4. Radionuclides

Chapter 4 from BASICS OF NUCLEAR PHYSICS AND OF RADIATION DETECTION AND MEASUREMENT – An open-access textbook for nuclear and radiochemistry students by Jukka Lehto

Primordial (primary) radionuclides, as well as other elements, were formed in the nuclear reactions following the creation of the universe and they have been present in the earth ever since of its birth some 4.5 billion years ago. Due to the high flux of energetic protons and alpha particles, a great number of heavy elements were created in these nuclear reactions. Those elements and nuclides with considerably shorter half-life than the age of the Earth have already decayed away and only those with half-lives comparable with the age of the Earth still exist. These primordial radionuclides can be classified into two cathegories:

• Parent nuclides of natural decay chains, ²³⁸U, ²³⁵U and ²³²Th
• Individual radionuclides of elements lighter than bismuth (Table IV.I.)

Table IV.I Lighter primordial naturally occurring radionuclides

Many of these very long-lived radionuclides were earlier considered as stable ones but as the measurement techniques have developed, their radioactive nature has become apparent. The isotopic abundances to these radionuclides are also presented, as in Table IV.I, because their fractions of the total element do not change in human observation time period. Considering radiation dose to humans the most important of these radionuclides is the ⁴⁰K having a very long half-life of 1.26·10⁹ years and an isotopic abundance of 0.0117%. Since humans have a practically constant potassium concentration in their bodies, their ⁴⁰K concentration is also constant, below 100 Bq/kg. ⁴⁰K contributes to several percentages (5% for Finns) of their total radiation dose.

The three primordial radionuclides ²³⁸U, ²³⁵U and ²³²Th are parent nuclides in decay chains, which end up through several alpha and beta decays to stable lead isotopes. In between there are a number of radionuclides of twelve elements. The half-life of ²³⁸U is 4.5·10⁹ y and it starts a series with 17 radionuclides and the ²⁰⁶Pb isotope is the terminal product (Figure IV.1.) This decay chain is called uranium series and as the mass numbers of the product are divided by four the balance is two.

Figure IV.1. The uranium decay chain, A = 4n+2 (http://www2.ocean.washington.edu/oc540/lec01- 17/).

From ²³⁵U, having a half-life of 7·10⁸ y, starts the A=4n+3 decay chain, called actinium series. There are altogether 15 radionuclides between ²³⁵U and the terminal product, the stable ²⁰⁷Pb isotope.

Figure IV.2. The actinium decay chain, A = 4n+3 (http://eesc.columbia.edu/courses/ees/lithosphere/labs/lab12/U_decay.gif).

The third decay chain starts from ²³²Th, with the half-life of 1.4·10¹⁰ y. This A = 4n chain is called thorium series and it has ten radionuclides between ²³²Th and the terminal product ²⁰⁸Pb.

Figure IV.3. The thorium decay chain, A = 4n (http://www2.ocean.washington.edu/oc540/lec01- 17/).

The uranium series has some important radionuclides with respect to radiation dose to humans. Most important of these is ²²²Rn with a half-life of 3.8 days. Part of the radon formed in the ground emanates into the atmosphere and also to indoor air. When inhaling radon-bearing air the solid alpha-emitting daughter nuclides ²¹⁸Po, ²¹⁴Bi and ²¹⁴Pb may attach to lung surfaces and give a radiation dose. In fact, radon in indoor air causes largest fraction of radiation dose to humans, for example, more than half in Finland. Other important radionuclides in the uranium series are ²³⁸U, ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po, which cause radiation dose to humans via ingestion of food and drinking water.

In nature, there has also been a fourth decay chain, starting from ²³⁷Np, but due to its relatively short half-life of 2.1·10⁶ y, more than three orders of magnitude shorter than the age of the Earth, it has decayed away long time ago. This chain consisted of seven radionuclides between ²³⁷Np and the end product ²⁰⁹Bi.

Cosmogenic radionuclides generate in the atmosphere through nuclear reactions induced by cosmic radiation. The main components of cosmic radiation are highly energetic protons and alpha particles. In the primary reactions of particles with atoms of the atmosphere, neutrons are also formed and these can induce further nuclear reactions. Altogether about forty cosmogenic radionuclides are known and some of them are listed in Table IV.II. These are formed in nuclear reactions of air gas molecules, especially oxygen, nitrogen and argon. The most important from the radiochemistry point of view are radiocarbon ¹⁴C and tritium ³H that are formed in the following reactions:

¹⁴N + n → ¹⁴C + p [IV.I]

¹⁴N + n → ¹²C + ³H [IV.II]

Table IV.II. Important cosmogenic radionuclides.

Since the intensity of cosmic radiation is in the long-term constant, the production of the cosmogenic radionuclides is rather constant. There is, however, great variation at different heights of the atmosphere and at different latitudes. Most energetic particles lose their energy already in the upper parts of the atmosphere. Gaseous cosmogenic radionuclides, such as ¹⁴C (CO₂) and ³⁹Ar, remain in the atmosphere while solid products attach to aerosol particles and deposit on the ground, especially with precipitation.

