Iodine isotopes deposition

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-... 

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. 

The radioactive tellurium isotopes decay to their isobaric iodine daughter products. This means that from each location where tellurium deposits have formed, radioactive iodine isotopes will be released to the steam flow. However, the magnitude of these iodine sources can be assumed to be small compared to the direct volatilization of iodine from the fuel. Thus, they can usually be ignored when evaluating iodine input into the containment, with the only exception possibly being the short-lived... 

The retention of radionuclides within the containment was little accounted for by the RSS, but ranges from little to very substantial because of agglomeration and deposition. This leads to a large over prediction of the iodine risk, but substantial agreement with RSS for some other isotopes. 

In Ref. [379], the isotopic carbon ( " C) atom distribution in the Ir (150-nm thick) layer, deposited on a SrTiOa substrate after a BEN step using C-methane, was investigated by elastic recoil detection (ERD) of a 170-MeV iodine atomic beam. The C atoms existed both at the substrate surface and near the Ir/SrTiOs interface. The fact that carbon diffuses into Ir to reach SrTiO is similar to that of Pt described in Section 12.1, although the diffusion rate was smaller for Ir. 

In other cases where the radioactive material is released, it can he deposited upon environmental surfaces or skin. It could also he inhaled or ingested. The radioactive material on skin and environmental surfaces can usually he washed away, but the close contact with skin may give high doses of radiation to the skin from those isotopes that emit alpha and beta radiation. When radioactive material is inhaled or ingested, it continues to emit radiation and gives the internal areas of the body exposure. If the radioactive material has a chemical affinity for a particular organ of the body, it may accumulate there and selectively irradiate that particular organ. Examples are radioactive iodine (accumulates in tlie thyroid), radioactive cesium (accumulates in the liver), or radioactive strontium (accumulates in bone). 

Figure 4.4. Release behavior of iodine and fission gas isotopes according to the diffusion and deposit models...

The radionuclides incorporated into the oxide layers, which lead to a radiation field in the surrounding area, are mainly the activated corrosion product nuclides, above all Co and Co. Out of the fission products present in the primary coolant during plant operation with failed fuel rods in the reactor core, iodine and cesium isotopes are not deposited into the surface oxide layers this reactor experience is consistent with the general chemical properties of these elements which do not allow the formation of insoluble compounds under the prevailing conditions (with the sole exception of Agl, see Section 4.3.3.1.2.). On the other hand, fission product elements that are able to form insoluble compounds (such as oxides, hydroxides or ferrites) in the primary coolant are incorporated almost quantitatively into the contamination layers (see Section 4.3.3.1.4.). However, because of the usually low concentrations of polyvalent fission products in the primary coolant, only in very rare cases will these radionculides make a measurable contribution to the total contamination level for this reason, they will not be treated in this context. 

III-39. In the case of noble gases, the fraction of the releases from the fuel that is discharged to the atmosphere is determined by the half-life of the isotope and the rate of depressurization of the reactor. For iodine and caesium nuclides, which are released in molecular form, deposition on reactor surfaces reduces the concentration in the coolant and hence the discharge to the atmosphere. It is necessary to take account of both deposition and subsequent desorption. Important factors determining the deposition and desorption rates are the variations of coolant flow rate and surface temperatures with time and the extent of mixing of coolant in the reactor. 

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