Corrosion product radionuclides coolant

The behavior of the corrosion product radionuclides in the primary coolant... 

Table 4.8. Activity concentrations of the main corrosion product radionuclides in PWR primary coolant (Bq/m ) (Average values, undisturbed constant-load operation)...

Primary coolant analyses show that a fraction of the corrosion product radionuclides is present in the dissolved state, while the remainder appears in the form... 

When corrosion products are deposited on the fuel surfaces, they are activated by neutron capture. Some of the most prominent of these activities are 55Fe, 63Ni, 60Co, 54Mn, 58Co, and 59Fe. These radionuclides will then be found in the reactor coolant. 

The primary motivation for predicting the electrochemical properties of the coolant circuits of water-cooled nuclear power reactors has been that of explaining and predicting tenacious operating problems that include SCC and CF, mass transport of corrosion products and subsequent fouling of heat transfer surfaces, activity transport due to the movement of neutron-activated radionuclides from the core to out-of-core surfaces that are not shielded, and, in the case of PWRs, the axial offset anomaly (AOA). This latter phenomenon results from the deposition of boron... 

The activation products of the coolant, with the sole exception of N, are not of substantial importance in plant operation in some cases, however, they have to be taken into consideration environmentally following release of off-gas or waste water. The fission products and the fuel activation products represent by far the greatest proportion of the radionuclide inventory in the reactor, from the viewpoint of radioactivity as well as from that of radiotoxicity. However, with the exception of severe accidents (which will be treated in Part C), during plant operation they are reliably confined within the fuel rods, so that only the very small amounts released from failed rods to the primary coolant are of interest in this context. Finally, the activated corrosion products are the origin of the buildup of radiation dose rates at the surfaces of the circuits, which potentially complicate the performance of operational procedures, in particular of inspection and repair work. 

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 appearance of non-volatile fission products or actinide isotopes in the coolant can indicate the presence of fuel rod defects with a direct contact between the fuel and liquid water. This can occur with large-sized defects, in particular in comparatively cold regions of the fuel rod at the vertical or horizontal periphery of the reactor core. However, any statement in this regard can only be based on radionuclides that are not present in the coolant as a remnant from preceding transients this means that in a PWR Cs or Cs are not appropriate indicators for such fuel rod failures. The requirements are in principle fulfilled by Np, which is a reliable indicator for defects with fuel-to-water contact, as are ruthenium and cerium isotopes, as well. However, because of the complex behavior of these radionuclides in the coolant (adsorption on suspended corrosion products and deposition on primary circuit surfaces), only qualitative assessments can be made, which means that a quantitative evaluation of the number of fuel rods showing... 

At the PWR primary coolant pH of 7 to 8, the fission product isotopes of the tri- and tetravalent elements show strong hydrolysis, resulting in very low solubilities. This macrochemical behavior is consistent with the observations made in coolant analyses that these radionuclides can be almost quantitatively isolated together with the suspended corrosion products by filtration. However, this behavior does not necessarily indicate the presence of particular oxides or hydroxides of these fission products, since due to their very low element concentrations in the coolant their solubility limits are probably not exceeded. Presumably, these element traces are attached to the corrosion product oxides either by adsorption onto their surfaces or by formation of mixed crystals. A significant fraction of the longer-lived tri- and tetravalent fission products, as well as of the actinides, is incorporated into the contamination layers which cover the primary circuit surfaces. However, because of the usually very low actiAuty concentrations of these radionuclides in the coolant and, consequently, in the contamination layers, their contribution to the contamination dose rates is negligible. 

Since both reactions are induced by fast neutrons having energies >10MeV, the total amount of Co released from the materials to the primary coolant of a 1300MWe PWR was calculated to be only on the order of 10" Bq per year (Sie-mens/KWU, unpublished). In this calculation, the essential radionuclide sources were assumed to be corrosion product deposits on the fuel rods as well as corrosion and/or wear from in-core materials. This low production rate raises the question of whether the y rays at 122 and 136 keV occasionally detected in waste water really belong to Co or whether they are caused by side-peaks originating from the decay of other radionuclides present in the samples. 

The interpretation of the results of experiments performed in recent years has yielded contradictory conclusions as to the sources and the mechanisms of contamination buildup. A th one exception, the measures taken on the basis of these results have not resulted in a clear success on the contrary, in some cases a deterioration of the situation has resulted. The question as to the reasons for such consequences emerges and it seems that the failure of many attempts has mainly been due to the fact that only macrochemical aspects (e. g. effect of pH and temperature on the solubility of the corrosion product oxides) have been taken into consideration. In reality, because of the very low mass concentrations of the essential radionuclides and their mother elements in the coolant, severe deviations in behavior from that of macroamounts are to be expected, an effect which is well known in radiochemistry. In particular, in the behavior of Co and Co trace-chemical mechanisms such as surface adsorption onto oxide particles, co-precipitation together with other elements, as well as ion exchange and isotope exchange with other constituents of the corrosion product oxides can be assumed to play an important role, but in most of the investigations performed up to now these have not been considered. 

