Radioactivity, its measurement, sources and dangers
Abstract: Radioactivity is the phenomenon whereby a nucleus spontaneously disintegrates, emitting particles of matter and energy. Discovered by Henri Becquerel in 1896, it has had important applications in medicine, the military and, in our case, electricity generation. Radioactivity has harmful effects on humans after certain doses, which vary according to the type of radiation.
What is radioactivity?
Radioactivity is the phenomenon whereby a nucleus spontaneously disintegrates, emitting particles of matter and energy. Not all atoms can do this; a certain instability is required, which only concerns around fifteen elements, the best-known of which is undoubtedly uranium. Preventing significant doses of radioactivity from leaking from nuclear power plants is the central challenge of nuclear safety.
Its first applications were medical: this is the principle of X-rays. X-rays are projected through the body, producing an image. Carbon-14 dating also relies on radioactivity. In fact, this carbon isotope is not stable and, once the organism is dead, its concentration decreases.
Measuring radioactivity
Radioactivity can be measured in different ways.
The objective quantity of radiation, or radioactive activity, is measured in becquerels. It is often compared to mass (Bq/kg) or volume (Bq/L), and is referred to asmass activity. In nuclear accidents, it is a means of quantifying the extent of radioactive releases.
Subjective radiation quantities can be used to assess the severity of irradiation:
- The absorbed dose is quantified in grays (Gy), which correspond to the energy received in J/kg. In the past, the rad was used. 1Gy = 100rad.
- The sievert (Sv) weights the absorbed dose according to the dangerousness of each type of radiation, as well as the risk of cancer. We speak of equivalent dose for an organ or tissue, or effective dose for an organism.
Types of ionizing radiation
When an unstable nucleus disintegrates, it releases radiation capable of transforming the molecules it touches into ions. This is known as ionizing radiation. There are several types:
- Α radiation, like a helium nucleus, composed of 2 protons and 2 neutrons. It’s a heavy particle with low penetrating power: a sheet of paper or even air are enough to stop it.
- Β rays are electrons (known as β- particles) or positrons (β ). Electrically charged, they are stopped by a sheet of aluminum foil or a pane of glass a few millimeters thick. When an electron encounters a positron, they annihilate to form two gamma photons.
- X and γ (gamma) radiation result from the expulsion of a photon, and are of a high-frequency, electromagnetic nature. They are extremely penetrating: it takes thicknesses of lead or concrete to stop them. The difference between X-rays and γ-rays is their origin: the former are produced by electronic transitions, while the latter are produced by nuclear transitions.
There are also neutrons, but they are present in nuclear reactors, so they don’t represent the radioactive risk: if you’re faced with a chain reaction, you’ll have other things to worry about…
A brief history of radioactivity
Radioactivity was discovered in 1896 by Henri Becquerel, while working on phosphorescence. His results were reminiscent of X-rays, discovered the previous year by Wilhelm Röntgen, but he, Pierre and Marie Curie and Ernest Rutherford realized that radioactivity was more complex.
The health risks of radioactivity were unknown for a long time. Many of its inventors, but not only, suffered as a result. Radioactive products were also marketed. Radium, popular as a “tonic”, was prescribed in the form of amulets. There was even a powder made from thorium and radium…
In France, in the 1950s, the discovery of uranium in France triggered a veritable “rush”:
“The hunt for uranium was on. With the French Atomic Energy Commission buying up every mineral discovered in France, a fever gripped the crowds and the Geiger counter, now in the hands of peddlers, became as commonplace as the ballpoint pen and the taxman’s note.INA, Journal Les Actualités Françaises – 10.01.1956, https://www.ina.fr/ina-eclaire-actu/video/afe85006555/la-ruee-vers-l-uranium-prospecteursa-vos-compteurs
Other radioactive antics included attempts at nuclear explosion mining and nuclear fracking. The Plowshare project in the USA had explored the possibility of using nuclear explosions for fracking. This was put into practice by the Gasbuggy project (1967), the Rulison project (1969) and the Rio Blanco project (1973) in Wyoming, USA. The story is in this video.
Exposure to radioactivity: orders of magnitude
Natural radioactivity
The world’s primary source of radioactivity is natural.
Average natural exposure of the French
- Cosmic radiation. From 0.5mSv/year at sea level, this rises to 50mSv/year at altitude and 1000mSv/year in space. Orano estimates that Thomas Pesquet received around 200mSv during his mission on the International Space Station.
