Paul Weisz's article on long−term energy supplies (Physics Today, July 2004, page 47) states that uranium resources with breeder reactors could provide the world's energy needs for "hundreds of years." That is a gross underestimate. The world's energy needs could be provided by uranium−fueled breeder reactors for the full billion years that life on Earth will be sustainable, without the price of electricity increasing by more than a small fraction of 1% due to raw fuel costs.1

The error in Weisz's calculation is that he is referring to uranium available at its present price, $10−20 per pound. But in breeder reactors, 100 times as much energy is derived from a pound of uranium as in present−day light water reactors, so we could afford to use uranium that is 100 times as expensive.

The cost of extracting uranium from its most plentiful source, seawater, is about $250 per pound—the energy equivalent of gasoline at 0.13 cent per gallon! The uranium now in the oceans could provide the world's current electricity usage for 7 million years. But seawater uranium levels are constantly being replenished, by rivers that carry uranium dissolved out of rock, at a rate sufficient to provide 20 times the world's current total electricity usage. In view of the geological cycles of erosion, subduction, and land uplift, this process could continue for a billion years with no appreciable reduction of the uranium concentration in seawater and hence no increase in extraction costs.

Reference

1. B. L. Cohen, Am. J. Phys. 51, 75 (1983).

Bernard L. Cohen
(blc@pitt.edu)
University of Pittsburgh
Pittsburgh, Pennsylvania

Weisz replies: My article examines the magnitude of available energy sources for the support of the growing human population. Expressing that magnitude in terms of human lifetimes indicates the urgency for remedial actions in technology and social behavior, since those actions themselves take a matter of lifetimes to accomplish.

Bernard Cohen and Eric Swager both point to the large potentials of nuclear energy. My article points out the large potential longevity capabilities of nuclear fission energy for "hundreds of years," and leaves the number of hundreds unspecifically large!

Cohen also mentions the potential of harvesting uranium from the ocean, where it is present in a few parts per billion concentration. However, important basic thermodynamic and mass transport rate constraints limit the economics and feasibility of concentrating highly dispersed matter. I have discussed those constraints relative to the analogous proposal by Fritz Haber, inventor of ammonia synthesis, for harvesting gold from the oceans to pay Germany's World War I debts.1 <SNIP>