Corrosion product radionuclides

Table 4.2. Important corrosion product radionuclides in light water reactors...

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... 

Table 4.9. Activity loadings of various corrosion product radionuclides on the inner surface...

In general, the results reported by different authors on the dependence of the deposition rates of corrosion product radionuclides on the nature of the pretreatment of the material surfaces do not show good agreement. Presumably, these discrepancies are due to differences in the details of the performance of the tests (e. g. pretreatment process, deposition testing) they reflect the fact that the essential details of the mechanisms of incorporation of radionuclides from the solution into the superficial oxide layers are not yet fully understood. On the other hand, the possibility cannot be ruled out that the nature of the superficial oxide layer plays only a minor role in the deposition of radionuclides from the solution. 

The concentrations of the corrosion product radionuclides in the reactor water depend on the same parameters as those that control the behavior of the total corrosion products and, in addition, on the intensity of neutron activation. As a consequence, the concentrations of radioisotopes may vary considerably from plant to plant and also within a plant. According to the observations reported by Anstine et al. (1984), the concentrations of Co and Co, the two most prevalent radioisotopes, do vary, but not as greatly as the iron concentrations. In the BWR plants examined in this context, the majority of Co and Co in the reactor water appeared in the dissolved state. On the average, the concentrations of dissolved Co increased for the first 3 to 5 fuel cycles and then leveled off to values on the order of 7 kBq/1, whereas the dissolved Co already reached its steady-state level in the same range of concentrations during the first fuel cycle. These differences in the time behavior of both cobalt isotopes probably are to be attributed to their different halflives. The concentrations of particulate Co and Co, on the other... 

Usually, the concentrations of the other corrosion product radionuclides such as Mn, Fe and Zn in the BWR reactor water are typically below those of Co. The high Cr concentrations (usually higher by a factor of more than 10... 

The carry-over of corrosion product radionuclides with the main steam in the direction of the turbine is effected, on the one hand, by droplet entrainment with the residual moisture content of the steam and, on the other, by steam volatility. Usually, droplet carry-over is the most significant transport mechanism however, the oxides of the primary system metals show a measurable solubility in steam even at BWR operating conditions. At different plants, concentrations of dissolved cobalt on the order of 60 ng/kg were measured in condensed samples of main steam, i. e. significantly higher than could be explained by droplet entrainment (e. g. Hepp et al., 1986). These observations are consistent with the fundamental results on steam volatility of weakly dissociated compounds under BWR operating conditions which were reported by Styrikovich and Martynova (1963). Since only non-dissociated substances are volatile with steam, it has to be assumed that a fraction of the cobalt present as dissolved ions in the reactor water at ambient temperature is converted to non-dissociated oxide, hydroxide or ferrite at the plant operating temperature. 

Because of the highly complex mechanisms of the production of corrosion product radionuclides and of contamination buildup, it cannot be expected that a single measure will result in an effective reduction of radiation levels. Rather a whole series of measures is necessary, aiming primarily at a reduction of the production, transport and deposition of Co, but in the second in an analogous reduction of the other corrosion product radionuclides. As can be concluded from the results discussed in the preceding sections, the dominant parameters responsible for contamination buildup are the concentration of dissolved Co in the reactor water and the readiness of the out-of-core oxide layers to incorporate Co from the reactor water. As with the PWR, in the BWR there are two possible main sources of corrosion product radionuclides the cobalt content of the in-core materials and the cobalt content in the corrosion products that are temporarily deposited on the surfaces of the fuel rods. Since the relative significance of these two mechanisms has not yet been analyzed to the same extent as in the PWR, the different measures taken up now have aimed at reducing both possible sources. 

The changes in the ambient conditions that occur in the course of a shutdown of the plant facilitate the release of zinc from the iron oxide fuel deposits, resulting in a Zn spiking similar in magnitude to that of the other corrosion product radionuclides. 

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