fission products radioactivity and their effects
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FISSION PRODUCTS RADIOACTIVITY AND THEIR EFFECTSK. Periasamy, M.Tech (Chem. Engg), Chennai 96. &&&&& Introduction The radioactivity of individual fission products and its effect on human being is a function of yield%, half life, the type of radioactive decay and its energy. When the yield % is high the quantity of that particular fission product is high and hence the radioactivity can be high. When the half life is high, the decay rate is slow. Hence, the radioactivity will be less for a given quantity of that particular fission product. The effect of various radioactivity types depend upon whether they are internal or external. For the given type of radioactivity, for the given type of exposure ( internal or external ), higher the energy level, the more will be the damage. Types of Radiation There are three types of radiation encountered in the Nuclear Fuel Cycle which consists of Uranium Mining, Ore Processing, Fuel Fabrication, Nuclear Reactor Operation, Spent Fuel Storage, Fuel Reprocessing and Waste Management. They are Alpha, Beta and Gamma. Alpha radiation is the least penetrating and it can be stopped by just a piece of paper or a few feet of Air. It is naturally emitted by large radioactive atoms like Pu 234, U 233, Ra 223, and Transuranics like Np 237, Am 241, U 235, Cm 243,
etc. The Alpha particle ionizes the medium through which it travels by knocking off the outer most electrons of the constituent Atoms of the medium. It deposits some energy when it is intercepted by any medium, including Air. Beta radiation can penetrate through few meters of air. But it does not deposit much energy when it is attenuated (i.e. stopped). Hence it is safer than alpha radiation. This also causes ionization but about 100 times lesser compared to Alpha. Both Alpha and Beta radiation are harmful only if the radioactive substances emitting these radiations enter our body and stay for long periods. This stay within the body
is measured in terms of another half life called Biological half life. For Cs137, the most problematic element, it is 70 days only. That is, even if the Cs137 enters our body through milk, meat, vegetables, etc. almost all Cs137 would have been biologically removed from our body by way of stool, urine, perspiration, respiration, etc. within about one year. Similarly Iodine which can get accumulated in Thyroid has biological half life of just 11 days for infants and 80 days for adults. Gamma Radiation is more intense than Alpha and Beta. Hence, they are able to not only knock off the outer most electrons, but are able to penetrate to the inner electron shells and further into the nucleus of the atom itself. During all these three stages they cause ionization by different mechanisms called, Compton Scattering, Photoelectric effect and Pair Production. In effect, when radiation interacts with matter, in particular living tissue, the main effect is ionization, by which process electrons of constituent atoms of the tissue are knocked off. radiation. Damage to DNA There has been a widespread belief that all radiation will cause human Cell damage, all Cell damages cause cancer and all cancers lead to genetic modifications ! This cancer issue needs to be understood in its right perspective. The human Cell consists of two types Somatic Cells and Germ Cells. Germ Cells constitute the Sperm and Ova. All other parts of the body are made of Somatic Cells. Somatic Cells contain 46 Chromosomes and Germ Cells contain 23 chromosomes. The combination of the 23 each in Sperm and Ova make up the 46 chromosomes required for the Somatic Cells. The chromosomes in turn are composed of linear sequence of genes. Mamalian Cells contain 60,000 to 70,000 genes. These are made up of DeoxyriboNucleic Acid ( DNA ) molecules. Each DNA molecule contains about 10 million atoms. It is made up of 2 molecular chains that are entwined around each other as a Double Helix as shown in the Figure. These two chains are interlinked by Hydrogen Bonds. This process of ionization is the cause for biological damage due to
Suppose a Cell in human body is exposed to radiation. One or more atoms are ionized as per the type and energy intensity of radiation. The ionizations can cause 1) Damage to the Hydrogen Bonds 2) Damage to one chain ( Single Strand Break) 3) Damage to both chains ( Double Strand Break ).
In many cases the Cells are able to repair these damages by themselves. Sometimes it is left unrepaired. Alpha particles attack the chains directly and damage them. Hence are more dangerous. This can be seen from the figure given below.
