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Fission products are the residues of Fission processes. __NOTOC__ PHYSICAL PROCESS OF NUCLEAR FISSION The sum of the Atomic Weight of the two atoms produced by the fission of one Atom is always less than the Atomic Weight of the original atom. This is because some of the mass is lost as free Neutrons and large amounts of Energy . MASS VS. YIELD CURVE If a graph of the Mass or Mole yield of fission products against the atomic mass of the fragments is drawn then it has two peaks, one in the area Strontium through to Palladium and one at Iodine through to Neodymium . This is due to the fact that the fission event causes the nucleus to split in an asymmetric manner. {Link without Title} Yield vs. Z - This is a typical distribution for the fission of uranium. Please note in the calculations used to make this graph the activation of fission products was ignored and the fission was assumed to occur in a single moment rather than a length of time. In this bar chart results are shown for different cooling times (time after fission). Because of the stability of nuclei with even numbers of Protons and/or neutrons the curve of yield against element is not a smooth curve. It tends to alternate. In general the higher the energy of the state that undergoes nuclear fission the more likely a symmetric fission is, hence as the neutron energy increases and/or the Mass Defect energy of the Fissile atom increases the valley between the two peaks becomes more shallow, for instance the curve of yield against mass for Pu-239 has a more shallow valley than that observed for U-235 when the neutrons are Thermal Neutron s. The curves for the fission of the later Actinides tend to make even more shallow valleys. In extreme cases such as 259Fm only one peak is seen. FISSION PRODUCTS IN POWER REACTORS Radioactivity of coolant In a Nuclear Reactor , the buildup of fission products as Reaction Poisons in the Fuel eventually leads to loss of efficiency, and in some cases to instability. They contribute most of the short and medium term radioactivity of high-level Nuclear Waste produced from spent reactor fuel. Depending on the quality of the fuel cladding, fission products can appear in the primary coolant. In a well-designed power reactor running under normal conditions, the radioactivity of the Coolant is very low. Light water reactors In the BWR reactors the bulk of the activity in the coolant is due to the activation of Oxygen to form O-19 and N-16. Because these are very shortlived radioisotopes, the simple erection of a shielding wall around the turbine is enough to protect the staff who anyway are excluded from the turbine hall while the reactor is in use. Gas cooled reactors If a fuel element leaks, then noble gas fission products will mix with the coolant. Some of these have half lives long enough to travel to the steam generators or the gas driven turbines (depending on the reactor setup) and there decay into solid elements like isotopes of Cesium (such as 137Cs) and strontium (such as 90Sr). Isotope signature of an atomic bomb vs. that of used reactor fuel The examination of the 134Cs/137Cs ratio is an easy method of distinguishing between fallout from a bomb and the fission products from a power reactor. Almost no Cs-134 is formed by nuclear fission (because Xenon -134 is stable). The 134Cs is formed by the Neutron Activation of the stable 133Cs which is formed by the decay of isotopes in the Isobar (A = 133). so in a momentary criticality by the time that the Neutron flux becomes zero too little time will have passed for any 133Cs to be present. While in a power reactor plenty of time exists for the decay of the isotopes in the Isobar to form 133Cs, the 133Cs thus formed can then be activated to form 134Cs only if the time between the start and the end of the criticality is long. THE CHEMICAL NATURE OF THE FISSION PRODUCTS For fission of Uranium-235 the most common radioactive fission products include isotopes of Iodine , Caesium , Strontium , Xenon and Barium . Many of the fission products decay through very shortlived isotopes to form stable isotopes, but also a considerable number of the radioisotopes have Half Lives longer than a day. Please see Fission Products (by Element) for a discussion of the main fission products. Some fission products are useful as Beta and Gamma sources in Medicine and Industry , see Common Beta Emitters and Commonly Used Gamma Emitting Isotopes for more details. Few fission products are Alpha Particle emitters, but a few Lanthanide Isotope s which are fission products can decay through alpha emission. The radioactivity in the fission product mixture is mostly short lived isotopes such as I-131 and 140Ba, after about four months 141Ce, 95Zr/95Nb and 89Sr take the largest share, while after about two or three years the largest share is taken by 144Ce/144Pr, 106Ru/106Rh and 147Pm. While 90Sr and 137Cs are the main long lived radioisotopes. COUNTERMEASURES AGAINST THE WORST FISSION PRODUCTS FOUND IN ACCIDENT FALLOUT The mixture of Radioactive fission products found in the Fallout from a Nuclear Bomb are very different in nature to those found in spent power Reactor fuel. This is because the reactor fuel will have had more Time for the short lived isotopes to decay. Iodine At least three isotopes of iodine are important. 