SAFE USE OF RADIOACTIVE SUBSTANCES
(DANGEROUS GOODS CLASS 7)
1. INTRODUCTION
Radioactive substances occur in many applications in industry. Radiography of welds, thickness measurement of pipe walls and electroplated coatings, irradiation of food and wool to kill pests, are just a few of these. Some products such as smoke alarms contain radioactive materials. Medical applications are probably the main area of use in industry.
Radioactive substances used in industry may be in the form of metals, powders or aqueous solutions of dissolved salts.
Radioactive substances are a group of Dangerous Goods, that are not usually controlled under the Dangerous Goods Act in Victoria. The Health Act and the Radioactive Substances Act apply to these materials.
To understand radiation safety, one should have an understanding of the structure of the atom.
It is believed, based on various evidence, that matter is made up of atoms, consisting of a nucleus, where mass is concentrated. This is surrounded by an area containing (a cloud of) electrons (negatively charged particles).
In ‘stable’ isotopes, the nucleus contains equal numbers of protons (positively charged particles), and neutrons (particles of no charge). The charge due to electrons around the atom, is normally ‘balanced’ against the total charge on the nucleus due to the protons.
The arrangement and number of electrons around the nucleus, gives the atom its ‘chemistry’. An element is identified by the chemical reactions it undergoes with other materials, as determined by the electrons. (An ‘element’ e.g. sodium, is believed to be unable to be reduced to a simpler form and is a basic building block of matter).
An element is normally composed of a number of isotopes, all of which emit radiation. The most common isotope of an element, has the longest ‘half-life’, and is considered to be ‘stable’. (All the isotopes of an element have the same chemistry.) This means that commonly occurring materials can be radioactive, if the elements in them are made up of radioactive isotopes which are ‘unstable’. This phenomenon can be caused by irradiating materials in a reactor.
Atoms, which have a different number of neutrons from the number of protons (in the nucleus), are considered to be ‘unstable’ isotopes, as they will often emit particles or other forms of radiation. Atoms, which spontaneously emit particles containing protons, are also considered to be ‘unstable’. In this case the ‘chemistry‘ changes (as the electron cloud adjusts to balance the charge on the nucleus), and a different element can be formed. These phenomena are called transformations.
Uranium, which can undergo transformations to various isotopes, forms different elements (through different pathways), eventually changing to lead. During the transformations a variety of particles, and other radiation, are emitted.
This transuranic series, is the basis of the atom bomb, and the fission reactor. These applications, are based on a transition of uranium 235 to other isotopes by emission of neutrons. The ‘slow’ neutrons emitted, can cause other uranium atoms to become unstable and emit more neutrons. This can result in an accelerating reaction, accompanied by a large release of energy. This chain reaction is known as a fission reaction as it involves ‘splitting the atom’.
Another transformation is based on hydrogen. Hydrogen undergoes a fusion reaction, to form a ‘heavier’ nucleus, with release of a large amount of energy. So far no way has been found to control this reaction, however it promises a way to develop ‘safe’ nuclear power, as the other isotopes formed during the reaction have very short ‘half lives’, compared with those from the uranium based materials.
A method of detonating a hydrogen bomb using a laser, has been developed by the French. This has potentially enabled use of the device, where a uranium-based bomb would be intolerable due to the long half-lives and amounts of isotopes formed during the explosion. The Sun is the largest local example of the fusion reaction of hydrogen.
It has been found that, for any particular element, the number of neutrons within the nucleus is not constant. Oxygen for example, consists of three nuclear species; one whose nucleus has eight neutrons, one of nine neutrons, and one of ten neutrons. The atomic masses of these three species are 16, 17, and 18 respectively. These three nuclear species of the same element are called ‘isotopes’. Most elements contain several isotopes. As we said before, these isotopes cannot be distinguished chemically, since they have the same electronic structure.
Radioactivity may be defined as spontaneous nuclear transformations that may result in formation of new elements. These transformations are accomplished by one of several different mechanisms, including alpha particle emission, beta particle and positron emission, neutron emission, and orbital electron capture.
Radioactivity and radioactive properties of nuclides are determined by changes within/to the nucleus only, and are independent of the chemical and physical states of the isotope.
2. TYPES OF RADIATION
An alpha particle is a highly energetic helium nucleus, emitted from the nucleus of a radioactive isotope. It is a positively charged, massive particle, consisting of an assembly of two protons and two neutrons.
