The term “radioactive waste” can have a chilling effect on listeners, conjuring up images of invisible danger and the high-level radioactive waste generated by nuclear power further evokes images of a hellish material that remains deadly for tens of thousands of years.
The truth is far less dramatic, as explained recently by Andrew Karam in the Bulletin of The American Council on Science and Health. Storage for moderate periods of time significantly tames even high-level waste.
There are 92 natural elements ranging from the lightest, hydrogen, to the heaviest, uranium. Elements are composed of atoms. Every atom has a central nucleus, composed of fundamental particles called protons and neutrons and surrounded by a cloud of electrons.
Almost all the mass of the atom resides in its nucleus. Atoms of the heavier elements have unstable nuclei that attempt to achieve stability by spitting out small bits of themselves in the form of particles/rays (alpha and beta particles and gamma rays). These unstable atoms are said to be radioactive.
An alpha particle comprises two protons plus two neutrons. A beta particle is a high-energy electron emitted by a neutron. Alpha particles would not penetrate through your skin and beta particles can penetrate only millimetres into human tissues. Gamma rays are very penetrating, easily passing through the body. Interaction of these particles/rays with human tissue can cause cancer and genetic effects.
When a radioactive atom (radioisotope) emits alpha and beta particles it changes its elemental nature into another element. The time taken for half of any collection of radioisotopes to disappear is known as the half-life. Thus, if the half-life of a radioisotope is one hour, after three hours only one-eighth of the original radioactive atoms remain.
Radioactive waste is generated in research laboratories, nuclear medicine centres and the nuclear industry. Radioisotopes used in medicine have very short half-lives and the radioactivity of such waste decays to very low levels in a matter of days/weeks. Waste from research activities is usually very low-level, posing negligible risk to human health.
Which brings us to the nuclear industry. Spent nuclear fuel from a reactor core is intensely radioactive and dangerous. This high-level waste is usually processed into a glassy state (vitrification) and the most popular option for its long-term storage is in stable underground geological formations.
The intense radioactivity reflects the preponderance of short half-lived radioisotopes that are disintegrating quickly. This high rate of nuclear decay means the level of radioactivity declines quickly – the radioactivity of spent nuclear fuel reduces to 10-20 per cent of its initial activity within six months of its removal from the reactor and within a few decades the radioactivity reduces by a further factor of two. Radioactive danger is largely gone within 100 years and within a few thousand years the stored spent fuel is little more radioactive that the uranium ore that first came out of the ground to be fabricated into new fuel rods.
Although some radioactivity remains in nuclear waste for over 10,000 years, these extremely long-lived radioisotopes are of low activity, many emitting alpha particles that cannot escape from the waste itself. Many people worry nuclear waste cannot be stored safely for millennia – how can we be confident that radioisotopes will not leach out from underground geological repositories?
But observations of uranium deposits in Oklo, Gabon, Africa reassure us that nuclear waste can be safely stored for millennia.
Scientists noticed in 1972 that radioisotope ratios in the Oklo uranium ore deposit were different from that seen anywhere else in the world. This isotope “fingerprint” indicated that this ore hosted a self-sustaining nuclear chain reaction two billion years ago. This natural nuclear reactor was sited in porous rocks saturated with water, conditions nobody would now choose for a nuclear waste repository. Nevertheless, studies revealed that virtually all the nuclear radioisotopes had remained in place for more than 2 billion years.
The bottom line is that high-level nuclear waste held for a relatively modest time after removal from the reactor is not the hellishly radioactive material of popular imagination and can be stored safely for millennia in geological repositories.
The main problems associated with nuclear waste disposal aren’t scientific or technical. They are mostly political and social, in particular the NIMBY phenomenon – nobody wants a radioactive waste repository close to their homes.
William Reville is an emeritus professor of biochemistry at UCC