Thermoacoustic Device for Nuclear Fuel Monitoring and Heat Transfer Enhancement
Randall A. Ali -- rza5052@psu.edu
Steven L. Garrett -- sxg185@psu.edu
Penn State Graduate Program in Acoustics
State College, PA 16802
James A. Smith -- james.smith@inl.gov
Dale K. Kotter
Fundamental Fuel Properties Group
Idaho National Laboratory
Idaho Falls, ID 83401
Popular version of paper 3aPA2
Presented Wednesday morning October 24, 2012
164th ASA Meeting, Kansas City, Missouri
In 2011, the Tōhoku region of Japan experienced the strongest earthquake ever recorded in the country. That earthquake, in conjunction with the inevitable powerful tsunami waves, resulted in considerable damage across Japan, including the Fukushima Dai'ichi nuclear disaster. Not only was there a failure of the nuclear reactor and boil out of the spent-fuel ponds, but the tsunami also disrupted the electrical power connection to the nuclear reactor and rendered the back-up electrical power generators, coolant pumps, and sensor systems useless. This created a situation where the nuclear plant's operators were not able to monitor the condition (e.g. temperature) of the fuel rods in the reactor and spent-fuel in the storage ponds. These events highlighted the need for self-powered sensors that could transmit data independently of electronic networks while taking advantage of the harsh operating environment of the nuclear reactor.
The field of thermoacoustics, which exploits the interaction between heat and sound waves, offers an attractive solution. A thermoacoustic engine produces sound from heat. A thermoacoustic device contains no moving parts and does not require external electrical power if the heat is available, such as that from the fuel in a nuclear reactor. The physical components of a thermoacoustic engine can be as simple as a closed cylindrical tube (e.g., the fuel-rod itself) and an entirely passive structure known as a "stack". The stacks in our devices are made from a ceramic material with a regular array of parallel pores that is manufactured as the substrate for catalytic converters, found in most automotive exhaust systems. The stack facilitates the transfer of heat to the gas in the resonator in a way that encourages the conversion of that heat to high-amplitude sound when there is a temperature difference along the stack. This process is shown in Figure 1.
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Figure 1 – Schematic view of the operation of a thermoacoustic engine of half wavelength (λ/2) that illustrates the motion of a parcel of gas experiencing the four stages during one cycle of motion within the “stack” of a thermoacoustic engine. 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, heat is transferred from the hot end of the stack to the gas, 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, heat is transferred from the gas to the stack, lowering the gas temperature and lowering the pressure. This sucks the gas back toward 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 Rayleigh1 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.”
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We built a laboratory version of this device and simulated the environment of a nuclear reactor using an electrical resistance heater much like the wire that glows in a toaster. The cylindrical tube of the thermoacoustic engine was manufactured by the Idaho National Laboratory (INL) from stainless steel that could withstand reactor temperatures, and of the same dimensions as a nuclear fuel rod. This tube was instrumented with pressure and temperature sensors and a ceramic stack that provided 1,100 parallel square channels per square inch. Heating was applied in two ways: an indirect method, which would be how the thermoacoustic engine would actually be used in the nuclear reactor, and a direct heating method in which there was more control over the heat source. Figure 2 shows the thermoacoustic engine and the direct electrical heating element.
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| Figure 2 – (Background) The cylindrical metal tube (resonator) used as the thermoacoustic engine is manufactured from Nitronic 60 stainless steel at INL. The larger-diameter PVC end on the resonator protects an instrumentation plate from the surrounding water. The plate contains sensors that measure gas temperatures and pressure (both static and acoustic). (Foreground) An electrical heater (glowing red) made from Nickel-Chromium wire, like that in a toaster, acts
as the laboratory version of a heat source. It is placed in contact with the stack which consists of a ceramic that has 1,100 parallel channels per square inch. The wires leaving the stack are the leads for two thermocouples that measure the temperature gradient across the stack. During the experiments, the electrical heater and stack are placed within the resonator and the entire system is submerged in water, as it would be in a nuclear reactor. |
Inserting one of these thermoacoustic engines into a nuclear reactor can be beneficial in two ways: one as a passive temperature sensor and the other for enhanced heat transfer from the nuclear fuel to the surrounding heat transfer fluid (usually water).
The high operating temperatures of the nuclear fuel rods or spent-fuel will create a sufficient temperature gradient across the stack to create an oscillating pressure wave within the engine. The frequency of the sound will be dependent upon the temperature within the resonator (i.e., fuel rod). This frequency can be propagated via sound radiation through the cooling fluid in the reactor and monitored at some distance away. This novel technique eliminates the dependence on electrical power for signal monitoring while actually taking advantage of the extreme operating conditions within the nuclear reactor.
The enhanced heat transfer from the nuclear fuel to the surrounding fluid is an additional bonus of the thermoacoustic engine with no additional complication. When the engine is in operation, the thermoacoustic effect produces an acoustically-driven streaming gas jet which will circulate hot gas away from the heat source (nuclear fuel) and along the walls of the engine and then into the surrounding cooling fluid. This acoustically-pumped gas flow increases the thermal contact between the gas and the water through the resonator walls. The current research indicates that this enhanced transfer can be on the order of a 20% increase.
It was also found that the thermoacoustic engine could be successfully operated if the heat is applied indirectly. In a nuclear reactor, the heat that is generated in the fuel rods will not be directly applied to the hot end of the stack, but will heat the closed end of the resonator by electromagnetic radiation. This is the same process by which the heat of the sun propagates through empty space to warm the Earth. The high temperatures of the nuclear fuel makes this radiative heat transfer effective enough to create the right conditions to produce sustained acoustic oscillations within the thermoacoustic engine.
In addition to Professor Steven Garrett and Randall Ali at the Pennsylvania State University, Dr. James Smith and Dale Kotter of INL have been working closely with the Penn State doing thermoacoustics experiments at INL to use thermoacoustically-generated sound to monitor microstructural changes in the nuclear fuel, measure gas mixture composition, and act as a failsafe device in emergency situations.
The funding for this work was provided by the Idaho National Laboratory. Randall Ali is a student in Penn State's Graduate Program in Acoustics. He is a Fulbright and Organization of American States Fellow from Trinidad and Tobago.
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1 J. W. Strutt (Lord Rayleigh), The Theory of Sound , Vol. II (Macmillian Co., 1896; reprinted Dover, 1945) §322.
2 Idaho National Laboratory (INL) is a science-based, applied engineering national laboratory dedicated to supporting the U.S. Department of Energy's missions in nuclear and energy research, science, and national defense.
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