So chemical of the week this week is my one of favourite elements – Number 92 uranium. I have some personal experience with uranium as in a previous life I worked on a project for BNFL (British Nuclear Fuels Limited) so right away that tells you we are dealing with a radioactive gem. The 1789 discovery of uranium in pitchblende is credited to Martin Heinrich Klaproth, who named the new element after the planet Uranus. Of interest, uranium is the last naturally occurring element in the periodic table hence every element after uranium is referred to as a transuranic element. During her studies Marie Curie investigated the radiation emitted from uranium salts and discovered that the intensity of the rays was in direct proportion to the amount of uranium in the sample. She also found that pitchblende was much more radioactive than uranium itself, and realised that it must contain a new radioactive element subsequently named polonium.
Uranium has two isotopes: uranium-235 and uranium-238 (great element to introduce when studying isotopes in KS4) and it’s uranium-235 that’s radioactive. This means it’s a fissile material that will decay when bombarded with neutrons splitting and producing a lot of energy and more neutrons. Off course if the reaction isn’t controlled (the nuclear energy harnessed) what you get is a chain reaction causing a nuclear explosion. Uranium was the component of one of the atom bombs used in WWII. The bomb, ‘Little Boy’, was made up of pieces of uranium which when fired together by an explosive made a single piece of the correct size to go critical. There is an excellent article this month in BBC Focus magazine about the explosion in Hiroshima giving an insight into the immense devastation caused.
This always makes me think of uranium as a chemical story of two halves – not only does it have the internally locked power to destroy and annihilate but it also can be harnessed successfully to give us clean energy. To illustrate this, complete combustion (or fission) produces ~8 kWh of heat from 1 kg of coal, ~12 kWh from 1 kg of mineral oil and around 24,000,000 kWh from 1 kg of uranium-235 – that means uranium roughly contains 3 million times the amount of energy as coal. If only everything was straightforward, as although there is a huge energy potential with uranium, it must be mined and processed before it is used as a fuel. There is also the huge safety implications of running a nuclear power plant ( terrorist / natural disaster risk) and then what to do with the waste.
Uranium has a half life of 4.5 billion years so it is not going to decay any time soon. Disposal methods range from transmutation ( changing uranium to another element), dissolving the uranium in concentrated nitric acid to the most basic waste management technique for low level waste – burial. Unfortunately this means nuclear power is always going to be contentious issue. Uranium is an element of both hope and destruction. It has profoundly shaped the past, will hopefully change the future and will exist long after we are gone.