Alpha particles are extremely limited in their ability to penetrate matter. The dead outer layer of skin is sufficiently thick to absorb all alpha radiations from radioactive materials. As a consequence alpha radiation from sources outside the body do not constitute a radiation hazard. In the case of internally deposited alpha-emitting isotopes however, the shielding effect of dead skin is absent, and the energy of the alpha radiation is dissipated in living tissue. For this reason alpha radiation is highly toxic, when allowed to enter the body. Radon gas encountered in uranium mines, is an example of an alpha emitter, which poses a significant risk when inhaled.
A beta particle is an ordinary electron that is ejected from the nucleus of a beta-unstable radioactive atom. It is believed that the beta particle is formed at the instant of emission, by the transformation of a neutron into a proton, in the nucleus.
Beta radiation has the ability to penetrate tissue to varying depths, depending on the energy of the beta particle, and may be an external radiation hazard. Beta rays give rise to highly penetrating X-rays called ‘Bremsstrahlung’ (secondary radiation) when shielding stops them. Unless shielding is properly designed, and other precautionary measures adopted, beta radiation may result an external radiation hazard through this effect. Any beta-emitting isotope is potentially hazardous when deposited in the body in amounts exceeding those considered to be ‘safe’.
A positron (positive electron) is a beta particle whose charge is positive, the nucleus may under certain conditions achieve stability by emitting a positron. Whereas negative electrons occur freely in nature, positrons have only a transitory existence. They occur in nature only as the result of interaction of cosmic rays with the atmosphere.
Since positrons are a type of electron, the radiation hazard from the positrons themselves, is similar to the hazard from beta particles. The gamma radiation (X-rays) resulting from the annihilation of the positron, however, makes all positron emitting isotopes potential external radiation hazards.
Gamma rays are single wavelength electromagnetic radiations, emitted from nuclei of excited atoms following radioactive transformation; they provide a mechanism for ridding excited nuclei of their excitation energy. X-rays are indistinguishable from gamma rays.
Gamma rays emitted from isotopes are usually low energy and normally only present a risk when allowed to enter the body.
Care must be taken when designing shielding for gamma rays (X-rays), as the radiation passing through the shielding consists of the higher energy radiation. The radiation is considered to be ‘hardened’.
Different types of radiations have different energies and other attributes, which change the way they affect human tissue etc., a feature known as quality factor (QF) is assigned to the various types. Alpha particles have a QF of 20, beta particles have a QF of 10, gamma rays have a QF of 5. This factor is used when calculating the dose equivalent for a given radiation dose.
The calculation of dose equivalent is as follows:
Dose Equivalent = Dose X Quality Factor X N
(where N is a number , 1 for all radiations except neutrons, when the value 10 is used)
3. HALF-LIFE
Different isotopes are transformed at different rates, and each isotope has its own characteristic transformation rate.
The time required for any given isotope to decrease to one half of its original quantity, is a measure of the speed with which the isotope undergoes radioactive transformation. This period of time is called the ‘half-life’. Half-lives of radioisotopes range from microseconds, to billions of years.
4. ACTIVITY
The unit for quantity of radioactivity is the becquerel. It is based on activity of the isotope. The becquerel is that quantity of radioactive material in which one atom is transformed per second. (An older unit used for this quantity is the curie.)
It must be emphasized that, although the becquerel is defined in terms of a number of atoms transformed per second, it is not a measure of rate of transformation. The becquerel is a measure only of quantity of radioactive material.
5. ABSORBED DOSE
The fraction of the energy in a radiation field that is absorbed by the human body, is energy dependent. It is necessary to distinguish between radiation exposure and radiation absorbed dose. ‘Exposure’ is assessed by measuring the degree of ionisation of air in a radiation field.
Radiation damage to tissue depends on the absorption of energy from the radiation, and is approximately proportional to the concentration of absorbed energy in the tissue. The basic unit of radiation dose, is expressed in terms of absorbed energy per unit mass of tissue. This unit is called the gray, and is defined as an absorption dose of one joule per kilogram.
(An older unit called the rad, which is equivalent of 100th of a gray, has been replaced by the gray, in common use.)
6. EFFECTIVE HALF-LIFE
The half-life of an isotope which is introduced into the body, is dependent on two factors:
In most instances biological elimination follows first-order kinetics (the rate of elimination is proportional to the amount originally introduced).
7. DOSE COMMITMENT
The dose commitment is defined as the absorbed dose from a given practice or from a given exposure. The commitment concept is applicable to external radiation, as well as to radiation from internally deposited isotopes.
8. DOSE-RESPONSE CHARACTERISTICS
Similarly with other noxious agents, radiation effects can be broadly grouped into two categories, namely stochastic and non-stochastic effects.
Stochastic effects are those, which occur by chance, and occur in unexposed individuals, as well as exposed persons. Stochastic effects are not unequivocally related to exposure to a noxious agent, as is acute poisoning in most cases. The main stochastic effects are cancer and genetic damage.
