Fission product iodine primary coolant

Coolant. Most fission products (excluding primary krypton and xenon) and all actinides escaping the fuel are soluble in the molten salt and will remain in the molten salt at very high temperatures. Cesium and iodine remain in the salt. Fluoride salts were chosen for the liquid-fueled molten salt reactor, in part because actinides and fission products dissolve in the molten salt at very high temperatures. ... 

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)...

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. 

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-... 

According to experiments performed under appropriate conditions, about 1% of the fission product iodine present in the flashed primary coolant volume is transported with the steam phase, about half of it in form of aerosols (Aim and Dreyer, 1980). This figure agrees well with that reported by Morell et al. (1985) and Hellmann et al. (1991), obtained in flashing experiments from small-diameter pipes (see Section 6.2.2.). In the experiments of Aim and Dreyer (1980), which were carried out in an uncoated steel vessel, only particulate iodide and elemental h were detected in the atmosphere of the vessel. The rate of plate-out of iodine from the atmosphere was found to depend on the specific geometric conditions of the experimental setup. In a comparatively small vessel, deposition halftimes of 8.5 hours for h and of 0.9 hours for aerosol iodide were measured within the first two hours after blowdown these values increased to 9.9 and 5.7 hours, respectively, during the following 22 hours. From other experiments markedly different values have been reported (e. g. CSE tests, see Section 7.3.S.3.8.). The reasons for these differences are not only due to the dimensions and true surface areas present in the respective experimental facility, but also to various other parameters, such as initial concentrations, turbulences in the atmosphere, rate of water droplet plate-out and of steam condensation. 

Fission product iodine present in the pool water is assumed to show a more complicated behavior. The iodide species originally present in the primary coolant can be considered as non-volatile and will behave, therefore, in the same manner as, for example, Li+ and Cs+ however, it cannot be ruled out that I will be in part oxidized by air, forming volatile I2. Since the compartments in the annuli (and also in the nuclear auxiliary building) are not closed systems, but have a normal air circulation, the principles of calculation of I2 volatilization using an equilibrium partition coefficient cannot be applied. Assuming an instantaneous iodine partitioning according to the equilibrium values would result in a drastic overestimation of iodine release, since I2 transport from the liquid to the gas phase is controlled by kinetics. The main characteristic in this context is a boundary layer at the interface between the sump and the gas phase, which is saturated with iodine in accor-... 

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). 

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. 

The pH of the aqueous solution collected in the containment sump after completion of injection of containment spray and ECCS water, and all additives for reactivity control, fission product removal, or other purposes, should be maintained at a level sufficiently high to provide assurance that significant long-term iodine re-evolution does not occur. Long-term iodine retention is calculated on the basis of the expected long-term partition coefficient. Long-term iodine retention may be assumed only when the equilibrium sump solution pH, after mixing and dilution with the primary coolant and ECCS injection, is above 7 (Reference...). This pH value should be achieved by the onset of the spray recirculation mode. 

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a piestressed concrete containment, the Konvoi pressurized water reactor with a steel primary contaimnent, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. The report concludes that the key accident sequences for European plant designs are transient events and small loss-of-coolant accidents, loss of cooling during shutdown, and containment bypass sequences. The most important chemical and transport phenomena are found to be revaporisation of volatile radionuclides from the reactor coolant system, iodine chemistry, and release paths through the plant. Additional research is recommended on release of fission products from the fuel, release of fission products from the reactor coolant system, ehemistry of iodine, and transport of radionuclide through plants. 

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. 

The escape rate coefllcients of the iodine and noble gas isotopes identified in one PWR or BWR plant can be directly applied to other plants of the same type, provided that the essential conditions, for example, fuel rod linear heat ratings, are comparable or can be corrected for. By this means it became possible to evaluate the number of failed fuel rods in the core of an operating reactor on the basis of the fission product activity concentrations measured in the coolant early attempts in this area were reported by Schuster et al. (1977). Although these estimates were based only on empirical data, they permitted a rather trustworthy prediction, as can be seen from Fig. 4.6., where the predicted numbers of failed fuel rods are compared with those detected in the course of post-cycle examinations. These techniques have been considerably improved, on the basis of experimental results as well as of model development and calculations. Because of the great number of parameters influencing the escape of fission products from defective fuel rods, evaluation of the number and type of defects from the measured activity concentration of fission products in the primary coolant is a difficult task which can be performed reliably only by specialists with considerable experience in this field. Attempts were, therefore, imdertaken to develop computerized expert systems that could be applied routinely. Lewis et al. (1992) described the development of such a system by means of which information can be obtained on the number of defects and their... 

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 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. 

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