Conclusion

As we have just seen throughout this book, industrial nuclear accidents can occur at any stage of the nuclear fuel cycle. They are particularly dangerous in nuclear reactors, as several serious accidents have shown. No nuclear industry is spared, nor is any country. Similarly, radiological, industrial and medical accidents do not spare any nation.

For a long time, the nuclear industry, imitating military nuclear power, has sought to hide accidents and minimize their environmental and health consequences. As a result, our knowledge of the Simi Valley, Windscale and many other accidents is very limited. The turning point was taken with the Three Mile Island accident and especially with the Chernobyl disaster. The fact that the USSR no longer existed a few years after the accident, and in particular the appropriation of part of the research in radioecology and radiation protection by academics and independent researchers, explain the spread of more and more varied knowledge.

The studies on the Chernobyl and Fukushima accidents are considerably more numerous than on all other accidents. The bibliographical references in this book bear witness to this. Studies carried out by university teams and independent nuclear research organizations ensure results without suspicion of conflicts of interest.

C.1. Comparison of the Chernobyl and Fukushima accidents

Both accidents are serious and have major environmental, health and socio-economic consequences. It is always difficult to compare two accidents, especially when one is older, such as Chernobyl, even though serious remedial measures still have to be implemented, and the other is at an even more uncontrolled stage. However, it is possible to make some provisional observations and the two major accidents classified at level 7 of the INES, generated different consequences [CHA 15].

Different circumstances

First, they occurred in different circumstances. Chernobyl was caused by a sudden and uncontrolled increase in the nuclear reaction (multiplied by one hundred), resulting in the explosion of the reactor core, the destruction of the building and the fire of the reactor graphite. Fukushima was the result of a loss of power supplies and cooling sources leading to the degradation of nuclear fuel and subsequent core meltdown of three reactors, followed by decompression of containment vessels and hydrogen explosions. The accidents resulted in very significant damage at the two nuclear sites (one reactor destroyed in Chernobyl and four severely damaged reactors in Fukushima).

Different air emissions

Unlike Chernobyl, atmospheric releases of low-volatility (e.g. strontium, ruthenium) and refractory (e.g. plutonium, zirconium, cerium) radionuclides have been limited to Fukushima. Follow-ups have detected low concentrations of 89.90Sr, 110mAg, 140La/140Ba, 95Nb and 238Pu in the samples from the most contaminated areas around Fukushima Daiichi, as well as ephemeral traces of 239Np measured only in Iitate on April 11, 2011.

The majority of authors agree that the Chernobyl accident has quantitatively higher environmental releases than the Fukushima accident. For example, Steinhauser et al. [STE 14] estimated that in almost every respect, the consequences of the Chernobyl accident far exceed those of the Fukushima accident. In both accidents, most of the radioactivity released was down to volatile radionuclides (noble gases, iodine, cesium, tellurium). However, the amount of refractory elements (including actinides) emitted during the Chernobyl accident was about four orders of magnitude higher than that of the Fukushima accident. For Chernobyl, a total release of 5,300 PBq (excluding noble gases) was established as the most commonly cited source term. For Fukushima, Steinhauser et al. [STE 14] estimated a total source term of 520 (340–800) PBq. According to Achim et al. [ACH 14], 10% of the radioactivity emitted into the atmosphere at Chernobyl was caused by the release of 131I and 137Cs.

Emissions into the atmosphere from Chernobyl increased in altitude and therefore spread more quickly worldwide. In contrast, those in Fukushima remained more localized over Japan and a large part (80%) dispersed over the Pacific Ocean near the Japanese coast. Estimates of the quantities of activity released to the environment vary depending on the source. For cesium 137, the estimates of the quantities released at the Fukushima plant shortly after the accident were about one-third of those released at Chernobyl. But gradually the quantities released at Fukushima were estimated to be increasing and would eventually be relatively close to Chernobyl. On the contrary, the quantities of plutonium and strontium released are lower in Fukushima than in Chernobyl and the opposite is true for rare gases such as xenon 133.

Atmospheric fallout and evacuation areas of various sizes

A “leopard spot” pattern of depositions were found throughout Europe following the Chernobyl accident, while atmospheric deposition only reached up to 250 km from Fukushima.

