Radiation incidents

Introduction

In August 1945, the first atomic fission bombs were detonated above the Japanese cities of Hiroshima and Nagasaki with devastating effects. Most radiation incidents, however, have been accidental with the most serious occurring in 1986 at Chernobyl in the former Soviet Union when a nuclear reactor unit exploded, dispersing radioactive material over a wide area. One hundred and thirty six people developed the acute radiation syndrome, of which 28 died. The majority of incidents, however, have involved small numbers of people and many have occurred as a result of deliberate bypassing of safety procedures.

There were no deaths from exposure to radiation or cases of radiation sickness following the 2011 Fukushima accident but over 160,000 people had to be evacuated from their homes to ensure this.

In Australia, the Australian Radiation Incidence Registry records all accidents where exposures occur that are not ‘within the limits known to be normal for the particular source of radiation and for the particular use being made of it’. Strict licensing and control systems, coupled with improving technology and training, have helped to minimize the number of Australian radiation incidents.

The advent of terrorism has increased the risk of multiple casualty incidents.

Radiation sources and incidents

Worldwide, the most common radiation sources are

• X-ray equipment: used for medical diagnosis and treatment, industrial and commercial inspections, irradiations and research.

• Accelerators: used for medical treatments, industrial irradiation, the production of radioisotopes and research.

• Radioactive materials: used for medical diagnosis and treatment, industrial radiography, quality control and tracing techniques, soil density and moisture tests, and research. Radioactive material may be unsealed or contained within sealed containers.

• Nuclear processing and reactor plants: used for processing uranium and plutonium for fuel purposes and nuclear weapons, power production and research.

With x-ray equipment and accelerators, the victim may be exposed to radiation but this does not make the tissues radioactive. These patients pose no threat to others, including medical attendants.

Unsealed radioactive material has the potential to cause radioactive contamination. This may be external on clothing or skin or internal following inhalation, ingestion or absorption through body orifices, mucous membranes and wounds. Following internal contamination, radioactive material may become incorporated into the patient’s tissues.

Other than for accidents involving nuclear processing and reactor plants, or nuclear explosions, incidents usually lead to either exposure or contamination.

There are no nuclear reactors in Australia except for the occasional visiting nuclear powered warship. These vessels are closely monitored while in Australian ports.

Measuring radioactivity

Radioactivity of an isotope is expressed as the average number of atoms that disintegrate per second. The becquerel (Bq) is the International System of Unit (SI unit) for one nuclear disintegration per second. The activity of a given mass of a radioactive substance with a short half-life will decrease with time.

Ionization in air caused by radiation can be measured by portable dosimeters to give an estimate of the levels of radioactivity at the site of an incident. This is used to calculate the exposure level of a patient with acute radiation illness. The units used are Roentgens. Dosimeters are also used in hospitals to measure the level of radiation to which staff members have been exposed or to monitor patients during decontamination.

The absorbed dose of radiation is the amount of ionization energy deposited in matter by ionizing radiation. One gray (Gy) is equivalent to one joule per kilogram. The effect of a given dose of radiation depends on the type of radiation emitted and the tissue type irradiated.

Pathophysiology

Radiation damages tissue both directly and indirectly by the production of free radicals from water molecules. Direct damage to cell membranes may cause changes in permeability and the release of lysosomes. Germinal, haemopoietic and gastrointestinal epithelial cells are relatively radiosensitive. The cells of bone, liver, kidney, cartilage, muscle and nerve tissue are relatively radioresistant. The delayed effects of radiation depend on whether the dose is lethal or sublethal to the tissue involved.

Lethal (deterministic) injuries are threshold dependent. Cells are killed when they receive more than a certain radiation dose, which varies with different tissues. Clinical expression occurs when the amount of cell killing cannot be compensated for by proliferation of viable cells. The acute and chronic radiation syndromes are deterministic. The earliest delayed effect of acute radiation injury, cataract formation at about 10 months, is an example of this type of injury.

For sublethal (stochastic) injuries there is no threshold level of radiation and the consequence is based on statistical probability. Sublethal injury to chromosomes is the most important effect of ionizing radiation. Double-strand breaks are not easily reparable, especially if the damage occurs simultaneously to both strands. This results in broken chromosomes with no template for repair. The exposed ends of chromosome fragments may join up at random, resulting in morphological chromosomal abnormalities. Sublethal damage to chromosomes is implicated in the development of tumours. Although the incidence of malignancy in adults is increased by radiation exposure, the age at which malignancies are clinically expressed does not change. The estimated increase in lifetime risk of fatal cancer is 0.008%/mSv of gamma radiation exposure. Therefore an individual who is exposed to 100 mGy (twice the acceptable Australian occupational annual exposure) has a 0.8% increase in the lifetime risk of fatal cancer.

Radiation exposure to the gonads may produce temporary or permanent infertility in men depending on the dose. With temporary infertility, there is preservation of the secondary sexual characteristics. In the female, however, all ova are present at birth and larger radiation doses are required to produce sterility. Radiation-induced infertility in females is associated with premature menopause.

Children are more prone to radiation-induced carcinogenesis because they have a higher number of future cell divisions and a longer life span. The fetus is exceptionally susceptible to radiation injury.

Acute radiation exposure

Radiation exposure accidents usually involve penetrating radiation, such as high-energy x-rays or gamma rays. The effects are primarily due to the loss of cells in the body. Acute exposure is more dangerous than chronic, as it does not allow time for cell replacement or tissue recovery. Clinically, radiation exposure may produce a generalized acute radiation syndrome or a localized irradiation injury.