The Application of Thermoacoustic Sensors to Examine Nuclear Fuel Rods: An Interview with Professor Steven L. Garrett

Steven L. Garrett, Professor of Acoustics and Senior Scientist at Penn State University talks to Kal Kaur at AZoSensors about the application of thermoacoustic sensors to examine nuclear fuel rods.

KK - Can you discuss the Inspiration behind the design and development of a thermoacoustic sensor for monitoring nuclear fuel rods?

SLG - The decision to consider the use of a thermoacoustic engine to act as a self-powered thermometer inside a nuclear reactor came about in a way that is unique in my 40 years as an academic research physicist. For the past ten years, we have made an inexpensive ($14) thermoacoustic engine demonstration kit available on the internet. In that time, we have sold over 1,000 kits to teachers, students, and researchers on every continent. Two years ago, Dr. James A. Smith, at Idaho National Laboratory, purchased one of those kits and after he built it he contacted me to ask if such a device might be used to sense temperature and other characteristics within nuclear reactors. That was the beginning of our on-going collaboration.

KK - Can you describe the method to monitor the status of nuclear fuel rods without relying on an electrical power supply?

SLG - It is possible to create a thermoacoustic engine within an acoustic resonator that is the same shape and made of the same material as a nuclear fuel rod if you can place a porous medium we call the �stack� within the fuel rod. The heat from the nuclear fuel will create the required temperature gradient across the stack supporting a high-amplitude, self-maintained acoustic oscillation. The frequency of that oscillation is related to the temperature inside the fuel rod. The sound propagates beyond the fuel-rod resonator into the surrounding heat transport fluid where it can be detected (we use an ordinary hydrophone) and the frequency of the radiated sound can be used to determine the temperature of the fuel rod or the surrounding heat transfer fluid.

KK - How does the thermoacoustic sensor work?

SLG - A standing-wave thermoacoustic engine is a �natural engine� in the sense that you do not have to provide any mechanical contrivances to properly phase the transport of heat during the thermodynamic cycle that converts the heat flow into mechanical work. In our case, that work appears as high-amplitude sound waves. In a typical engine, like the one in your car, there are cams, push-rods, rocker-arms, valves, a fly-wheel, a timing belt, etc., that are required to force the compressions and expansions, ignition and exhaust, all to occur at the correct phase of the cycle.

The standing sound wave corresponds to a �sloshing� of the gas which is accompanied by compressions that heat the gas and rarefactions that cool the gas. During half of the acoustic cycle, gas sloshes in one direction where it is compressed and heated adiabatically, but the temperature of the stack is higher than the heated gas, so the gas warms further, thereby increasing its pressure. As it sloshes in the other direction, it cools itself (again, adiabatically) but at the end of its motion the gas finds itself over a portion of the stack that is even cooler. Some heat leaves the gas, going to the stack, and therefore the pressure is further reduced.

This effect was observed by J. W. Strutt (Lord Rayleigh) in the late-1800�s, although in a slightly different context. Although a good quantitative description of thermoacoustics effects was not available until the 1970�s through the work of N. Rott, Rayleigh�s qualitative explanation appeared in his book, The Theory of Sou

nd, Vol. II: �If heat be given to the air at the moment of greatest condensation, or be taken away from it at the moment of greatest rarefaction, the vibration is encouraged.� In our standing-wave thermoacoustic engine, both the �giving� and �taking� are operative during each cycle.Schematic of a thermoacoustic sensor

Heat is applied to the Hot Heat Exchanger end and creates a temperature gradient across the stack. The Cold Heat Exchanger maintains the temperature of the rest of engine at ambient or another desired value.

As the gas moves to the left (step ?), heat is transferred from the hot end of the stack to the gas during step ?, increasing the gas temperature and pressure. The pressure increase pushes the gas back by a little more each cycle.

When the gas moves to the right (step ?), heat is transferred from the gas to the stack (step ?), lowering the gas temperature and pressure. This sucks the gas back toward the hot end of the stack by a little more each cycle.

Eventually, the amplitude of the sound wave grows to a steady-state level where the acoustic power dissipated during each cycle is equal to the acoustic power generated by the thermoacoustic process. The result is that an acoustic pressure wave is sustained within the engine.

This process of conversion of heat to sound was understood by Lord Rayleigh near the end of the 19th century when he stated that a �vibration is encouraged when heat is added during compression and removed during rarefaction.�

This is analogous to what occurs in an optical laser. A flash-lamp puts the atoms in the lasing medium into a non-equilibrium distribution � in a thermodynamic system, non-equilibrium corresponds to the presence of a temperature gradient that exists across our �stack�. In a laser there is an electromagnetic standing wave in a cavity terminated at one end by a mirror and at the other with a partially-reflecting mirror. That standing wave controls the phasing of the atomic transitions from an excited state back to their ground state. The thermoacoustic processes and the analogy to lasers is covered in greater detail in S. Garrett and S. Backhaus, �The Power of Sound�, American Scientist 88(6), 516-525 (2000).

