in. by 2 in. by about 67 in. long. Fissioning of 235U in the fuel occurs as it flows in 0.4-in. by 1.2-in. channels that are formed by grooves in the sides of the bars. There are about 1140 of these passages.

The nominal core volume within the 55-in.-diam by 64-in-high core structure is 90 ft3, of which 20 ft3 is fuel and 70 ft3 is graphite. At 10 MW, and with no fuel absorbed by the graphite, 1.4 MW of heat is generated in the fuel outside the nominal core, 0.6 MW is generated in the graphite, and 8.0 MW is generated in the fuel within the core. This corresponds to an average fuel power density of 14 kW/liter in the nominal core. The maximum fuel power density is 31 kW/liter.

Flow in the coolant channels is laminar, but both the graphite and the fuel have good thermal conductivities, so the maximum temperature of the graphite is only about 60°F above the mixed mean temperature of the adjacent fuel. The nuclear average and the maximum temperatures, respectively, of the graphite are estimated to be about 1255°F and 1300°F. The temperature of the fuel leaving the hottest channel in the core is about 1260°F.

Fuel leaves the top of the reactor at 1225°F and 7 psig through the side outlet of a special fitting designed as an access port for insertion of graphite and metal samples and for three 2-in-diam control rod thimbles. The poison elements in the control rods are short hollow cylinders of gadolinium oxide 1 in. in diameter, clad with Inconel and arranged on a flexible Inconel hose to permit passage through two bends that form an offset in each thimble. The control rods and drives are cooled by circulation of cell atmosphere through the flexible hoses and thimbles.

A 1-1/2-in.-diam outlet line is provided at the bottom of the reactor vessel for discharging salt to the drain tanks.

2.4.2 Fuel Pump

The fuel salt from the reactor flows directly to the centrifugal sump-type pump shown in Fig. 2.3. The pump has a vertical shaft and overhung impeller and operates at a speed of 1160 rpm to deliver 1200 gpm at a discharge head of 49 ft. The pump bowl is about 36 in. in diamter, and the pump and 75-hp motor assembly is about 8 ft high.

Devices are provided in the pump bowl to measure the liquid level as a means of determining the inventory of salt in the system. Small capsules can be lowered into the bowl to take a 10-g sample of salt for analysis or to add 120 g of fuel to the system. About 65 gpm of the pump output is circulated internally to the pump bowl for release of entrained or dissolved gases from the salt.

Helium flows through the gas space in the bowl at a rate of about 200 ft3/day (STP) to sweep the highly radioactive xenon and krypton to the off-gas disposal system. The helium also acts as a cover gas to exclude air and water vapor.

The pump is equipped with ball bearings that are lubricated and cooled with oil circulated by an external pumping system. The oil is confined to the bearing housing by mechanical shaft seals. A helium purge enters below the lower seal. A small part of this helium flows upward along the shaft and leaves just below the lower seal, carrying with it any oil vapors that leak through the seal. The remainder flows downward along the shaft to the pump bowl and subsequently to the off-gas system. This prevents radioactive gases from reaching the oil.

Cooling oil is also circulated through a metal block above the pump bowl which shields the lubricating oil and the pump motor.

The motor and the bearing shaft and impeller assembly are removable separately to facilitate maintenance.

An overflow tank of 5.5-ft3 volume is installed below the pump to provide sufficient volume for free expansion of salt under all foreseen conditions.

2.4.3 Heat Exchanger

Salt discharged by the fuel pump flows through the shell side of the horizontal shell-and-tube heat exchanger shown in Fig. 2.4, where it is cooled from 1225°F to 1175°F. The exchanger is about 16 in. in diameter and 8 ft long and contains one hundred sixty-three 1/2-in-OD U-tubes with an effective surface of 259 ft2. The coolant salt circulates through the tubes at a rate of 850 gpm, entering at 1025°F and leaving at 1100°F.

2.4.4 Coolant Pump

The coolant salt is circulated by a centrifugal pump identical in most respects to the fuel pump. The pump has a 75-hp, 1750-rpm motor and delivers 850 gpm against a head of 78 ft.

2.4.5 Radiator

The radiator is shown in Fig. 2.5. Seven hundred square feet of cooling surface is provided by 120 tubes 0.75 in. in diameter by 30 ft long. Cooling air is supplied to the radiator by two 250-hp axial blowers with a combined capacity of 200,000 cfm. Salt enters the radiator at 1100°F and leaves at 1025°F. The temperature rise of the air is 200°F at design power. To guard against freezing the salt in the radiator tubes on sudden reduction of reactor power, quick-closing doors are provided to shut off the air flow, and the radiator is heated by electrical heaters inside the enclosure. The opening of the doors can be adjusted, and some of the air can be bypassed around the radiator to regulate the heat removal rate.

