Corrosion shutdown spiking

In evaluating fuel rod failures from the presence of these fission products in the primary coolant, it has to be considered that some of them are also formed by neutron activation of metallic core materials, e. g. Zr/ Nb, Mo/ Tc, Sb. Determining which of these two production mechanisms is responsible for the coolant activity of the isotopes just mentioned is often difficult. In some cases, the shutdown spiking can be used to identify their origin from failed fuel rods however, in most cases this effect is not unequivocal since the radioactive corrosion products also show such a spiking (see Section 4.4.). 

In the determination of the mass of the deposited corrosion products there is still the question of whether or not the shutdown corrosion product spiking (see... 

Some observations made at operating PWR plants have suggested the idea of influencing the primary circuit dose rates by appropriate modalities of the reactor shutdown procedures, e. g. in the course of refuelling, either to achieve minimum corrosion product redistribution during the shutdown spiking or to achieve a certain decontamination of the out-of-core surfaces. In this context, different procedures carried out at PWR plants have been reported however, when evaluating the results obtained one has to bear in mind that in some of the reported procedures it is not possible to correlate cause and effect properly therefore, the possibility of pure coincidence cannot always be ruled out. 

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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