The most important cosmogenic radionuclide is ¹⁴C that is taken up by plants as CO₂ in photosynthesis. It is widely used in age determination of carbonaceous materials. Furthermore, measurement of some other cosmogenic radionuclides has been utilized in evaluation of transfer and mixing processes in the atmosphere and in the oceans.

During the last 70 years, more than 2000 artificial radionuclides have been produced. These have been obtained in the following ways:

• in nuclear weapon production and explosions
• in nuclear power production
• in production of radionuclides with reactors and accelerators

Nuclear explosions create a wide variety of fission products, transuranium elements and activation products, which are essentially the same as formed in nuclear power reactors. Most important of these are the fission products ⁹⁰Sr and ¹³⁷Cs and isotopes of transuranium elements Pu, Am and Cm. Underground nuclear weapons tests leave the radioactivity mostly underground but in the atmospheric tests the radioactivity first spreads in the atmosphere and eventually deposits on the ground. In the 1950's to 1980's more than four hundred nuclear weapons tests were carried out by the USA, Soviet Union, China, France and the UK. These tests resulted in a heavy local and regional contamination. The explosion clouds of the most powerful tests entered the upper part of the atmosphere, the stratosphere, from where the radioactive pollutants have deposited on a global scale. The highest radiation dose to humans have so far has resulted from radioactive cesium nuclide ¹³⁷Cs but in the long-term the largest contribution to the radiation dose comes from ¹⁴C created from atmospheric nitrogen by neutron activation.

In nuclear weapon production, a source of radionuclides is plutonium production, which is done by irradiating ²³⁵U-enriched uranium in a nuclear reactor. In uranium weapon material production no new radionuclides are formed since only ²³⁵U is enriched with respect to ²³⁸U. The radionuclides formed in plutonium production are essentially the same as in nuclear explosions and in nuclear power reactors. After radiochemical separation of plutonium for weapons material the rest, the high-active waste solution, contains the fission products and other radionuclides than Pu and U. These waste solutions are stored in tanks in the USA and they still wait to be treated before final disposal. In the former Soviet Union, only part of the waste solutions are in tanks while a large fraction was discharged into the environment at the Majak site, first to Techa river and later to Karachai lake. This has resulted in a huge contamination of the area. In nuclear weapons production, there has been two major accidents leading to large environmental contamination. The first occurred in 1957 in Sellafield in the UK where a plutonium production reactor caught fire and released large amounts of radioactivity, especially radioactive iodine. In the same year, a high-active waste tank exploded at the Majak site in Russia and large areas, fortunately mostly inhabited, were contaminated with radionuclides.

The 99.99% of the radioactivity created in nuclear power production is in spent nuclear fuel of which 96% is uranium dioxide, 3% is fission products and 1% is transuranic elements, mainly plutonium. Spent nuclear fuel will be disposed of either after reprocessing or as such into geological formations. In reprocessing the nuclear fuel is dissolved in nitric acid and uranium and plutonium is separated for further use as a fuel while the rest, fission products and minor actinides, remain in the high-active waste solution which is vitrified for final disposal. In addition to fission products (¹³⁵Cs, ¹²⁹I, ⁹⁹Tc, ⁷⁹Se etc.) and transuranium elements the spent nuclear fuel contains long-lived activation products, such as ¹⁴C, ³⁶Cl, ⁵⁹Ni, ⁹³Mo, ⁹³Zr and ⁹⁴Nb, formed in impurities in the nuclear fuel and in the metal parts surrounding the fuel. In addition to radionuclides in spent fuel, also activation and corrosion products, such as ⁶⁰Co, ⁶³Ni, ⁶⁵Zn, ⁵⁴Mn, are formed in nuclear power plants in steel of their pressure vessel and impurities in the primary coolant. These end up in the low and medium active waste and are disposed of in repositories constructed for them. Nuclear power plants also release rather small amounts of liquid and gaseous radionuclide-containing discharges into the environment. Liquid releases contain same radionuclides as found in low and medium active waste while air releases contain gaseous radionuclides, such as ¹⁴C and ⁸⁵Kr. From the final disposal of nuclear waste, the radiation dose to humans will be very small.

There have been, however, three major accidents in nuclear power plants resulting in a large release of radionuclides into the environment. The first one occurred in 1979 in Harrisburg, USA, but only noble gases and other gaseous radionuclides were released from the damaged reactor and no long-term contamination of the surrounding area took place. The second and the largest accident took place in Chernobyl, Ukraine, where a power reactor exploded and caught fire in 1986. This accident caused a severe environmental contamination, not only in Ukraine, Belorussia and Russia, but also in many other countries in Europe. In 2011, several reactors damaged due to tsunami in Fukushima in Japan. Large radioactive releases, about one tenth of that from the Chernobyl accident, ended up to the Pacific Ocean and also to a large area inlands northwest of the plant.

A wide range of radionuclides for research and medical use are being produced in reactors and accelerators. After use, they are mainly either aged or released into the environment. Some of the most important radionuclides used in medical and biosciences and in clinical use are listed in Table IV.III.

Table IV.III. Some important radionuclides used in bio and medical sciences.