Radioactive contamination refers simply to the pre­sence of radioactive material (as opposed to radiation) in places we would rather it was not there.

Ideally, if we are able to contain or seal all the radioactive material on a power station so that there is no possibility of escape, then there would be no contamination to worry about. There would still be the possibility of radiation emitted by the contents penetrating the walls of the container causing a pos­sible external radiation hazard if the radiation is pene­trating enough. It is only when radioactive material is not contained that contamination is present. For example in fuel elements, fission product radioiso­topes are constantly being formed (e. g., Sr-90, 1-131, Cs-137 and the radioactive isotopes of the gases Xe and Kr). Also, radioactive activation products may be produced by neutron bombardment of a number of materials in the vessel, as described earlier. Some of these radioisotopes may eventually escape into the carbon dioxide pressure circuit, e. g., a leaking fuel element can, or corrosion of metal containing the ac­tivated isotopes. Thus the gas circuit becomes contami­nated with radioactive gases and particulate activity.

The contaminated gas is contained within the pres­sure circuit, however, if there is a leak somewhere along the line, the gas with its radioactive contents will contaminate the surrounding area, causing air contamination if it remains^airbofne or surface con­tamination if it settles on surfaces, e. g., walls, floors pipes, etc.

^The maximum permissible doses referred to earlier aVe ‘the total doses from external and internal sources. The CBGB has adopted the policy of measuring doses from external radiation, and where the risk of internal intake of radioactive material occurs, personnel are required to follow laid-down procedures for working in contaminated areas and to use respiratory or other protection where necessary.

The chief danger with contamination is the possi­bility of the radioactive material entering the body and emitting radiation inside the body. Radioactive material can enter the body in the following ways:

• Inhalation — contaminated air breathed in through the mouth or nose and into the lungs, where the contamination may settle out and irradiate the lungs or be absorbed into the bloodstream and taken to various parts of the body. Hence the requirement for breathing protection above certain concentra­tions of contamination.

• Ingestion — into the stomach by eating contami­nated food or by transfer of contamination via the fingers to the mouth and subsequent swallowing. Once in the stomach the radioactive material may be absorbed through the intestinal walls into the bloodstream and carried to various parts of the body. This leads to the ban on eating and drinking inside the controlled area.

• Injection — directly into the bloodstream by contamination settling on a skin break, or by cut­ting oneself with a contaminated object. There is a requirement that no one is allowed to enter a contamination zone without medical department permission.

• Absorption — directly through the skin without a break being present. An example is tritiated water which is known to be absorbed through the skin

walls and into the bloodstream. This leads to the

special requirements for tritium protection.

The method of entry and the contamination hazard depends on the physical form of the contaminant. Dry and dusty contamination presents not only a di­rect contact hazard but also may become airborne. Wet contamination is more easily transferred by con­tact, but wetting dry contamination reduces the air­borne hazard and is an effective method of control. Solid contaminated objects may present little hazard if the contamination is fixed, but there is the danger that if the material is worked, e. g., polished, ground, milled or abraded, the contaminant may become air­borne.

Problems of both radiation and contamination con­trol can be largely dealt with by good design. The ultimate principle in design is to consider every source of radiation and contamination and reduce the levels to minimise the risk to personnel and the public. Un­fortunately this costs money and so a cost benefit analysis is usually performed. In this, a balance is struck between any radiation dose saved by increasing the precautions, and the cost of introducing these.