SAFE USE OF RADIOACTIVE SUBSTANCES

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 isotopes.

9. RADIATION EFFECTS

1. Acute effects.

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:

• Nausea and vomiting
• Malaise and fatigue
• Increased temperature
• Blood changes.

In addition to these effects, numerous other changes are seen.

1. Delayed effects

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:

• Tissue cancer
• Leukemia
• Bone cancer
• Lung cancer

1. Genetic effects

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.

2. Hazard and toxicity

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 thetoxicity 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).

3. Life shortening

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.

4. Cataracts

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.

9. RADIATION PROTECTION

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.

Safety Procedures for Radiation

Radiation, like fire, is a resource that can be both helpful and, if used incorrectly, extremely dangerous. Radiation therapy, a procedure in which cancer patients receive measured doses of radiation, has proven to be an effective means of treating a wide variety of malignancies. This therapy is effective because the radiation destroys the DNA of the cancerous cells, killing them. Unfortunately, the radiation cannot distinguish between healthy cells and malignant cells. Therefore, it is crucial that the medical facility administering the radiation take careful safety precautions to limit the danger.

Equipment Check

The first safety procedure before radiation therapy is administered is a thorough check of all radiation-emitting equipment. Beam output should be carefully monitored. This will help ensure that the patient does not receive too much radiation due to mechanical error. It is also standard procedure to double-check and verify all dosing instructions prior to treatment. This helps lessen the chance that human error will result in a life-threatening overdose of radiation.

Protecting the Patient

Precautions should be put in place to ensure effective and safe treatment while the radiation is being administered. Radiation therapy is generally given five days a week for anywhere between two and eight weeks. This schedule allows for small, daily doses of radiation. The radiation is directed by angled beams toward the cancerous area of the patient's body. Both of these procedures help minimize the patient's exposure to radiation, as well as limiting damage to healthy cells.

Protecting the Medical Staff

Precautions should be taken to protect those administering the radiation. The procedure should be done in an isolated room. After adjusting the machine and ensuring patient comfort, the medical professional administering the therapy will then leave the room. During the procedure, no one but the patient is in the therapy room. This is a very important precaution because it helps the medical professional to avoid exposure to radiation. Otherwise, on a daily basis over a long period of time, the medical professional would be in danger of developing complications such as cancer or radiation poisoning.

Personal Protection Equipment

Pictures How To Handling Radioactive Isotopes