Circuit surfaces

After charging, the decay of the open-circuit surface potential is measured. From these measurements, important information can be extracted. In the past few decades, the xerographic probe technique has become a very popular and unique means to characterize electronic gap states. In particular, a map of states near mid-gap is determined by a time-resolved analysis of the xerographic surface potential. 

The deposition of fission products on primary circuit surfaces and, in particular, on the reformer tube walls causes difficulties during maintenance and catalyst refilling procedures, if the activity is intolerably high. Cesium and silver isotopes released during reactor operation are of major concern. Particularly silver diffuses easily out of the fuel elements at operating temperature conditions into the coolant and migrates easily into metal surfaces and is difficult to remove in decontamination operations [32]. 

When the UME is moved close to an insulating surface, the current drops to a lower value Ij because the surface and the insulating sheath of the UME block transport of active species O. This effect is sometimes called negative feedback and is further enhanced by the fact that no reoxidation of R can occur at insulating parts of the surface. Approaching a conductive surface kept at an electrode potential where reoxidation of R is possible causes an opposite effect (positive feedback) and Ij is enhanced with a closer distance. Both possibilities are schematically depicted in Fig. 7.11. A similar effect may be observed with an unbiased (not kept at any specific potential, but instead at open circuit) surface. Because the large surface area is in contact with the solution containing a supply of O, the surface electrode potential is essentially controlled by the Nernst equation. At the potential established by the concentration of O, the reduced species R created at the UME will be reoxidized, whereas further O is reduced elsewhere on the surface. 

An example of recent EX>w Coming Corporation studies of this type has been given by Gentle and Baney. They describe a new concept in corrosion protection based on silica/silicone nanocomposite Aims. Thin films of a silsesquioxane material are applied to a substrate such as aluminum or a microelectronic circuit surface by spin or dip coating... 

Initial impurity content due to oxygen absorption on the circuit surface is 3.4 g/m (I circuit) and 2.2 g/m (II circuit), that is in a good agreement with the experimental data (1.4-2.4 g/m ). Total amount of oxygen absorbed is about 30 kg. 

The shear ratio is defined to be the ratio of the transient wall shear stress to the wall shear stress during steady-state irradiation, it is used to determine the amount of radioactive plate out that is lifted off from the circuit surfaces and entrained in the coolant circuit. 

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... 

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. 

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. 

The Cora code was developed in the US it is based on the principal aspects of corrosion product transport in a PWR plant and it permits a time-dependent calculation of the radionuclide activities and concentrations, of the corrosion product masses and the activity concentrations they contain, as well as of the radiation levels at the primary circuit surfaces. As can be seen from Fig. 4.41., where a schematic diagram of the improved code Cora-II is shown (Kang and Sejvar, 1985), this code uses 8 nodes for the activity transport and 2 additional nodes for the mass transport, taking into account the total masses inside and outside the neutron field. In each node the general mass balance is defined according to... 

As has been discussed above in detail, °Co and Co are usually the radionuclides that determine the radiation dose rates at the primary circuit surfaces. In isolated cases, however, other radionuclides, usually activation products of other primary circuit materials, may also be of significance. Fission products normally do not contribute to a significant extent to contamination buildup. 

Other full-system decontaminations (e. g. of the Steam Generating Heavy Water Reactor) have been performed using the Rjrco reagent or the Lomi process with and without the NP pretreatment step. The results of these operations were satisfactory as regards decontamination factors and materials compatibility at the circuit surfaces, as well as at the fuel assemblies. 

In general, it can be assumed that the reaction between silver and iodine species in the gas phase, as well as the reaction of iodine vapor with silver aerosol or with silver deposited on the primary circuit surfaces, is only of minor significance for iodine behavior in the course of a severe accident. The main reasons are the rather short residence time of the silver aerosols in the gas phase, the fact that iodine and silver volatilization from the reactor core may differ considerably over time and, finally, the small proportion of elemental I2 and of HI (compared with the Csl fraction) assumed to be present in the gas phase during transport through the primary circuit. In contrast, Agl formation is expected to proceed to a significant extent later on in the containment sump water (see Section 7.3.3.3.3.). 

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