The evacuation area where radioactivity was about 37 MBq.km−2 has a radius of 30 km, an area of 2,830 km2 and includes 135,000 inhabitants in Chernobyl. The same area has a radius of 20 km with an area of 628 km2 and 85,000 inhabitants in Fukushima [RAM 11]. The IRSN [IRS 11b] estimated that, on the 66th day after the accidents, the surface area of soils contaminated by cesium 134 and 137 to more than 555 kBq.km−2 was respectively 10,700 km2 and 874 km2 in Chernobyl and Fukushima (8.5% in Chernobyl) and affected 270,000 and 70,000 inhabitants respectively (26% in Chernobyl). The major problems in Chernobyl are the contamination of the environment and the health status of local populations. The major problems in Fukushima are the radioactive waters that contaminated the ocean until April 2011 and the population’s suffering [CHA 15].

A single accident versus multiple accidents

There is also a significant difference between the two accidents. The Chernobyl accident is a unique accident, having taken place over a short period of time, while the Fukushima accident is multiple involving several reactors and pools and took place over a much longer period of time, still not being definitively completed in fact.

Contrasting radioactive contamination for the environment

However, for some consequences, the Fukushima accident seems more serious than Chernobyl. This is particularly the case for the volume of radioactive waste. Just remember that in Fukushima, three reactors melted and three coriums have to be processed over several decades. This is not to mention the solid and liquid waste still present on the site or in the immediate environment, nor the uncertain fate of three pools containing very large quantities of uranium rods. In addition, the environmental consequences of the Fukushima accident for wildlife appear to be more severe than those of Chernobyl. Unlike Chernobyl, where the terrestrial environment was particularly affected, in Fukushima, as a result of the meteorological conditions and the position of the plant, it was the marine environment that was heavily contaminated, both at the start of the accident by atmospheric fallout, and in the medium and long term by run-off and underground infiltration from the site where many leaks are still present. At soil and forest level, Pumpanen et al. [PUM 16] compared 137Cs concentrations in conifer needles and soils between 1995 and 2013 at two sites affected by the two Chernobyl and Fukushima accidents, one in southern Finland (Hyytiälä) and the other in Fukushima. The vertical distribution of deposited 137Cs in soils was similar in Hyytiälä and Fukushima with the highest concentrations of this radionuclide in the upper layers of the soil. These concentrations have decreased exponentially over time. However, the behavior of cesium in the soil was different between surface and depth in both 1995 and 2013. This shows that 137Cs is still mobile in the upper layers of the soil 27 years after the Chernobyl accident. On the contrary, this radionuclide is more stable in the aerial parts of trees than in the ground and, despite its mobility and active role in metabolism, 137Cs remains in the trees for decades. This means that there is not much exchange of 137Cs between the tissues of the heartwood and those of the bark of the stem. The fate of 137Cs in the physical environment therefore changes over time. Based on an initial analysis of approximately 3,000 articles, Gralla et al. [GRA 14] identified 121 publications that measured cesium 137 levels in food, forage and wood. They found that the global mapping of the impacts of nuclear accidents on the provision of ecosystem services requires a significant increase in peer reviews, including the evaluation of existing gray literature. Assessments should follow a consistent protocol, providing consistent information on the location of sampling and the identification of ecosystem services. In addition, accident studies are necessarily transdisciplinary [GRA 15].

Contrasting effects for flora and fauna

Mousseau and Møller [MOU 14a] summarized observations of effects on wildlife in Chernobyl and Fukushima. Recent advances in genetic and ecological studies of wildlife populations have demonstrated significant genetic, physiological, developmental and physical effects resulting from exposure to radioactive contaminants. The few genetic studies conducted in Chernobyl generally show high rates of genetic damage and high mutation rates. All the main taxonomic groups studied (birds, bees, butterflies, locusts, dragonflies, spiders, mammals) showed a small population size in the highly radioactive parts of the Chernobyl exclusion zone. In Fukushima, population censuses of birds, butterflies and cicadas suggested that abundances were also negatively affected by exposure to radioactive contaminants, while populations of other groups (dragonflies, grasshoppers, bees, spiders) did not decrease significantly, at least during the first summer following the disaster. To date, there is insufficient information to assess the effects on the life history of the various zoological groups, with the exception of insects and birds. The differences observed between Fukushima and Chernobyl may be down to the fact that the maximum exposure was felt at different seasons (early spring and late winter). In addition, the accumulation of multigenerational mutations in populations is higher in Chernobyl than in Fukushima. There has been considerable variation between taxa in their apparent sensitivity to ionizing radiation and this partly reflects life history, physiology, behavior and evolutionary history.

