Nuclear radiations: how harmful

Friday 08 April 2011

HC Goel Recent nuclear reactor disaster that occurred at Fukushima Daiichi power station in Japan has provoked planners, leaders, environmentalists and intellectuals of various countries to rethink about the nuclear energy option.

The point whether one should go for nuclear power has been debated for a long time. In this context the most important concerns are of health and ecology which can make large parts of the globe unfit for survival. Radiation effects depend on a number of factors like radiation dose, dose rate, intensity of radiation, criticality and radio-sensitivity of the exposed parts. Radio-nuclei like iodine-131, cesium-137 and strontium-90 can be released into the atmosphere as a result of explosion or breakdown of a nuclear reactor. These radionuclides enter the body through inhalation or ingestion. These radionuclides then reach their target organs, like iodine is deposited in thyroid, strontium and cesium in bones and lungs. This results in damage to the DNA and leads to mutations which can then cause diseases like cancer.

These radionuclides also contain charged particles like photons, alpha particles and electrons. The density of ionisation and excitation caused by these particles is high and thus results in high damage to human tissues. For example, electrons are light and their energy loss per unit distance travelled is much smaller than that of alpha particles of the same energy. So the energy of electrons is distributed into a greater volume and the impact they cause on human tissues is much smaller. Photons on the other hand reach deep in tissue and require many centimetres of lead to provide an effective shield. Neutrons are uncharged and interact only with other nuclei but they induce changes in atom and molecules they hit. When cell organelles and molecules are hit, many abnormalities may be caused due to ionisation. Water, about 80 per cent of total contents of a cell, gets ionised to form free radicals which interact with target molecules like membranes, biomolecules and biomacromolecules like DNA and cause damages.

The sensitivity of different organ systems to radiation also differs significantly. The assessment of risk is done in terms of weighting factor (wT). The maximum wT is for gonads 0.2, followed by bone marrow, stomach, colon, and lung (0.12 for each), bladder, breast, liver, thyroid and esophagus (0.05 for each), skin and bone surface (0.01) while it is 0.05 for the rest of the body.

The Biological damage rendered after a whole body irradiation is categorised into three: acute, delayed and genetic effects. The acute effects are expressed in short time to few days. Nausea, vomiting, diarrhoea, loss of appetite and uneasiness are initially noticed and depending on the radiation dose, the severity of symptoms may increase. The radiation dose yielding 50 per cen lethality (LD50) in the exposed human population, is considered at about 3.0 - 3.5 Gray (Gy) and people engaged in rescue operations are often allowed to receive doses limiting to 0.5 to 1.0 Gy. By two weeks time after an exposure of 3.0- 4.0 Gy the symptoms of the failure of bone marrow may start appearing distinctly and may lead to death within a month due to haemopoietic failure. Higher doses (more than 5 Gy) may lead to the failure of gastrointestinal system leading to acute diarrhoea and ultimately death within two weeks as happened in case of the exposure of the scrap dealer at Mayapuri in Delhi in 2010. Higher radiation doses lead to failure of central nervous system and death ensues within 2-3 days.

Delayed effects include the damage manifesting in few weeks to months and years after the exposure. The radiation exposure to a mother carrying a foetus may cause abortion, organ abnormalities especially of the bones and brain, depending on the time of exposure during embryogenesis. The effects on the eye are especially on the lens which gets cataract due to loss of transparency. The other delayed effects include cancer in thyroid, bones, breast, lungs and skin.

The genetic effects due to radiation exposure appear in the next generation. Higher incidence of congenital defects like abnormalities in bones, malformation of vital organs including reproductive organs, leukemia, thyroid and bone cancers. Fortunately a number of deleterious abnormalities in the genetic material are lost from expression in terms of living beings due to death of such abnormal embryos within the womb itself. This is an important safeguard against the increasing accumulation of genetic mutations. The frequency of mutations occurring in the genome of a population is called the ‘Genetic load’. In the natural environment the background radiation cause mutation yield. Radiation accidents lead to a further increase in the genetic load. At present there are no methods available to reverse the increased genetic load.

It is also well known that radiations and radionuclides are exploited for useful activities like energy generation, sterilization of medical and food products, diagnostics and radiation therapy of cancer etc. However radiations can also cause a real disaster to life and environment especially the ecosystem. It can be a great threat not merely to the present generation but also the future generations and human race itself.

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