Synthesis of noble metals

Synthesis of noble metals refers to the realization of the age-old dream of alchemists—to artificially produce noble metals. The goal of this could be to achieve greater economic gain when compared to traditional methods of obtaining noble metals. Synthesis of noble metals is only possible with methods of nuclear physics, either using nuclear reactors or by particle accelerators. Particle accelerators require huge amounts of energy, while nuclear reactors produce energy, so only production methods utilizing a nuclear reactor are of economic interest.

Rhodium, Ruthenium

Rhodium and ruthenium are noble metals produced by nuclear fission, as a small percentage of the fission products. The radio-isotopes of these elements with the longest half-life, which are generated by nuclear fission have half-life times of 45 days and 373.59 days for rhodium and ruthenium, respectively. This makes their extraction from spent nuclear fuel possible, although they must be checked for radioactivity before use.

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

Ruthenium

Each kilo 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. Ru 103 has a half life of about 39 days meaning that within 390 days it will have effectively decayed to ground state, well before any reprocessing is likely to occur. Ru 106 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 to ground state.

Rhodium

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

Each kilo 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 then the 13.3 grams of rhodium metal would contain 1.3 kBq of ¹⁰²Rh and 500 kBq of ^102mRh. At first glance the rhodium might be adding to the resource value of reprocessed fission waste, but the cost of the separation of the rhodium from the other metals needs to be considered.

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 time of 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.

ilver

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 palladium, rhodium and ruthenium (price of silver in 2005: about 200 €/kg, rhodium and ruthenium: about 300,000 €/kg) the economics change substantially. Silver becomes a byproduct of platinoid metal recovery from fission waste.

Gold

The artificial production of gold is the age-old dream of the alchemists. It is possible in particle accelerators or nuclear reactors. Since there is only one stable gold isotope, Au-197, nuclear reactions must create this isotope in order to produce usable gold.

Gold synthesis from Mercury

Gold obtained by mining has copper and silver as impurities. Gold of higher purity can be made through the photoneutron process:

:Mercury 198 + 6.8MeV gamma ray $o$ 1 neutron + Mercury 197 (half-life 2.7 days $o$ Gold 197)

These energy levels allow a more efficient neutron source than the Spallation Neutron Source.

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 that will be transmuted into Gold, Platinum and Iridium, which are lower in atomic number.

Gold synthesis in a nuclear reactor

Gold was first synthesized from mercury by neutron bombardment in 1941, but the isotopes of gold produced were all radioactive [cite journal | author=R. Sherr, K. T. Bainbridge, and H. H. Anderson | title=Transmutation of Mercury by Fast Neutrons | journal=Physics Review | volume=60 | issue=7 | year=1941 | pages=473-479| format = HTML | url = http://prola.aps.org/abstract/PR/v60/i7/p473_1 | doi =10.1103/PhysRev.60.473 ] .

Gold can currently be manufactured in a nuclear by irradiation either of platinum or mercury. Since platinum is more expensive than gold, platinum is economically unsuitable as a raw material. Only the mercury isotope Hg-196, which occurs with a frequency of 0.15% in natural mercury, can be converted to gold by neutron capture, and following K+-decay into Au-197 with slow neutrons. Other mercury isotopes are converted when irradiated with slow neutrons into one another or formed mercury isotopes, which beta decay into thallium. Using fast neutrons, the mercury isotope Hg-198, which is contained to 9.97% in natural mercury, can be converted by splitting off a neutron and becoming Hg-197, 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 get the Hg-197. However such high-energy neutrons can be produced only by particle accelerators.

References

External links

* [http://www.sns.gov Spallation Neutron Source]
* [http://www.nndc.bnl.gov/nudat2/reCenter.jsp?z=80&n=117 Mercury 197]
* [http://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=197HG Mercury 197 decays to Gold 197]