|MadSci Network: General Biology|
Nuclear wastes are safely disposed with maximum precautions. It is a very specialized subject to discuss on the disposal methods. The radioactive waste materials are sealed in multiple layer containers so that neither radiation nor radioactive material can escape out of the container beyond permissible levels. Further, most of the radio-nuclides produced in nuclear fission are short lived. Hence, the waste is stored at the site of the reactors for some time before disposing so that it is easier to handle the waste with lesser activity. Before disposing off the nuclear waste, most of the important radioactive materials are recovered, which are useful in many peaceful applications of radiation in medicine, industries, agriculture and research. High dose exposures to human being is possible only in fatal nuclear accidents and warfare. Most of the human data available now are from such unfortunate incidents. Epidemiological survey of exposed people in such accidents reveals many effects. Depending on the nature, the effects are classified as deterministic and stochastic effects. Deterministic effects are those, which have direct causal relationship with the radiation exposure. They can be usually observed only at high doses such as 1 Sv and more. It includes induction of cataract, radiation sickness, skin erythema (reddening of skin and skin burns), haematopoietic syndromes (blood related), gastro-intestinal syndromes and central nervous system syndromes. All deterministic effects exhibit threshold doses below which there is no clinical manifestation of the effects. The thresholds are different for the above-mentioned effects. None of the deterministic effects are observed below 1 Gy. The severity of the effects increases with exposure. Stochastic effects are only probabilistic in nature and there is no way to distinguish them from normal incidence. All cancers (solid cancers as well as leukaemia) and hereditary effects are stochastic effects. It is found that radiation exposure increases cancer risk. Increased level of all types of cancer is observed in exposed human populations. However, it is interesting to note that no increase in hereditary effects is observed in exposed population. The risk estimation is based purely on animal experiments and it seems to be over estimated. It is also interesting to note that no increase in induction is cancer is evident in occupationally exposed people to low levels of radiation and also in people living in high natural backgroud radiation around the world. The probabilities of cancer induction is very less and inconclusive in such doses. The risk estimates are based on high dose data from nuclear accidents and atomic bomb survivors of Hiroshima and Nagasaki. No doubt, radiation is harmful to all life. Nevertheless, it is possible to suppress the harm by regulatory procedures. ICRP-60 has estimated the risk of radiation as 5% per Sv (sievert) for general public and 4% per Sv for the occupational workers in the age group of 18 - 65 years. Sv is the unit used for estimating radiation quantity. It is a biological equivalent of physical dose unit Gy (gray = 1 joul/kg). For sparsely ionising radiations such as X-rays, gamma rays, beta rays, both Sv and Gy are equivalent. Certain densely ionising radiations such as alpha rays, especially when the activity is ingested or inhaled may cause more damage. For such radiations, more waitage is given and the weighting factor is multiplied to the physical dose to estimate the equivalent dose i.e., Sv. All risk estimates are purely based on stochastic effects. The limits are set such that deterministic effects are completely avoided for both general public and occupational workers. Further, the limits are evaluated by cancer and genetic effects risk estimates so that the nuclear industry can be declared as "safe". 20 mSv per year, averaged over 5 years and not exceeding 50 mSv in any given year is the occupational limit as suggested by ICRP-60. The limits for general public are about 1/20th compared to the nuclear industry workers. Further information: Radiobiology. Eric J. Hall ICRP-60 http://www.icrp.org/
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