2.4.6 Drain Tank Systems

Four tanks are provided for safe storage of the salt mixtures when they are not in use in the fuel- and coolant-salt circulating systems. Two fuel-salt drain tanks and a flush-salt tank are connected to the reactor by means of the fill and drain line. One drain tank is provided for the coolant salt.

A fuel drain tank is shown in Fig. 2.6. The tank is 50 in. in diameter by 86 in. high and has a volume of about 80 ft3, sufficient to hold in a non-critical geometry all the salt that can be contained in the fuel circulating system. The tank is provided with a cooling system capable of removing 100 kW of fission-product decay heat, the cooling being accomplished by boiling water in 32 bayonet tubes that are inserted in thimbles in the tank.

The flush-salt tank is similar to the fuel-salt tank except that it has no thimbles or cooling system. New flush salt is like fuel salt but without fissile or fertile material. It is used to wash the fuel circulating system before fuel is added and after fuel is drained, and the only decay heating is by the small quantity of fission products that it removes from the equipment.

The coolant-salt tank resembles the flush-salt tank, but it is 40 in. in diameter by 78 in. high and the volume is 50 ft3.

The tanks are provided with devices to indicate high and low liquid levels and with weigh cells to indicate the weight of the tanks and their contents.

2.4.7 Piping and Flanges

The major components in the salt circulating systems are interconnected by 5-in. sched-40 piping. Flanged joints between units in the primary system facilitate removal and replacement of components by remotely operated tools. These flanges, called freeze flanges, utilize a frozen salt seal between the flange faces as well as a conventional O-ring-type joint to form a helium-buffered, leak-detected type of closure.

The fill and drain lines are 1-1/2-in. sched-40 piping and contain the only "valves" that come in contact with salt. The valves, called freeze valves, have no moving parts, unmodulated flow control being achieved by freezing or thawing salt in a short, partially flattened section of pipe that can be heated and cooled.

2.4.8 Heaters

All parts of the salt-containing systems are heated electrically to maintain the salts above the liquidus temperature of 840 to 850°F. The equipment is preheated before salt is added and the heaters are energized continuously during reactor operation to make sure that there is no uncontrolled freezing in any of the piping and that the salt can be drained when necessary. The total capacity of the heaters is about 1930 kW, but the actual power consumption is somewhat less than half of this. About 300 kW of heat can be provided by the diesel electric emergency power supply.

2.4.9 Materials

The salt-containing piping and equipment are made of INOR--a special high-nickel and molybdenum alloy having a good resistance to attack by fuel and coolant salts at temperatures at least as high as 1500°F. The mechanical properties are superior to those of many austenitic stainless steels, and the alloy is weldable by established procedures. The chemical composition and, some of the physical properties are given in Table 2.2. Most of the INOR equipment was designed for 1300°F and 50 psig, with an allowable stress of 2750 psi.

Stainless steel piping and valves were used in the helium supply and in the off-gas systems.

2.4.10 Cover- and Off-Gas Systems

A helium cover-gas system protects the oxygen-sensitive fuel from contact with air or moisture. Commerical helium is suppled in a tank truck and is passed through a purification system to reduce the oxygen and water content below 1 ppm before it is admitted to the reactor systems. A flow of 200 ft3/day (STP) is passed continuously through the fuel pump bowl to transport the fission product gases to activated charcoal adsorber beds. The radioactive xenon is retained on the charcoal for a minimum of 90 days, and the krypton for 7-1/2 days, which is sufficient for all but the 85Kr to decay to insignificant levels. The 85Kr is maintained well within tolerance, the effluent gas being diluted with 21,000 cfm of air, filtered, monitored, and dispersed from a 3-ft-diam by 100-ft steel-containment ventilation stack.

The cover-gas system is also used to pressurize the drain tanks to move molten salts into the fuel and coolant circulating systems. Gas from these operations is passed through charcoal beds and filters before it is discharged through the off-gas stack.

