Synthesis of precious metals

The synthesis of precious metals, a symbolic goal long sought by alchemists, is possible with methods utilizing nuclear physics, currently involving either nuclear reactors or particle accelerators. Since particle accelerators require huge amounts of energy while nuclear reactors produce energy, production methods using a nuclear reactor are considered more economically feasible. Often the goal of synthesis is to produce an element at a cost significantly less than the standard methods of production. Recovery of rare elements from spent fuel rods is also anticipated to help offset the cost of reprocessing.

Precious metals occurring as fission products

Ruthenium, rhodium

Ruthenium and rhodium are precious metals produced by nuclear fission, as a small percentage of the fission products. The radioisotopes of these elements generated by nuclear fission with the longest half-lives have half-lives of 373.59 days and 45 days for ruthenium and rhodium, respectively. This makes their extraction from spent nuclear fuel possible, although they must be checked for radioactivity before use.^[1]

The radioactivity in MBq per gram of each of the platinum group metals which are formed by the fission of uranium. Of the metals shown, ruthenium is the most radioactive. Palladium has an almost constant activity, due to the very long half-life of the synthesized ¹⁰⁷Pd, while rhodium is the least radioactive.

Until now no facility has been reprocessing spent nuclear fuels for ruthenium and rhodium; however, Japan is planning to do so in their new spent fuel reprocessing facility, which will help offset the cost of reprocessing.

Ruthenium

Each kilogram of the fission products of ²³⁵U will contain 63.44 grams of ruthenium isotopes with halflives longer than a day. Since a typical used nuclear fuel contains about 3% fission products, one ton of used fuel will contain about 1.9 kg of ruthenium. The ¹⁰³Ru and ¹⁰⁶Ru will render the fission ruthenium very radioactive. If the fission occurs in an instant then the ruthenium thus formed will have an activity due to ¹⁰³Ru of 109 TBq g⁻¹ and ¹⁰⁶Ru of 1.52 TBq g⁻¹. ¹⁰³Ru has a half-life of about 39 days meaning that within 390 days it will have effectively decayed to the only stable isotope of rhodium, ¹⁰³Rh, well before any reprocessing is likely to occur. ¹⁰⁶Ru has a half-life of about 373 days, meaning that if the fuel is left to cool for 5 years before reprocessing only about 3% of the original quantity will remain; the rest will have decayed.^[1]

Rhodium

It is also possible to extract rhodium from used nuclear fuel: 1 kg of fission products of ²³⁵U contains 13.3 grams of ¹⁰³Rh. At 3% fission products by weight, one ton of used fuel will contain about 400 grams of rhodium. The longest lived radioisotope of rhodium is ^102mRh with a half-life of 2.9 years, while the ground state (¹⁰²Rh) has a half-life of 207 days.^[1]

Each kilogram of fission rhodium will contain 6.62 ng of ¹⁰²Rh and 3.68 ng of ^102mRh. As ¹⁰²Rh decays by beta decay to either ¹⁰²Ru (80%) (some Positron emission will occur) or ¹⁰²Pd (20%) (some gamma ray photons with about 500 keV are generated) and the excited state decays by beta decay (electron capture) to ¹⁰²Ru (some gamma ray photons with about 1 MeV are generated). If the fission occurs in an instant then 13.3 grams of rhodium will contain 67.1 MBq (1.81 mCi) of ¹⁰²Rh and 10.8 MBq (291 μCi) of ^102mRh. As it is normal to allow used nuclear fuel to stand for about five years before reprocessing, much of this activity will decay away leaving 4.7 MBq of ¹⁰²Rh and 5.0 MBq of ^102mRh. If the rhodium metal was then left for 20 years after fission, the 13.3 grams of rhodium metal would contain 1.3 kBq of ¹⁰²Rh and 500 kBq of ^102mRh. Rhodium has the highest price of these precious metals ($25,000/kg in 2015), but the cost of the separation of the rhodium from the other metals needs to be considered.^[1]

Palladium

Palladium is also produced by nuclear fission in small percentages, amounting to 1 kg per ton of spent fuel. As opposed to rhodium and ruthenium, palladium has a radioactive isotope, ¹⁰⁷Pd, with a very long half-life (6.5 million years), so palladium produced in this way has a very low radioactive intensity. Mixed in with the other isotopes of palladium recovered from the spent fuel, this gives a radioactive dose rate of 7.207x10⁻⁵ Ci, which is well below the safe level of 1x10⁻³ Ci. Also, ¹⁰⁷Pd has a very low decay energy of only 33 keV, and so would be unlikely to pose a hazard even if pure.

