Fission product iodine circuit

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. 

Some of the iodine species (e. g. Csl) will condense on surfaces or will form aerosols in the reactor pressure vessel or during their residence time in the pipes and components of the primary system they can be transported together with the primary aerosols or deposited within the primary system, but in each case they may undergo further chemical reactions potentially leading to other, more volatile products. Other species such as elemental iodine h are transported in gaseous form inside the primary circuit after having reached the containment free volume, they may enter into a partition equilibrium between sump water and containment atmosphere. For this reason, it is of considerable interest to know the chemical forms of fission product iodine that can be generated within the primary circuit. 

Already the first measurements performed in the containment about one day after the onset of the accident showed that the major fraction of fission product iodine was plated out in the sump water, while only a very smaU fraction was airborne in the containment atmosphere. Taking the different voliunes of both phases into account, an integral iodine partition coefficient of about 2 1(F was calculated from these data (Pelletier, 1980). The pH of the siunp water was about 8.6 (due to sodium hydroxide solution which was automatically injected into the containment sump to improve iodine retention in the liquid phase) the value of the partition coefficient is consistent with the data obtained in the CSE experiments, when the lower pH of the sump water in these experiments is taken into account. This high value indicates that in the TMI-2 accident the bulk of the fission product iodine was released from the primary circuit to the containment in the form of an iodide compound and not as elemental I2. This assumption is consistent with the observation made later on that only about 1% of the iodine present in the sump water was in the form of iodate it is also consistent with the redox conditions in the reactor pressure vessel which were mentioned above For such an H2 H2O... 

Garland, J.A., Wells, A.C., Higham, E.J. Smith, I.C. (1984) Experimental study of the deposition of iodine and other fission products in the coolant circuit of a CAGR. In Fifth International Meeting on Thermal Nuclear Reactor Safety, compiled G. Bork H. Rininsland, vol. 3, pp. 1423-41. Karlsruhe Nuclear Research Center. 

The higher fuel temperatures under normal operating conditions compared with the electricity generating variant result in a higher fission product contamination of the primaiy circuit outside the active core, a factor of 3 for iodine and a factor of 4 for cesium. In the case of a depressurization through a large leak, it is the dust-borae cesium and... 

The coolant does not boil if die primary circuit loses its tightness, and has die property to retain iodine, as a rule, its radionuclides represent the major factor of radiation danger just after die accident as well as the other fission products (inert gases are exception) and actinides. This reduces sharply a scale of radiation consequences of that accident in comparison with pressurized water reactors. 

With regard to long-lived radionuclides that show a low decontamination factor on the purification system (e. g. Cs in PWR primary coolant containing LiOH), the purification constant e also has to include the coolant losses via water exchange or leakages. On the other hand, fission products which form insoluble compounds or can be adsorbed onto non-dissolved corrosion product particulate matter may be removed from the coolant by plate-out onto the primary circuit surfaces. These and other parameters which are liable to affect the activity concentrations of the radionuclides in the primary coolant are the reason why a trustworthy calculation of source strengths can only be made using specific radionuclides such as fission product noble gas isotopes and iodine isotopes. 

When failed fuel rods are present in the reactor core, fission product cesium isotopes will also appear in the primary coolant in significant activity concentrations. The high solubility of the cesium compounds deposited in the gap of the fuel rod facilitates the transport to the coolant which, however, is only possible via the liquid phase. This means that under constant-load operating conditions a significant cesium transport will only occur when such fuel rod failures are present in the core that allow a direct contact between fuel and liquid coolant in addition, the shutdown spiking results in a considerable cesium transport to the coolant with almost all types of fuel rod defects. The comparatively low cesium retention on the primary circuit purification resins which are saturated with LiOH occasionally leads to the buildup of activity concentrations of cesium isotopes in the coolant on the same order of magnitude as that of the iodine isotopes 1 and 1, even at comparatively low cesium source strengths or those which are not constant over time. 

Beslu, P., Leuthrot, C., Brissaud, A., Harrer, A., Beuken, G. Solid fission products and actinides release and deposition inside PWR s primary circuits. Proc. 5. Intemat. Meeting on Thermal Nuclear Reactor Safety, Karlsruhe 1984, (KfK-3880) p. 1387-1397 Bishop, W. N. Iodine spiking. Report EPRI NP-4595 (1986)... 

In most cases, the accumulated radionuclide content will consist of activated material such as Co, Co, Cr, Mn and Fe (depending upon the composition and corrosion rates of the material used in the primary circuit). Fission products such as isotopes of iodine, caesium and strontium may be important if failure of fuel cladding occurs. 

Figure 5 shows the elemental fission product density histories at the exit of the circuit into the containment predicted by VICTORIA. Silver, indium, and cadmium releases have several peaks, which correspond to bursts of control rod segments predicted by CORSOR. Most of the fission product densities tail off at about 7000 s, which corresponds to shutdown of the reactor. However, cesium and iodine densities remain relatively high due to revaporization within the circuit. 

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