About Uranium
Have you ever heard of the element uranium? It’s sometimes talked about in the news. It’s an important fuel in some power plants that make electricity.
Tyrone Mineo, Uranium [a book for children], 2014
The first specimen ever to be accurately identified was carbonaceously black; but only because we got to it too late. The silvery luster of freshly cleaned uranium metal is rapidly converted first to a golden yellow, and then to black oxide-nitride film within three or four days. And perhaps the purity left something to be desired. In due time we learned that this element manifested a peacock’s worth of pretty colors in its compounds: Uranium trifluoride was dark purple, the tetrafluoride green, the pentafluoride either greyish-white or yellowish-white, depending. Uranium carbide, which did our thermodynamic work in sodium- or lead-cooled reactors, was a dark grey solid with a metallic luster. Uranium trioxide, another nuclear fuel, could be anywhere from brick-red to yellow.
You may recall that carbon made up 200 parts per million in our planet’s crust. Uranium was less conspicuous, at two parts per million. Even so, its busy irradiations contributed to the warmth of the place we once called Hell. And whatever it did down there it might as well do up here for us, steaming water in order to rotate turbines, so that we could keep the lights on.
To be sure, it presented certain difficulties. The Encyclopedia of Chemical Technology called it a general cellular poison which can potentially affect any organ or tissue. It specialized in harming the liver and kidneys.
Since it only came in at number 48 of all the elements for abundance, it rarely did us mischief. (As that children’s book explained: Everyone is exposed to a small amount of radiation every day. This isn’t harmful.) Volume for volume, “natural uranium” was 1/1,000 as “hot” as radium-226—which, by the way, was literally hot, emitting 180 BTUs per hour for every pound. (As for plutonium, that feels hot, like a live rabbit.)—Uranium rock did not even feel warm.*—We ate a microgram or two of uranium every day in our food crops. If it got into our blood, it ended up in our skeletons. Otherwise we mostly pissed it out.—Inhaled uranium dust, of course, was not so good.
Following human nature, we set out to concentrate the nasty stuff.* We had already been unknowingly doing so, thanks to agriculture. Uranium’s natural occurrence in soil was 0.7 to 11 parts per million. The phosphate fertilizers we spread not only helped warm the atmosphere,* they also boosted cropland’s uranium levels to 15 parts per million. And indeed, one way to get uranium was to soak those phosphates in a hydrocarbon solvent: organic esters dissolved in that traditional oil well product, kerosene. Thus even this very first manufacturing stage begins to reveal the ironic fact that nuclear fuel, touted as the anti-carbon ideology, required carbon-based feedstocks—and considerable carbon-combustion-derived electric power.
(Come to think of it, our fossil fuels frequently contained relatively high levels of uranium as impurities. Thus we added value to the fields, roads, back yards and streams near power plants.)
But mostly we left those phosphates to agrochemists, mining the conglomerate rocks and sandstone in which 90% of uranium could be found,* as a mix of two isotopes* of significance: U-235 and U-238. U-235 is capable of continuous energy release through fission; U-238 is not, at least not without help. In fact, U-238 inhibits the fission of U-235. In 1997, U-235 comprised only 0.72% of all uranium; and since it decayed more rapidly than U-238, in your time it will be slightly harder to come by.
Our task became to “enrich” uranium rock until it held an increased proportion of U-235.—Time to burn more fossil fuels!
First we had to dig up the pitchblende, uranite and other ores which contained decent concentrations of what we were after. I hate to pooh-pooh the good old days when any solvent American could buy a pocket-sized Geiger counter from Sears and Roebuck, with earphone head set, radio-active material for testing and AEC booklet “Prospecting for Uranium”—but in fact what we were about to undertake required heavy machinery with internal combustion engines: Now came pulverization, which we preferred not to accomplish with hammers and human slaves. Some ores are highly refractory and require intensive processing, while others break down between the mine and the mill. We fed the refractory rock into jaw crushers, at an undisclosed cost in BTUs per hour, some of which we paid up front in coal or oil, while the rest we left to you zero-interest beings of the future. Meanwhile the miners paid in their own way, for these operations emitted the gas radon-222 (half-life: four days), which is highly unstable and highly dangerous because of its intense radioactivity. For that we established a brilliant work-around: Tailings got placed in large piles and covered to prevent a local health problem.
We ground up our coarse heap in rod mills, hammer mills, whatever it took. Next came the oxidizing roast, which required heat inputs (again provided by coal or oil, of course). Having solubilized the powder, we leached it in an oxide-laced bath of acid or alkali. Most mills use acid leaching, which completely extracts uranium. Because of its low cost, sulfuric acid is preferred . . . (To make sulfuric acid, which might have been our favorite chemical,* we often roasted sulfur, with fossil fuels for heat inputs.)