Silver

Silver is produced as result of nuclear fission in small amounts (approximately 0.1%). Because of this, extraction of silver from highly radioactive fission products would be uneconomical, but when recovered with ruthenium, rhodium, and palladium (price of silver in 2011: about 880 €/kg; rhodium; and ruthenium: about 300,000 €/kg) the economics change substantially; silver becomes a byproduct of platinoid metal recovery from fission waste.

The total value of the above metals as of August 2015 is approximately $35,000 per ton of spent fuel, compared to the wholesale value of the electricity produced by the fuel of approximately $20 million.

Precious metals produced via irradiation

Ruthenium

In addition to being a fission product of uranium, as described above, another way to produce ruthenium is to start with molybdenum, which has a price averaging between $10 and $20/kg, in contrast with ruthenium's $1860/kg.^[2] The isotope ¹⁰⁰Mo, which has an abundance of 9.6% in natural molybdenum, can be transmuted to ¹⁰¹Mo by slow neutron irradiation. ¹⁰¹Mo and its daughter product, ¹⁰¹Tc, both have beta-decay half-lives of roughly 14 minutes. The end product is stable ¹⁰¹Ru.

Rhodium

In addition to being a fission product of uranium, as described above, another way to produce rhodium is to start with ruthenium, which has a price of $1860/kg, which is much lower than rhodium's $39,900/kg. The isotope ¹⁰²Ru, which forms 31.6% of natural ruthenium, can be transmuted to ¹⁰³Ru by slow neutron irradiation. ¹⁰³Ru then decays to ¹⁰³Rh via beta decay, with a half-life of 39.26 days. The isotopes ⁹⁸Ru through ¹⁰¹Ru, which together form 44.2% of natural ruthenium, could also be transmuted into ¹⁰²Ru, and subsequently to ¹⁰³Ru and then ¹⁰³Rh, through multiple neutron captures in a nuclear reactor.

Rhenium

The cost of rhenium as of January 2010 was $6,250/kg; by contrast, tungsten is very cheap, with a price of under $30/kg as of July 2010.^[3] The isotopes ¹⁸⁴W and ¹⁸⁶W together make up roughly 59% of naturally-occurring tungsten. Slow-neutron irradiation could convert these isotopes into ¹⁸⁵W and ¹⁸⁷W, which have half-lives of 75 days and 24 hours, respectively, and always undergo beta decay to the corresponding rhenium isotopes.^[4][5] These isotopes could then be further irradiated to transmute them into osmium (see below), increasing their value further. Also, ¹⁸²W and ¹⁸³W, which together form 40.8% of naturally-occurring tungsten, can, via multiple neutron captures in a nuclear reactor, be transmuted into ¹⁸⁴W, which can then be transmuted into rhenium.

Osmium

The cost of osmium as of January 2010 was $12,217 per kilogram, making it roughly twice the price of rhenium, which is worth $6,250/kg. Rhenium has two naturally occurring isotopes, ¹⁸⁵Re and ¹⁸⁷Re. Irradiation by slow neutrons would transmute these isotopes into ¹⁸⁶Re and ¹⁸⁸Re, which have half-lives of 3 days and 17 hours, respectively. The predominant decay pathway for both of these isotopes is beta-minus decay into ¹⁸⁶Os and ¹⁸⁸Os.^[6][7]

Iridium

The cost of iridium as of January 2010 was $13,117/kg, somewhat higher than that of osmium ($12,217/kg). The isotopes ¹⁹⁰Os and ¹⁹²Os together make up roughly 67% of naturally-occurring osmium. Slow-neutron irradiation could convert these isotopes into ¹⁹¹Os and ¹⁹³Os, which have half-lives of 15.4 and 30.11 days, respectively, and always undergo beta decay to ¹⁹¹Ir and ¹⁹³Ir, respectively.^[8][9] Also, ¹⁸⁶Os through ¹⁸⁹Os could be transmuted into ¹⁹⁰Os through multiple neutron captures in a nuclear reactor, and subsequently into iridium. These isotopes could then be further irradiated to transmute them into platinum (see below), increasing their value further.

