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3.3.3 Radioactive contaminants

Contents
3.3.3.1 Introduction
3.3.3.2 Natural, cosmogenic and anthropogenic radioactivity
3.3.3.3 Natural or primordial radionuclides
3.3.3.4 Cosmogenic radionuclides
3.3.3.5 Anthropogenic radionuclides
3.3.3.6 Summarising considerations

3.3.3.1 Introduction

Some objectives of the different surveys include identifying site contaminants, determining relative ratios of contaminants, and establishing DCGLs and conditions for the contaminants which satisfy the requirements of the responsible agency(ies). Identification of potential radionuclide contaminants at the site is generally performed through laboratory analyses, such as alpha and gamma spectrometry. These analyses are used to determine the relative ratios of the identified contaminants, as well as isotopic ratios for common contaminants like uranium and thorium.

Most of this section is based on general technical and statistical concepts for the characterizations of radioactively contaminated sites and/or groundwater for remediation purposes, however, much of the guidance can still be applied to other types of regulations or standards. The purpose of this paragraph is to provide the information required to understand the investigation process described in this section. This information:

  • Summarizes and defines the differences between natural, cosmogenic and anthropogenic radioactivity to understand the differences in origin of the radioactive contaminants.
  • Deals with types of (possible) radioactively contaminated sites and/or groundwaters and with possible sources of contamination.

3.3.3.2 Natural, cosmogenic and anthropogenic radioactivity

Many materials contain some radioactivity, although typically at such a low level that sensitive instruments are required to detect them. The radioactivity occurs in the form of radionuclides derived from two sources:

  • Natural ionising radiation pervades our environment, naturally occurring radionuclides, which can further be classified as either primordial radionuclides (with half-lives comparable to the age of the earth) or cosmogenic radionuclides (produced by the interactions of cosmic radiation with matter).
  • Anthropogenic radionuclides, i.e., those produced by man, can be, as a result of site-specific activities, resulting in radioactive contamination. Notwithstanding this, anthropogenic radionuclides are widely distributed in the environment as a result of different mankind actions.

3.3.3.3 Natural or primordial radionuclides

The most commonly encountered naturally occurring radionuclides are in the decay series originating from 238U, 235U and 232Th. The dominant naturally occurring isotope of uranium is 238U (99.28% natural abundance by mass). 235U constitutes essentially all of the remaining 0.72% by mass of natural uranium. The activity ratio of 238U/235U is approximately 22.

Crystal rocks, on average, contain approximately 25 Bq/kg 238U (equivalent to 2 ppm U) and 30 Bq/kg 232Th (equivalent to 7 ppm Th). Some rocks, such as the granites, can contain significantly higher levels of U: typically of order 16 ppm (200 Bq/kg 238U). The activity of uranium in soils is also variable, and is influenced by the nature of the parent material, the mineralogy of the soil and the geochemical conditions in the soil column.
In the context of this guidance, as examples:

  • A key long-lived member of the 238U decay chain is 226Ra that was used extensively to produce luminising paint. 226Ra decays to 222Rn, a short-lived radioactive gas which, in turn, decays to stable lead via a series of short-lived, predominantly alpha-emitting, radionuclides.
  • 40K is a lighter radionuclide and has a half-life of 1.28×109 years, with an isotopic abundance of 0.0118%. This leads to natural potassium being radioactive, and having an activity of approximately 30 kBq/kg.

Data for concentrations of significant primordial radionuclides in soil is presented in Table 3.8.

Radionuclide Median value in Europe (Bq/kg)
Mean Range

40K 400 140 – 850
238U 35 16 – 110
226Ra 32 17 – 60
232Th 30 11 – 64

Table 3.8 Concentration in soil of significant primordial radionuclides in Bq/kg.

3.3.3.4 Cosmogenic radionuclides

The interactions between neutrons and protons associated with cosmic radiation and atoms of nitrogen, oxygen and argon produces a series of radionuclides, the most abundant of which are 39Ar, 14C, 7Be and 3H (see Table 3.9).

