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Radioactive Heat
July 26, 2011
Radionuclides emit
heat even when they're not lumped into huge piles, as in a
nuclear fission reactor. Reasonably small quantities of
radioactive material are used in devices called
radioisotope thermoelectric generators that convert the emitted heat to useful
electrical power. This is usually done using the
Seebeck effect in which an array of
thermocouples is employed with their hot junctions at the radioisotope, and their cold junctions at a suitable low temperature sink.
As can be imagined, the efficiency of such devices depends as much on the availability of cold as they do on the heat of the radioisotope. This is yet another example of the
second law of thermodynamics at work (For and exposition of the second law, see my
previous article, Second Law of Thermodynamics, February 7, 2011). This makes such generators ideal in a
space environment where the cold of deep space works to their favor.
NASA has developed a number of radioisotope thermoelectric generators, one of the more powerful being the
General Purpose Heat Source Radioisotope Thermoelectric Generator, used most famously on the
Cassini-Huygens spacecraft to
Saturn. This generator contains 7.8
kg of
plutonium-238 (
238Pu) as the heat source. This generator produced 300
watts of electrical power while the spacecraft was in
Earth's vicinity. Since the
efficiency of thermoelectric conversion is so low, 4,400 watts of thermal energy are required to produce these 300 electrical watts.
Also capitalizing on the availability of a cold sink, the
Soviet Union deployed such units for
lighthouse power in the
Arctic Ocean. However, the problems of having such a compact source of radioactive material available to
terrorists have discouraged such terrestrial usage. Even after the devices have outlived their usefulness as a power source, when the material has too far decayed, the remaining material still poses a great hazard.
The Earth, itself, contains huge quantities of radioactive elements in its
mantle and
crust. It's aways been thought that
decay of these radionuclides helped to keep the Earth warm. However, the exact percentage of heat that radioactive decay supplies compared with the residual heat of Earth's formation was not really known. A recent paper posted on the
Nature Geoscience website, and soon to be published in the journal, provides evidence that the decay of
potassium,
uranium and
thorium provides about half of Earth's internal heat.[1-3]
Sitting on a furnace.
The heat flux from the Earth into space is about 44 trillion watts.
(Lawrence Berkeley National Laboratory Image))
To say that this research was performed by an international collaboration is somewhat of an understatement. This work was performed by the "KamLAND Collaboration," which includes scientists from fifteen organizations in
Japan, the
United States and
The Netherlands. By my count, there are eighty scientists who contributed to this project. KamLAND stands for
Kamioka Liquid-scintillator Antineutrino Detector.
KamLAND was designed for another purpose; namely, detection of
antineutrinos from nuclear reactors in Japan in order to study
neutrino oscillation.
Electron neutrinos are produced in nuclear reactors, but they oscillate into the two other neutrino "flavors,"
muon and
tau as they travel.[2] This same detector will detect geoneutrinos, neutrinos released during the decay of radioactive elements in the Earth, and thus measure the decay rates and types. The neutrino detector inside Japan's Mount Ikenoyama collected data from March 2002 through November 2009.[3] This study also used data from the
Borexino detector in
Italy.
The KamLAND detector, as shown in the figure, is a sphere containing a thousand
metric tons of
scintillating mineral oil surrounded by more than 1,800
photomultiplier tubes. It's located underground near
Toyama, Japan.[2] Detection is aided by a double-scintillation that occurs when an antineutrino interacts with a
proton in the fluid. This reaction converts the proton to a
neutron and a
positron. The positron is then annihilated by an
electron, causing an initial scintillation. A few microseconds later, the neutron binds with a proton to emit a
gamma ray with emission of a second scintillation. Careful analysis of the scintillations allows discrimination of such signals from a cosmic ray background.[2]
The KamLAND antineutrino detector is a spherical vessel, filled with scintillating mineral oil, and lined with photomultiplier tubes. It's located underground, near Toyama, Japan. The scale can be seen from the seated technicians pictured in the control room.
(KamLAND Collaboration Image, via Lawrence Berkeley National Laboratory))
The data show that uranium-238 and thorium-232 together account for about 20 terawatts of Earth's heat flux. Potassium-40 is a big contributor, but it was below the detection limit. Potassium-40 is estimated to contribute about four terrawatts. These three elements generate about half Earth's heat flux
in toto.[1] Because of the inevitable radioactive decay process, the Earth is currently cooling at a rate of approximately 100 degrees Celsius each 1 billion years.[3]
Statistics are hard for neutrinos. Of those detected, 485 were produced from manmade sources, such as nuclear reactors and nuclear waste. Another 245 were likely produced by
cosmic ray interaction with
Earth's atmosphere. Just 111 neutrinos could be associated with Earth's natural radioactivity, of which only 106 were definite candidates.[3]
The team estimates that the overall neutrino flux from uranium-238 and thorium-232 decay at Earth's surface is 4.3 million per square centimeter per second.[3] It's a good thing that neutrinos don't interact much with matter (they interact only via the
weak force and
gravity), or we wouldn't be here! This flux points to a 54% contribution of radioactive decay to Earth's heat, with eight terawatts from uranium-238 (
238U), eight terawatts from thorium-232 (
232Th), and four terawatts from potassium-40 (
40K).[2]
References:
- The KamLAND Collaboration, "Partial radiogenic heat model for Earth revealed by geoneutrino measurements," Nature Geoscience, Published online 17 July 2011; Supplementary Information.
- Paul Preuss, "What Keeps the Earth Cooking? Berkeley Lab scientists join their KamLAND colleagues to measure the radioactive sources of Earth’s heat flow," Lawrence Berkeley Laboratory Press Release, July 17, 2011
- Half Of Earth's Internal Heat Comes From Radioactive Decay, RedOrbit, July 18, 2011.
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Linked Keywords: Radionuclides; heat; nuclear fission reactor; radioactive material; radioisotope thermoelectric generator; electrical power; Seebeck effect; thermocouple; second law of thermodynamics; outer space; NASA; GPHS-RTG; General Purpose Heat Source Radioisotope Thermoelectric Generator; Cassini-Huygens spacecraft; Saturn; kilogram; kg; plutonium-238; watt; Earth; efficiency; Soviet Union; lighthouse; Arctic Ocean; terrorist; mantle; crust; rdioactive decay; Nature Geoscience; potassium; uranium; thorium; Lawrence Berkeley National Laboratory; Japan; United States; The Netherlands; Kamioka Liquid-scintillator Antineutrino Detector; antineutrinos; neutrino oscillation; electron neutrino; muon neutrino; tau neutrino; Borexino; Italy; metric tons; scintillation; mineral oil; photomultiplier tube; Toyama, Japan; proton; neutron; positron; electron; gamma ray; KamLAND Collaboration; statistics; cosmic ray; Earth's atmosphere; weak force; gravity.