Most biological effects fall into the category of non-stochastic effects. These effects are characterised by three qualities:
Because of the minimum-dose that must be exceeded, before an individual shows the effect, non-stochastic effects are also called threshold effects.
In an experiment to determine a dose-response curve, the 50% dose (the dose to which 50% of exposed animals respond), is considered to be a statistically reliable measure of the relative effectiveness of a particular agent, in eliciting a particular response.
When death of the experimental animal is the biological end point, this 50% dose is called the LD-50 dose. If 50% of the experimental animals die within 30 days, we refer to the LD-50/30 day dose. This index is widely used by toxicologists to designate the relative toxicity of a substance.
Acute whole body radiation affects all the organs and systems of the body. Since not all organs and organ systems, have equal sensitivity to radiation, the response (or disease syndrome) in an overexposed individual depends on the size of the dose.
Certain common effects include:
In addition to these effects, numerous other changes are seen.
The delayed effects of radiation may be due to a single large overexposure or continuing low-level exposure.
Continuing overexposure can be due to external radiation fields, or can result from inhalation or ingestion of a radioisotope, which becomes fixed in the body (e.g. the ‘bone-seeker’, strontium 90).
Delayed effects include:
Genetic information necessary for the production and functioning of a new organism is contained in the chromosomes of the germ cells – the sperm and the ovum. All the cells in the human body contain the same genetic information.
The units of information in the chromosomes are called the genes. The genes consist of chemical building blocks called amino acids which make up an enormously complex macromolecule called deoxyribonucleic acid (DNA).
The genetic information can be altered, by various chemical and physical agents. These ‘mutagens’ can disrupt the sequence of amino acids in the DNA molecule. (There is a genetic repair mechanism, based on biological ‘backup’ of the genetic code, which can rectify the disruption in many cases, sometimes this does not seem to work.)
If the disrupted molecule is in the germ cell , and is subsequently fertilised, the new individual will carry a genetic defect, or mutation. Such a mutation is called a point mutation, as it results from damage to one point on a gene. Most geneticists believe the majority of such mutations in man are undesirable or harmful. This effect however, is probably responsible for biodiversity on earth, and may provide the basis for adaptation to a changing environment. Notwithstanding this, the proposal that ‘a little radiation may be good for you’, is probably an unsustainable argument.
Experiments in which lung tumors resulted from radioactivity implanted surgically in the lung, clearly cannot serve as a measure of the hazard from radioactive dusts. They can only serve to indicate the toxicity of a radioactive material after the radioactivity is located at the site of its toxic action.
The hazard from inhaled radioactive dusts (or any other toxic material) must include consideration of the likelihood that the toxic substance will reach the site of its toxic action. The deposition of particles within the lung depends mainly on the particle size of the dust. The retention in the lung, depends on the physical and chemical properties of the dust, as well as the physiological status of the lung (hence an association with cigarette smoking as a confounding factor in many epidemiological studies).
Cancer resulting from overexposure to radiation usually shortens the life span of persons thus overexposed. Radiation in large doses may shorten life span by increasing the rate of physiological aging. Some data suggest an increased death rate from non-specific causes among users of X-rays, however radiation exposure at the levels encountered by radiologists is not high enough to accelerate the aging process to a degree that will cause a statistically significant shortening of life span.
A much higher incidence of cataracts, has been observed among physicists in cyclotron laboratories, who have been exposed to relatively low radiation fields, intermittently and over a long period of time. Atomic bomb survivors, who were exposed to a single large dose of radiation, show similar effects.
The ratio of the amount of energy of 200keV X-rays, required to produce a given effect, to the energy required of any radiation to produce the same effect is known as the relative biological effectiveness (RBE).
The RBE of a specific radiation is established under experimental conditions, however a conservative upper limit for the most important effect, known as the Quality Factor, is used as a normalising factor in adding doses from different radiations.
The sievert, Sv, is the unit of radiation dose equivalent, that is used for radiation protection purposes, for engineering design criteria, and for legal and administrative purposes.
The dose equivalent expressed in sievert, considers the ‘quality factor’ of the radiation as well as absorbed dose, and other factors such as non-uniform distribution (applies to ‘bone-seekers’) and which may influence the biological effect of a given absorbed dose.
An older unit the rem, is 100th of a sievert.
Public policy in dealing with potential risks from technological innovation depends on the perceived risk as well as on the real risk.
In the case of risks from low level exposure to ionising radiation, the stochastic nature of the adverse effects of overexposure, together with their very low probability, implies that the magnitude of the risk can be measured only by studying large population groups.