In order to avoid the difllculties that can arise in obtaining a representative coolant sample, attempts have been made to develop non-invasive, real-time analysis procedures. Unfortunately, the possibilities of such techniques are limited. For example, chemical composition of the corrosion products, as well as radionuclides without appropriate y emission, are usually not accessible to these procedures. One possible application is the determination of the concentration of particles suspended in the coolant for this task, optical measuring devices have bron developed which are able to operate on-line even at 290 °C, 9 MPa and to count the number of particles with a size down to 1 pm (e. g. Schenker et al., 1992). 

There is no doubt that the generation of radionuclides from the corrosion product elements can only occur in the neutron field, i. e. inside the reactor pressure vessel (RPV). On the other hand, the radionuclides which cause the radiation fields which potentially complicate work during plant normal operation as well as during inspection and repair work are those deposited on the inner surfaces of the out-of-RPV primary circuit piping and components, regions they are transported to by the primary coolant. This means that contamination buildup in the PWR primary circuit is a complex process. It can be roughly divided up into three stages (see Fig. 4.26.), each of which raises its particular questions ... 

The investigations cited above (like others performed in this field) yielded results on the amounts of corrosion products formed under PWR operating conditions, but not on the amounts of the associated radionuclides. As was emphasized above, Co source must not be confused with the term of essential interest Co source . Moreover, it has to be pointed out that the conclusions drawn from such evaluations of corrosion rates are only fully valid if the deposition of corrosion products on the fuel rod surfaces and subsequent neutron activation there, i. e. mechanism 1 in Fig. 4.26., were the most important contributor to the production of Co carried in the coolant. If this assumption does not apply, as will be discussed below, then these arguments would be of less significance. In order to correlate the two figures Co supply to the coolant and Co supply to the coolant , the activation period and the neutron flux density to which the different potential sources are exposed also have to be taken into account. Such calculations are comparatively... 

On the other hand, the amount of radionuclides (in particular of longer-lived ones) produced in the corrosion products that are deposited on the surfaces of the fuel rods depends on the deposited mass of oxides, on their elemental composition, and on their residence time in the neutron field. Because of the great importance of the extent of radionuclide production in such deposits, detailed studies have been carried out to throw light on the mechanisms which control deposition. According to the results obtained, this process may be basically initiated by two different mechanisms by deposition of solid suspended matter (i. e. non-dissolved corrosion product oxides), on the one hand, and by precipitation of dissolved substances from the coolant on the other. 

The hydrazine chemistry regime employed in some VVER plants is reported to result in a reduced deposition of corrosion products on the fuel assembly surfaces and, consequently, in less production of radionuclides (Pashevich et al., 1992). Changing from KOH-NH3 chemistry to hydrazine chemistry in an operating plant is claimed to effect a peeling-off of deposited oxides from the core surfaces, with the consequence of temporarily enhanced corrosion product and activity levels in the primary coolant. However, because of the rather limited experience that has been had with this type of water chemistry so far, no detailed informations about its effects on the contamination levels in the primary circuit could be given. 

The second method for distinguishing between the two mechanisms responsible for contamination buildup is radiochemical and is based on the measured element specific activities of appropriate radionuclides such as °Co/Co, Ni/Ni, and Fe/Fe. These ratios can be determined in the corrosion products collected from the fuel rod surfaces as well as in those isolated from the primary coolant (see Fig. 4.25.). From these analytical results and from the neutron flux density known from the design of the reactor core, apparent residence times in the neutron field... 

During shutdown of a PWR plant and, somewhat less pronounced, also during its startup, a strong increase in the concentrations of corrosion products and the associated radionuclides in the primary coolant is observed, an effect which will be discussed in more detail in Section 4.4.3.3. This process causes a dissemination of the radionuclides over the entire primary circuit and, as a possible consequence, results in increased radiation dose rates in the area surrounding it. For this reason, the origin of these radionuclides is of interest, with their possible source being the resuspension of activated corrosion products previously deposited either inside or... 

To get a closer look at the mechanisms that lead to the partitioning of the corrosion product elements (and their radionuclides) between dissolved and particulate species, it seems advisable to give some short remarks on the general behavior of metal atoms which are released from metallic surfaces to a high-temperature coolant. 

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