Different health effects

Ahmed and Aly [AHM 14] estimated and compared the cancer risk resulting from radionuclides released into the atmosphere by the Fukushima and Chernobyl accidents. A screening method using the near-field air dispersion pathway was used. The simulation was performed using 131I, 134Cs, 137Cs and 85Kr. The analysis of the results showed that the cancer risk resulting from the Chernobyl accident is higher than that of Fukushima for different atmospheric stability classes (according to the 1961 Pasquill classification [PAS 61] ranging from extremely unstable to moderately stable) and for various downwind distances. At the same downwind distance, the cancer risk for 131I is higher than for 137Cs, 134Cs and 85Kr. Therefore, it is necessary to take into account the characteristics of atmospheric stability in the development of emergency preparedness and response strategies.

C.2. Consequences of nuclear accidents on the physical environment

The first consequence of a serious nuclear accident is large-scale soil contamination. The result is an evacuation of populations and an abandonment of agricultural soils and economic activities over a large area. The most tragic disadvantage is that this situation will last for decades and even, for the most radioactively contaminated lands, for several centuries.

C.3. Ecological consequences of nuclear accidents

In retrospect, now after more than 32 years and 7 years respectively, the major nuclear accidents at the Chernobyl and Fukushima plants provide abundant ecological data. However, ecological studies conducted on fauna and flora chronically exposed to ionizing radiation often yield contradictory conclusions. This is probably caused by the complexity of interspecific relationships within polluted ecosystems, as well as the need for multidisciplinary approaches to interpret observed effects, unbiased but difficult to deploy [IRS 16c].

However, some consequences are obvious, such as the morphological anomalies observed in pine trees in both Chernobyl and Fukushima. The frequency of occurrence of these anomalies should be correlated with the dose absorbed by the trees, but dosimetric estimates are uncertain and do not allow robust dose–response relationships to be established. Any decline in abundance of terrestrial invertebrate populations depends on the species groups studied. Differences in radiosensitivity between species and effects related to chronic exposure over several generations are the main explanations for these differences. A significant number of ecological studies focus on bird communities, including long-term observations in the Chernobyl exclusion zone and in the contaminated territories of Fukushima. The decrease in abundance observed at both sites was correlated with the increase in the ambient dose rate. A dosimetric reconstruction in Fukushima birds showed that the observed effects were consistent with knowledge on the decrease in reproductive capacity related to the increase in absorbed doses [GAR 15]. The abundance of large mammals in the evacuated areas has been the subject of controversy. Some studies observed a decrease, others an increase in abundance. These discrepancies are probably related to observation bias and the fact that individual species have a relatively large range including both highly contaminated areas and possibly others that are much less radioactive.

C.4. Adaptation of organisms to radiation

Møller and Mousseau [MOL 16] looked in the literature for evidence that adaptation to natural radiation or after a nuclear accident exists for some species. They found only a total of 17 suspected cases of adaptation, most based on field experiments with organisms derived only from two or three sampling sites and a single experimental study showing evidence of improved radiation resistance. Finally, these same authors could not find any evidence to support the existence of the phenomenon of low radiation hormesis (a biphasic dose response to an environmental agent characterized by a low-dose stimulation or beneficial effect and a high-dose inhibitory or toxic effect).

