Amory Lovins and Long-Lived Radioactivity

The politicians of the USA, whether right or left, have still not abandoned the wickedness of being the world's major rogue nuclear weapons owner. Ours is the only country that has actually used them.

But a large proportion of my fellow liberals are fanatically against civilian nuclear power. They are entirely wrong. It is in fact the only way to put an end to the disastrous, poisonous outpourings with which every coal powered plant in the world is polluting the atmosphere. The ratio of fuel to energy is one millionth of that of coal, and could be even better.

Amory Lovins of the Rocky Mountain Institute writes a considerable amount of nonsense about how the common isotope of uranium, U-238, remains radioactive for billions of years, and some of the daughter products are fiercely radioactive.
It is indeed radioactive for thousands of millions of years, and produces even more radioactivity from its decay products, some of them (e.g. Radon and Radium and Polonium) very much more radioactive. This is true, but the picture that you get, of vastly increasing radioactivity, is entirely false.

The annual total production of daughter isotopes of U-238 is 155 micrograms per tonne, that's 155 parts per million million. That's the same as half of the uranium decaying in 4500 million years -- note that it's a negative exponential, not linear. The equilibrium quantity of radium produced by 4.5 tonnes of U-238 is about 1.6 grams.

Likewise, when he mentions "long-lived radioactive isotopes" of iodine and technetium, he seems to display utter ignorance of the fact that the radioactivity of a sample of unstable isotope is inversely proportional to its half life. So Iodine-129, this long lived menace, half-life 15.7 million years, produces about 0.69/15.7 millionths of its total radioactivity in a period of a year. (0.693... is the natural logarithm of 2.0) That's 44 parts per American billion. About 0.66% of the uranium-235 fissioned in a reactor yields this isotope. Since the total annual production of nuclear power in the USA involves the actual fission of about 60 tons of U-235, it must produce about 0.4 tons of I-129, of which 0.044 millionths per year emit the dreaded radiation.

The first daughter product of U-238 is Th-234, isotope 234 of thorium, with a half life of 24.1 days, compared with its parent's half life of 4,468 million years. When the quantity of Th-234 is equal to that of the parental U-238, times (24.1 days) divided by (4,468 million years), the thorium is decaying (into Protactinium-234) as fast as it is being produced. The Pa-234 is radioactive, half of it changing into uranium 234 (U-234) in just over 6 hours. The radioactivity of the equilibrium quantities of these decays is comparable with that of the U-238, although the quantity of the decay products is minuscule. (25 days)/(4,468 million years) = 25/(365*4,468 million) is less than one part in 15 billion.
The half-life of U-234 produced by the Pa-234 is 233 thousand years, so its equilibrium quantity is a fraction 233/(4468 thousand) of the U-238. That's about 51.8 grams per tonne At a production rate of 155 micrograms per tonne of the great-grandparent U-238, it takes about a third of a million years for the radioactivity of the Th-234 daughter to equal that of the U-238.

It takes even longer for significant quantities of radium, radon and polonium to appear. But in fact, most of the radioactive daughter products of "depleted uranium", U-238, are already present in any ore or ash containing uranium, except for the radon because it is a gas. So the radioctivity of your sample of uranium is already about as high as it is likely to get. The radioactivity of U-238 and its daughter products can never exceed or even equal that of the radioactive elements in the original ore.
Indeed, the original ore, for instance pitchblende, does indeed emit more than ten times the radiation of the U-238 within it. It has been decaying into radioactive daughter products like radium, radon, and polonium, over millions of years. But if you start with chemically refined uranium, the rejected portion, unless you deliberately harvest the radioactive residue as valuable, will carry as much radioactivity as the uranium will create in the next few thousands of millennia.

But the radon that may migrate through the soil from any uranium-containing rocks, having a half-life of less than 4 days, needs only the tiniest quantities to give off as much radiation as the tons of uranium that were its long-ago parent.

Helen Caldicott seems to be as ignorant of the importance of actual numbers as Amory Lovins is.

Valid HTML 4.0 Transitional