2.4.11 Instrumentation and Control Systems

Nuclear and process control are both important to the operation of the MSRE. The reactor has a negative temperature coefficient of 6.4 to 9.9 x 10^-5 (delta-k/k)/°F, depending on the type of fuel that is being used. The excess reactivity requirements are listed in Table 2.3, and they are not expected to exceed 4 x 10^-2 delta-k/k at the normal operating temperature. The three control rods have a combined worth of 5.6 to 7.6% delta-k/k, depending upon the fuel composition. Their major functions are to eliminate the wide temperature variations that would otherwise accompany changes in power and xenon poison level and to make it possible to hold the reactor subcritical to a temperature 200 to 300°F below the normal operating temperature. They have some safety functions, most of which are concerned with the startup of the reactor. Rapid action is not required of the control rods; however, a magnetic clutch is provided in the drive train to permit the rods to drop into the thimbles with an acceleration of 0.5 g as a convenient way of providing insertion rates that are more rapid than the removal rates. Burnup and growth of long-lived fission product poisons is compensated by adding fuel through the sampler enricher. Complete shutdown of the reactor is accomplished by draining the fuel.



When the reactor is operated at power levels above a few hundred kilowatts, the power is controlled by regulating the air flow, and thereby the rate of heat removal, at the radiator. The power level is determined by measuring the flow rate and temperature difference in the coolant salt system. The control rods operate to hold the fuel outlet temperature from the reactor constant, and the inlet temperature is permitted to vary with power level. At low power the control rods operate to hold the neutron flux constant, and the heat withdrawal at the radiator or the input to the heaters on piping and equipment is adjusted to keep the temperature within a specified range.

Preventing the salts from freezing, except at freeze flanges and valves, and protecting the equipment from overheating, are among the most important control functions. Over one thousand thermocouples are installed throughout the fuel and coolant salt systems, and about three-fourths of these serve indication, alarm, or control functions. The heating and cooling equipment is controlled to maintain temperatures (throughout the systems) within specified ranges.

Digital computer and data handling equipment are included in the instrumentation to provide rapid compilation and analysis of the process data. This equipment has no control function but gives current information about all important variables and warns of abnormal conditions.

2.5 Fuel Processing

Batches of fuel or flush salt which have been removed from the reactor circulating system can be processed in separate equipment to permit their reuse or to recover the uranium.

Salts that have been contaminated with oxygen to the saturation point (about 80 ppm of O2), and thus tend to precipitate the fuel constituents as oxides, can be treated with a hydrogen-hydrogen fluoride gas mixture to remove the oxygen as water vapor. These salts can then be reused.

A salt batch unacceptably contaminated with fission products, or one in which it is desirable to drastically change the uranium content, can be treated with fluorine gas to separate the uranium from the carrier salt by volatilization of UF6. In some instances the carrier salt will be discarded; in others uranium of a different enrichment, thorium, or other constituents will be added to give the desired composition.

The processing system consists of a salt storage and processing tank, supply tanks for the H2, HF, and F2 treating gases, a high temperature (750°F) sodium fluoride adsorber for decontaminating the UF6, several low-temperature portable adsorbers for UF6, a caustic scrubber, and associated piping and instrumentation. All except the UF6 adsorbers are located in the fuel processing cell below the operating floor of Bldg. 7503, as shown in Fig. 4.4.

After the uranium has been transferred to the UF6 adsorbers, they are transported to the ORNL Volatility Pilot Plant at X-10, where the UF6 is transferred to product cylinders for return to the AEC production plants.

2.6 Plant Arrangement

The general arrangement of Building 7503 is shown in Fig. 4.3. The main entrance is at the north end. Reactor equipment and major auxiliary facilities occupy the west half of the building in the high-bay area. The east half of the building contains the control room, offices, change rooms, instrument and general maintenance shops, and storage areas. Additional offices are provided in a separate building to the east of the main building.

Equipment for ventilating the operating and experimental areas is located south of the main building. A small cooling tower and small buildings to house stores and the diesel-electric emergency power equipment are located west of the main building.

The reactor primary system and the drain tank system are installed in shielded, pressure-tight reactor and drain tank cells, which occupy most of the south half of the high-bay area. These cells are connected by an open 3-ft-diam duct and are thus both constructed to withstand the same design pressure of 40 psig, with a leakage rate of less than 1 vol % per day. A vapor-condensing system, buried in the ground south of the building, is provided to keep the pressure below 40 psig during the maximum credible accident by condensing the steam in vapors that are discharged from the reactor cell. When the reactor is operating, the reactor and drain tank cells are sealed, purged with nitrogen to obtain an atmosphere that is less than 5% oxygen, and maintained at about 2 psi below atmospheric pressure.

The reactor cell is a carbon steel containment vessel 24 ft in diameter and 33 ft in overall height. The top is flat and consists of two layers of removable concrete plugs and beams, for a total thickness of 7 ft. A thin stainless steel membrane is installed between the two layers of plugs and welded to the wall of the steel vessel to provide a tight seal during operation.

