A possible nearer-term use for gaseous helium in the energy conversion enterprise is in the high-temperature gas-cooled reactor. This type of fission reactor is fueled with a mixture of graphite and fuel-bearing elements. The coolant consists of helium gas pressurized to about 100 atm. Helium, which is radiologically inert, passes through interstices in the array of fuel and graphite elements. These reactors can operate at extremely high temperatures, as graphite has a high sublimation temperature and helium is chemically inert. The hot helium can then be directly used either as the working fluid in a high-temperature gas turbine or as the heat source to generate steam. The advantages of such a reactor are that it is meltdown-proof, nearly 50 percent more efficient than current water-cooled reactors, more proliferation-resistant since it uses ceramic fuel, and an efficient plutonium burner, and also that it produces less high-level waste. A joint U.S./Russian program is developing and constructing reactors for the destruction of Russian weapons-grade plutonium and is looking at their possible commercialization and marketing. Each reactor would require an inventory of 100,000 scf (2,800 scm) of helium and a reserve of 200,000 scf (5,600 scm). The inventory is expected to be drawn down at 25 percent per year. Depending on scenarios for deployment, the cumulative requirement for helium by 2020 could be as high as 75 million scf (2.1 million scm).

Cryogenic Wind Tunnels

Other potentially important uses for helium are in the operation of high-Reynolds-number wind tunnels, which would facilitate testing the behavior of aircraft and ships, and high-Rayleigh-number Bénard cells, which could lead to the realistic modeling of convective behavior in weather patterns and other studies of turbulent and convective phenomena having astrophysical and geophysical significance. These facilities would require liquefiers of the scale used by the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, so they could become large-scale users of helium.

The Reynolds number is a figure of merit that characterizes the flow of a fluid around an object such as an airplane or a ship. It is proportional to the product of a length of the object, the density of the fluid, and its velocity, and is inversely proportional to the fluid's viscosity. When a scale model is tested in a wind tunnel, the test is realistic only if the flow over the model is the same as the one the real plane would experience in flight. To achieve this, the Reynolds numbers of the real object and the test object must be the same. This can be difficult if the real object is much larger than the model, because the Reynolds number depends on the size of the object under test. One solution to the problem has been to increase the speed of the gas passing over the model. Indeed there are supersonic wind tunnels that use helium gas. There are limits to this approach, however. Building larger models and larger wind tunnels is too expensive, and thus the highest number achieved is typically 10 million. A submarine moving in water can have a Reynolds number as high as a billion. Liquid helium just above its superfluid transition has a very low viscosity and can be used to achieve very high Reynolds numbers.

Standard wind tunnels achieve Reynolds numbers of 106. A liquid-helium flow tunnel can achieve values of 109. Using cold, gaseous helium, Rayleigh numbers of 1016 to 1020 can be achieved, depending on the scale. For purposes of comparison, the Reynolds number of a Boeing 747 fuselage is 5 × 108 and that of a nuclear submarine is 109. The Rayleigh numbers of the atmosphere, ocean, and Sun are 109, 1020, and 1021, respectively.



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