C.5. Health consequences of nuclear accidents

In the event of a major accident at a nuclear reactor anywhere in the world, more than 90% of the 137Cs emitted would be transported beyond 50 km and approximately 50% beyond 1,000 km before being deposited. Based on a dispersion model [LEL 12], the risk of contamination by 137Cs particle deposits (> 40kBq.m−2.yr−1) would be greater than 2% per year in the north-eastern United States, Western Europe and Japan. As a result, a considerable number of people would be exposed to contamination, estimated to be greater than 5.106 Bq.m−2.yr−1 in the north-eastern United States, Western Europe, Japan and East Asia. The results are similar for gaseous emissions of iodine 131 [LEL 12]. As already noted in the medical surveillance of survivors of Hiroshima and Nagasaki [AMI 19], there are apparently no specific pathologies related to exposure to ionizing radiation. The effects of radiation are multiple and are responsible for a statistically significant excess of many cancers and pathologies, including those of the cardiovascular system. However, these pathologies may have other causes, which make it very difficult for victims to obtain compensation.

C.6. Social consequences and perceived risk of nuclear accidents

The most serious accidents require the evacuation of a large population. This has had health, sociological and economic consequences that have been largely underestimated in the past. Indeed, evacuated individuals must change their physical and social environment and lifestyle habits. The result is profound psychological imbalance, frequently leading to anxiety, then mental distress and often a drift towards the use of licit and/or illicit drugs. This cost, which is difficult to quantify precisely, is enormous.

The societal impact is not limited to areas directly affected by the nuclear accident. For example, Visschers and Siegrist [VIS 13] conducted two surveys on nuclear power plants for Swissnuclear, one 5 months earlier and another directly after the accident in Fukushima. The longitudinal study was conducted in Switzerland with 790 respondents. Four variables were studied: acceptance, perceived risks, perceived benefits and trust associated with nuclear power plants. These four variables were correlated with each other and determined essentially identical responses before and after the accident. Even after a serious accident, the public can still consider the benefits to be relevant, and trust remains important in determining their perceptions of risks and benefits. However, it should be noted that on May 21, 2017, the Swiss approved by vote a new energy law, which aims to gradually switch from nuclear to renewable energy. Several other countries (Germany, Belgium, Italy) have decided to phase out the use of civil nuclear energy.

C.7. Probability of a new nuclear accident

The question of the probability of a new nuclear accident is still topical and worrying. Several studies are trying to answer this question. In a sample of five random years, the probability of having a major nuclear accident worldwide (INES level 7) is 14%. The risk of at least one nuclear accident, with an INES level greater than or equal to 4, is 67%. However, these figures are subject to uncertainty because of the vague definition of a nuclear accident [HAD 14]. Taking into account the accidents actually observed to date, the probability of at least one serious accident in the next 10 years is about 63% [RAF 88]. According to calculations by Rose and Sweeting [ROS 16], the overall probability of a core meltdown accident in the next decade, in a world with 443 reactors, is nearly 70%. Owing to statistical uncertainty, however, the probability could range from 28% to 95%. The United States, with 104 reactors, has an approximate 50% probability of experiencing a fusion accident within the next 25 years. After the Fukushima disaster, Rose and Sweeting [ROS 16] analyzed all reactor core meltdown accidents and estimated a failure rate of 1 in 3,704 reactor years. This rate indicates that more than one such accident could occur somewhere in the world in the next decade. According to UNSCEAR [UNS 00a], it should be noted that since 1975, the number of accidents that have occurred, with or without clinical consequences for nuclear workers, has been steadily decreasing (Figure C.1). These various probabilities of nuclear accidents do not fully take into account the efforts required, and currently partially realized, by the regulatory agencies to builders following the Chernobyl and Fukushima disasters.

C.8. Costs of civil nuclear accidents

The cost of a nuclear accident is difficult to estimate and varies greatly depending on whether direct costs or all indirect costs are taken into account. According to Sovacool [SOV 08], nuclear accidents caused 4,100 direct deaths and cost more than US$18 billion (2006 value). However, these expenses only concern the repair of the reactor sensu stricto. The actual costs of a serious nuclear accident are significantly higher. They must take into account environmental costs, such as the loss of ecological services, economic costs, such as those generated by the abandonment of local industries and the loss of agricultural land, for several centuries, health costs related to both physical and mental health, and the costs of building new cities to house and treat displaced populations. All of these costs are spread over several generations and are several hundred billion dollars.