The reactor cell vessel is located within a 30-ft-diam steel tank. The annular space is filled with a magnetite sand and water mixture, and there is a minimum of 2 ft of concrete shielding around the outer tank. In addition to this shielding the reactor vessel is surrounded by a 14-in.-thick steel-and-water thermal shield.

The drain tank cell adjoins the reactor cell on the north. It is
a 17-1/2-ft by 21-1/2-ft by 29-ft-high rectangular tank made of reinforced concrete and lined with stainless steel. The roof structure, including the membrane, is similar to that of the reactor cell.

The coolant cell abuts the reactor cell on the south. It is a shielded area with controlled ventilation but is not sealed.

The blowers that supply cooling air to the radiator are installed in an existing blower house along the west wall of the coolant cell.

Rooms containing auxiliary and service equipment, instrument transmitters, and electrical equipment are located along the east wall of the reactor, drain tank and coolant cells. Ventilation of these rooms is controlled, and some are provided with shielding.

The north half of the building contains several small shielded cells in which the ventilation is controlled, but which are not gas-tight. These cells are used for storing and processing the fuel, handling and storing liquid wastes, and storing and decontaminating reactor equipment.

The high-bay area of the building over the cells mentioned above is lined with metal, has all but the smaller openings sealed, and is provided with air locks. Ventilation is controlled and the area is normally operated at slightly below atmospheric pressure. The effluent air from this area and from all other controlled-ventilation areas is filtered, and monitored before it is discharged to the atmosphere. The containment ventilation equipment consists of a filter pit, two fans, and a 100-ft-high fuel stack. They are located south of the main building and are connected to it by a ventilation duct to the bottom of the reactor cell and another along the east side of the high bay.

The vent house and charcoal beds for handling the gaseous fission products from the reactor systems are near the southwest corner of the main building. The carbon beds are installed in an existing pit that is filled with water and covered with concrete slabs. The vent house and pit are also controlled-ventilation areas. Gases from the carbon beds are discharged into the ventilation system upstream of the filter.

Maintenance of equipment in the fuel circulating and drain tank systems will be by removal of one or more of the concrete roof plugs and use of remote handling and viewing equipment. A heavily shielded maintenance control room with viewing windows is located above the operating floor for operation of the cranes and other remotely controlled equipment. This room will be used primarily when a large number of the roof plugs are removed and a piece of highly radioactive equipment is to be transferred to a storage cell.
Equipment in the coolant cell cannot be approached when the reactor is operating, but since the induced activity in the coolant salts is short lived, the coolant cell can be entered for direct maintenance shortly after reactor shutdown.

OK, that's all I plan to post, but you can read the rest in the original document...enjoy!

It's a very interesting document! The reactor, as probably any such big facility, has formidable complexity.

I was just skimming through various parts, and what caught my attention was the part about water vapor, and how they made sure the helium had very little of that.

What would water vapor do with the fuel salt?

There is a slow reaction between water and uranium tetrafluoride to uranium oxide and hydrogen fluoride. The uranium oxide is solid and will precipitate out of the salt, possibly accumulating somewhere and causing a hot spot. That's why they included zirconium in the MSRE salt--to preferentially form zirconium oxide instead of uranium oxide.

After a few years of operation, they realized there were better ways to manage chemistry control than including a neutron absorber like zirconium. I don't remember off the top of my head what the plan was, but the molten-salt chemist I talked to said a future fluoride reactor won't need zirconium for oxidation control.

One or two of the common Zr isotopes are in fact quite low neutron absorbers.
Might be worthwhile separating those out, and using them as the oxygen "getter."

.

Hi Kirk,
I am new to this forum and have been reading up a lot of posts. While looking for information on the net, I bumped into the following link. Sounds like an accident at the experimental plant after it was shut down. Can you comment on this?

http://members.aol.com/doewatch/msre.html

The fluorine releases that came from the MSRE came from the fact that the uranium was not removed from the salt after shutdown. I talked to some of the old-timers at ORNL and they said that they didn't do it (fluorinate the salt to remove uranium) right after shutdown because they entertained some hope of restarting the reactor on yet-to-be-acquired funds.

After a few years, the salt froze and then radiolytic decomposition of some of the fluorides occured. Nature formed its own little fluorinators, and they fluorinated UF4 to UF6, which is gaseous and some of which evolved out of the salt into the lines of the system.

They should have removed the uranium when they shut down--this would not happen in an operational fluoride reactor and at any rate took about a decade to happen in this reactor.

on another topic....

This Phelps guy is pretty odd. I saw the same thing and poked around his site. Seems that fluorine is the root of all evils to him. Hope he still brushes his teeth.

"Fluoridation, Mandrake...the most monstrously conceived and dangerous commie plot we have ever had to face..."