129I, 131I and 132I. An overview of iodine exposure in the USA (resulting from bomb tests) can be seen at {Link without Title} A counter measure against the shortlived iodine isotopes (such as 131I), is to take Potassium Iodide by mouth. This overloads the body with iodide, and so decreases the amount of activity absorbed by the Thyroid . Unfortunatly, the dose of potassium iodide required will make most people feel sick and possibly vomit. These are symptoms of radiation overexposure, and can make the case more confusing. A rule of thumb discovered at Chernobyl is that if within 15 minutes of being exposed to radiation you feel sick or Vomit , then the dose you have got will do you serious harm. An alternative to potassium iodide for iodine sensitive persons is Sodium Perchlorate . Radioiodine is a particularly dangerous radionuclide because the body concentrates it in the thyroid gland. Potassium iodide cannot protect against other causes of Radiation Poisoning , however, nor can it provide any degree of protection against a Dirty Bomb unless the bomb happens to contain a significant amount of Radioactive Iodine . The purpose of radiological emergency preparedness is to protect people from the effects of radiation exposure after an accident at a Nuclear Power Plant . Evacuation is the most effective protective measure in the event of a radiological emergency because it protects the whole body (including the thyroid gland and other organs) from all radionuclides and all exposure pathways. However, in situations when evacuation is not feasible and in-place sheltering is substituted as an effective protective action, administering potassium iodide reduces the effects of radio iodine by 99%, and is a prudent, inexpensive supplement to sheltering. Potassium iodide protects the thyroid gland against internal uptake of radioiodines that may be released in the event of a nuclear reactor accident or from fallout. When potassium iodide is ingested, it is taken up by the thyroid gland. In the proper dosage, and taken at the appropriate time, it will effectively saturate the thyroid gland in such a way that inhaled or ingested radioactive iodines will not be accumulated in the thyroid gland. The risk of thyroid effects is reduced. Such thyroid effects resulting from radioiodine uptakes due to inhalation or ingestion, or both, could result in acute, chronic, and delayed effects. Acute effects from high doses include thyroiditis, while chronic and delayed effects include hypothyroidism, thyroid nodules, and thyroid cancer. The FDA has approved potassium iodide as an over-the-counter Medication . As with any medication, individuals should check with their doctor or pharmacist before using it. A low-cost alternative to commercially available iodine pills is a Saturated Solution of potassium iodide. It usually possible to obtain several thousand doses for prices near US$ 0.01/dose. Long term storage of KI is normally in the form of reagent grade crystals, which are convenient and available commercially. The purity is superior to "pharmacologic grade". Its concentration depends only on temperature, which is easy to determine, and the required dose is easily administered by measuring the required volume of the liquid. At room temperature, the U.S. standard adult radiological protective dose of 130mg is four drops of a saturated solution. A baby's dose is 65mg, or two drops. It's normally administered in a ball of bread, because it tastes incredibly bad. Use is contraindicated in individual known to be allergic to iodine, for such persons sodium perchlorate is one alternative. (see chap 13, Kearney). #Cresson Kearny, Nuclear War Survival Skills, available on line at Oregon Institute of Science and Medicine , created with the permission of the author. The information on KI is near the end of chapter 13. This manual has proven technical info on expedient fallout shelters, and assorted shelter system needs that can be created from common household items. OISM also offers free downloads of other civil defense and shelter information as well. Cesium In Livestock farming an important countermeasure against 137Cs is to feed to animals a little Prussian Blue . This Iron Potassium Cyanide compound acts as a Ion-exchanger . The cyanide is so tightly bonded to the iron that it is safe for a human to eat several grams of prussian blue