C.9. Future of civil nuclear power

After a severe nuclear accident, large geographical areas are unfit for human activities for several centuries and many generations of human populations are displaced. As the consequences of such an accident are immense, if nuclear energy is to have a future, it is essential and urgent that those in charge of this sector make research efforts to understand nuclear accidents, their triggers and their changes. Efforts must also be made to prevent these accidents. Finally, there is a need to significantly improve the management of the acute crisis, corresponding to the course of the accident, as well as post-accident management.

These research and management efforts will be the subject of the third volume of this series on radioactive risk.

images

Figure C.1. Temporal changes in the numbers of accidents and the number of nuclear workers suffering clinical consequences (modified from [UNS 00a]). For a color version of the figure, see www.iste.co.uk/amiard/industrial.zip

Glossary

Absorbed dose:
when radiation penetrates matter, it interacts with it and transfers energy to it. The dose absorbed by the material characterizes this energy transfer. It therefore corresponds to the amount of energy absorbed at one point per unit mass of matter (inert or living). It is directly measurable and is expressed in grays (Gy) (1 Gy = 1 J.kg−1).
Actinide:
heavy elements, with an atomic number between 89 (actinium) and 103. This group corresponds to the filling of the electron subshells 5f and 6d. Example of actinides: uranium, plutonium, thorium, neptunium, curium and americium. All of these elements are radioactive and are alpha emitters. Most actinides have significant radiological and chemical toxicity. Their properties are similar and they are also similar to lanthanides or rare earths (with an atomic number between 57 and 71). Four actinides exist in their natural state: actinium (89), thorium (90), protactinium (91) and uranium (92). It is common to separate the major actinides (uranium and plutonium), present or formed in fuel, from the minor actinides found in the reactor by successive neutron capture.
Aerosol:
suspension of very fine solid or liquid particles in a gas.
Alpha radiation (α):
emission of alpha particles (helium−4 nuclei, highly ionizing but not very penetrating) from the nucleus of an unstable atom (radionuclide). Since particles only transfer their energy over a very short distance and cannot penetrate the outer layer of the skin, alpha radiation can lead to internal radiation.
Americium:
an actinide with atomic number 95 belonging to the transuranic family. The atomic mass isotope 241 can be derived from the decay of 241Pu (14.4-year period) by beta radiation emission.
Atom:
a basic constituent of matter. It is composed of a nucleus (neutrons + protons) and a cloud of electrons. Depending on their electron envelope, atoms can be neutral (normal state), excited (excess energy of some electrons that are in a distant orbit) or ionized (the atom has a number of electrons, too many or too few). At normal temperature, atoms are assembled into molecules.
Becquerel (Bq):
a legal and international unit of measurement used for radioactivity. The becquerel (Bq) is equal to one disintegration of a radioactive nucleus per second regardless of the number, nature and energy of the radiation emitted. The old unit, the curie (Ci), was equivalent to 3.7 x 1010 disintegrations per second or 37 GBq.
Beta radiation (β):
emission of electrons or positrons from the nucleus of an unstable atom (radionuclide). Beta particles can penetrate one or two centimeters of biological tissue. They constitute a danger of internal and external radiation.
Biologically effective dose (BED):
the physical dose corrected by a biological efficiency factor.
Burn-up:
total energy released per unit of mass in a nuclear fuel in megawatts per day and per ton (MW.d−1.t−1).
Chain reaction:
at each fission, 2.5 neutrons are ejected on average, which in turn are likely to cause further chain fissions. If the losses are minimal, we will then have a fantastic energy released in a few fractions of a second. This is the principle of the A-bomb. If, on the contrary, at each fission, there is only one neutron formed, the control of the chain reaction is achieved and this is the principle of the nuclear reactor.
Charge number (Z):
the number of protons contained in the nucleus. It is referred to as Z. It is equal to the atomic number.
Combustion rate:
it corresponds to the percentage of heavy atoms (uranium and plutonium) that have undergone fission during a given period. It is expressed in megawatt days per ton (M.d−1.t−1).