The equilibrium activity of these cosmogenic radionuclides is controlled by their production rate in the atmosphere and their residence times in the atmosphere, in the oceans and in the sub-surface.

All living matter contains carbon of which a proportion is 14C. The relative concentration of 14C is approximately 0.23 kBq/kg of carbon. On the death of the organism, continued accumulation of 14C, and the remaining unsupported 14C decays (with a half-life of 5,730 years); Tritium (3H) is produced naturally in the atmosphere by interactions of fast neutrons with nitrogen.

Radionuclide Concentration in troposphere (mBq/m3)

3H 1.4
7Be 12.5
14C 56.3
39Ar 6.5

Table 3.9 Concentration of cosmogenic radionuclides in the troposphere

3.3.3.5 Anthropogenic radionuclides

Anthropogenic radionuclides are produced as a result of (see also Section 3.3.3.2):

  • Nuclear fission: the splitting of a heavy nucleus, such as uranium or plutonium, by spontaneous reaction, bombardment with neutrons or bombardment with charged particles. This is the process that occurs in a nuclear reactor to generate energy.
  • Activation: the result of irradiation by neutrons. Many radionuclides for medical and industrial use are also produced by this process. Further, in a nuclear reactor, these reactions occur with the fuel, leading to the production of different isotopes, e.g., americium, and with the structural components (e.g., steels and graphite), leading to the production of unstable isotopes such as 60Co and 14C.

3.3.3.6 Summarizing considerations

Each nuclide is characterized by the name (or symbol) of the element and the nuclide’s atomic mass; for example Nitrogen-14 (14N) or strontium-90 (90Sr). Nuclides of the same element with different atomic masses for example, uranium-235 (235U) and uranium-238 (238U) are known as isotopes of the element. Most elements have more than one known isotope, so the total number of nuclides is several times greater than the number of elements.

Most nuclides found in nature are stable, but some nuclides that occur naturally as well as some produced by humans; exhibit the property known as radioactivity. These nuclides are referred as radionuclides.

A nuclide that is radioactive is unstable. The atomic nucleus spontaneously decays, that is it changes into the nucleus of a different nuclide, emitting radiation in the process. This is a random process that cannot be predicted exactly, but the average rate at which nuclei decay and the type of radiation they emit are both characteristic of the radionuclide.

The rate at which a radionuclide decays is called its activity and is equal to the average number of decays per second. The unit of activity is decays per second, which SI-unit is given the special name Becquerel (Bq). 1 Bq is defined as: 1 decay per second.

1 Bq is a very small level of activity, so values are often quoted in multiples such as kBq (103), MBq (106) and GBq (109), although background radiation activities might be expressed in smaller units such as mBq (10-3).

A related measure of the rate at which a radionuclide decays is the half-life, also a constant characteristic of the radionuclide. This is the average time it takes for one-half of the atoms in a sample of the radionuclide to decay. After two half-lives, one-quarter of the atoms will remain, after three half-lives there will be one-eighth left, and so on. After 10 half-lives, the activity will reduce to about one-thousandth of the initial value. Half-lives of known radionuclides range from tiny fractions of a second to many millions of years.

A radionuclide will eventually decay into a stable nuclide, this may take one step or many steps. For some natural radionuclides this “decay chain” can extend through many intermediate radioisotopes, known as daughters, before a stable state is achieved. In some situations the relative half lives of parent and daughter radionuclides are such that daughter radionuclides can quickly accumulate and end up being the same activity as the parent. This is known as secular equilibrium. This is particularly important for some radionuclides such as 137Cs. 137Cs (parent-nuclide) is a beta emitter, however its daughter-nuclide 137mBa is a gamma emitter. Parent- and daughter-nuclides are often treated together when they occur in secular equilibrium.