Material of the month – Plutonium

Materials World magazine
,
29 Mar 2016

‘It is easier to denature plutonium than to denature the evil spirit of man.’ - Albert Einstein, 1946

In 1934, the year Adolf Hitler became Führer, and the SS and Gestapo carried out the Night of the Long Knives, Italian physicist Enrico Fermi and his group at the University of Rome, Italy, announced the discovery of elements 93 and 94. By bombarding uranium with neutrons, the uncharged sub-atomic particles, which had been discovered by Englishman James Chadwick two years earlier, they believed they had discovered two new elements. Fermi named them Ausonium and Hesperium, both after Greek names for Italy, as a tribute to his mother country.

However, not everyone was convinced by Fermi’s results. Ida Noddack, a scientist from Germany, publically suggested that rather than creating heavier elements, Fermi may have actually broken up the nucleus into several fragments. This line of thinking was pursued by Otto Hahn, Lise Meitner and Fritz Strassmann in Berlin. The occupation and annexation of Austria into Nazi Germany in the Anschluss of 1938 led Meitner, an Austrian Jew, to flee to Stockholm. She maintained correspondence with Hahn in Berlin, and by December that year, Hahn told her that he and Strassmann had proof that bombarding uranium with neutrons forms a product containing barium, throwing Fermi’s claim to elements 93 and 94 into dispute. 

This result was perplexing since, until then, the largest fragments to be chipped off a nucleus were alpha particles, yet here was an element created from uranium but with an atomic mass 40% less than the parent material. Meitner, along with her nephew Otto Frisch, calculated that if two new nuclei were formed, they would have a combined mass that was slightly smaller than the parent uranium nucleus – by about one fifth of the mass of a proton. She realised that, with nowhere else to go, this mass must be converted to energy during the splitting of the atom, according to Einstein’s famous formula, E=mc2. They had discovered nuclear fission.

Word quickly spread across the pond and, realising the terrifying possibilities of this new technology, a letter was written to President Roosevelt by physicists Leó Szilárd and Eugene Wigner and signed by Albert Einstein himself. They warned that Germany had the capabilities to develop atomic bombs, and urged the President to begin America’s own nuclear programme in earnest. On 9 October 1941, Roosevelt approved the atomic programme – the Manhattan Project had begun.

Elements 93 and 94 were finally correctly produced, isolated and chemically identified in that same year by a team at the Berkeley Radiation Laboratory in California. Glenn T Seaborg, Edwin McMillan, Joseph W Kennedy and Arthur Wahl succeeded where others had failed by bombarding uranium with deuteron (a deuterium nucleus made up of one proton and one neutron)         in their 60-inch cyclotron. 

McMillan had named the first transuranic element 93 neptunium after the planet Neptune, since it followed uranium, after Uranus. He thought it therefore made sense for the next in the series (element 94) to be named after Pluto. Seaborg called element 94 ‘plutonium’. Other ideas mooted at the time were ultimium or extremium, in the belief these were the final elements on the periodic table. In actual fact, 25 new elements have been added since, including Seaborgium, Meitnerium, Fermium, Einsteinium and Curium. Indeed, four new elements were added at the beginning of 2016, numbers 113 – ununtrium, 115 – ununpentium, 117 – ununseptium, and 118 – ununoctium, although each name is currently a placeholder. 

Plutonium production

It took Fermi and colleagues less than a year to successfully prove the theory of fission in practice, achieving the first self-sustaining nuclear chain reaction in a graphite and uranium pile known as CP-1. However, the need for a headquarters for the Manhattan Project was becoming clear, and Los Alamos National Laboratory was completed in December 1942. Roughly concurrently, DuPont began to construct a plutonium-production plant at Oak Ridge in Tennessee. The air-cooled X-10 Graphite Reactor which extracted plutonium from enriched uranium was based on Fermi et al.’s CP-1 pile but on a much grander scale. Scientists at Los Alamos received the first sample of plutonium from Oak Ridge in April 1944, but it was quickly found that it had a higher concentration of plutonium-240 than the cyclotron-produced plutonium they had been working with up until this point.

Twenty isotopes of plutonium exist, such that atoms can have anything between 228 and 247 nucleons (neutrons and protons). The isotope used by the Manhattan Project and in today’s nuclear reactors is plutonium-239. It’s a highly fissile material, meaning that the nuclei can be easily broken apart by slow moving neutrons, and in doing so produce further neutrons to sustain a chain reaction. The chain reaction can only be sustained, however, if plutonium-239 is in a large enough quantity with the right geometry to form what is called a critical mass. During fission, a fraction of the binding energy, which held the nuclei together, is released as a large amount of electromagnetic and kinetic energy, and much of the latter is rapidly converted to thermal energy. The fission of 1kg of plutonium-239 can explode with the power equivalent to 20,000 tonnes of TNT. It was these sorts of numbers that were scaring scientists.