Committed dose:
internal dose received by an organ or tissue within 50 years of the incorporation of a radionuclide.
Confinement:
the physical and/or chemical ability to maintain radioactive and/or toxic elements in a given place or object.
Core:
the central area of a nuclear reactor, containing the fuel assemblies, the heat-transfer system and the moderator, where the chain reaction occurs.
Core:
the central part of the atom, positively charged and composed of protons and neutrons. These two components are strongly linked by attraction forces known as nuclear forces. There are 274 types of stable nuclei and approximately 1,500 unstable nuclei.
Corium:
magma resulting from the fusion of the core elements of a nuclear reactor. It consists of the nuclear fuel, the elements of the fuel assembly and the various core elements with which it comes into contact. Corium only forms during nuclear accidents.
Cosmic rays:
naturally occurring nuclear rays, derived from particles that circulate, endowed with enormous energies, in interstellar space. A distinction is made between primary cosmic rays and secondary rays, derived from the former.
Criticality:
the criticality of an event is the product of the frequency of occurrence and the severity factor. It is also the characteristic configuration of a mass of material that contains fissile elements and geometry such that a fission chain reaction is self-sustaining.
Critical mass:
It is defined as the minimum fuel mass for the exact maintenance of the chain reaction in a reactor.
Curie (Ci):
an old official unit corresponding to the number of disintegrations per second of one gram of radium. 1 Ci = 37.109 Bq and 1 Bq = 2.7.10−11 Ci.
Defense in depth:
a mechanism integrated into the system that limits the production or spread of failures.
Depleted uranium:
it refers to uranium whose isotope 235 content, the only fissile isotope, is lower than its natural level (0.72% by mass). It is mainly obtained, on the one hand, as a co-product of an enrichment operation (approximately 0.3% of 235U) and, on the other hand, as a by-product (1% of 235U) of a spent fuel treatment after passing through the reactor.
Disembodiment:
the extraction of a toxic agent from the organism into which it has entered (incorporation).
Dose coefficient or committed effective dose per unit of incorporation in Sv.Bq1:
a coefficient that takes into account the two weighting factors (Wr and Wt, see Effective dose) as well as the retention functions (biological half-life).
Effective dose:
a dose that takes into account the late risk resulting from the exposure of several organs or tissues of different radiosensitivities. It is the sum of the equivalent doses delivered to the different organs and tissues of an individual, weighted by a factor specific to each organ or tissue (WT). It is expressed in Sievert (Sv).
Electron:
an elementary particle with the smallest electrical charge. It often refers to the negative electron (negatron) that is part of the atom’s electron cloud. The positive electron, or positron, is the symmetrical particle, the antiparticle of the negative electron. It appears in particular in disintegration and in the creation of pairs.
Enriched fuel:
a fuel in which the amount of fissile material is greater than the amount of fuel in its natural state.
Enriched uranium:
in nuclear reactors in the light water sector (where water is the heat carrier), it is necessary to increase the proportion of uranium 235 isotope to 2–5% from 0.72% by mass (its natural content) for fission chain reactions to occur. Enrichment is the process (physical or chemical depending on the process) that separates the isotopes from the uranium to collect an enriched flux (235U isotope content greater than 0.72%, mining uranium content) and a depleted flux.
Equivalent dose:
the product of the dose absorbed into a tissue or organ by a weighting factor taking into account the biological effect related to the nature and energy of the radiation (WR). It is expressed in Sievert (Sv).
Exchangeable organically bound tritium (OBT-E):
tritium present in organic molecules where it has replaced hydrogen in typical radicals (-OH, -SH, =NH). The tritium atoms in the exchangeable organic fraction are very unstable (weak covalent bonds) and can be extracted by simple isotopic exchange.
Fast neutrons:
neutrons released during fission, moving at very high speed (20,000 km.s−1). Their energies are in the order of a million electron volts.
Feedback:
an organization that aims to learn from incidents, malfunctions, accidents or crises to avoid their recurrence. There is no universally accepted definition. Feedback requires a wide range of practices and procedures.