A study conducted in 1982 by the U.S. National Academy of Sciences Committee, considered mainly somatic effects (cancer). Agreement was reached on dose response relationships for high doses. They also agreed that any increased incidence of cancer due to background radiation would be masked by other factors, to an extent, which would prohibit statistical correlation in studies.
The chief sources of data on which risk estimates are based, are the Hiroshima and Nagasaki nuclear bomb survivors, patients who were exposed to therapeutic doses of radiation for ankylosing spondylitis and for other diseases, and occupationally exposed populations, such as radium dial painters and uranium miners.
The range of risk estimates for fatal cancers from low LET radiation, are as follows:
|
Source |
Cancer deaths per million man-rem |
|
BEIR III (1980) |
|
|
Majority |
67-226 |
|
Minority a. (1 member) |
158 – 501 |
|
Minority b. (1 member) |
10 – 28 |
|
BEIR I (1972) |
115 – 621 |
|
ICRP (1977) |
125 |
|
UNSCEAR (1977) |
100 |
Risk management is very relevant to use of radiation hazards in the workplace. The normal processes involving radioactive materials, have a ‘quality risk’ – the outcome of the process must satisfy customer needs, an ‘environmental risk’ – a risk of causing environmental damage due to release of the substance, and a ‘security risk’ – the risk of theft of radioactive materials. Control of these risk areas must be compatible with the risk of injuring workers. Workers should be conscious of any ‘trade-offs’.
This aspect should be reflected in any administrative control, used when conducting processes involving radioactive substances.
The limitations of the philosophy applied to disposal of radioactive waste – ‘Delay and Decay’ (for long half-life isotopes), or ‘Dilute and Disperse’ (for short half-life isotopes), should always be taken into consideration, when there is a probability of producing a long half-life product from a nuclear reaction. Production of waste containing these materials presents a unique problem which might not be solved.
Engineering control of the environment by occupational hygienists, and by public health personnel is usually based, in the case of non-stochastic effects, on the concept of a threshold dose.
If the threshold dose of a toxic substance is not exceeded, then it is assumed that the normally operating physiological systems can cope with the biological insult from that substance.
This threshold is usually determined from a combination of data from experiments with animals, and clinical activities. It is then reduced by an appropriate factor of safety, which leads to the maximum allowable concentration (MAC), for the substance. The MAC is used as the criterion of safety in environmental control.
The MAC was defined by the International Association on Occupational Health in 1959: ‘The term maximum allowable concentration shall mean that average concentration in air, which causes no signs or symptoms of illness, or physical impairment in all but hypersensitive workers during their working day on a continuing basis, as judged by the most sensitive internationally accepted tests.’
A different philosophy underlies the control of environmentally based agents, such as ionising radiation and radioactive isotopes, which lead to increased probability of cancer and genetic effects.
For the purpose of setting safety standards for radiation, as well as for chemical carcinogens and mutagens, there is no threshold dose for stochastic effects. The dose-response curves for carcinogenesis and mutagenesis, are assumed to be linear down to zero dose.
It is assumed that effects are independent of the dose rate, and that only the total dose is of biological significance. This means that every increment of dose, no matter how small, increases the risk of an adverse effect by a proportional amount.
The basis for control of man-made radiation is the limitation of the radiation dose to a level to a level that is compatible with the benefits that accrue to society and to individuals from the use of radiation.
The system of dose limitation is founded on three basic tenets:
It is emphasized that point number 2, urges that actual operational dose limits for any radiological activity be more restrictive than the maximum recommended dose limit. This means that processes, equipment (such as shielding, ventilation, etc), and other operational factors be designed so that workers do not exceed the operational dose limit.
This operating philosophy is known as the ALARA (As Low As Reasonably Achievable) concept.
To apply the ALARA concept it is recommended that cost-benefit analyses of alternative lower operational dose-limits be made, and selection of that level of radiation protection that optimises the cost of detrimental effects of the radiation, verses the benefits to be derived from the radiation practice.
For individuals in the general public, the ICRP recommends a whole body dose-equivalent limit of 5 mSv (500 mrem) in a year. It is believed that the average dose to members of an exposed group will be less than the dose limit, using this criterion.
The current (1990) ICRP 60 recommendations do not specify either maximum permissible body burdens or maximum permissible concentrations. They specify annual limits on the intake (ALI) of radioisotopes.
The ALI is defined as the annual intake that would lead to an effective committed dose (a 50 year dose commitment) not exceeding 50 mSV (5 rem), and an annual dose equivalent to any single organ or tissue not exceeding 500 mSv (50 rem).
Alan Cotterell
Acotrel Risk Management Pty Ltd
24th August 1999
