Authors: Ben Hale1, Mathew Tsui1, Betsy Collins2, Kim Smith2
1 Winchester College, 2 St Mary’s, Ascot
The misuse of nuclear weapons by nations or terrorists has become a serious possibility. How can we verify the source of any radioactive material seized?
Nuclear forensics allows chemists to work out the origins and attributes of an individual radioactive sample based on the fact that each one will have a unique isotopic composition. Mass spectrometry, optical microscopy, gamma/alpha spectroscopy are some of the techniques used. It is used to trace the origins of unknown specimens, to see whether they originate from bombs, fuel grade or naturally occurring uranium. The specimens are often gathered from accidents, illegal dumping, traces from declared activities, abandoned nuclear material or illicit trafficking. The US State Department used nuclear forensics, in 2006, to trace the origins of nuclear material from China, India and Austria, which they believe were sold to Iran.
Nuclear materials can be sorted into three types:
- Reactor fuel
- Commercial radioactive sources.
Special nuclear material includes the basic building blocks of nuclear weapons: weapons-grade plutonium and highly enriched uranium. Reactor fuel includes low-enriched uranium, reactor- and fuel-grade plutonium, and mixed oxide-grade plutonium. (Mixed oxide fuel is a mixture of plutonium and depleted uranium oxides and can be a substitute for low-enriched uranium.) Commercial radioactive sources are used in medical diagnostics, thermoelectric generators, food irradiators, and radiography equipment.
The first thing when a sample arrives in the laboratory is that it is screened for alpha and gamma particles. By the energy spectrum of the emissions, certain isotopes can be identified and the relative intensities can give an idea of their relative abundances. This can only be used if a significant amount of radiation is given off.
Mass spectrometry is then used to determine the chemical and isotopic ratios. This will give information about the major elements as well as the daughter elements and impurities. The presence of certain impurities provides clues as to where the samples were produced. Every reactor and enrichment plant has different standards and resources so that almost every sample produced will have a different composition making each and every one unique. The ratio of major elements to daughter elements can also be used to determine the age of the sample, by the utilisation of the equation of exponential decay.
Non-destructive techniques are also used, such as optical and scanning electron microscopy, Fourier-transform infrared spectrometry and X-ray fluorescence. Since these do not destroy the sample, these techniques are preferred to mass spectrometry as the sample needs to be dissolved and vaporised.
Many plants have created databases where all the samples produced were recorded by percentage composition and all the fingerprinting so that the sample could be traced in future by forensic analysis.
Analysis does not need to be done on the sample alone. It is vital to track the route the sample has taken, so even tracking the container is a key piece to the puzzle. In one instance, a man in Bulgaria was found with a 2.4 kg lead case with several grams of a fine black powder. Documents with it described it as 99.99% Uranium 235. It was in a glass ampoule wrapped in paper and wax. Analysis on the wood on the outside of the container could be traced to a certain species of tree found in small areas of Eastern Europe. The wax was identified as paraffin, and the yellow colour was due to barium chromate which is an additive rarely used in Western countries: mainly in Brazil, China, India and Eastern Europe.
In the example of a bomb, residue from such an explosion would contain important clues to a material’s origin. These clues include the physical, chemical, and isotopic characteristics that distinguish it from other nuclear or radiological materials and enable researchers to identify the processes used to create a material. After a detonation, however, hair, fibres, and other material near the blast would become contaminated with radioactive material. The problem is to preserve the conventional evidence while removing any radioactive contaminants. Analyzing the materials that accompany a radioactive sample is very important. These so-called route materials-such as containers, fingerprints, fibres, and pollen-supply attribution information about who has handled a sample or the journey it has travelled.
A detailed example
Location: Hong Kong
Situation: an 8.96 kg specimen of a dark grey metal was involved in a sale of nuclear weapons. 10 years later it re-emerged in a US Consulate and it was sent for analysis.
Analysis and Results: On initial inspection it was obvious that the sample was radioactive and there were wear patterns on the surface of the metal. After ?-analysis with an HPGe spectrometer it was shown that the main component of the sample was uranium which was depleted in U-235.
The surface ?-emission rate was 1000 counts per minute. This was almost 10 times lower than would normally be expected from the surface of plain Uranium.
The density of the sample was 17 g cm¬¬-3¬, a value which was considerably less than the theoretical density of Uranium.
The wear on the surface showed that the sample had not been stored in a protective atmosphere. Under these conditions pure Uranium would have been oxidised to such an extent that the surface would no longer appear metallic.
A 9 MeV x-ray radiograph of the interior showed no apparent voids. Qualitative x-ray fluorescence spectrometry using a 241Am excitation source was further complicated as photons were present not just from the source but also emitted from the specimen. However, this did show that the sample was coated in Nickel. As this had only a very small effect on the self absorption of low energy photons it appeared that the Nickel coating had to be 1mm thick or less.
The majority of the material was depleted Uranium that contained 0.3% 235U. The sample was however an alloy that contained 90% U and 10% Mo. Radiochronometry showed that the date of the last chemical purification of the substance would have been in around 1961. The part was found to be made by the national Lead Company based in Albany, New York. The hoax arose as pieces of metal such as this were supposedly almost pure 235U and were often sold for prices in excess of $10,000.
A few shorter examples:
Location: Ulm, Germany
Situation: 202 pellets of radioactive substance were discovered in a bank safe by the police.
Results: the shape suggested that they were nuclear fuel from a light water reactor. The tests showed that the substance was 4.8% uranium-235. Two nuclear fuel plants were found to use such pellets; however the texture of the fuel’s surface allowed the correct plant to be identified.
Situation: A piece of metal found in a scrap yard, was found to be radioactive stainless steel.
Results: It could be identified as part of a reactor. However, it had 87.8% uranium-235, instead of the normal 19%. Over 85% can be used for nuclear weapons – the material found was, therefore, potentially weapons grade uranium that had been illegally dumped.
Nuclear forensics has many important uses – it can be used to determine the origins of radioactive materials and is often used to trace the origins of a sample of uranium. This is a vital use as it can then be determined whether the uranium was of weapons grade standard, enriched or commercial grade. As every sample will be unique due to the conditions in which it has been contained and purified the sample can then hopefully be traced back to its source and, if a felony has been committed, it may eventually be possible to identify the perpetrators.
In today’s society, the misuse of nuclear weapons by nations or – in rare circumstances – individuals has become more of a serious possibility. Advancing and refining the techniques used in nuclear forensics can be seen as critical as this will allow the origins of radioactive materials to be determined with even greater accuracy. Increased detection may eventually restrict the smuggling, dumping and illegal sale of uranium (of various grades) which would help to increase security.
1. “Nuclear Forensic Analysis” – Kenton J. Moody, Ian D. Hutcheon and Patrick M. Grant http://www.iaea.org/NewsCenter/index.html – The International Atomic Energy Agency
2. “Nuclear forensics-a methodology providing clues on the origin of illicitly trafficked nuclear materials” – Klaus Mayer, Maria Wallenius and Ian Ray