Fertile material:
it can produce a fissile isotope after a nuclear reaction: thorium 232 captures a neutron, giving thorium 233, which is transformed into uranium 233.
Fissile element:
an element that is able to undergo nuclear fission.
Fissile material:
an isotope is said to be fissile if it can be so under the impact of neutrons. Uranium 235 is the only natural fissile isotope. Uranium 233 is a fissile isotope produced from thorium.
Fissionable nucleus:
a nucleus that can undergo fission caused by the absorption of a slow neutron.
Fission product:
during fission, the heavy nucleus that fission breaks into two pieces of unequal size are the fission products. These new nuclei contain too many neutrons to be stable and are therefore generally radioactive, emitting gamma rays, beta rays and sometimes neutrons.
Fuel:
a substance that constitutes the core of a reactor, containing the fissile elements that support the chain reaction within it.
Fuel cycle:
all the steps followed by the fuel from the extraction of the ore to the storage of waste.
Fusion:
two light nuclei fuse to form a heavier nucleus with the ejection of a proton or neutron, or simply a gamma emitter.
Gamma radiation (γ):
emission of photons (electromagnetic radiation), carriers of energy but without charge, by an unstable atom (radionuclide). Gamma radiation is the most penetrating of all radiation.
Gray (Gy):
the unit of absorbed dose, measuring the amount of radiation absorbed by the exposed individual (or object). 1 gray corresponds to an absorbed energy of 1 joule per kilogram of material. International System of Units. The old unit is rad (1 Gy = 100 rad).
Heavy nuclei:
the name given to the isotopes of elements whose number of protons is equal to or greater than 80 (e.g. actinides).
Ion:
an ionized atom (or molecule), having lost one or more electrons, or on the contrary, having gained an additional electron or electrons.
Ionizing:
it refers to radiation capable of ionizing the matter it passes through, i.e. removing electrons from it. Alpha, beta, gamma or neutron radiation is ionizing.
Ionizing radiation:
electromagnetic or particulate radiation with an associated energy greater than 10 electron volts (eV). This associated energy is sufficient to create ions in the matter through which they pass, by adding or removing an electron from a molecule, or by cutting the molecule in half. Below this energy value, radiation is called “non-ionizing” and includes ultraviolet radiation or very low frequency electromagnetic radiation.
Irradiation:
the propagation of radiation to a living organism or material substance.
Linear energy transfer (LET):
the rate of energy loss per unit of trajectory length. It is expressed in KeV.μm−1.
Mass number (A):
the total number of neutrons and protons in the nucleus. It is equal to the atomic mass.
Moderator:
a material formed by light nuclei that slow down neutrons by elastic collision.
Neutron:
a subatomic particle. As its name suggests, the neutron is neutral and therefore has no electrical charge (neither positive nor negative). Neutrons, together with protons, are the components of the atom’s nucleus. For an atom, we denote Z as the number of protons (called the atomic number) and A as the number of protons + neutrons. The number of neutrons is therefore N or A-Z.
Neutronics:
the theoretical and experimental study of the neutron as a nuclear bombing particle.
Non-exchangeable organically bound tritium (TOL-NE or OBT-NE):
tritium directly bound to carbon atoms in organic molecules. The extraction of the organic tritium fraction in the non-exchangeable position requires the breaking of carbon-hydrogen bonds (strong covalent bonds) by combustion.
Nuclear energy:
energy released in nuclear fission and nuclear fusion.
Nuclear fission:
a nuclear reaction in which a heavy nucleus divides into two lighter nuclei. Fission is accompanied by the emission of neutrons, γ rays and a high amount of energy. These forces are enormous but are only exerted at infinitesimal distances (approximately 10−15 m).
Nuclear reactor:
a device in which a nuclear fission chain reaction can be controlled.
Nucleon:
a generic term for the components of the atomic nucleus (i.e. protons and neutrons). The number of nucleons per atom is generally noted “A”, and called the “mass number”.
Nuclide:
it corresponds to a type of atom, or possibly a type of nucleus. A nuclide is characterized by its exact constitution (atomic number Z and mass number A). There are 325 naturally occurring nuclides.
Plutonium:
