Throughout this book an attempt has been made to point out that radiation is of significance to our health. The emphasis, so far, has been placed on detrimental effects, but the positive attributes of radiation and radioactivity have also been discussed. Because of the negative effects, radioactive isotopes have been called poisonous or toxic. The toxicity is related to the types of radiations emitted.
The nuclear power industry is the largest source of radioactive isotopes but plays only a minor role in delivering radiation doses to humans. The medical uses of radiation along with natural background radiation deliver the largest doses.
A number of chemical compounds are dispersed to the environment accidentally or sometimes deliberately. Some of these compounds are very hard to break down. It is, therefore, difficult to rank the toxicity of the different compounds. One of the major problems is the shape of the dose-effect curves at small doses. Environmental problems created by chemical compounds that are not radioactive are often similar to those that are radioactive.
Plutonium has sometimes been classified as the most dangerous element in the world. If plutonium is concentrated into one lump, large enough to become critical, it represents a serious threat. The fact is that, when plutonium is thinly dispersed around the globe, it is not dangerous.
Pu-239 emits α-particles. This means that plutonium does not represent any radiation problem when it is outside the body. The range of α-particles in air is only a few cm. In tissue the range is only a few cells but the ionization density is high, a characteristic of high-LET radiation. Plutonium can enter the body via two routes:
1. Consumption of food.The plutonium received in food will mainly be excreted from the body. The uptake from the intestine into the blood stream is small. Consequently, the plutonium in food is of minor importance.
2. Inhalation.Inhalation of air containing plutonium can lead to plutonium in the lungs and the bronchial tubes. How this isotope is distributed in the respiratory tract and the rest of the body is a function of the size of the particulates and their chemical state. Some of the plutonium will remain in the respiratory tract with a short half-life, some with a long half-life. The more water-soluble the plutonium compound, the more of the isotope that will dissolve in lung fluids and be taken up by the circulating blood. Of that which is taken up by the blood, 45% will be deposited in bone with a biological half-life of 100 years and 45% in the liver where it will have a half-life of 40 years. A very small percentage is taken up by the gonads where it will remain indefinitely.
The conclusion is that it is inhaled plutonium that delivers damage to the body. The most important long-term effect is the risk of cancer (cancer of the lungs, liver and bone). Experiments have been carried out with dogs, rats and rabbits that have breathed air containing plutonium. The doses involved were very large and resulted in lung cancer.
These small animal experiments gave the very important finding that the latent period depends on the dose. The smaller is the dose, the longer the latent period. If (from these experiments) the dose-latent period curve is extrapolated down to a dose region that may occur in a human population exposed to a plutonium accident, the latent period would be significantly longer than the life expectancy of a human.
4.1. Plutonium and the environment
In the Chernobyl accident there was a release of plutonium with a fallout that was concentrated in a region within 30 km of the reactor. Under weather conditions with strong winds, this plutonium dust could be picked up into the air and consequently, present a radiation risk.
Another source for plutonium pollution comes from the many nuclear tests performed in the atmosphere in the 1960s. A certain fraction of the plutonium was not fissioned and resulted in fallout that is assumed to be about 6 tons altogether. If we assume that this plutonium is distributed evenly around the world, over land and sea, this amount of plutonium represents a pollution of approximately 26 Bq/m2. Since most of the atmospheric tests were performed on the northern hemisphere, the plutonium pollution in that hemisphere may be up to 50 Bq/m2.
With regard to plutonium, the following can be concluded:
1. Plutonium dispersed into the environment represents a relatively small or nonexistent health problem.
2. Plutonium material gathered in a lump that can be made critical is extremely dangerous.
3. Plutonium used in reactors can be an important energy resource that does not contribute greenhouse gases to the environment.
In some countries, resources and monies are available for environmental actions. It is important to judge the different proposals for action carefully in order to use resources in the best way. A cost-benefit analysis should always be made.
In the case of radiation, large amounts of money have been spent to reduce the radioactivity in meat. It is known that animals feeding on grass or lichen in polluted areas will take up this activity. For example, Cs-137 was taken up by sheep and reindeer after the Chernobyl accident. The radioactivity reached (in some parts of Scandinavia) several thousand becquerel per kilogram (for the reindeer it was measured up to 100,000 Bq/kg).
Since the biological half-life for Cs-137 in sheep is of the order of 3 weeks, it is possible to feed the animals for a few weeks on nonradioactive food before slaughtering. Considerable amounts of money have been spent in these feeding actions. The radioactivity in the meat was reduced and a threshold limit of 600 Bq/kg was set for selling the meat.
A critical question was not addressed. What is the cost-benefit of this remedial action? The authorities thought the benefits justified the cost, but from a radiobiology point of view, this was a waste of money. The extra radiation doses involved were already very small, even without any remedial action. The only argument that favors this remedial action is a psychological one. People were given the impression that authorities took action in behalf of their safety.