- Radon is a radioactive gas produced by the decay of traces of uranium in granite rock. The average radon level in France is 1.43 mSv/year. However, there are major disparities: granitic areas such as Brittany and the Massif Central are far more exposed.
- Telluric radiation comes from radioactive elements in the earth’s crust. These represent an average of 0.62mSv/year.
- Some foods contain radioactive elements, such as potassium 40 or carbon 14. Their irradiation represents 0.55mSv/year.
On average, the French are naturally exposed to 3mSv/year.
Volcanic eruption: a nuclear accident?
Note that the eruption of the Eyjafjöll volcano, from April 14 to 16, 2010, released considerable quantities of radioactive material: “400 tonnes of uranium, and 1,300 tonnes of thorium for a thorium/uranium ratio equal to 3.3, i.e. a source term of 20 TBq of uranium and 16 TBq of thorium“(IRSN)
Banana equivalent dose (BED)
To put the quantity and severity of radioactivity into perspective, we often refer to the “banana equivalent dose” (named after the excellent blog by a nuclear safety engineer, Tristan Kamin) or DEB: a banana contains relatively high levels of potassium 40, which is radioactive and represents an average of 130 becquerels per kg, or 19.5bq for a 150g banana. Its ingestion results in an exposure of 0.12µSv. This is a unit of measurement that immediately shows whether exposure is insignificant.
Artificial radioactivity
In addition to natural radioactivity, there is artificial radioactivity, produced by human activity. For example, a
- A lung X-ray represents an irradiation of 0.3 mSv/year.
- Average irradiation from nuclear power plants in France is 0.01mSv/year.
This represents an average of 1.5mSv per year. All in all, French people are usually exposed to 4.5mSv per year.
The regulatory dose limit for nuclear professionals is 20mSv/year. This is a safety limit, not a health limit. In 2019, 5 people exceeded this threshold, including 4 for … medical and veterinary activities (= scanners, etc.) and none in the nuclear sector. 393,000 workers are monitored, 1/4 have a role involving exposure, and their average individual dose is 0.95mSv/year.
Artificial sources of radioactivity include cigarette smoke and smoke from coal-fired power stations.
Nuclear safety is very demanding when it comes to these things. For example, swimming on the surface of a (modern) pool containing spent nuclear fuel would pose no risk (the heat is normally 25-35°C). Of course, the water cooling system must be functional). Water absorbs radiation and heat. Getting too close to the fuel would be fatal. It’s a bit like swimming with a caged shark: there’s no risk as long as you don’t go into the cage. [To be verified]
Accidental exposure
The effect of radioactivity on humans
Radioactivity alters the state of cells by ionizing their molecules. This can lead to malformation or destruction. The impact varies according to the dose received. The skin begins to redden after 1 Gy. Burn marks appear at 5 Gy and necrosis at 15 Gy. The crystalline lens is damaged if the dose exceeds 4 Gy. Blood cells begin to be affected as early as 1Gy. From 10Gy onwards, the digestive tract is affected, followed by the nervous system at around 40Gy.
“For levels below 100 mSv, no long-term health effects have been demonstrated.” Above 1000 mSv, radioactivity “has a direct effect on health and implies a risk to the life of the person exposed in the weeks and months that follow.” (IRSN)
The main long-term risks considered are the generation of cancers. For example, the Chernobyl accident is thought to have caused around 4,000 thyroid cancers in local children and teenagers at the time of the disaster, due to iodine 131. Iodine tends to contaminate the thyroid gland, an organ that is often poorly developed in children.
Further information
- IRSN, L’exposition de la population française à la radioactivité, https://www.irsn.fr/FR/connaissances/Sante/exposition-population/exposition-population-france-metropole/Pages/1-Exposition-population-France-moyenne-et-variabilite.aspx
- IRSN, What is ionizing radiation? How to protect yourself, https://www.irsn.fr/FR/connaissances/Sante/rayonnements-ionisants-effets-radioprotection-sante/effets-rayonnements-ionisants/Pages/1-rayonnement-ionisant.aspx
- CEA, Radioactivity quantities and units, https://www.cea.fr/comprendre/Pages/radioactivite/essentiel-sur-grandeur-unites-radioactivite.aspx
- AFIS, Radioactivity: what doses? https://www.afis.org/Radioactivite-quelles-doses
- Tristan Kamin, Disintegration: radioactivity and fission, https://doseequivalentbanana.home.blog/2021/05/06/desintegration-radioactivite-et-fission/