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REPOSITORY DOSES AND RISKS |
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Once the disposed radionuclides have migrated through to the biosphere, they have the potential to provide a radiological hazard to any future human populations that exist near the discharge point. The nature and extent of the hazard depends on the type of human behaviour that takes place in the vicinity of the discharge. |
Previous: Radionuclide transport
Exposure Pathways
Once radioactive materials have succeeded in reaching the biosphere, there exists the potential for those radionuclides to give rise to a radiation dose to any humans who come into contact with them. The radiation dose that is received depends critically on the radionuclides present and the nature of the human behaviour that takes place.
For a terrestrial discharge of radionuclides, the most significant hazards are usually to a future generation that sets up a farm community in the vicinity of the discharge. If a farm is set up near the discharge, then the possibility arises that radionuclides could be taken up by grass and crops, and these in turn could be incorporated into animal products (e.g. milk, beef, lamb) if the animals are allowed to graze the grass. The human population could then be contaminated through eating the contaminated crops or animal products.
An important task is to identify all the possible exposure pathways for exposure scenarios such as the farming scenario. In this scenario, some of the most important exposure pathways are:
1. Inhalation of contaminated dusts;
2. Inadvertent ingestion of contaminated soils;
3. Ingestion of contaminated crops;
4. Ingestion of contaminated animal products;
5. Ingestion of contaminated water abstracted from a well;
6. External irradiation;
7. Dermal contact;
8. Uptake of radionuclides through cuts or other wounds.
The relative importance of these pathways varies between the different radionuclides that are present in the contamination.
A diagrammatic summary of the exposure pathways in the terrestrial environment is shown in the following figure.
Where a marine discharge is expected, the exposure pathways can similarly be stated. Some of the most important pathways are:
1. Ingestion of contaminated marine products (e.g. fish, molluscs, shellfish)
2. External dose arising from bathing in the sea;
3. Doses to folk who make use the of any beaches that are near the contaminated waters.
Other Exposure Scenarios - Human Intrusion
The discharge of radionuclides into the terrestrial or marine environments via transport in groundwater is a significant mechanism by which repository radionuclides can return to the biosphere, but it is unlikely to be the only possible return mechanism. Other return mechanisms can exist, depending on the nature of the repository and its location. For example, at the proposed Yucca Mountain site for a repository in Nevada, USA, the possibility of future volcanic activity also needs to be considered, because the Yucca Mountain region of Nevada is a volcanically active region.
Volcanic activity is minimal in the UK, and as such this scenario does not need to be considered for UK repositories. However, there is a class of return mechanisms that needs to be considered for virtually every repository concept, and that class of return mechanisms is referred to as "human intrusion".
Human intrusion is basically what the name says. It is the exposure of future human populations who dig or drill through the repository and come into contact with the disposed wastes. It is always assumed that the diggers or drillers are not aware of the presence of the repository - those who deliberately drill into the repository, with full knowledge that it is there, are excluded from safety analyses. (Why? Because it is assumed that if they have located the repository and know that it is there, they will take appropriate precautions to protect themselves).
So why would people intrude inadvertently into a radioactive waste repository? Bearing in mind that repositories have to store the disposed wastes for thousands or millions of years, it is inevitable that there will come a time when memory of the repository is lost. No-one within a future generation thousands of years from now is likely to know that it is there. Thus all human intrusion at that time is likely to be inadvertent. (The possibility that techniques for detecting the repository are available in the future is neglected). There are various reasons why a human intrusion might occur, the major one being drilling during resource exploration (e.g. oil, precious metals, etc). For this reason, repository locations are chosen in regions of low mineral and fuel resources, so as to minimise the possibility.
Two hazards exist when a human intrusion occurs:
1. The hazard to the intruders themselves;
2. The hazard to future populations who come into contact with drilled materials that are not properly disposed by those who carried out the intrusion.
Both of these exposure scenarios are routinely considered in repository safety assessments. Indeed, for near-surface repositories, for which human intrusion is considerably more important than deep repositories, these scenarios are often the most significant in terms of doses and risks to future populations. For a deep repository, the likelihood of an intrusion is much lower (one of the advantages of deep disposal), and for these the terrestrial discharge scenarios considered at the start of this article are usually more important.
Dose and Risk
When judging the severity of the radiation exposure to the human population, two measures can be used - radiation dose and radiological risk. Radiation dose is (in very simple terms) a measure of the extent of the radiation damage induced in the body's cells and DNA.
Expressing
the size of a radiation dose is most conveniently done by specifying the amount
of energy deposited by the incident radiation.
The basic measure of radiation dose is called absorbed dose and is equal to the amount of energy deposited in a
human organ or
body, divided by the mass of the organ or body volume irradiated.
The unit is the Gray, and one Gray = one joule per kilogram.
One joule is quite a small amount of energy (to heat 1 litre of water by
1 degree Celsius requires 4200 joules of energy), but 1 Gray of
radiation dose is quite a substantial dose.
Other
more sophisticated measures of radiation dose take account of the differing
abilities of the different types of radiation to induce cellular damage (for
example, alpha
particles are more effective than beta particles and gamma rays), and the
different sensitivities of different tissues in the body.
Of particular importance is a measure called the
"Sievert".
The Sievert defines a quantity called
"effective
dose", which takes into account both of these considerations.
Radiological risk assists in the quantification of the major health impact arising from low levels of radiation exposure - the induction of cancer. The risk is basically a measure of the expected probability of a cancer induction in an exposed individual. When radiation doses are sufficiently low (less than about 0.5 Sv), the risk of cancer induction is proportional to the radiation dose received:
Risk = Constant × Dose Received
The constant of proportionality between dose and risk depends on the human population group under consideration. For a "mix" of adults and children, the constant is around 0.06 Sv-1.