Iodine nuclides

C22-0005. Predict whether each of the following is stable or unstable. If you predict that it is unstable, give your reason (a) the nuclide with 94 protons and 150 neutrons (b) the iodine nuclide with 73 neutrons (c)... 

Fuel leakages are qualitatively indicated by a sudden increase in the concentrations (spiking) of fission products, especially noble gases and iodine nuclides. The appearance of less volatile nuclides in the coolant (e.g., transuranium nuclides) suggests the occurrence of open defects. 

Releases from open defects are characterised by a low concentration ratio of to assuming the relatively free leakage of aU iodine nuclides. 

These alkali and alkaline-earth iodides would presumably have low solubilities in Bi and, as a result, have a tendency to leave the U-Bi fuel and collect on unwetted solid surfaces. These iodides also transfer heavily to the salt in the FPS-removal proi ess, but the rate of processing would be too slow to extract significant quantities of P and, in fact, most of the other iodine nuclides. Thus there appear to lie two predominant modes by which I departs from the fuel physical expulsion in the form of iodides and radioactive decay. 

Half-lives span a very wide range (Table 17.5). Consider strontium-90, for which the half-life is 28 a. This nuclide is present in nuclear fallout, the fine dust that settles from clouds of airborne particles after the explosion of a nuclear bomb, and may also be present in the accidental release of radioactive materials into the air. Because it is chemically very similar to calcium, strontium may accompany that element through the environment and become incorporated into bones once there, it continues to emit radiation for many years. About 10 half-lives (for strontium-90, 280 a) must pass before the activity of a sample has fallen to 1/1000 of its initial value. Iodine-131, which was released in the accidental fire at the Chernobyl nuclear power plant, has a half-life of only 8.05 d, but it accumulates in the thyroid gland. Several cases of thyroid cancer have been linked to iodine-131 exposure from the accident. Plutonium-239 has a half-life of 24 ka (24000 years). Consequently, very long term storage facilities are required for plutonium waste, and land contaminated with plutonium cannot be inhabited again for thousands of years without expensive remediation efforts. 

The abundances of krypton and xenon are determined exclusively from nucleosynthesis theory. They can be interpolated from the abundances of neighboring elements based on the observation that abundances of odd-mass-number nuclides vary smoothly with increasing mass numbers (Suess and Urey, 1956). The regular behavior of the s-process also provides a constraint (see Chapter 3). In a mature -process, the relative abundances of the stable nuclides are governed by the inverse of their neutron-capture cross-sections. Isotopes with large cross-sections have low abundance because they are easily destroyed, while the abundances of those with small cross-sections build up. Thus, one can estimate the abundances of krypton and xenon from the abundances of. v-only isotopes of neighboring elements (selenium, bromine, rubidium and strontium for krypton tellurium, iodine, cesium, and barium for xenon). 

The results of the release measurements are given in Table I as a function of temperature and annealing time. For the unirradiated samples, typical release data for the metallic nuclides are included for comparison. The data clearly show that the releases of iodine and xenon have been unaffected by irradiation. The releases of barium and strontium, on the other hand, have been enhanced by a factor of about five at 8.5% FIMA and 10 at 24% FIMA. These data are interpreted in the... 

Numerous studies by other workers (I, 10) have shown that the releases of iodine and the noble-gas fission products from pyrolytic carbon-coated fuel particles are controlled by diffusion of these nuclides through grain boundaries, cracks, and defects in the isotropic pyrolytic carbon coating. When coatings are intact, however, the release of these fission product nuclides is low. However, the pyrolytic carbon coating constitutes only a delaying barrier to the metallic nuclides barium and strontium through which they diffuse with diffusion coefficients of the order of 10 9 cm.2/sec. (at — 1400°C.). The steady-state release of these metallic nuclides is controlled instead by diffusion out of the fuel kernel,... 

