Читать книгу Industrial and Medical Nuclear Accidents - Jean-Claude Amiard - Страница 14

To answer this question, it is necessary to define some essential terms in nuclear physics. An atom, an elementary unit, is made up of a nucleus and electrons that gravitate around it. Each nucleus consists of protons (Z) and neutrons (N), the sum constituting the nucleons (A=Z+N). For the same element, the composition of the nucleus may differ in terms of the number of protons and neutrons, each composition constituting an isotope. Some elements can have many isotopes.

The simplest of the elements, hydrogen, consists of a very abundant isotope with a single proton and a single electron. There are two other isotopes: deuterium (nucleus comprising a proton and a neutron) and tritium (nucleus consisting of a proton and two neutrons). The great difference in the constitution of the nucleus explains the exceptional attribution of a particular name. Hydrogen is the most abundant isotope (99.98%), the other two isotopes being very much in the minority. A contrasting example is cesium, where 40 isotopes coexist with a number of nucleons ranging from 112 to 151.

For a given element, some isotopes are unstable and therefore radioactive. To regain their atomic stability, they must get rid of an electron, a helium nucleus or a photon. These unstable isotopes are called radionuclides.

In hydrogen elements, only the tritium isotope is unstable and radioactive. It recovers its stability by losing an electron and thus transforming into helium. In the case of cesium, only one isotope is stable, cesium 133 (¹³³Cs) containing 78 neutrons. All other isotopes are radionuclides, two of which are important: cesium 134 (¹³⁴Cs) and cesium 137 (¹³⁷Cs) because they are fission products of uranium 235 (²³⁵U).

The return to stability is not always a one-step process. There are often several descendants. For example, there are three main natural families whose leaders are uranium 238 with 14 descendants, uranium 235 and thorium 232 with 11 descendants each.

The toxicity of radionuclides is twofold, chemical toxicity resulting from their elementary characteristics and radiotoxicity from their emission of ionizing radiation. With the exception of uranium, which has a high chemical toxicity, radiotoxicity is considered, for other radionuclides, to be the most dangerous.

As a chemical element, the radionuclide will participate in the usual chemical reactions, disperse into various compartments of the environment and enter living organisms through various pathways (including food). This results in radioactive contamination of the environment and living beings (external and internal contamination).

By definition, any event affecting the entire atom will qualify as atomic. When the event takes place at the nucleus level, the event will be nuclear. The distinction between atomic and nuclear events was abandoned in the 1950s because the term atomic had too negative a connotation and only the term nuclear remained for civil activities. An accident involving releases of radionuclides to the environment will therefore be a nuclear accident. When only ionizing radiation reaches the living being, the event qualifies as radiological. This is the case when radioactive contamination remains external and confined. An accident with an intact radioactive source or linear accelerator will therefore be a radiological accident.

In the case of a nuclear accident, the International Atomic Energy Agency’s (IAEA) International Nuclear Event Scale (INES) will be used as a calibration to estimate the severity of the accident. In the case of a radiological accident, another scale such as the ASN-SFRO or radiological scale should be used instead.