Molten salt reactors for the production of electrical power were studied at Oak Ridge
National Laboratory from 1957 to 1960. The Molten Salt Reactor Experiment. MSRE. Preliminary design
of the reactor began in May 1960. The molten salt fuel for this reactor is radically different.
Salt flows from a pipe which is heated to keep the salt molten at 1200 degrees Fahrenheit.
The salt does not react with air. It flows like water, but does not boil. MSRE is a
circulating fuel graphite moderated single region reactor during Operation Fuel which is
nearly as fluid as water. It enters the flow distributor near the top of the vessel and
spirals downward in turbulent flow through the space between the vessel and core can.
The fuel is then directed upward and laminar flow through channels, machined and graphite
bars of the core matrix, where heat is generated by the fissioning of uranium-235.
A simple control system of three flexible control rods helps to regulate the power
and temperature. The salt, which is now 1210 degrees Fahrenheit, is discharged by the fuel
pump and flows through the primary heat exchanger and then back to the reactor.
Gaseous fission products are removed from the circulating salt stream in the pump
tank. Coolant salt is circulated through the heat exchanger and leaves at 1070 degrees Fahrenheit.
Heat from the cooling salt transferred to air in the radiator. Cooling air is forced across
the radiator and upper stack. The molten salt is held in the whole system by special freeze valves.
The fuel is composed of lithium, beryllium, zirconium and uranium chloride salts. The
coolant is a chloride salt mixture like the fuel, but it does not contain uranium or zirconium.
Piping and components are fabricated from Hastelloy-N, a special nickel molybdenum alloy
developed at Oak Ridge, which has good resistance to corrosion attack by fluoride salts, heads for
the reactor vessel or a cold pressed from one end 18 inch thick has to lie in alloy plates.
The reactor vessel, its fuel inlet, was also cold formed as were most
of the Hastelloy-N alloy parts. Assembly of the reactor vessel began in mid 1962.
The reactor core of nearly 600 vertical graphite bars was carefully assembled on a graphite lattice
within a clean room. The five foot diameter reactor with core and control rod assembly weighs
about nine tons Tanks provide safe storage of salt mixtures with bayonet tubes and steam dome,
in which fission product decay heat is removed by flashing water to steam.
The thermal shield as a double walled stainless steel vessel which surrounds the reactor.
The annulus is filled with water and carbon steel balls for a primary shielding
special freeze flanges in the fuel salts circulating piping
permit the removal and replacement of major components such as the heat exchanger.
All components and connecting piping and the salt circulating systems
are heated electrically to maintain the system above the salt freezing point of 840 degrees.
It is planned to remove impurities and recover uranium from the fuel salt in this onsite
processing facility. The Sampler And Enricher is a unique device for sampling and adding fuel
during full power operation of the reactor. A power driven cable
remotely lowers a small bucket into the liquid salt of the pump boil
and then raises the bucket into a manipulator area for transfer to a shielded container.
More than 300 samples have been obtained routinely by this method. Fuel is enriched by lowering a
bucket of enriched U-235 and dissolving it into the circulating fuel of stream. This
eliminates the need for excess fuel reactivity Another feature of the MSRE is its high speed
digital computer. With the final edition of 680 grams of enriched uranium into the fuel stream
by the Sampler and Enricher, the Molten-Salt Reactor Experiment when critical on June 1st 1965.
After final preparations, power operations began early in 1960 61 of the highest temperature
reactors operating today. It has been demonstrated that MSRE can be operated safely and reliably.
Maintenance of radioactive equipment accomplished with minimum difficulty. Nuclear characteristics
were extremely close to predicted values and the system was dynamically stable at all power levels.
Graphite and Hastelloy-N specimens similar to these have been removed from the reactor core
examination indicated no deleterious corrosion or structural change after
salt had been circulated in the fuel system for more than 6000 hours.
The fuel proved to be stable to radiation and heat. The laboratory's important goals
of nuclear reactor development converge the search for a stable fuel at high temperature
and the capability of providing an infinite supply of electrical power at low costs.
The laboratory initiated a program to develop component maintenance and replacement through
remotely controlled operation mockup of a small reactor system to use thermal megawatts. Inside
the reactor is connected to the primary piping circuit by two 6 inch mechanical joints. The
freeze flange type specially designed adjustable springs supports permit all piping to expand or
contract freely without overstretching the pipe to remove and replace the component.
It must be possible to break and remake pipe connections
to extremely leak type specification. Freeze flange is used at such a joint
takes advantage of fuzed salt properties to form a frozen seal between flange faces.
This seal is backed up by a conventional metal O-ring which seals in the fission gases
and serves as a leak monitor. Flanges are held together by two suction wedge clamps.
A 16 inch gap exists between the faces. It is in this volume that the salt seal is formed. PIN
and funnel guides align the flanges during remote assembly and removing the motor pump and reactor.
The motor is first disconnected electrically by the remote manipulator, three bolts holding
the motor and pumped four units together, loosened with a pneumatically operated impact.
The motor is lifted from the pump, removed from the cell. Four jack bolts are actuated to break
loose the seals between the pump rotary and stationary elements. An eyebolt is inserted
in the pump shaft to facilitate its removal by the crank. The pump flange O-ring gasket
is the final piece of equipment removed in this setup. Total time for removal
of the motor and pump 6 hours and removing the reactor core vessel.
23 thermocouple and heater leads are first disconnected. Jack control handles the top
heat exchanger dollies are turned to separate the flange races and permit lifting of the
reactor vessel following attachment of a lifting jig. The reactor vessel is carefully raised from
its support. Stand and remove another component designed for remote installation as the electric
preheated unit heat loop piping to 1200 degrees Fahrenheit prior to filling the loop with fuel.
As the pre heat or lifting handle is pulled, the hinged sections spread out
and the unit is withdrawn. The replacement heater is then lowered and locked into position
by pushing down on the hand. Operations indicate it is feasible to remotely
remove and replace components of a molten salt reactor without direct or semi direct
manual assistance. The life expectancy of reactor components is estimated to be years.
The 36 hours required in replacing the most difficult component in the mockup therefore
constitutes a negligible cost item. If such efficiency can be achieved
on a real molten salt reactor.