Our treasure remained no good to us until we could precipitate it out of solution. One method was solvent extraction. It takes energy and usually petrochemicals to make a solvent, which then adds to global warming by emitting volatile organic compounds.*
The other way involved ion exchange, with the help of ammonia. (You may remember that in 2012, carbon dioxide from ammonia production caused 0.4% of all greenhouse emissions from the EU-15 countries.) Or, instead of ammonia, we could aim for a higher uranium concentration by adding the powdered white oxide called magnesia, whose manufacture generally required high heat, and therefore, considerable energy, very likely from fossil fuels.
But enough on distracting subjects; at last we had our yellowcake—which was not necessarily yellow—and at the bottom of a corrugated round tunnel we now glimpse, courtesy of a photo in that same children’s book (which unlike any other source absolutely guarantees the following: METAL MANIA! All the uranium on earth is the result of a large star exploding more than 5 billion years ago), yes, that powdered yellowcake, while a bespectacled technician rests his chin on his hand, peering in at us through what one trusts is a heavy glass window.—It was refining time.
So we dissolved the yellowcake in nitric acid (whose manufacture had already cost our climate in nitrous oxide emissions*), then extracted it back again with tributyl phosphate—in a kerosene or hexane diluent, which must have originated either in a coal mine or an oil well—and after re-dissolution, evaporation, pyrolysis and reduction, all of which I shall spare you, we had uranium dioxide (brown to copper-coloured), one of the most common reactor fuels.
Oh, but it was far from ready yet! It had not yet been “enriched.” As that children’s book reminds us: It takes a lot of energy to create enriched uranium. The uranium isotope needed for nuclear energy makes up less than 1 percent of all uranium!
So through two or three more stages we converted it into uranium hexafluoride. Unlike the prettier incarnations of uranium, whose hues might appeal to many a child, “hex” was an extremely corrosive, colorless, crystalline solid, which sublimes with ease at room temperature.
Permitting it to sublime, we diffused it through a membrane of silvered zinc. The U-235 and U-238 within the invisible gas had very slightly different molecular weights. After the “hex” had gasified, we sent half of it through the membrane, passed half of that through another membrane, etcetera, each time pumping the undiffused half back to go through again; and of course at each sifting a smidgeon more of the lighter isotope came through. In case you are wondering how many repetitions were required, well, it depended on the enrichment. The first time, at Oak Ridge, when the purpose was to spread destruction and terror to Japan, thereby not only wrapping up the war but also cowing our Russian allies, the slide rule crew worked out that raising the proportion of U-235 atoms from 0.72 to 99% would take 4,000 diffusion cells. We later settled on a 96–97% enrichment for weapons-grade fuel, and 2 to 5% for reactor fuel.* Even the latter (whose “cascade” was modest at more than a thousand separation stages) cost considerable thermodynamic work. Gaseous diffusion units are enormous in size, often covering hundreds of acres, and requiring huge amounts of electric power to operate. At Oak Ridge’s enrichment facility, K-25, the coal-fired power plant designed to generate electricity to run the barrier diffusers for an atomic bomb was rated at 238,000 kilowatts—more than 800 million BTUs an hour. If it ran at capacity, during that hour it would have burned 97 tons of coal!
Eventually we learned that centrifugal separation could be more efficient, although the results were highly corrosive. Moreover, efficient or not, those gas centrifuges must have drawn considerable power in their own right, with their high velocity . . . attained under temperature and pressure conditions that favor evaporation.*
At any rate, that was how we enriched uranium. Then we reconverted the enriched hexafluoride back into dioxide, or one of the other nuclear fuels: pure uranium metal, or sulfates, silicides, nitrates, carbides, and molten salts. What quantity of climate-altering emissions we made along the way I cannot begin to reckon. Here is one cryptically suggestive passage from a European Union greenhouse report, pertaining to France—the only mention of uranium in the whole document:
Uranium tetrafluoride: N2O emissions data is taken directly from annual statements of pollutant emissions since 1990 and emissions are derived from continuous measurements since the 2012 submission.
By 1943 we had even discovered how to irradiate that most common and supposedly unfissionable isotope, U-238, feeding slow neutrons to its greedy nuclei. The resulting “capture reaction” transformed it into U-239, which speedily decayed into neptunium-239—which expeditiously became plutonium-239. This isotope possessed a magnificent shelf life. After 24,360 years, half of its atoms would still be Pu-239. While that might not have been a desirable feature in fuel waste, it was awfully convenient from a military point of view. Furthermore, Pu-239 readily fissions, as was so empirically proved at Nagasaki.
The milder plutonium-238 powered certain machines on the Apollo 14 spacecraft; while Pu-239 presently entered use in the “mixed-oxide” (MOX) reactors containing plutonium and uranium in a ratio of about 1:17. Wasn’t thermal efficiency the ideal in our steam turbines? As the World Nuclear Association promised us: A single recycle of plutonium in the form of MOX fuel increases the energy derived from the original uranium by some 12%, and if the uranium is also recycled this becomes about 22%. Well, viva MOX reactors!—One of those exploded at Fukushima, but plutonium contamination remained almost indetectable throughout the red zones, so I would call the World Nuclear Association right on the money, wouldn’t you?*
Back when we were alive, we liked to sell our inventories of everything as rapidly as possible, so that we could sell some more. MOX possessed a very agreeable quality in this regard: The more quickly somebody put it to work, the better!
