Special section: Chain Reaction - Potential nuclear energy source
Increasing world population and improved standards of living have led to a rise in demand for renewable and nuclear sources for fuel. Kevin Hesketh and Jeremy Davison of the UK’s National Nuclear Laboratory examine the potential of uranium and plutonium as energy sources.
The world’s industrial economies are unavoidably dependent on energy, and since the Industrial Revolution fossil fuels have dominated world energy supply. Although fossil fuels are a finite resource, the more pressing limitation is now the capacity of the atmosphere to absorb the CO2 produced by carbon combustion. The threat of global climate change has driven many countries to invest in renewables and nuclear, both of which are low-carbon energy sources. Although both have limitations, with populations on the increase and standards of living constantly improving, future demand for energy is pressing enough that renewables and nuclear will be needed.
Nuclear power has been around since the mid1950s and, despite many setbacks that have slowed expansion, it currently contributes about 16% to world electricity production and about 7% to world primary energy production. The view in many countries is that nuclear capacity must increase to keep pace with demand while keeping CO2 emissions to a minimum. If nuclear expansion becomes a reality, the world will become increasingly dependent on the principal nuclear fissile materials (materials that can sustain a nuclear chain reaction with neutrons of any energy): uranium and plutonium.
Most reactors in the current nuclear reactor fleet use uranium as fuel, which explains why it is such an important material to the global economy. Naturally occurring uranium consists of two main isotopes, U-235 and U-238, with abundances of 0.7% and 97.3% respectively. Only U-235 is fissile and this generates most of the fission energy output. In most power reactors the initial U-235 content is increased by an isotopic separation process, such as gas centrifugation. The U-238 is not entirely passive, and a small proportion of the isotope fissions via fast neutrons with a high kinetic energy of more than one million electron volts. More importantly, U-238 captures neutrons to form a short-lived nuclide U-239 that subsequently undergoes two radioactive decays to Pu-239, which is also fissile. Through this fertile conversion mechanism, nonfissile U-238 is converted to fissile Pu-239, which typically provides 30–40% of the total energy output in current reactors. So although most current reactors are fuelled with uranium, the energy actually comes from a mix of uranium and plutonium.
Not all of the U-235 and Pu-239 is used up during the lifetime of the nuclear fuel, and useful quantities can be recovered if the fuel is recycled in a reprocessing plant. Reprocessing involves the dissolution of spent nuclear fuel and chemical separation of the uranium and plutonium from the highly radioactive fission products accumulated during in-reactor irradiation. All of the plutonium recovered in this way can be recycled, as can a proportion of the U-235. As a result, the amount of energy that can be extracted from uranium can be increased. In fast reactor designs (some of which are currently being pursued in international collaborations) that use high energy neutrons to sustain the fission chain reaction, the useful energy output could, theoretically, be enhanced by about 50 times through indefinitely repeating the cycle of in-reactor irradiation, reprocessing and recycling of the fuel. This is why plutonium, already important in the current reactor fleet, may assume a much higher level of strategic importance in the future. Safe handling will be a key requirement of a future recycle-based nuclear resurgence, but to achieve this it is necessary to gain a better understanding of the materials in question.
Over the last few years, attempts have been made to make videos about each of the elements in the periodic table through a project at the University of Nottingham. Where these could be made, the videos provide good visual impact, and bring greater understanding of both physical and chemical properties to a global audience in a much more dynamic manner.
As materials go, it is often those that are more exotic or difficult to obtain that generate wider interest, whether it is due to their intrinsic rarity value or for their scarcity among the general population. Uranium, and especially plutonium and other related nuclear materials such as neptunium, americium or thorium, fit nicely into this category.
While this was relatively easy to achieve for much of the periodic table, it was difficult to gain access to a number of elements in order to create a useful video. With assistance from the UK’s National Nuclear Laboratory (NNL), videos were made of four radioactive elements – uranium, plutonium, americium and neptunium. Working with small quantities of each element, it was possible to capture their different forms on film.
The aim was to view and handle more than just the metallic elements themselves, which required some preparation. Due to the fissile and radioactive nature of the materials, some routine measures were required alongside some additional precautions, in order to film the elements. State-of-the-art gloveboxes with shielding allowed the materials to be handled and to perform experimental work within them. Further safety requirements included ensuring that the video cameras and other equipment remained clean, with all equipment checked and monitored out of the active area of the laboratory to prevent contamination outside the controlled area.
Some unique footage of rare elements in different forms was captured and subsequently uploaded to the YouTube website, allowing the images to be viewed around the world. The most recent upload captures plutonium on film, one of the first videos of the nuclear material to become available online. You can view this and all of the other videos, including one of uranium, at www.youtube.com/periodicvideos. Further information