KK - What challenges were the research team faced with during the design process on this sensor technology?

SLG - Most of the technical challenges in converting a simple lecture demonstration device to a sensor that was compatible with the form-factor of a nuclear fuel rod were related to the fairly small size of the fuel-rod. This required that we use higher frequencies and smaller stacks with smaller pore sizes.

Since such fuel rods are typically surrounded by a heat transfer liquid (we used distilled water), we had to protect our auxiliary sensors (e.g., thermocouples, pressure sensors) and their leads from being damaged by the water when submerged. Of course, our hydrophone was perfectly happy since it was designed for operation in water.

KK - What is the thermoacoustic sensor made of and how does this benefit the functional principle to this sensor type?

SLG - The only difference between an ordinary nuclear fuel rod and the thermoacoustic fuel-rod sensor is that the sensor must contain a �stack� material. Our thermoacoustic fuel-rod resonators were fabricated at Idaho National Laboratory using the same materials used for ordinary nuclear fuel rods.

The stack material we use is an extruded cordierite ceramic, called Celcor®, produ

ced commercially by Corning�s Environmental Products Division. It is used worldwide as a catalytic converter substrate in automobile exhaust systems. As such, it is inexpensive (before the catalyst is coated on the surface) and is designed to work under harsh conditions at the high temperatures characteristic of the exhaust from internal combustion engines.

Unlike piezoelectric ceramics, the Celcor® is not electrically polarized so it is tolerant of both high temperatures and the high fluxes of energetic and ionizing particles that permeate nuclear reactors.

KK - How will this technology work in an evolving world where efforts to reduce pollution and the carbon footprint is heavily stressed?

SLG - Thermoacoustic sensing could provide a fail-safe means of monitoring reactor condition even if electrical power is interrupted. Of course, nuclear power is non-polluting (in the conventional sense) and has no �carbon footprint� associated with the production of electrical power.

KK - How do you plan on developing the thermoacoustic sensor technology in terms of function and application?

SLG - Our next step is to power the sensor with nuclear fuel. To this point, all experiments have simulated the heating of the stack by the nuclear fuel with conventional electrical heating elements. We also look forward to using the thermoacoustic sensors to detect changes in fuel properties and to detect the evolution of gases associated with radioactive decay, particularly krypton and xenon. We have also discovered that the acoustically-driven streaming of the gas within the resonator enhances the heat transfer between the fuel rod and the surrounding coolant. This heat transfer enhancement can be significant, although it requires no additional structures, external energy source or pumps.

KK - Can you discuss some emergency situations where this technology is likely to be applied?

SLG - In any emergency situation where a nuclear power plant, or nuclear fuel stored in a spent fuel pool, loses electrical power, a self-powered, acoustically-telemetered thermoacoustic sensor can provide critical status information (e.g., fuel and coolant temperature) that might not be otherwise available.

KK - How do you plan on developing this technology for wider application?

SLG - We have filed a provisional patent covering both thermoacoustic sensing and thermoacoustically enhanced heat transfer and are seeking a commercial collaborator within the nuclear industry. During that process, we are planning further tests at Idaho National Laboratory that will allow us to produce a nuclear-power thermoacoustic fuel rod sensor and monitor its operation acoustically in their Advanced Test Reactor.About Professor Steven L. Garrett

Steven Garrett received his Ph.D. in Physics from the University of California, Los Angeles in 1977. He continued research in quantum fluids at the University of Sussex in England, followed by two years in the Physics Department at the University of California at Berkeley as a Fellow of the Miller Institute for Basic Research in Science. Dr. Garrett joined the faculty of the Naval Postgraduate School (NPS) in 1982 where his research efforts were concentrated on the development of fiber-optic sensors and thermoacoustic refrigerators. He left NPS in 1995 to assume his current position as a Professor of Acoustics in the Graduate Program in Acoustics, and as Senior Scientist in the Applied Research Laboratory, both at Penn State.

In 2001, he was a Fulbright Fellow at the Danish Technical University and in 2008 began working for the US State Department where he is a Senior Science Advisor for the Bureau of East Asian and Pacific Affairs. Professor Garrett is a fellow of the Acoustical Society of America and recipient of the Popular Science Magazine Award for Environmental Technology, the Helen Caldecott Award for Environmental Technology, and the Rolex Award for Enterprise (environment category). He has been issued over two dozen patents.