Possible countermeasures against BWR contamination buildup

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 shift in the relative significance of the different sources also seems to affect the status of °Co in the reactor water while in older BWRs about 70% of the Co activity is attributable to crud and about 30% to ionic species, in more modem plants this ratio is reversed (Asakura et al., 1989). 

Occasionally, changes in the design of the plant effect more a reduction of personnel radiation exposure than of the contamination levels on the surfaces of the piping and components. As an example, the replacement of the external recirculation systems by internal axial pumps has to be mentioned from Fig. 4.18. the benefits of this measure can be clearly seen (Reitzner et al, 1993). 

Besides reduction of the cobalt inventory of the primary systems and minimization of the generation of corrosion products by appropriate coolant chemistry control, prevention or at least reduction of the transport of non-radioactive as well as of radioactive corrosion products is another effective countermeasure, independent of the predominant source of radionuclide production. The reactor water cleanup system in its usual design is not efficient enough for a significant reduction in the concentrations of non-dissolved and dissolved corrosion products in the 

Over the entire shutdown period, the reactor water cleanup system has to be operated at its maximum throughput in order to remove as much of the radionuclides as possible from the primary system and to minimize radionuclide redeposition on the out-of-core surfaces. 

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