Fission product iodine

The retention of fission product iodine and xenon by unirradiated and irradiated pyrolytic-carbon-coated (Th,U)C2 fuel particles has been studied in annealing experiments and has been compared with similar studies of the release (or retention) of barium and strontium. The objective was to study the effects of irradiation on the retention of the two types of fission products and to determine the mechanism of release which could account for the observed behaviors. In both unirradiated and irradiated particles, iodine and xenon were found to be retained highly by the impervious isotropic pyrolytic coating which was unaffected by the irradiation. In contrast, the fuel kernel which controls the release of the metallic species is damaged severely by the irradiation, resulting in a marked decrease in its ability to retain the metals. 

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. 

Of particular interest is the behavior of iodine. Fission product iodine exists in the salt in the reduced form— iodide —and is not volatile. After proper accounting for precursor transport, this behavior was confirmed in the irradiation tests and during reactor operation. An extensive chemical study showed that iodine can be removed from the salt only by extremely oxidizing conditions that promote iodide to elemental iodine,or by displacement with fluorine under oxidizing conditions. ... 

Figure 3.21. Distribution of fission product iodine, cesium and tritium in irradiated LWR fuel pellet...

Castleman, A. W.jr., Tang, I. N., Munkelwitz, H. R. The chemical states of fission product iodine emanating into high temperature aqueous environment. J. Inorg. Nucl. Chem. 30, 5-13 (1968)... 

Collins, J. L., Osborne, M. F., Lorenz, R. A., Malinauskas, A. P. Fission product iodine and cesium release behavior under light water reactor accident conditions. Nucl. Technology 81, 78-94 (1988)... 

During steady-state operation of the reactor and as long as there is no change in the number and type of fuel rod failures, the source strengths of the iodine isotopes and their activity ratios are constant over time. This observation suggests that the amount of fission product iodine deposited in the gap shows an equilib-... 

Figure 4.7. R B ratios of fission product iodine isotopes as a function of the decay constant in the case of the simultaneous presence of fuel rod defects and uranium contamination PWR primary coolant...

Figure 4.9. Fission product iodine activity concentrations in PWR primary coolant during reactor shutdown (P reactor load Pc coolant pressure coolant temperature Ca activity concentration)...

As was discussed in Section 4.3.1.1., the fission product noble gas isotopes "Kr, Kr, Kr, Xe and Xe (besides the fission product iodine isotopes) are valuable indicators for evaluating the number and type of fuel rod failures present in the reactor core. 

The level of the activity concentrations of the fission product iodine isotopes in the primary coolant depends on the number and the size of the fuel rod failures in the reactor core. The isotopes I and are always the main contributors to the total iodine activity, whereas the shorter-lived isotopes and are only... 

The lighter homologue of iodine in the Periodic System of the Elements, namely bromine, is also produced by nuclear fission. Due to the comparatively low fission yields of the relevant bromine isotopes and their short halflives, they are only present in the primary coolant in very low activity concentrations and are of no significance in practice. The only bromine isotope that should be mentioned here, Br, is a neutron emitter quite similar to the heavy fission product iodine isotopes. 

In the water—steam circuit, the fission product iodine which is carried by the main steam is distributed between different water and steam flows. A substantial fraction of it is plated out in the cyclone downstream of the high-pressure part of the turbine, and is transported back with the separated condensate to the feedwater storage tank. Similar washdown of iodine occurs at the other locations where condensates are separated from the remaining steam, so that only a small fraction reaches the main condenser. Here as well the major part is plated out to the main condensate water phase and is retained in the ion exchangers of the condensate polishing system the fraction of iodine which passes over to the condenser off-gas represents only a few percent of the amount originally carried by the main steam. 

The isotopic composition of fission product iodine present in the BWR reactor water in the case of failed fuel rods in the reactor core is quite similar to that in the PWR primary coolant. Since the iodine purification factor of the reactor water cleanup system is on the order of 100, i. e. virtually identical to that of the PWR primary coolant purification system, this similarity in isotopic composition demonstrates that the release mechanisms of iodine isotopes from the failed fuel rods to the water phase are virtually identical under both PWR and BWR operating conditions. On the other hand, the resulting chemical state of fission product iodine in the BWR reactor water is quite different from that in the PWR primary coolant. The BWR reactor water usually does not contain chemical additives (with the possible exception of a hydrogen addition, see below) as a result of water radioly-... 

In the case when defective fuel rods are present in the reactor core, the BWR reactor water contains the other fission products and the activation products released from the fuel in concentrations well below those of fission product iodine. This applies as well for fission product cesium, which is retained on the ion exchangers of the reactor water cleanup system with a decontamination factor of about 100. As far as it is known, cesium in the reactor water is present as the Cs ion, whereas large proportions of most of the polyvalent fission products and of the actinides are attached to the corrosion product particles suspended in the water as yet, there is no detailed knowledge on the chemical state of these elements (i. e., adsorbed to the surfaces or incorporated into the Fe203 lattice). It was reported that the strontium isotopes as well as Np appear in the reactor water in the dissolved cationic state, while Tc was found in the reactor water as a dissolved anionic species, most likely Tc04 (Lin and Holloway, 1972). According to James (1988), discrete fuel particles were not detected in the BWR reactor water. 

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