Iodine carry-over

Figure 4.15. Iodine carry-over vs. copper concentration in BWR feedwater...

Figure 4.16. Iodine carry-over as a function of H2/H2O mole ratio in steam...

When evaluating the experimental results it should not be overlooked that steam sampling from a saturated steam flow is generally susceptible to errors which are caused by a non-representative composition of the collected sample. In order to deal with this problem, one has to make provision for isokinetic sampling in order to preclude separation of suspended droplets from the steam phase. Moreover, one has to take into consideration that the walls of the system pipes are always covered by a liquid water film in which radionuclides may be present in higher concentrations than in the steam flow itself. Therefore, when evaluating results of measurements of iodine carry-over it is important to examine closely the experimental techniques which were applied for sampling. 

The iodic acid is heated in an oil-bath to about 150° C., and a measured volume of the gas mixture to be examined is passed through at a velocity of about 10 ml. per minute. The iodine liberated by the carbon monoxide is carried over by the gas stream and bubbles into the potassium iodide solution. After the measured volume of gas has been passed, pure air is drawn through for some minutes to ensure complete absorption of the iodine which remains in the connecting tubes. 

Conservation of momentum dictates that the H atom must carry over 99% of the energy released resulting in an initial velocity of 22kms (In fact the H atom velocity distribution is bimodal as the iodine atoms can also be formed in the first excited state). Wolfrum and coworkers [129, 130] have carried out a number of hot H atom studies for example the endothermic reactions... 

Impurities in the rare metals produced by the iodide process can generally be reduced to a few tens of parts per million or less, for each element, provided care is taken in the selection of materials of construction. This initial advantage over other processes arises from the fact that the rare metals are produced without direct contact with a crucible or other container. Elements with volatile iodides should clearly be avoided in locations where the temperature is appropriate for attack by iodine vapour. Similarly, the crude feed should be as free from such elements as possible. For example, the whole of the zirconium impurity in a crude titanium feed would be carried over into the product, and vice versa, since the iodide process is equally suitable for the impurity as for the rare metal being purified. A large fraction of the iron and aluminium would be transferred, but decontamination factors from other elements such as nickel, chromium, carbon silicon and nitrogen are usually of the order of 10 to 100. 

In those BWR plants in which all condensates from the turbine cycle are directed to the condensate polishing system, the equations mentioned above can be used with the modification that allowance must be made for the carry-over constant, which describes the loss of some iodine and other non-gaseous fission products from the coolant to the steam phase. The carry-over constant A is added to the term (X + 8), making it (X -I- 8 + A) A is calculated according to... 

Linden, E., Turner, D. J. Carry-over of volatile iodine species in some Swedish and American BWRs. Proc. 3. BNES Conf. Water Chemistry of Nuclear Reactor Systems, Bournemouth 1983, Vol. 1, p. 111-119... 

Because of its potential to form volatile species, the behavior of fission product iodine is of particular significance in this context. In Section 4.3.3. it was pointed out that both in the primary coolant and in the steam generator secondary-side water iodine is present as non-volatile iodide the measured carry-over rates to the main steam are identical with those of fission product cesium, indicating that carryover is exclusively effected by droplet transport (entrainment). 

In both accident scenarios I is the most important radionuclide because of its radiotoxicity and its potential volatility in comparison, the release of other radionuclides from the resulting sump due to droplet or aerosol carry-over can be ignored. The postulated identical release fractions of iodine and non-volatile radionuclides can only be explained by the assumed presence of iodine in the solution as iodide ion. However, such a high droplet entrainment seems to be very unlikely, since even from boiling water surfaces a significantly smaller fraction of droplets (on the order of 0.1% and less) is formed and transported over distances... 

The equilibrium partition state is not established instantaneously, not only because of its dependence on the disproportionation of HOI, but also because of the mechanisms of iodine transport in the liquid and gas phases. Under isothermal conditions, i. e. when both phases are at the same temperature, I2 molecules present in the water phase have to be transported by diffusion to the gas-liquid boundary layer from where they can pass over to the gas phase. When, however, the liquid phase shows a higher temperature than the atmosphere, diffusion transport will be supported by convection in the water phase. In the case of a boiling iodine solution the rate of I2 carry-over to the gas phase is greatly enhanced as a consequence of the vigorous convection within the solution. Usually, the time taken to reach the equilibrium state of iodine partitioning is not of significance for the situation inside... 

Although the analysis demonstrates that no fuel rods are damaged, and that there is therefore no release to the reactor coolant, a conservative analysis has been performed assuming 10 percent of the rods are damaged. Activity carried over to the secondary side because of primary-to-secondary leakage is available for release to the enviromnent viathe steam line safety valves or the power-operated relief valves. The significant radionuclide releases due to the locked rotor accident are the iodines, alkali metals and noble gases. 

Though iodimetric titration of iron (III), which involves the reaction of iron (III) with acidified iodide and titration of the liberated iodine with standard thiosulphate, has been recommended in British Pharmacopoeia, 1968, the most frequent [20] difficulties are encountered due to recurring end-points, as the reaction is very slow near the stoichiometric end-point. Extraction of the liberated iodine into chloroform or carbon tetrachloride has been recommended to push the reaction to completion [20]. However, such titrations involving two immiscible phases are always cumbersome and the endpoint is often carried over. 

Potassium iodide [7681-11-0] M 166.0, pK -8.56 (for HI). Crystd from distilled water (0.5mL/g) by filtering the near-boiling soln and cooling. To minimise oxidation to iodine, the crystn can be carried out under N2 and the salt is dried under vacuum over P2O5 at 70-100°. Before drying, the crystals can be washed with EtOH or with acetone followed by pet ether. Has also been recrystallised from water/ethanol. After 2 recrystns ACS/USP grade had Li and Sb at <0.02 and <0.01 ppm resp. 

Iodine is a very good electrophile for effecting intramolecular nucleophilic addition to alkenes, as exemplified by the iodolactonization reaction71 Reaction of iodine with carboxylic acids having carbon-carbon double bonds placed to permit intramolecular reaction results in formation of iodolactones. The reaction shows a preference for formation of five- over six-membered72 rings and is a stereospecific anti addition when carried out under basic conditions. 

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