The plutonium-240 in the samples from Oak Ridge was a big problem. With a high spontaneous fission rate, plutonium-240 raises the background neutrons in the material to such an extent that there is a high risk of pre-detonation of a device made from it, a process named ‘fizzle’. Today, the amount of plutonium-240 dictates its grade – super-grade contains between 3–4%, weapons grade contains less than 7%, fuel grade     7–18% and reactor grade more than 18%.

Nuclear weapons

The first gun-type nuclear weapon, designed at Los Alamos, USA, and code-named Thin Man, was abandoned since the likelihood of pre-detonation was too high. They needed a new strategy. Fat Man, the second bomb design was a more complicated implosion device, involving the compression of a plutonium core into a critical mass by conventional explosive lenses.

The most pressing task for the metallurgists at this point was to find a way to cast plutonium into a sphere for the core of the Fat Man bomb. Inconveniently for them, plutonium has six different allotropes (and another exists at high temperatures and a limited pressure range), all with very different densities and crystal structures, rendering it extremely difficult to machine due to the significant volume changes during phase transitions. The brittle alpha phase occurs at room temperature, but the malleable delta phase, which normally exists between 300–450°C, can be stabilised at room temperature by the addition of certain alloying elements. Aluminium was ruled out since it would amplify the pre-ignition problem through releasing neutrons when bombarded with alpha particles, but gallium was a success. This plutonium-gallium alloy could be hot-pressed into the desired shape, and was coated with nickel to prevent corrosion. 

Dangerous grounds

The first atomic bomb test was detonated at 05:30 16 July 1945 in New Mexico and resulted in an explosion equivalent to 20,000 tonnes of TNT, causing a shockwave that was felt more than 100 miles away and a mushroom cloud 7.5 miles high. It also formed a crater of radioactive glass called trinitite in the desert 75m wide. Following the test, Robert Oppenheimer of the University of California, Berkeley, said, ‘We knew the world would not be the same. A few people laughed, a few people cried. Most people were silent’.

The same Fat Man design with a plutonium core was dropped on Nagasaki on 9 August 1945. It is thought to have killed up to 80,000 people, either instantly in the blast or later from longer-term radiation sickness. Only after this event shocked the world was the identity of this new lethal material introduced to the public in the Manhattan Project’s Smyth Report. 

Toxicity

Back at the Berkeley Radiation Laboratories, Hamilton was studying the effect of plutonium on animals. What started with rats eventually diversified to mice, rabbits, fish and even dogs. Then, from 1945 to 1947, 18 human test subjects, supposedly already with fatal illnesses, were injected with plutonium without informed consent. When word eventually broke out, the public were enraged, but it took until the 1990s for the US government to evaluate the ethics of the programme.

Two things make plutonium harmful – its radioactivity and its heavy metal poisoning effects. Although plutonium is not easily absorbed by the body upon ingestion, inhalation of plutonium dust can cause it to pass into the bloodstream, where it is sent to the bone marrow and liver to be processed. Here, it can remain for decades, and the ionising radiation from plutonium in the body causes radiation sickness, genetic damage, cancer, and eventually, death. Today, miniscule traces of plutonium can be found in all of us, as a direct result of atmospheric and underwater nuclear tests, and a small number of nuclear accidents, but         the levels are harmlessly low. 

Despite the difficulties and dangers associated with handling plutonium, even without the motivation of a nuclear arms race, other applications have been found. The deceleration of alpha particles which plutonium produces within its own bulk produces heat which makes the metal unnervingly warm to the touch. This self-irradiation also causes plutonium to fatigue over time, although the heat produced also partly counteracts this effect by annealing. This emitted heat makes plutonium very useful as a green heat source or thermoelectric generator, particularly in the remote location of space – various space probes as well as the Mars Curiosity Rover have all been fitted with such devices. Also, much closer to home, there was a time when plutonium-238 was used to power pacemakers, although it has now been largely replaced by batteries. 

The nuclear energy stored in plutonium and uranium can be harnessed safely in nuclear reactors to provide millions around the world with low-carbon energy, and today nuclear power provides almost 11.5% of the global energy demand. Although the safe storage of nuclear waste has traditionally been a hindrance of the industry, the PUREX process (Plutonium-Uranium Extraction) reprocesses spent nuclear fuel to form so-called mixed oxide fuel, which can be reused in nuclear reactors, thereby reducing the amount of spent nuclear fuel requiring storage. 

Plutonium is a material with a short and extremely chequered history, but responsible use in nuclear power stations could make it a critical material in the future sustainable energy landscape. As we have seen, the huge amounts of energy which can be derived from plutonium is double-edged sword, with the ability to both destroy and, arguably, to save the planet.

To read a longer version of Anna Ploszajski’s history of plutonium, download the app at app.materialsworld.org