In Advanced Gas Cooled (AGR), Pressurised Water (PWR) and Boiling Water (BWR) reactors, and in the Russian RMBK, the fuel is U02. Experiments in the UK and USA, reviewed by Farmer Beattie (1976), showed less than 1% release of fission product iodine and caesium from punctured U02 fuel cans at about 1000°C in air or steam, rising to 10-50% release at 1800°C. At 2800°C, the U02 melted and there was nearly complete release of volatile nuclides (I, Te, Cs, Ru) but only small release of refractory alkaline earth and rare earth nuclides. 

Iodine is lost from herbage by the same processes which cause field loss of Sr, 137Cs and other nuclides (Section 2.13). There is also the possibility of revolatilisation of iodine. If XG is the rate constant of field loss (fraction of iodine per unit area of ground lost from vegetation per second) and X1 the rate constant of radioactive decay, the combined apparent or effective loss rate is XE = XG + Xt. The effective half-life is Te = 0.693/A . The use of the term half-life implies that field loss is exponential and TE invariant with time, which is not always true. 

In environmental waters, the most important oxidation states are iodide ( — 1) and iodate ( + 5). Most published methods for the analysis of radioiodine aim only to convert all species to one chemical form in order to determine a total concentration value for the particular nuclide of interest. However, some specialist methods designed for the analysis of the stable element such as that recently described by Woittiez et al. (1991) for the determination of iodide, iodate, total inorganic iodine and charcoal-absorbable (organic) iodine in seawater could presumably be adapted to provide information about the speciation of radioiodine as well. More difficult to adapt would be techniques such as polarography which have been useful in the measurement of the iodide/iodate system (e.g. Liss et al., 1973). 

If 50 MeV protons arc available, the (p, 5n) reaction with is the most favourable, because iodine has only one stable nuclide. However, measurable amounts of Xe are produced by the (p, 3n) reaction. The volatile products formed by irradiation of h, Lil, Nal, KI or CH2I2 arc transferred by He. In a first trap, cooled to -ITC, directly formed iodine and iodine compounds arc separated and in a second trap, cooled to -196°C, Xc and Xe are collected. The second trap is taken off and after about 5 h the iodine formed by decay of - Xe is dissolved in dilute NaOH. During this time - Xe has largely decayed, but Xe (t]/2 = 16.9 h) only to some extent. The yields are of the order of 100 to 500 MBq per pAh, and the relative activity of I is of the order of 0.1%. 

Antigens, haptens, and antibodies radiolabeled with or are commonly used as tracers in immunoassay. These nuclides can be introduced directly into functional groups normally present in proteins and other macromolecules or into suitable derivatives that can be synthesized by a variety of chemical procedures. The most widely used iodination methods have been direct chemical or enzymic substitution of hydrogen in tyrosine or related groups using chloramine-T or lactoperoxidase, respectively. These methods are described in separate chapters in this volume. 

The experiments have proved that membrane distillation can be applied for radioactive wastewater treatment. In one-stage installation the membrane retained all radionuclides and decontamination factors were higher than those obtained by other membrane methods. The distillate obtained in the process was pure water, which could be recycled or safely discharged into the environment. It seems the process can overcome various problems of evaporation such as corrosion, scaling, or foaming. There is no entrainment of droplets, which cause the contamination of condensate from thin-film evaporator. Operation at low evaporation temperature can decrease the volatility of some volatile nuclides present in the waste, such as tritium or some forms of iodine and ruthenium. The process is especially economic for the plants, which can utilize waste heat, e.g., plants operating in power and nuclear industry. 

Generally, the more unstable a nuclide is, the shorter its half-life is and the faster it decays. Figure 15 shows the radioactive decay of iodine-131, which is a very unstable isotope that has a short half-life. 

Example Consider the spontaneous fission of americium-244. Two of the daughter nuclides formed are iodine-134 and molybdenum-107. The reaction is ... 

Other examples of nuclides that decay by negative p-emission are carbon-14( C), iron-59 ( Fe), and iodine-131 ( 1). Negatively charged P-particles are smaller in mass and interact less with matter than p-p articles, easily penetrate paper and cardboard, but are absorbed by metal sheets. 

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