Thursday, January 19, 2012

Nuclear Technology Basics: Part 9 Molten-Salt Reactors

Introduction

Part 1

Part 2

Part 3

Part 4


Part 5

Part 6

Part 7

Part 8

Molten-salt reactors represent the most versatile and efficient types of reactors ever designed in the history of nuclear power engineering. Molten-salt reactors do not store their fuel in solid fuel rods. Instead, the nuclear fuel is chemically bonded with a halogen such as chlorine or fluorine to form a mineral salt. This mineral salt is liquid at the operating temperature of the reactor, which is pumped through the reactor systems. The molten salt mixture has a much higher operating temperature, allowing molten-salt reactors to take advantage of the Brayton cycle, and it is also unpressurized which removes the need for high pressure ductwork and allowing for a simpler reactor design. Conversely, it is impossible for these reactors to experience a meltdown since the fuel mixture is already molten and the density of the molten core decreases with increasing heat, and it pushes the amount of material needed to sustain critical mass out of the reactor core, slowing down the reaction.

There have been molten-salt reactors that have been built in the past as working experimental reactors, but there has yet to be an existing commercial example of an MSR-type reactor. The FUJI reactor project in Japan is a planned prototype that has been based on MSR technology, but funding for the project has been stalled at the moment. Of all of the Generation IV designs, the MSR reactors show the most promise.

1. Liquid Salt Very High Temperature Reactor (LS-VHTR)

This is a molten salt variant of the VHTR (Very High Temperature Reactor) design.

Liquid-Salt Very High Temperature Reactors are similar to the VHTRs discussed previously in part 7 of this series of posts, except that they have a liquid core instead of a solid core. The liquid core allows for a greater degree of control over the fission reaction as well as enabling more effective removal processes for the reaction poisons in the fission by-products. In addition, the liquid core achieves higher ratios of thermal efficiency because of its greater conductivity of heat compared to solid-cored reactors. Finally, the physical properties of the liquid core make a runaway fission event impossible because the liquid core expands when heated. If the core becomes overheated, it will expand to the point to where the core's density will be too low in order for fission to continue.

The core is contained within a housing where fission takes place and the entire core is submerged in a pool of molten salt to regulate the temperature of the reactor core. A heat exchanger removes excess heat from the pool of molten salt to maintain a stable equilibrium within the coolant pool.

As fission takes place in the reactor, heat from fission heats up the material in the core where the hot liquid is pumped out of the core chamber into a duct near the top of the core in the primary coolant loop. It is carried to a heat-exchanger where a secondary loop of molten-salt coolant is heated up and is then sent to the turbine and generator. The cooled down salt in the primary coolant loop returns to an intake at the bottom of the reactor core where it is then heated up again to complete the circuit.

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2. Liquid Fluoride Thorium Reactor (LFTR)

This is an even more revolutionary reactor design than the LS-VHTR. The reactor has a core of molten salt rather than a solid core, except it uses thorium as its nuclear fuel rather than uranium. It is much more efficient in terms of fuel utilization than a traditional thorium-based reactor and it can also be used as a breeder to create more fuel. This reactor design is very flexible in terms of what types of radioisotopes it can use to carry out fission and could even use the spent material left over from LWRs as fuel. Finally, its high operating temperature could be used as a heat source for many industrial processes rather than having to rely on natural gas and could hypothetically be an economically viable source of hydrogen production.

The reactor design is similar to that of the LS-VHTR, except that a circuit at the bottom of the reactor core allows part of the molten salt to be diverted to a chemical processing plant for fuel breeding purposes and to remove reactor poisons. The reactor uses a gas turbine to take advantage of the Brayton cycle for higher thermal efficiencies instead of a steam turbine. Instead of being submerged in coolant, the core of the reactor has valve that is sealed by a plug of frozen material that would defrost in the event of the reactor overheating which would allow the molten salt to be dumped into an emergency holding tank for the molten salt to cool down and solidify in.

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3. Liquid Chloride Thorium Reactor

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4. Liquid Fluoride Uranium Reactor

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5. Liquid Chloride Thorium Reactor

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Technology: Generation IV

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