Platinum

The cost of platinum as of October 2014 was $39,900 per kilogram, making it equally as expensive as rhodium. Iridium, by contrast, has only about half the value of platinum ($18,000/kg). Iridium has two naturally occurring isotopes, ¹⁹¹Ir and ¹⁹³Ir. Irradiation by slow neutrons would transmute these isotopes into ¹⁹²Ir and ¹⁹⁴Ir, with short half-lives of 73 days and 19 hours, respectively; the predominant decay pathway for both of these isotopes is beta-minus decay into ¹⁹²Pt and ¹⁹⁴Pt.^[10][11]

Gold

Chrysopoeia, the artificial production of gold, is the symbolic goal of alchemists. Such transmutation is possible in particle accelerators or nuclear reactors, although the production cost is currently many times the market price of gold. Since there is only one stable gold isotope, ¹⁹⁷Au, nuclear reactions must create this isotope in order to produce usable gold.

Gold synthesis in an accelerator

Gold synthesis in a particle accelerator is possible in many ways. The Spallation Neutron Source has a liquid mercury target which will be transmuted into gold, platinum, and iridium, which are lower in atomic number than mercury.

Gold synthesis in a nuclear reactor

Gold was synthesized from mercury by neutron bombardment in 1941, but the isotopes of gold produced were all radioactive.^[12] In 1924, a Japanese physicist, Hantaro Nagaoka, accomplished the same feat.^[13]

Gold can currently be manufactured in a nuclear reactor by irradiation either of platinum or mercury.

Only the mercury isotope ¹⁹⁶Hg, which occurs with a frequency of 0.15% in natural mercury, can be converted to gold by slow neutron capture, and following electron capture, decay into gold's only stable isotope, ¹⁹⁷Au. When other mercury isotopes are irradiated with slow neutrons, they also undergo neutron capture, but either convert into each other or beta decay into the thallium isotopes ²⁰³Tl and ²⁰⁵Tl.

Using fast neutrons, the mercury isotope ¹⁹⁸Hg, which composes 9.97% of natural mercury, can be converted by splitting off a neutron and becoming ¹⁹⁷Hg, which then disintegrates to stable gold. This reaction, however, possesses a smaller activation cross-section and is feasible only with un-moderated reactors.

It is also possible to eject several neutrons with very high energy into the other mercury isotopes in order to form ¹⁹⁷Hg. However such high-energy neutrons can be produced only by particle accelerators..

In 1980, Glenn Seaborg transmuted several thousand atoms of bismuth into gold at the Lawrence Berkeley Laboratory. His experimental technique, using nuclear physics, was able to remove protons and neutrons from the bismuth atoms. Seaborg's technique would have been far too expensive to enable the routine manufacture of gold, however.^[14][15]

See also

• Nuclear transmutation
• Precious metals

References

External links

• Spallation Neutron Source
• Mercury 197
• Mercury 197 decays to Gold 197
• Kolarik, Zdenek; Renard, Edouard V. (2005). "Potential Applications of Fission Platinoids in Industry" (PDF). Platinum Metals Review 49 (2): 79. doi:10.1595/147106705X35263. 
• Kolarik, Zdenek; Renard, Edouard V. (2003). "Recovery of Value Fission Platinoids from Spent Nuclear Fuel. Part I PART I: General Considerations and Basic Chemistry" (PDF). Platinum Metals Review 47 (2): 74–87. 
• Kolarik, Zdenek; Renard, Edouard V. (2003). "Recovery of Value Fission Platinoids from Spent Nuclear Fuel. Part II: Separation Process" (PDF). Platinum Metals Review 47 (2): 123–131.