an actinide with atomic number 94 belonging to the transuranic family. It is formed by neutron capture during the spontaneous fission of uranium.
Rad:
the former official unit of the irradiation dose. 1 Gy = 100 rad and 1 rad = 10−2 Gy.
Radioactive decontamination:
the partial or total elimination of radioactive contamination by means of controlled recovery of contaminants.
Radioactive material:
any material with a specific activity greater than 70,000 Bq.kg−1.
Radioactive release:
releases of radioactive substances into the environment either during normal operation (subject to prior authorization) or in the event of an accident.
Radioactivity/radioactive:
the property of certain chemical elements whose nuclei spontaneously disintegrate to form other elements by emitting particles and/or ionizing radiation (α, β or γ). The terms natural radioactivity and artificial radioactivity are often used to describe the origin of radioactive nuclei.
Radioelement:
a chemical element, natural or artificial, in which all isotopes are capable of disintegrating by emitting radiation.
Radioisotope:
an unstable form of an element that only differs in its number of neutrons, but is chemically identical.
Radiological dose:
energy absorbed per unit mass of biological tissue exposed to ionizing radiation, measured in grays (Gy).
Radionuclide:
a radioactive isotope of an element. It is an atom whose nucleus is unstable (like uranium 238) and whose disintegration is accompanied by the emission of electromagnetic radiation.
Radioprotection:
all measures designed to protect the health of the general population and workers from the harmful effects of ionizing radiation.
Radiotherapy:
a method of local treatment of cancers, using radiation to destroy cancer cells by blocking their ability to multiply. The purpose of irradiation is to destroy all tumor cells while sparing, if possible, healthy peripheral tissues.
Radium:
a chemical element with the symbol Ra and atomic number 88. It is the densest of all alkaline earth metals. There are 25 isotopes of radium. Its most stable radioactive isotope is radium 226 with a physical half-life of 1,620 years.
Radon:
a radioactive gas from the decay of uranium and thorium in the earth’s crust.
Relative biological effectiveness (RBE):
the ratio of the dose of reference radiation (x-rays, 60Co gamma radiation) to the physical dose of the studied radiation producing the same biological effect.
Relative risk (RR):
the ratio of the risk of disease or death in exposed people to the risk of disease or death in unexposed people.
Roentgen:
an old dose unit, physical measure of energy received per cm2 of irradiated tissue.
Sievert (Sv):
the unit of measurement of the effective dose, which makes it possible to report the biological effect produced by a given absorbed dose. From the energy received per unit of mass, the effective dose is calculated by applying coefficients depending on the nature of the radiation and the organs or tissues irradiated. Its most frequently used sub-multiple is the millisievert (mSv).
Source:
a radioactive material that produces radiation for experimental, medical or industrial purposes.
Spent (or irradiated) fuel:
fuel assemblies extracted from a reactor after a period of useful energy production.
Tank:
a container containing the core of a reactor and its heat transfer fluid.
Thermonuclear reaction:
fusions of light nuclei depend on the global agitation of the nuclei, which in turn depends on the increase in temperature, hence their name thermonuclear reactions.
Transuranics:
the name given to chemical elements with an atomic number higher than that of uranium (92). There are seven families of isotopes: uranium, neptunium, plutonium, americium, curium, berkelium and californium.
Tritium:
an isotope, natural or artificial, super-heavy hydrogen (1 proton, 2 neutrons) used in particular in fusion reactions (H-bombs, ITER).
Uranium:
an element with the atomic number Z = 92. A silvery, radioactive, malleable and ductile metal that tarnishes in air and is attacked by steam and acids. It is used as a nuclear fuel and in the manufacture of nuclear weapons. It naturally occurs as a mixture of three main isotopes always associated in the same proportions: 238U (99.27%), 235U (0.72%) and 234U (0.0053%).
Y:
yttrium. Strontium 90 disintegrates into two successive emissions – the first gives rise to yttrium 90 and the second to stable zirconium 90.
Zircaloy:
an alloy of zirconium and one or more other metals (tin, iron, chromium, nickel), which is particularly resistant from a mechanical and chemical point of view. It is used for fuel sheaths in water reactors.