Contamination buildup

At the point when a contaminant buildup plateau has been reached, the postflushing stage can begin. 

Random-contamination buildup —> Flush column with strong solvent clean up sample use HPLC grade solvent. 

Regarding compliance to GMP, the main issue is related to avoid trace-contaminant buildup in the recycle loop. In fact, this risk is rather limited, as the makeup flow rate for replacing the fluid consumption is generally higher... 

In order to minimize contamination buildup in the plant primary system, addition of zinc to the reactor water has been proposed. Laboratory investigations have... 

It can be assumed that the radioactive cesium isotopes are present in the coolant as Cs" ions, but as yet there is no direct experimental proof of this assumption. Very low cesium activities which are occasionally detected in filtered suspended corrosion products are probably attached by occlusion or by adsorption at the surfaces of the solids. Since cesium is not able to form insoluble compounds in the primary coolant, it does not participate noticeably in the contamination buildup on the primary circuit surfaces. 

The radionuclides of principal significance in contamination buildup, their origins and their production reactions are summarized in Table 4.2. These radionuclides are all gamma emitters with a comparatively long halflife. In virtually all plants, the radiation dose rates in the area surrounding the circuits and components are predominantly due to °Co, because of the high energies of its y quants of 1.17 and 1.33 MeV and the transition probability of both of them of 100%. Besides Co, additional activation products of other constituents of austenitic steels and nickel-based alloys, in particular Co, have to be mentioned. Only in special cases... 

At BWR plants, the situation with regard to contamination buildup is quite similar insofar as the development of dose rates over time differs considerably from one plant to the other. As an example, dose rate levels at the recirculation lines of a number of plants are shown in Fig. 4.21. (EPRI, 1983) after comparable periods of operation differences by factors of 5 and more can be observed. Saturation levels of the radiation dose rates such as reported from PWR plants cannot be derived from this data. 

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. 

It has to be pointed out that due to the great interest in this topic, the Uterature dealing with contamination buildup in light water reactor plants is very extensive for this reason, only a selection from this literature is given below (in particular of papers published in recent years). 

Analytical methods applied in contamination buildup studies... 

Analytical determination of the chemical as well as the radiochentical composition of the different substances that play a role in the production as well as in the transport and deposition of the radionuclides of interest, is a very important tool for the elucidation of the mechanisms leading to contamination buildup. For this reason, a short survey will be given in the following of the available anal dical methods with particular emphasis on those that can be used with the coolants and components of the primary systems of nuclear power plants. 

Dose rate measurements yield information about the level of gross y radiation but without giving details as to the specific contributions of the individual radionuclides. They are, therefore, not able to give useful information on the mechanism of contamination buildup. This disadvantage can be overcome by application of non-invasive y-spectrometry measurement using a transportable, shielded, high-... 

The different analytical techniques mentioned above are able to yield answers to specific questions which are of importance for clarification of the individual stages that play an essential role in the process of contamination buildup. There-... 

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 ... 

Production and transport of corrosion products are the main reasons for contamination buildup in the PWR primary circuit. Basically, in a PWR the generation of radionuclides from elements present in the primary circuit materials can proceed according to two different mechanisms, as is schematically shown in Fig. 4.26. 

Pathway 2 The second possible source of radionuclides are the materials which are permanently located inside the neutron field. During operation of the plant, they become highly activated from their surfaces metal atoms are also released to the coolant by corrosion or wear and are transported to the primary circuit where they contribute to contamination buildup. 

There is a clear distinction to be made as to the relative importance of both types of potential sources (in-core and out-of-core) for contamination buildup. While the in-core materials generally contribute only a small proportion to the total element input of corrosion products to the coolant (compared to the out-of-core surfaces), they could prove to be the predominant source of radioactive nuclides, due to the high specific activities of their corrosion products, resulting from their long-term exposure to the neutron field. As will be shown in the following, this question has been and still is the subject of debate. 

As a very important topic in contamination buildup, the question is still open to what extent the data on corrosion product solubilities in the primary coolant are of importance for the behavior of trace amounts of cobalt. It seems to be still questionable whether cobalt ferrites as a well-defined compound with properties similar to the nickel ferrites can exist under PWR primary coolant conditions, whether cobalt atoms can be incorporated into a nickel ferrite lattice or whether traces of cobalt may be deposited onto the surfaces of the reactor core by adsorption on other, already deposited oxides. Such adsorption processes may occur even on the Zircaloy oxide films in the absence of any net deposition of corrosion products. Experimental investigations of the interaction of dissolved cobalt with heated Zircaloy surfaces (Lister et al., 1983) indicated that at low crud levels in the coolant cobalt deposition on surfaces is dominated by processes involving dissolved species, with adsorption/desorption processes being the responsible mechanisms. The extent of cobalt deposition is controlled by the type of oxide present on the Zircaloy surface thin black films of zirconium oxide will pick up less cobalt from the solution than thick white oxide films, even when the differences in the available surface areas of both types of oxides are taken into account. The deposition process seems to be little affected by the heat flux in the exposed metal. According to Thornton (1992), such adsorption-desorption exchange processes provide a pathway for radioactive species to be transported around the circuit even when the net movement of corrosion products is minimized this means that under such circumstances the processes of activity transport and of corrosion product transport may be decoupled. They may provide a pathway for target nuclides such as Co to be adsorbed onto fuel rod surfaces even in a core which is virtually free of deposited corrosion product particles. 

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