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March 23, 2005

Nuclear Heat On Tap

Fully Automated Reactors

I have long argued in favor of nuclear energy as the only way to go, eventually, for supplying the needs of an advanced, industrialized civilization. Energy density is the key not only to doing familiar things better, but also to being able to do new things altogether. The densities available from transitions of the atomic nucleus are thousands of times greater than those involved in rearranging the outer shells of atoms, which forms the basis of all chemical processes, and hence offer efficiencies in generating electricity and process heat that can't be attained from conventional forms of combustion or from solar. Nuclear technology has the potential of opening up revolutionary approaches to such things as materials extraction and processing, transmutation of elements, space transportation, desalination of seawater, and the total recycling of all forms of waste. Although such issues as radiation fears, accidents, and waste disposal have been shown to be vastly exaggerated, media sensationalism and ideological opposition have conspired to keep public fears at a level not justified by the realities.

Baseless fears can still be real, however. One approach that has been proposed to addressing the various concerns that continue to do the rounds is to design the reactor as a self-contained unit that would reside underground, operating fully automatically for its entire lifetimeThe concept is described in a study entitled "Completely Automated Nuclear Reactors For Long-Term Operation" by Edward Teller, Muriel Ishikawa, Lowell Wood, Roderick Hyde, and John Nuckolls, posted some years ago by the Lawrence Livermore National Laboratory at www-phys.llnl.gov/adv_energy_src/ICENES96.html.

A 2 gigawatt heat source -- suitable for producing 1 gigawatt, or 1,000 megawatts of electrical power, i.e. the output of a large power plant -- is situated 100 meters below ground and requires no human access, precluding the possibilities of hazard due to error or abuse. The heat engine and electrical generators are above ground, the only connection being via coolant conduits carrying high-temperature helium gas up to the turbines. The heat-source takes the form of a cylinder 10 meters long and 3 meters in diameter containing thorium or uranium fuel functioning as a breeder reactor, with a centrally positioned ignitor module. Ignition creates an expanding "burn wave" front behind which moderately enriched fuel is produced and the reaction is concentrated. The burn wave diverges radially outward and then propagates gradually toward the two ends. The process continues for a design life of 30 years until reactivity ends with fission product accumulation and depletion of fertile material.

Multiple automatic thermostats control local flows of lithium to regulate the neutron flux, maintaining the helium coolant temperature at 1,000 degrees K. In addition, triple independent, passive (i.e. self-activating through gravity on power loss) energy dumping systems remove heat from core in the event of loss-of coolant for any reason and at the end of operational life. On shut down, the site is flooded with a neutron-absorber to nullify beta-decay. No maintenance or replacement of fuel elements is called for, and the site becomes its own waste repository, obviating the need to transport spent fuel.

Economic performance is indicated as a result of achieving high safety without the overhead of expensive safety mechanisms, personnel, and regulations, and low operating and maintenance combined with high conversion efficiency. The projections for 21st-century fuel needs dictate that a breeder approach will be required eventually. Thorium is widespread and cheap, while the use of unenriched fuel minimize requirements for isotope separation. Once people take to the idea (and why not? There's probably enough gasoline alone in most cities to kill the inhabitants several times over) an urban location would reduce power transmission costs and make the waste heat available for industry and space heating.

 
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