The power conversion system is an essential element in any nuclear reactor design, converting the high-temperature thermal energy generated by the reactor into usable shaft work and ultimately into electrical energy for further distribution. The efficiency of the power conversion system is broadly limited by thermodynamic considerations, such as the average temperature at which thermal energy is added to and rejected from the conversion system and efficiencies in the components themselves.

For many reasons, a closed-cycle gas-turbine has been chosen as the power conversion system for the liquid-fluoride reactor. Conventional gas-turbines, familiar to many as jet engines, use chemical combustion as the heating source and use air as their working fluid, exhausting it in a “open-cycle.” Closed-cycle gas turbines are heated and cooled externally and can use fluids other than air. The cycle we have chosen is heated by hot fluoride salts and uses supercritical carbon dioxide as the working fluid. It is very compact and highly efficient.

Closed-cycle gas turbines of this configuration are not currently in use in industry today, primarily due to lack of a suitable non-combustion heat source. Nevertheless, the thermodynamic and practical advantages of the closed-cycle gas turbine are so great that they are being considered for a variety of advanced reactor concepts, such as sodium-cooled fast reactors and very-high temperature gas-cooled reactors. There are subtle differences between how a closed-Brayton cycle can be implemented to maximum advantage in each of these reactor types, but there are overall similarities that can be observed.

In each case, the working fluid gas is compressed at relatively low temperatures and at the minimum pressure of the cycle. Compression is executed at low temperature to minimize the work input to the gas during compression. After compression the gas flows to a recuperator, which is essentially a counter-current heat exchanger. In one channel of the recuperator the high-pressure gas from the compressor exhaust is heated to much higher temperatures by the flowing, low-pressure exhaust of the turbine. Recuperation is a feature commonly found in high-performance closed-cycle gas turbines and improves the thermodynamic efficiency of the system markedly.

After exiting the recuperators at much higher temperatures than it enters, the high-pressure gas is then heated to its maximum temperature. In this reactor design, this heating is achieved by passage through a counter flow heat exchanger where hot fluoride coolant salt, used to cool the reactor, is used to heat the working-fluid gas to its maximum cycle temperature. This is sometimes referred to as an “indirect cycle” gas turbine. In other nuclear gas-turbine concepts the gas might be directly heated in the reactor vessel itself, which is sometimes called a “direct-cycle” gas turbine. By using the indirect approach the high-pressure operation of the gas turbine can be isolated from the reactor. After being heated, the gas is then expanded through a turbine, generating shaft work. A substantial portion of this shaft work is used to drive the compressors but the remainder can be used to drive an electrical generator, where shaft work is converted into electrical energy at high efficiency (>95%).

After passage through the turbine, the gas is at a lower pressure and overall energy. It then passes directly into the high-temperature recuperator and heats the high-pressure gas on the other side, thus “pre-heating” the incoming high-pressure stream to improve thermodynamic efficiency.

Upon exiting the recuperators, the gas must be further cooled by contact with an external cooling fluid before reaching the lowest temperature of the cycle and entering the compressor. This cooling fluid might be ambient air or water. Air has the profound advantage of being available as a heat sink no matter where the reactor might be operated, but is disadvantaged in the large volumetric flow rates and large heat exchange surface required for cooling. A dense fluid like water is a much better cooling medium, but depending on the application, water might be in short supply. Another consideration might involve using cooling water for desalination purposes, if the reactor is located near to a body of salty water.

The design of the thermodynamic cycle has profound ramifications for the overall reactor design, and the reactor design sets constraints on the thermodynamic cycle design. For a reactor employing a liquid fluoride salt as a cooling medium, the high temperature capability of the salt allows very impressive thermodynamic conversion efficiencies to be readily achieved, on the order of 45%, depending on specifics of cycle design.

The decision to use a liquid fluoride salt also levies constraints on the cycle design as well, the most pronounced of which is the need to keep the minimum temperature of heat addition in the cycle above the freezing point (liquidus) of the salt. Typical liquidus temperatures in the fluoride salt are such that a regenerated cycle is practically a requirement for any practical machine.

We are developing cycle design tools that allow such system trades to be evaluated quickly and accurately. The final cycle design that will be chosen for the power conversion system is broadly bounded by the need to add heat to the gas at temperatures above the freezing point of the coolant salt and the need to reject heat to the environment at as low a temperature as possible.

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