Even with low Dose, the Cell Surviving fraction drastically falls for Alpha. But the other two radiations cause damage by indirect means through generation of radicals and the Cell Surviving fraction does not fall even at high Doses. The effect of radiation on human body is measured in terms of equivalent dose. Alpha radiation has 20 times higher effect on human body than that of X-Ray, Gamma Ray, and Beta Radiation. This is an important aspect to take cognizance of. Among the three types of radioactive substances, we have the least possibility for Alpha emitting particles coming into public domain, even under accident conditions. These DNAs left unrepaired, especially the damaged chains can combine in uncommon positions leading to new type of Cells. The damaged Cells can multiply in the body with damaged information. This kind of proliferation of damaged Cells which do not behave like normal Cells is the root cause of cancer.
If the DNA of Germ Cells gets damaged and is not repaired in time, and if they happen to meet the Sperm or Ova as the case may me, then there could be offsprings carrying the Cells containing this damaged information. It need not always be deformed limbs, insane kids, double headed babies, etc as projected by common media. This is the essence of radiation effect on human Cells. As explained earlier, Alpha radiation is the most dangerous in terms of cancer, as they break the main chain. Also the Alpha emitting atoms being larger they will be sitting at one place and bombarding the same cells again and again unlike in case of Cs-137 which is mobile within the body. But, fortunately, the atoms which emit Alpha particles ( U-235, Pu-239, Am-241, Np-237,etc) seldom come in picture even under accident situations due to their very low volatility. Even if they come out of the reactor, it will be in the form of heavy particles and it will fall close to the reactors. Hence, there is very little possibility for it to be spread wider. Most of the Cells which are damaged are automatically repaired by the self repair mechanism like in any other self defense mechanism available in the body for various diseases. We shall understand that, for cancer to manifest, several coincident probabilities are required like 1) The radiation particles / atoms shall attack the sensitive organs 2) Sustained presence and attack of particular Cells 3) Exception from natural death of attacked Cells 4) Exception from death of Cells by excessive radiation itself 5) Possibility of damaged Cells joining at wrong locations 6) Possibility of the wrongly formed Cells not being attacked by the radiation again, etc. So, it is not that as and when there is radiation, there will have to be cancer ! As and when there is cancer, there has to be deformed baby births ! Hence, there is no surprise that the radiation effect on human beings is much less compared to the predictions made by experts who simply keep arguing that every radiation causes cancer, every radiation kills people, there is no threshold lower limits for any effect on human beings for radiation, etc. They do not want to understand these basic concepts.
This has been proved beyond doubt in Hiroshima, Nagasaki and Chernobyl. We have data for almost 60 years now. Even in cancer treatment by radiation, we are exposing the good Cells adjacent to the cancer affected Cells to very high doses. We dont see any cancer development in those Cells. Those Cells simply die in most cases. Similar thing happens in other radiation exposures also. OK, but why there is so much fear for radiation if it is not all that dangerous ? History of Nuclear Radiation Fear There has been a gross misunderstanding over the years on Nuclear power leading to some sort of fear psychosis. This misunderstanding has been generated and sustained by the ill informed and half truth media hypes created over several decades. Since historically Nuclear Energy has always been linked to defense applications apart from power, Nuclear energy has always been kept under the wrap of Government secrecy throughout the world. Even when the media was writing many absurd stories on nuclear energy, the scientists and officials associated with Nuclear energy were asked not to react. With the result, media writings gained more credibility in the minds of general public, especially the educated lot who are not directly associated with Nuclear Energy. There is another dimension to the whole issue. Since the consequences of radioactive contamination / accidents are higher compared to the conventional chemicals / pollutants / accidents, etc, and it is invisible, the nuclear scientists have always kept the safety on top priority. For every new employee who enters the nuclear industry, the first training they undergo is on the dangers of radioactivity. In fact, they are intoxicated with a fear for radiation so that they do not take the safety issue as a casual one as is normally done in other industries. Sometimes exaggerated phrases are used during the training to sensitize the workers on the importance of sticking to the procedures in their duties. These phrases are transferred verbatim to the media by the lower and middle level employees in their enthusiasm to display the importance / critical nature of their work. The media in turn t