per day. The prussian blue reduces the Biological Half Life (different than the Nuclear Half Life ) of the cesium. The physical or nuclear half life of 137Cs is about 30 years. This is a constant which can not be changed but the biological half life is not a constant. It will change according to the nature and habits of the organism it is expressed for. Cesium in humans normally has a biological half life of between one and four months. An added advantage of the prussian blue is that the cesium which is stripped from the animal in the droppings is in a form which is not available to plants. Hence it prevents the cesium from being recycled. The form of prussian blue required for the treatment of humans or animals is a special grade. Attempts to use the Pigment grade used in Paint s have not been successful. 137Cs is an isotope which is of long term concern as it remains in the top layers of soil. Plants with shallow root systems tend to absorb it for many years. Hence Grass and Mushroom s can carry a considerable amount of 137Cs which can be transferred to humans through the Food Chain . One of the best countermeasures in dairy farming against 137Cs is to mix up the soil by deeply Ploughing the soil. This has the effect of putting the 137Cs out of reach of the shallow Root s of the grass, hence the level of radioactivity in the grass will be lowered. Also after a nuclear war or serious accident the removal of top few cm of Soil and its burial in a shallow trench will reduce the long term gamma dose to Human s due to 137Cs as the gamma Photon s will be attenuated by their passage through the Soil . The more remote the trench is from humans and the more deep the trench is the better the degree of protection which will be afforded to the human population. More details about the cesium release from the and Iodine than the Cerium and Plutonium were released. Strontium Also by the addition of Lime to soils which are poor in Calcium the uptake of Strontium by plants can be reduced, likewise in areas where the soil is low in Potassium , the addition of a potassium Fertiliser can discourage the uptake of cesium into plants. However such treatments with either Lime or Potash should not be undertaken lightly as they can alter the soil chemistry greatly so resulting in a change in the plant Ecology of the land. FISSION PRODUCTS WITHIN THE BACK END OF THE NUCLEAR FUEL CYCLE Cesium It is known that the isotope responsible for the majority of the External gamma exposure in Fuel Reprocessing plants (and the Chernobyl site in 2005) is Cs-137 . 137Cs does appear to be an indicator of nuclear fission, as it is only formed by nuclear fission of an actinide. 137Cs is often removed from waste waters in the nuclear industry by means of solid Ion Exchange rs. A range of Zeolite s can be used for this task. In nuclear reactors both 137Cs and 90Sr are found in locations remote from the Fuel , this is because these isotopes are formed by the beta decay of noble gases (xenon-137 {halflife of 3.8 minutes}and krypton-90 {halflife 32 seconds}) which enable these isotopes to be deposited in locations remote from the fuel (eg on control rods and in the space inside a Fuel Pin between the fuel and the cladding) Iodine 133I decays by Beta Particle decay (with a Half Life of 20.8 hours) to 133Xe which in turn decays by Beta decay (with a half life of 5.2 days) to 133Cs. The isotopes which decay to 133I are very short lived. 129I is very long lived and this is one of the major radioactive elements which enter the sea from reprocessing plants. Fission products which form anions Some fission products are very long lived, examples of these include Iodine -129 and Technetium -99. Both of these are very mobile in solid/water as they form Anionic species (Iodide and 99TcO4-). Absorption of fission products on metal surfaces Tc It is interesting to note that in common with Chromate and Molybdate that 99TcO4- ion can react with steel surfaces to form a Corrosion resistant layer. In this way these metaloxo anions act as Anodic Corrosion Inhibitor s. The formation of 99TcO2 on steel surfaces is one effect which will retard the release of 99Tc from nuclear waste drums and nuclear equipment which has become lost prior to decontamination (eg Submarine reactors which have been lost at sea). This 99TcO2 layer renders the steel surface passive, it inhibits the Anodic Corrosion reaction. I In a similar way the release of iodine-131 in a serious power reactor accident could be retarded by absorption on University of Technology in Sweden .
A lot of other work on the iodine chemistry which would occur during a bad accident has been done. {Link without Title} {Link without Title} {Link without Title} REFERENCES Radioactivity, Ionizing Radiation and Nuclear Energy, by J. Hala and J.D. Navratil |
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