Plutonium from reprocessed fuel is usually fabricated into MOX as soon as possible to avoid problems with the decay of short-lived plutonium isotopes. In particular, Pu-241 (half-life 14 years) decays to Am-241 which is a strong gamma emitter, giving rise to a potential occupational health hazard if separated plutonium over five years old is used in a normal MOX plant (where radiation levels are very low). The Am-241 level in stored plutonium increases about 0.5% per year . . .
As for the “hex,” someday we might even address the problem of the disposition of 500,000 [metric] tons of depleted UF6 stored at Paducah, Portsmouth, and Oak Ridge. We could always blend in plutonium oxide, and whip up a fresh batch of MOX. Or we could do something else, sometime, in some ecosystem somewhere. But why not put all that off, just as we did with such other trivialities as global warming? Let the future worry, we said to ourselves. Why not live in the present? For Carbon Ideologies now delights to inform you that unlike its three main rival fuels, nuclear could be fun! The daughter of a uranium processing worker once assured me: “During the early nuclear testing times in Nevada the Sands Hotel used to pack lunches for guests to enjoy with their families while out watching the atomic testing in the desert outside of town. “The Chamber of Commerce in Las Vegas would print when and where the tests were going to be, along with the best viewing site on their yearly calendars.”
Back to “atoms for peace”: How efficient was plutonium? Well, from a thermodynamic point of view it fully justified the extravagant energies poured into its production. One pound of it would have carried out every last BTU of the hourly work for which that coal plant at Oak Ridge was rated—non-stop for almost three years.
CALORIFIC EFFICIENCIES OF COAL, OIL, NATURAL GAS, URANIUM-235 AND PLUTONIUM-239,
in multiples of the thermal energy of coal
as simplified from the table of Calorific Efficiencies (here)
All levels expressed in [BTUs per pound].
All comparative efficiencies greater than 10 have been rounded to the nearest whole number. All figures after U-235 rounded to 2 significant digits.
1
Coal, based on West Virginia Coal Association average [12,500 BTUs/lb].
1.54
“Oil,” calculated from average for diesel fuel [19,250].
1.95
Natural gas, dry American, A.D. 1980 [24,381].
689,503
Uranium-235, “in theory” [8,618,790,909].
1.48 million
Estimate of uranium burned in a 1-million-kWh Japanese reactor [18.60 billion].
1.54 million
Plutonium-239, “used in a conventional nuclear reactor” [19.25 billion].
To complete the preparation of nuclear fuel we shaped our uranium dioxide, MOX, or whatever else pleased our fancy, into spheres and pellets, furnace-fired them, then dropped them into long* zirconium-jacketed rods.—For marine reactors we actually alloyed the uranium with zirconium. This latter element resembled uranium in its ductile silveriness, but resisted corrosion and disdained to absorb neutrons, which create radioactive byproducts.—Those built up all the same.
One might imagine that once a reactor began to run, the cost of the energy it generated would fall to near nothing. What did anyone need to do then but let those long-lived fuels go right on fissioning?—Strange to say, nuclear power cost about the same as coal.
The lifetime of a fuel rod was two to five years. After that, the rod grew so contaminated with those byproducts (or, if you’d rather, “daughter nuclides”) as to impede neutron travel: No neutrons, no fission.
I liked the way they put it: The used or spent fuel contains a large inventory of fission products. They sounded like salespeople laying out their wares. But they were more generous than that. Wherever possible, they would give their fission products away—to you.
We had dreamed up three methods of waste disposal, depending on the danger: dilute and disperse, wait out short-term decay and concentrate and contain.
Here was a typical to-do list:
Submerge spent fuel in a water pool for a decade.
Transfer it to a dry storage facility.
Sweetly bundle it in titanium or stainless steel.
Lovingly jacket it in concrete or steel.
The fundamental safety criterion for the permanent repository is that the engineered package retain complete integrity for at least 1000 yr. (In Japan, where everything was better and brighter, a shorter interval sufficed, at least for “low level nuclear waste,” which must be kept apart from residential settlement for up to 400 years.)
The daughter nuclides cesium-137 and strontium-90 provide most of the radioactivity, radiation and heat during the early years of a disposal facility. Another fission product was iodine-131—more short-lived than the other two, but particularly dangerous to children. We will meet all three of them at Fukushima.
Depleted uranium (0.2% U-235) offered twice the density of steel. It was 19 times denser than water, 11 times denser than lead! So it made for efficient ballast in ships. And the military loved it, because DPU shells could self-sharpen during armor penetration by failure along adiabatic shear bands.*