This is another in a series of diaries that I have been writing on the innovative developments in nuclear energy now underway in India, currently either the most populated or second most populated country in the world, depending on how closely you look. Most recently, I wrote on India's intention to recover the precious metal palladium from used nuclear fuel in the diary Indians Publish A Method To Recover High Purity Palladium From Used Nuclear Fuel..
Someone asked about the value of palladium in (US) used nuclear fuel, and I guessimated somewhere around $300,000,000, but it turns out I had a spreadsheet in my files, based on fission yields from U-235 (more on that below) from a few years back, and at current prices, ($7,400/kg or $244/tr. oz) there's about $450,000,000 worth of Palladium in used nuclear fuels. I would now like to talk about the Indian approach to a far more valuable and rare metal, rhodium, which it turns out, is more readily available from used nuclear fuels than it is from naturally occurring ore and the value of which is correspondingly higher.
(Only about 3 MT/year of rhodium is now produced from mines, mostly from Russia and South Africa.) Rhodium prices spiked as high as $9,776/tr. oz or $296,000/kg as recently as June of 2008, but have fallen dramatically to $1344/tr. oz. or $40,000/kg as of April of this year.
It can be shown that the value of rhodium in used nuclear fuel in the United States has thus ranged between $5 billion dollars and $34 billion dollars.
Organic chemists who have worked in asymmetric synthesis - the difficult synthesis of just one of a pair of molecules that differ in the way left and right hands differ - in a broad way or have studied it in a broad way are familiar with rhodium catalysts which are some of the best catalysts for this purpose. Chemists will be appreciate some of the asymmetric reactions that are catalyzed by rhodium including ring rearrangements, alkylations, hydrogenations and arylations (using boronated aryl species).
There are many other examples of rhodium catalyzed reactions.
Like palladium, rhodium is also used in the manufacture of catalytic converters that are supposed to mitigate the immediate toxicity of exhaust gases, dangerous fossil fuel wastes, from dangerous fossil fuel powered dangerous cars, although it does nothing really to help with the most dangerous of these dangerous wastes, carbon dioxide.
Thus the collapse of the dangerous car industry - about which I can only personally feel ambivalent at best - has affected immediate rhodium prices. However no matter what happens, rhodium is going to remain an extremely important material for industrial use for however long humanity may survive its own stupidity.
(On the other hand, the use of nuclear energy, which produces rhodium as a side product has been the single most effective means on this planet for more than 3 decades - and will remain the most effective means for decades to come - at mitigating the release of dangerous carbon dioxide, but that's another story.)
In my last diary on nuclear fuel, the one linked in the intro, I noted that India intends to recover palladium from its nuclear fuel and I also noted that India needs to develop new fuel cycles for its switch from uranium based nuclear power to thorium based nuclear power, which is ideal given India's resources and its CANDU/PHWR based nuclear fleet.
The palladium based chemistry, as I noted, involves what is called PUREX chemistry, which involves the dissolution of used nuclear fuel in nitric acid and extraction of the fuel with various chelating agents suspended in organic compounds derived from dangerous fossil fuels. This process has always been used for the uranium/plutonium fuel cycle but will need modification or replacement for the thorium/U-233/(possibly plutonium and neptunium) cycle.
Superior chemistry to Purex was developed in the United States for the IFR program that was canceled by the Clinton administration in one of its less-than-wise acts. (This reactor was designed to consume plutonium and heavier actinides such as americium and curium, as I recall, but I'm not an expert on the reactor itself although I was very interested in the chemistry of fuel processing developed by it.) This chemistry was known as pyroprocessing as well as electrorefining and it did not involve the use of dangerous fossil fuels at all, which is, of course, a good thing.
Other nuclear chemistry that is superior to Purex chemistry - which was invented in the United States in the mid 20th century - that has also been developed in the United States, involves materials known as ionic liquids, which are salts that are molten at room temperature or near room temperature. (This work is still going on at Brookhaven National Laboratory but is under funded because we'd rather subsidize things like dreams of dangerous electric cars, even though we are still producing the bulk of our electricity from dangerous fossil fuels. I met one of the leading chemists on this technology - Jim Weichart - although I doubt he remembers me or has a clue about my nom de guerre, NNadir.)
India is exploiting both electrorefining and ionic liquid chemistry in its approach to recovering rhodium from used nuclear fuel. (For the record, Japan also is recovering palladium, rhodium and ruthenium at its Purex based nuclear fuel recycling facility, which is relatively new. The chemistry I will discuss below is not the only approach to accomplishing this task.)
The paper I will be discussing today is entitled "Electrochemical behavior of rhodium(III) in 1-butyl-3-methylimidazolium chloride ionic liquid" and the reference is Electrochimica Acta 53 (2008) 2794–2801. The authors are Srinvasan and his co-workers at the Indira Ghandi Centre for Atomic Research at Kalpakkam.
To repeat what I have said above:
Rhodium is a noble metal. At present, there is a huge demand for rhodium and its compounds in various industries owing to its unique physical and chemical properties . The abundance of rhodium in the earth crust is very low and occurs only in few countries. The price of rhodium in international market is escalating steeply and it is likely to increase further due, partially, to the heavy demand of rhodium and largely to the increased cost of mining from already depleted natural resources. This cost and the added costs arising from separation, purification up to conversion of rhodium in to a required form may become exorbitant after few decades and further mining would be an unworthy exercise and uneconomical for trading rhodium in global market. It is therefore necessary to look for other alternate sources of rhodium that is abundant and easily accessible. One such source is a spent nuclear fuel [1–3].
Rhodium is one of the by-products of nuclear fission reaction and therefore spent nuclear fuel is a valuable resource of man-made rhodium [1–3]. Significant quantities of platinum group metals (PGMs, palladium, rhodium, ruthenium) are produced as fission products in nuclear reactor and 103Rh is the major non-radioactive isotope produced by fission reaction . The fission yield of other rhodium isotopes, 102Rh and 102mRh with half-life of 2.9 years and 0.57 years is very low and may vanish if the fuel is cooled over three decades. It was estimated that by the year 2030 AD, the amounts of man-made rhodium produced by nuclear reactor operation all over the world will match with the amounts of rhodium available at that instant in the earth crust . Therefore, recovery of rhodium from spent nuclear fuel may provide appreciable incentives in view of its growing demand and widespread applications in various chemical, pharmaceutical and electronic industries [4,5].
The work is on going, but one of authors conclude that the electrorefining of palladium by this process inevitably involves getting the more valuable metal, rhodium, as a side product, about 10%.
There is some talk about cooling times because of the presence (in low nuclear yields, fortunately) of Rh-102 and Rh-102m, radioactive isotopes that occur with non-radioactive Rh-103, but this is a trivial matter. After 5 years cooling the specific activity of Rh obtained from nuclear fuels is less than a millicurie a gram, and after a few decades, irrelevant.
I want to add a brief note on this concept. If one graphs the distribution (by mass number) of fission products for most readily available fission fuels (uranium-233, uranium-235, plutonium-239, and the under appreciated plutonium-241) one will see a line that looks rather like two camel humps. Fission is asymmetric for these elements. (Symmetric fission does take place in elements heavier than californium, like say, fermium, but that is of no commercial or practical interest.)
As it happens, the hump on the right (heavier elements from say tellurium to gadolinium) always stays pretty much in the same place no matter what fission fuel one is using, but the distribution drifts for the various fuels. What I mean can be shown by looking at the graph here.
India intends to go - and I approve - to a U-233 fuel cycle derived from transmuting thorium into this isotope. Their yields of rhodium, ruthenium and palladium will be lower than for the yields from plutonium burning. Still they are significant.