The need for greater amounts of electricity is driven by economic development and population growth across the world, and in large part this demand has been met by increasing use of fossil fuels, particularly natural gas. But the energies of the nucleus of an atom are roughly a million times greater than the chemical energies released from combustion, and this motivates us to strongly consider greater use of nuclear energy.

Today’s reactors use pressurized water to cool uranium fuel and to carry thermal energy to a steam turbine, generating electricity. This pressurized water reactor technology was originally developed to power nuclear submarines. The temperature at which energy is generated inside a reactor is important, because it determines how efficiently we can convert that energy into useful work, most often electricity. The higher the temperature at which we generate thermal energy, the more we can convert to electricity and the less we have to reject as waste heat. So achieving higher and higher operating temperatures has always been a goal of nuclear operations.

fundamental-reactor

To understand some of our options, let us simplify a nuclear power system to its essential components. There is the reactor itself, where thermal energy is generated from over a million trillion fission reactions each second. This thermal energy is carried away by a coolant fluid to a heat exchanger, where that thermal energy is transferred to another fluid that will pass through the power conversion system. The power conversion system has a turbine which turns a shaft connected to a generator, making electricity. About 2/3rds of the thermal energy is rejected as waste heat to the environment. In a PWR, the primary coolant is pressurized water and the working fluid is water raised into steam, with a steam turbine as the power conversion system. But there are other possible ways to build a reactor if different coolants are considered.

Nuclear reactors have generally considered four different kinds of coolants, and we group them in a two by two matrix here, with operating pressure across the top axis and operating temperature along the side axis. Ideally, we would like a coolant that lets us reach the highest temperatures at the lowest pressures, and this can be achieved by considering the use of liquid fluoride salts as a coolant. Pressurized water, by contrast, must operate at very high pressures to achieve modest temperatures, which limits the efficiency at which it can be used to produce electricity.

But there is another attribute that should be considered very early in reactor design, and that is the volumetric heat capacity of the coolant, in other words, how much thermal energy a unit volume of the coolant can hold. Volumetric heat capacity is the product of the specific heat of the material and its density. In this category we see that water is an exemplary choice, but that a fluoride salt composed of lithium and beryllium fluorides can do even better. Volumetric heat capacity is very important because it is the basic yardstick that sizes the reactor vessel, the piping, and the primary heat exchanger. The higher the volumetric heat capacity, the more compact the reactor can be. Gas-cooled reactors are particularly disadvantaged in terms of the volumetric heat capacity of their coolant because of its very low density.

Fluoride salts, as a class of materials, embody many advantages. They are the most chemically stable of all materials, which gives them a tremendous liquid temperature range of roughly a thousand degrees Celsius. This is far in excess of the few hundred degrees Celsius of liquid range we can achieve with water under great pressure. Their ionic bonding structure also makes them entirely impervious to radiation damage from neutrons or gamma rays. This is again contrasted with water which is continuously broken apart by radiation into hydrogen and oxygen and must be recombined.

In our goal to reduce construction costs, it is highly desirable if we are able to get materials in the reactor to perform multiple functions. The pressurized-water reactor is a good example of this, in that it uses water to both cool the fuel rods and to slow down, or moderate, the high-energy neutrons from fission.

Liquid-fluoride salts can also be more than just a coolant. The mixture of lithium fluoride and beryllium fluoride, sometimes called “flibe”, can be used as a solvent to carry nuclear fuels like uranium, plutonium, and thorium. This is a picture of liquid flibe salt with uranium tetrafluoride dissolved in it. When fluoride salts carry salts of nuclear fuels in them, they can fulfill the goal of having a material that serves multiple purposes in the reactor, and this has the potential to simplify the reactor and reduce costs.

In a liquid-fluoride reactor, fuel salt containing fissile material passes through piping into the reactor vessel, where graphite slows down (moderates) neutrons, increasing the probability that they will cause fission reactions in fissile material. Fission reactions deposit thermal energy in the fuel salt that increases its temperature. As the heated fuel salt passes out of the reactor vessel, fission reactions are no longer possible because the fuel salt has been separated from the graphite. The fuel salt heats a coolant salt in an external heat exchanger and returns to the reactor vessel. The coolant salt passes out of the reactor containment region and heats the gaseous working fluid of a gas-turbine power conversion system, analogous to the gas turbines used in today’s jet engines. The hot, high-pressure gas expands in a turbine, generating shaft work that turns a generator and produces electricity while also turning a compressor. The turbine gas exhaust, now at low pressure, is cooled either by external air or water. This cooling process can serve as the thermal input for a seawater desalination process if the reactor is located near salt water. The cold, low pressure gas is then compressed in the compressor using shaft work from the turbine and is ready to be heated again to generate work and power.

A liquid fuel enables a remarkable passive safety feature to be implemented that can solve perhaps the most vexing problem in reactor safety. For many years, we have been concerned about how to cool the solid fuel of a reactor in the event of an accident in order to prevent a meltdown. Various approaches have been proposed, including some newer and more innovative ideas. But the use of liquid fluoride fuel enables a simple and remarkable solution, based on the melting temperature of the fuel, which is about 400 degrees Celsius. The reactor is fitted with a drain line that is kept plugged by a frozen slug of salt. This plug kept frozen by an active flow of coolant over the outside of the drain line. In the event of a complete loss of power, the salt plug melts, and the fuel salt in the reactor drains through the line into a dedicated tank called a drain tank. This tank is configured to maximize the passive rejection of decay heat to the environment. This enables the reactor to dispense with a multitude of emergency core cooling systems that are required in solid-fueled reactors, particularly those that operate at high pressure. Because the frozen salt plug requires active cooling, in the event of complete power loss, that cooling will be interrupted and the plug will melt, causing the fuel to drain and the reactor to completely shut down. This remarkable safety feature is a compelling argument for consideration of the liquid fuel approach.

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