Technology

In order to maintain a critical mass that can produce heat from uranium, a moderator is required. The moderator can either be a liquid, solid or gas. The most common moderator in use today is water and the second most common is graphite. 

Water is simultaneously used to remove heat from the reactor and as a moderator to sustain a nuclear chain reaction. Through a series of pumps, valves, pipes and other materials or systems, the heat from the reactor is used to produce steam which is used to turn a turbine to create electricity. This creates a complex system and suggests that if the moderator function can be separated from the heat removal function then a great deal of complexity can be removed. A solid moderator combined with a separate means of heat extraction could meet the small and simple criteria inherent in the proposed design. 

Solids such as graphite have the characteristics that can enable a chain reaction and critical mass in order to provide a stable source of heat. Graphite could also enable a reactor to have as simple and small of a footprint as possible. 

One of the most common ways of converting heat to electricity is the steam cycle. The steam cycle, however, is large and complex. If two requirements are to build a nuclear system as simple and small as possible, then another technology must be utilized. That technology already exists in the form of solid-state thermoelectrics. Thermoelectrics (TEs) have been around for more than a century and use the Seebeck effect to convert heat directly into electricity. This process eliminates the need for pumps, valves, pipes and other materials or systems that exist with the steam cycle. 

NASA is experienced with TEs and have used them for decades to power satellites that turn the decay heat from plutonium into electricity. TEs have also become a commercial industry where any consumer can purchase modules to convert heat to electricity. Depending on the heat source design and temperature, hundreds of different metal combinations can be used for a thermoelectric module including silicon, magnesium, bismuth, antimony, nickel and germanium, to name a few.

Thermoelectrics produce power ranging from microwatts to kilowatts, operate in any configuration, and can be very compact leading to many flexible design options.  A TE module is typically a few inches in length and width and produces anywhere from one to fifty watts of power. It can be a few centimeters thick with one side touching the heat source and the other side touching a cold source in order to create a temperature difference. The difference in temperature is what produces an electric current. The greater the difference in temperature, the greater the current produced. Many modules can be lined up together in any number of orientations and quantities to generate the most electricity possible.

Thermoelectrics are arguably the simplest and smallest options for converting heat to electricity. 

Today’s nuclear plants use water to remove the heat from the reactor, convert the hot water to steam, use the steam to turn a turbine to generate electricity, and then dissipate the excess energy to the air or a water source like ponds, rivers and oceans. The use of water (or any fluid) to cool a reactor requires a number of systems and backup systems to operate. This is not a simple process.

Further, today’s nuclear plants need to be near a substantial water source. If nuclear energy is to be as prevalent as fossil fuels, then it can’t be dependent on water because adequate water sources aren’t everywhere. A nuclear system needs to be able to expel heat anywhere in order to provide energy everywhere. Therefore, the only alternative for a nuclear heat sink is air. 

Nine reactors from the 1940s to the 1960s were air-cooled. These air-cooled reactors used fans to expel the excess heat to the environment. The second reactor to operate in the world, X-10 at Oak Ridge National Laboratory, was graphite moderated and used two large fans. The use of fans to push air to expel excess heat from a reactor is perhaps the simplest heat exchanger possible.